5. Introduction to Anatomy and Physiology

A BRIEF SUMMARY OF MEDICAL HISTORY

Studies of illness and aging of the human body are major components in the field of medicine. The study of “modern” medicine began in the fifth century BC.

Hippocrates, who was born in approximately 460 BC on the island of Cos, Greece, is recognized as the “Father of Medicine.” After his death, all the existing writings on medicine were gathered into a work called the Hippocratic Collection and attributed to him whether he wrote them or not. The Hippocratic Oath, which is still in use today, is from the Collection. Hippocrates concluded that illness had rational explanations instead of being caused by evil spirits or disfavor of the gods. This freed medicine from supersti­tion and allowed for scientific study.

In 1249, Roger Bacon invented eyeglasses, bringing better vision to many. Leonardo da Vinci advanced the understanding of human anatomy by carefully dissecting corpses and making detailed anatomic drawings during the fifteenth century. William Harvey published An Anatomical Essay on the Motion of the Heart and Blood in Animals in 1628, detailing how blood was pumped from the heart throughout the body and then returned to the heart and recirculated. This work showed that food was not converted into blood by the liver and then consumed as fuel by the body, as was widely alleged at that time.

A Dutch cloth merchant, Antony van Leeuwenhoek dis­covered blood cells in 1670. This discovery was made pos­sible by his microscope. Although the microscope had been invented by Robert Hooke a few years earlier, van Leeuwen­hoek, by grinding his own glass lenses, greatly improved the microscope’s design and achieved magnifications of greater than 270 diameters. He also observed bacteria, yeast cells, spermatozoa, and protozoa. In addition, his microscope allowed capillaries to be observed, thus showing the link between arteries and veins and confirming Harvey’s theory of blood circulation.

Edward Jenner, an English microbiologist, is known as the “Father of Immunology.” He observed that people who had contracted cowpox seemed immune to the deadly smallpox. He theorized that deliberately infecting people with cowpox would protect them from smallpox. He tested his theory on a young boy in 1796 and then demonstrated that the lad was indeed immune to smallpox.

William Beaumont, while serving as an army post surgeon, treated a patient who had been blasted by a musket at close range. The resulting large wound affected part of his lung, two ribs, and his stomach. Beaumont treated the wounds but was unable to get the hole in the stomach to completely close; repeated bandaging was required to prevent food and drink from coming out. He quickly real­ized this was an opportunity to study the digestion process. He tied small pieces of food with silk string and dangled them through the hole in the patient’s stomach, removing the items at 1-hour intervals. He published his observations in 1833.

Medicine truly came of age during the second half of the nineteenth century. Louis Pasteur and Robert Koch estab­lished the germ theory of disease. Florence Nightingale showed the importance of hygiene and sanitation to reduce hospital infections. Sir Humphry Davy discovered the anes­thetic properties of nitrous oxide, and Joseph Lister pio­neered the use of carbolic acid as an antiseptic to clean wounds and surgical instruments. His antiseptic technique reduced deaths from infection after surgery from about 60% to under 4%. Then in 1895 a German scientist named Wilhelm Roentgen discovered the x-ray. Medicine has never been the same since.

The twentieth century continued to build on all these discoveries. The pharmaceutical industry mushroomed with the development of thousands of new medicines. Vaccines were developed to prevent polio, measles, mumps, and many other diseases. Cardiac pacemakers and defibrillators were invented. Medical imaging advanced with the develop­ment of computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), ultrasound, and many other techniques. Numerous advances occurred in surgery, including open-heart surgery and organ transplants. Radiation therapy and chemotherapy for the treatment of cancer were developed. Kidney dialysis machines were invented, and hospital intensive care units (ICUs) were established to better treat very sick patients. The list of medical advances goes on and on and is added to every year.

THE HUMAN BODY

The human body is an awesome masterpiece. Imagine bil­lions of microscopic parts, each with its own identity, working together in an organized manner for the benefit of the total being. The human body is more complex than the greatest computer, yet it is personal. The study of the human body is as old as history itself because people have always had an interest in how the body is put together, how it works, why it becomes defective (illness), and why it wears out (aging).

The study of the human body is essential for those plan­ning a career in health sciences, just as knowledge about automobiles is necessary for those planning to repair them. How can you fix an automobile if you do not know how it is put together or how it works? How can you help fix a human body if you do not know how it is put together or how it works?

Anatomy and Physiology

Human anatomy is the study of the shape and structure of the human body and its parts. It encompasses a wide range of study, including the development and microscopic orga­nization of structures, the relationship between structures, and the interrelationship between structure and function. Gross human anatomy deals with the large structures of the human body that can be seen through normal dissec­tion. Microscopic anatomy deals with the smaller struc­tures and fine detail that can be seen only with the aid of a microscope.

Human physiology is the scientific study of the func­tions or processes of the human body. It answers how, what, and why anatomic parts work. Anatomy and physi­ology are interrelated because structure and function are always closely associated. The function of an organ, or how it works, depends on how it is put together. Con­versely, the anatomy or structure provides clues to under­standing how it works. The structure of the hand, with its long, jointed fingers, is related to its function of grasp­ing things. The heart is designed as a muscular pump that can contract to force blood into the blood vessels. By contrast, lungs are made of a thin tissue and function to exchange oxygen and carbon dioxide between the outside environment and the blood. Imagine what would happen if the heart were made of thin tissue and the lungs were made of thick muscle. Structure and function are always related.

Levels of Organization

Among the most outstanding features of the complex human body are its order and organization—how all the parts, from tiny cells to visible organs, work together to make a functioning whole. The organizational scheme of the body has six levels (Figure 5-1).

Starting with the simplest and proceeding to the most complex, the six levels of organization are chemical, cellular, tissue, organ, body system, and total organism. The structural and functional characteristics of all organisms are determined by their chemical makeup.

Chemical Level The chemical level deals with the interac­tions of atoms, such as hydrogen and oxygen, and their combinations into molecules, such as water. Molecules contribute to the makeup of a cell, which is the basic unit of life.

Cells Cells, discussed later in this chapter, are the basic living units of all organisms. Estimates indicate that there are about 75 trillion dynamic, living cells in the human body. These cells represent a variety of sizes, shapes, and structures and provide a vast array of functions.

Tissues Cells with similar structure and function are grouped together as tissues. All of the tissues of the body are grouped into four main types: epithelial, connective, muscle, and nervous. The tissue level of organization is discussed later in this chapter.

Organs Two or more tissue types that form a more complex structure and work together to perform one or more functions make up organs, the next higher level of organization. Examples of organs include the skin, heart, ear, stomach, and liver.

Body Systems A body system consists of several organs that work together to accomplish a set of functions. Some examples of body systems include the nervous system, the digestive system, and the respiratory system.

Total Organism Finally, the most complex of all the levels is the total organism, which is made up of several systems that work together to maintain life.

Organ Systems

The human body has 11 major organ systems, each with specific functions, yet all are interrelated and work together to sustain life. Each system is described briefly here and then in more detail in later chapters. The organ systems are illustrated and summarized in Table 5-1.

Integumentary System Integument means skin. The integumentary (in-teg-yoo-MEN-tar-ee) system consists of the skin and the various accessory organs associated with it. These accessories include hair, nails, sweat glands, and seba­ceous (oil) glands. The components of the integumentary system protect the underlying tissues from injury, protect against water loss, contain sense receptors, help in tempera­ture regulation, and synthesize chemicals to be used in other parts of the body.

Skeletal System The skeletal (SKEL-eh-tull) system forms the framework of the body and protects underlying organs, such as the brain, lungs, and heart. It consists of the

Figure 5-1 Organizational scheme of the body. From simple to complex, the levels are chemical, cellular, tissue, organ, body system, and total organism.

bones and joints along with ligaments and cartilage that bind the bones together. Bones serve as attachments for muscles and act with the muscles to produce movement. Tissues within bones produce blood cells and store inor­ganic salts containing calcium and phosphorus.

Muscular System Muscles are the organs of the muscular (MUS-kyoo-lar) system. As muscles contract, they create the forces that produce movement and maintain posture. Muscles can store energy in the form of glycogen and are the primary source of heat within the body.

Nervous System The nervous (NER-vus) system consists of the brain, spinal cord, and associated nerves. These organs work together to coordinate body activities. Nerve cells, or neurons, are specialized to transmit impulses from one point to another. In this way, body parts can commu­nicate with each other and with the outside environment. Some nerve cells have special endings called sense receptors that detect changes in the environment.

Endocrine System The endocrine (EN-doh-krin) system includes all the glands that secrete chemicals called hor­mones. These hormones travel through the blood and act as messengers to regulate cellular activities. The endocrine and nervous systems work together to coordinate and regulate body activities to maintain a proper balance. The nervous system typically acts quickly, whereas the endocrine system acts slowly but with a more sustained effect. The endocrine system also regulates reproductive functions in both males and females.

Cardiovascular System The cardiovascular (kar-dee-oh- VAS-kyoo-lar) system consists of the blood, heart, and blood vessels. The blood transports nutrients, hormones, and oxygen to tissue cells and removes waste products such as carbon dioxide. Certain cells within the blood known as white blood cells defend the body against disease. The heart acts as a pump to create the forces necessary to maintain blood pressure and to circulate the blood. The blood vessels serve as pipes or conduits for the flow of blood.

Table 5-1 Organ Systems of the Body

Lymphatic System The lymphatic (lim-FAT-ik) system consists of a series of vessels that transport a fluid called lymph from the tissues back into the blood. In addition to lymph, the system includes lymph nodes and lymphoid organs, such as the tonsils, spleen, and thymus, that filter the lymph to remove foreign particles as a protection against disease. Lymphoid organs also function in the body’s defense mechanism by enhancing the activities of cells that inactivate specific pathogenic agents. The lymphatic system is some­times considered to be a part of the cardiovascular system.

Table 5-1 Organ Systems of the Body—cont’d 
Lymphatic System Digestive SystemRespiratory System

Digestive System The organs of the digestive (dye-JES- tiv) system include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine (colon), which make up the digestive tract. Accessory organs of this system include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. The functions of this system are to ingest food, process it into molecules that can be used by the body, and then eliminate the residue.

Respiratory System The respiratory (reh-SPY-rah-tor-ee or res-per-ah-TOR-ee) system brings oxygen, in the form of air, into the lungs; removes the carbon dioxide; and provides a membrane for the exchange of these gases between the blood and lungs. The system consists of the nasal cavities, pharynx, larynx, trachea, bronchi, and lungs.

Urinary System The kidneys, ureters, urinary bladder, and urethra make up the urinary (YOO-rin-air-ee) system. The kidneys remove various waste materials, especially nitrogenous wastes, from the blood and help to regulate the fluid level and chemical content of the body. The product of kidney function is urine, which is transported through the ureters and urethra. The urinary bladder serves as a reservoir or storage area for the urine.

Reproductive System The purpose of the reproductive (ree-pro-DUK-tiv) system is the production of new indi­viduals. The primary organs of the system are the gonads, which produce the reproductive cells. These are the ovaries in the female and testes in the male. In addition to gonads, there are accessory glands, supporting structures, and duct systems for the transport of the reproductive cells. In the female the reproductive system produces ova or eggs, receives sperm from the male, and provides for the support and development of the embryo and fetus. The male repro­ductive system is concerned with the production and main­tenance of sperm and the transfer of these cells to the female.

Homeostasis

Homeostasis refers to the constant internal environment that must be maintained for the cells of the body. The word is derived from two Greek words: homeo, which means “alike” or “the same,” and stasis, which means “always” or “staying.” Putting these together, the word homeostasis means “staying the same.” When the body is healthy, the internal environment always stays the same. It remains stable within limited normal ranges.

Everyone is familiar with aspects of the external environment—whether it is cold or hot, humid or dry, smoggy or clear. The internal environment is not quite as obvious. It involves the tissue fluid that surrounds and bathes every cell of the body. Normal functional activities of the cell depend on the internal environment being main­tained within limited normal ranges. The chemical content, volume, temperature, and pressure of the fluid must stay the same (homeostasis), regardless of external conditions, so that the cell can function properly. If the conditions in the tissue fluid deviate from normal, mechanisms respond that try to restore conditions to normal. If the mechanisms are unsuccessful, the cell malfunctions and dies. This leads to illness and disease. Ultimately the goal of medical treatment is to restore homeostasis.

Negative and Positive Feedback

Any condition or stimulus that disrupts the homeostatic balance in the body is a stressor. When a stressor causes internal conditions to deviate from normal, all the body systems work to bring conditions back to the normal range.

This is usually accomplished by a negative feedback mech­anism in which a stimulus initiates reactions that reduce the stimulus. This mechanism works similarly to a thermostat connected to a furnace and an air conditioner. When the temperature in the room decreases (stressor) below the ther­mostat setting (normal), the sensing device in the thermo­stat detects the change and causes the furnace to add heat to the room. When the room becomes too warm, the furnace stops and the air conditioner begins to cool the room. Negative feedback mechanisms do not prevent varia­tion, but they keep variation within a normal range.

An example of a physiologic negative feedback mecha­nism in the human body involves blood pressure. When blood pressure decreases below normal, body sensors detect the deviation and initiate changes that bring the pressure back within the normal range. When the pressure increases above normal, changes occur to decrease the pressure to normal. Variations in blood pressure occur, but homeostatic mechanisms keep them within the limits of a normal range.

The nervous and endocrine systems work together to control homeostasis, but all the organ systems in the body help maintain the normal conditions of the internal envi­ronment. The brain contains centers that monitor tempera­ture, pressure, volume, and the chemical conditions of body fluids. Endocrine glands secrete hormones in response to deviations from normal conditions, and these hormones affect other organs. The changes required to bring condi­tions back to the normal range are mediated by various organ systems. Good health depends on homeostasis. Illness results when the negative feedback mechanisms that main­tain homeostasis are disrupted. Medical therapy attempts to assist the negative feedback process to restore balance, or homeostasis.

Anatomic Terms

Certain basic terms need to be understood to communicate effectively in the health care profession. In other words, you have to speak the language. This section explains some basic terms that relate to the anatomy of the body. They are used to describe directions and regions of the body.

Anatomic Position

If directional terms are to be meaningful, there must be some knowledge of the beginning position. If you give a person directions to go somewhere, you must have a starting reference point. When using directions in anatomy and physiology, it is assumed that the body is in anatomic posi­tion. In this position, the body is standing erect, the face is forward, and the arms are at the sides with the palms and toes directed forward. Figure 5-2 illustrates the body in anatomic position.

Directions in the Body

Directional terms are used to describe the relative position of one part to another. Note that in the following list of directional terms, the two items in each pair of terms are opposites.

Figure 5-2 The body in anatomic position.

Superior means that a part is above another part, or closer to the head. The nose is superior to the mouth. Infe­rior means that a part is below another part, or closer to the feet. The heart is inferior to the neck.

Anterior (or ventral) means toward the front surface. The heart is anterior to the vertebral column. Posterior means that a part is toward the back. The heart is posterior to the sternum.

Medial means toward, or nearer, the midline of the body. The nose is medial to the ears. Lateral means toward, or nearer, the side, away from the midline. The ears are lateral to the eyes.

Proximal means that a part is closer to a point of attach­ment, or closer to the trunk of the body, than another part. The elbow is proximal to the wrist. The opposite of proximal is distal, which means that a part is farther away from a point of attachment than is another part. The fingers are distal to the wrist.

Superficial means that a part is located on or near the surface. The superficial (or outermost) layer of the skin is the epidermis. The opposite of superficial is deep, which means that a part is away from the surface. Muscles are deep to the skin.

Figure 5-3 Transverse, sagittal, and frontal planes of the body.

Visceral pertains to internal organs or the covering of the organs. The visceral pericardium covers the heart. Parietal refers to the wall of a body cavity. The parietal peritoneum lines the wall of the abdominal cavity.

Planes and Sections of the Body

To aid in visualizing the spatial relationships of internal body parts, anatomists use three imaginary planes, each of which is cut through the body in a different direction. Figure 5-3 illustrates these three planes.

The sagittal plane refers to a lengthwise cut that divides the body into right and left portions. This is some­times called a longitudinal section. If the cut passes through the midline of the body, it is called a midsag­ittal plane, and it divides the body into right and left halves.

The transverse plane or horizontal plane is perpendicular to the sagittal plane and cuts across the body horizon­tally to divide it into superior and inferior portions. Sections cut this way are sometimes called cross sections.

The frontal plane divides the body into anterior and posterior portions. It is perpendicular to both the sagittal plane and the transverse plane. This is some­times called a coronal plane.

Body Cavities

Spaces within the body that contain the internal organs or viscera are called body cavities. The two main cavities are the dorsal cavity and the larger ventral cavity, which are

Figure 5-4 The two major cavities in the body and their subdivisions.

Figure 5-5 Abdominopelvic quadrants that are formed by a midsagittal plane and a transverse plane through the umbilicus.

Figure 5-6 Nine abdominopelvic regions formed by two sagittal planes and two transverse planes.

illustrated in Figure 5-4. The dorsal cavity is divided into the cranial cavity, which contains the brain, and the spinal cavity, which contains the spinal cord. The cranial and spinal cavities join with each other to form a continuous space.

The ventral cavity is much larger than the dorsal cavity and is subdivided into the thoracic (tho-RAS-ik) cavity and the abdominopelvic (ab-dahm-ih-noh-PEL-vik) cavity. The thoracic cavity is superior to the abdominopelvic cavity and contains the heart, lungs, esophagus, and trachea. It is sepa­rated from the abdominopelvic cavity by the muscular dia­phragm. Although there is no clear-cut partition to divide it, the abdominopelvic cavity is separated into the superior abdominal cavity and the inferior pelvic cavity. The stomach, liver, gallbladder, spleen, and most of the intestines are in the abdominal cavity. The pelvic cavity contains portions of the small and large intestines, the rectum, the urinary bladder, and the internal reproductive organs.

To help describe the location of body organs or pain, health care professionals frequently divide the abdomino­pelvic cavity into regions using imaginary lines. One such method uses the midsagittal plane and a transverse plane that passes through the umbilicus. This divides the abdomi­nopelvic area into four quadrants, illustrated in Figure 5-5. Another system uses two sagittal planes and two transverse planes to divide the abdominopelvic area into the nine regions illustrated in Figure 5-6. The three central regions are, from superior to inferior, the epigastric (ep-ih-GAS- trik), umbilical (um-BIL-ih-kal), and hypogastric (hye-poh- GAS-trik) regions. Lateral to these, from superior to inferior, are the right and left hypochondriac (hye-poh-KAHN-dree-

Regions of the Body

The body may be divided into the axial (AK-see-al) portion, which consists of the head, neck, and trunk, and the appen­dicular (ap-pen-DIK-yoo-lar) portion, which consists of the limbs. The trunk, or torso, includes the thorax, abdomen,

Table 5-2 Body Area Terms
Abdominal (ab-DAHM-ih-nal)Portion of the trunk between the thorax and pelvis; celiac region
Antebrachial (an-te-BRAY-kee-al)Region between the elbow and wrist; forearm; cubital region
Antecubital (an-te-KYOO-bih-tal)Space in front of elbow
Axillary (AK-sih-lair-ee)Armpit area
Brachial (BRAY-kee-al)Arm; proximal portion of upper limb
Buccal (BUK-al)Region of cheek
Buttock (BUT-tuck)Posterior aspect of lower trunk; gluteal region
Carpal (KAR-pal)Wrist
Celiac (SEE-lee-ak)Abdomen
Cephalic (seh-FAL-ik)Head
Cervical (SER-vih-kal)Neck region
Costal (KAHS-tal)Ribs
Cranial (KRAY-nee-al)Skull
Crural (KROO-rahl)Portion of lower extremity between knee and foot; leg
Cubital (KYOO-bih-tal)Forearm; region between elbow and wrist; antebrachial
Cutaneous (kyoo-TAY-nee-us)Skin
Femoral (FEM-or-al)Thigh; part of lower extremity between hip and knee
Frontal (FRUN-tal)Forehead
Gluteal (GLOO-tee-al)Buttock region
Groin (GROYN)Depressed region between abdomen and thigh; inguinal
Inguinal (IN-gwih-nal)Depressed region between abdomen and thigh; groin
Leg (LEG)Portion of lower extremity between knee and foot; also called crural region
Lumbar (LUM-bar)Region of lower back and side between lowest rib and pelvis
Mammary (MAM-ah-ree)Pertaining to the breast
Navel (NAY-vel)Middle region of abdomen; umbilical region
Occipital (ahk-SIP-ih-tal)Lower portion of the back of the head
Ophthalmic (off-THAL-mik)Pertaining to the eyes
Oral (OH-ral) or (AW-ral)Pertaining to the mouth
Otic (OH-tik)Ears
Palmar (PAWL-mar)Palm of hand
Pectoral (PEK-toh-ral)Chest region
Pedal (PED-al)Foot
Pelvic (PEL-vik)Inferior region of abdominopelvic cavity
Perineal (pair-ih-NEE-al)Region between anus and pubic symphysis; includes region of external reproductive organs
Plantar (PLAN-tar)Sole of foot
Popliteal (pop-LIT-ee-al or pop-lih-TEE-al)Area behind knee
Sacral (SAY-kral)Posterior region between hip bones
Sternal (STIR-nal)Anterior midline of the thorax
Tarsal (TAHR-sal)Ankle and instep of foot
Thigh (THIGH)Part of lower extremity between hip and knee; femoral region
Thoracic (tho-RAS-ik)Chest; part of trunk inferior to neck and superior to diaphragm
Umbilical (um-BIL-ih-kal)Navel; middle region of abdomen
Vertebral (ver-TEE-bral or VER-teh-bral)Pertaining to spinal column; backbone

From Applegate E: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders.

and pelvis. In addition to these terms and the nine abdomi­nopelvic regions identified in the previous section, there are numerous other terms that apply to specific body areas. Some of these are listed in Table 5-2 and are identified in Figure 5-7.

and on and on, until the adult human body has an esti­mated 75 trillion cells. Cells are the structural and func­tional units of the human body. Homeostasis depends on the interaction between the cell and its environment.

During development, cells become specialized in size, shape, characteristics, and function, resulting in a large variety of cells in the body. For descriptive purposes it is convenient to imagine a typical, generalized cell that con­tains the components of all the different cell types. Not all the components of a “generalized” cell are present in every cell type, but each component is present in some cells and has its particular function to maintain life. A generalized

CELL STRUCTURE AND FUNCTION

Structure of the Generalized Cell

Every individual begins life as a single cell, a fertilized egg. This single cell divides into two cells, then four, eight, 16,

Figure 5-7 Terms for selected regions of the body.

cell is illustrated in Figure 5-8, and the structure and functions of the cellular components are summarized in Table 5-3.

Plasma Membrane

Every cell in the body is enclosed by a plasma (cell) mem­brane. The plasma membrane separates the material outside the cell (extracellular) from the material inside the cell (intracellular). If the membrane breaks, the cell dies. The plasma membrane determines what can go into, or out of, the cell. It is selectively permeable, which means that some substances can pass through the membrane but others cannot. The main structural components of the plasma membrane are phospholipids and proteins.

Cytoplasm

The cytoplasm (SYE-toh-plazm) is the gel-like fluid inside the cell. The cytoplasm has numerous small structures, called organelles, suspended in it. These organelles (or-guh- NELZ), or “little organs,” are the functional machinery of the cell, and each organelle type has a specific role in the metabolic reactions that take place in the cytoplasm.

The cytoplasm is primarily water known as the intracel­lular fluid. About two thirds of the water in the body is in

the cytoplasm of cells. The intracellular fluid contains dis­solved electrolytes, metabolic waste products, and nutrients such as amino acids and simple sugars.

Nucleus

The nucleus (NOO-klee-us) is the control center that directs the activities of the cell. All cells have at least one nucleus at some time during their existence; some, however, such as red blood cells, lose their nucleus as they mature. Other cells, such as skeletal muscle cells, have multiple nuclei.

The nucleus is a relatively large, spheric body that is usually located near the center of the cell (see Figure 5-8). It is enclosed by a double-layered nuclear membrane that separates the cytoplasm of the cell from the nucleoplasm, the fluid portion inside the nucleus.

The nucleus contains the genetic material of the cell. In the nondividing cell, the genetic material, deoxyribonucleic acid (DNA), is present as long, slender, filamentous threads called chromatin (see Figure 5-8). When the cell starts to divide or replicate, the chromatin condenses and becomes tightly coiled to form short, rodlike chromosomes. Each chromosome, composed of DNA with some protein, con­tains several hundred genes arranged in a specific linear order. Human cells have 23 pairs of chromosomes that

Figure 5-8 Generalized cell.

Table 5-3Structure and Function of Cellular Components 
ComponentStructureFunction
Plasma membraneBilayer of phospholipid and protein moleculesMaintains integrity of cell; controls passage of materials into and out of cell
CytoplasmWater; dissolved ions and nutrients; suspended colloidsMedium for chemical reactions; suspending medium for organelles
NucleusSpheric body near center of cell; enclosed in a membraneContains genetic material; regulates activities of cell
Nuclear membraneDouble-layered membrane around nucleus; has poresSeparates cytoplasm from nucleoplasm; pores allow passage of material as needed
ChromatinStrands of DNA in nucleusGenetic material of cell; becomes chromosomes during cell division
NucleolusDense, nonmembranous body in nucleus; composed of RNA and protein moleculesForms ribosomes
MitochondriaRod-shaped bodies enclosed by a double-layered membrane in cytoplasm; folds of inner membrane form cristaeMajor site of adenosine triphosphate synthesis; convert energy from nutrients into a form that is usable by body
RibosomesGranules of RNA in cytoplasmProtein synthesis
Endoplasmic reticulumInterconnected membranous channels and sacs in cytoplasmTransports material through cytoplasm; rough endoplasmic reticulum aids in synthesis of protein; smooth endoplasmic reticulum involved in lipid synthesis
Golgi apparatusGroup of flattened membranous sacs usually near nucleusPackages products for secretion; forms lysosomes
LysosomesMembranous sacs of digestive enzymes in cytoplasmDigest material taken into cell, debris from damaged cells, worn-out cell components
CytoskeletonProtein microfilaments and microtubules in cytoplasmProvides support for cytoplasm; helps in movement of organelles
CentriolesPair of rod-shaped bodies composed of microtubules; located near nucleus at right angles to each otherDistribute chromosomes to daughter cells during cell division
CiliaMembrane-enclosed bundles of microtubules that extend outward from cell membrane; short and numerousMove substances across surface of cell
FlagellaSimilar to cilia, except usually long and singleCell locomotion

together contain all the information necessary to direct the synthesis of more than 100,000 different proteins.

The nucleolus (noo-KLEE-oh-lus) (“little nucleus”) appears as a dark-staining, discrete, dense body within the nucleus (see Figure 5-1). It has no enclosing membrane, and the number of nucleoli may vary from one to four in any given cell. The function of the nucleolus is to produce ribonucleic acid (RNA) and combine it with protein to form ribosomes. Ribosomes function in protein synthesis, as described later in this section. In growing cells and other cells that are making large amounts of protein, the nucleoli are large and distinct.

Cytoplasmic Organelles

Cytoplasmic organelles are “little organs” that are suspended in the cytoplasm of the cell. Each type of organelle has a definite structure and a specific role in the function of the cell.

Mitochondria

Mitochondria (mye-toh-KON-dree-ah) are elongated, oval, fluid-filled sacs in the cytoplasm that contain their own DNA and can reproduce themselves (see Figure 5-8). Enzymes necessary for the production of adenosine triphos­phate (ATP) are located inside the mitochondria. ATP is a chemical that stores chemical energy within the cell and provides energy for use by the body cells. Mitochondria could be called the “power plant” of the cell because they convert energy from nutrients into ATP.

Ribosomes

Ribosomes (RYE-boh-sohmz) consist of small granules of RNA located in the cytoplasm. The RNA in the ribosomes is from the nucleolus, and when fully assembled, ribosomes function in protein synthesis. Some ribosomes are found free in the cytoplasm. These ribosomes function in the synthesis of proteins for use within that same cell. Other ribosomes are attached to the membranes of the endoplas­mic reticulum (ER) and function in the synthesis of pro­teins that are exported from the cell and used elsewhere.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) (end-oh-PLAZ-mik reh- TICK-yoo-lum) is a complex series of membranous chan­nels extending throughout the cytoplasm. The interconnected membranes form fluid-filled flattened sacs and tubular canals. The membranes are connected to the outer layer of the nuclear membrane, to the inner layer of the cell mem­brane, and to certain other organelles. The ER provides a path to transport materials from one part of the cell to another.

Some of the membranes of the ER have granular ribo­somes attached to the outer surface (see Figure 5-8). This is called rough endoplasmic reticulum (RER) and, because of the ribosomes, it functions in the synthesis and transport of protein molecules. Other portions of the ER lack the ribosomes and appear smooth. This is the smooth endoplasmic reticulum (SER), which functions in the syn­thesis of certain lipid molecules, such as steroids.

Golgi Apparatus

The Golgi apparatus (GOL-jee ap-ah-RAT-us) is a series of four to six flattened membranous sacs, usually located near the nucleus, and is connected to the ER (see Figure 5-8). It is the “packaging and shipping plant” of the cell.

Proteins and lipids are carried through the channels of the ER to the Golgi apparatus. Within the Golgi appa­ratus, the proteins become surrounded by a piece of the Golgi membrane. Then they are pinched off the end of the apparatus to become a secretory vesicle, a temporary inclu­sion in the cytoplasm. The secretory vesicles move to the cell membrane and release their contents to the exterior of the cell.

The Golgi apparatus are especially abundant and well developed in glandular cells that secrete a product, but they also function in nonsecretory cells. In these cells they appear to package intracellular enzymes in the form of lysosomes. Because of the vesicles pinching off the ends of the flattened membranous sacs, the Golgi apparatus is sometimes described as looking like a stack of pancakes with syrup dripping off the edge.

Lysosomes

Lysosomes (LYE-soh-sohmz) are membrane-enclosed sacs of various enzymes that have been packaged by the Golgi apparatus. When cells are damaged, these enzymes destroy the cellular debris. They also function in the destruction of worn-out cell parts. The enzymes break down particles, such as bacteria, that have been taken into the cell. When a white blood cell phagocytizes or engulfs bacteria, the enzymes from the lysosomes destroy the bacteria. Lysosomal activity also seems to be responsible for decreasing the size of some body organs at certain periods. Atrophy of muscle because of lack of use, reduction in breast size after breast­feeding, and decrease in the size of the uterus after parturi­tion all seem to be caused by lysosomal function.

Filamentous Protein Organelles

Several types of protein filaments are considered to be cel­lular organelles. The cytoskeleton and centrioles are in the cytoplasm, but the cilia and flagella project outward, away from the cell surface.

Cytoskeleton

The cytoskeleton helps to maintain the shape of the cell. At times it anchors certain organelles in position, but it may also move organelles from one position to another. Some parts of the cytoskeleton may move a portion of the cell membrane, whereas others may move the entire cell. The cytoskeleton also plays a role in muscle contraction.

The cytoskeleton is made up of protein microfilaments and microtubules. Microfilaments are long, slender rods of protein that support small projections of the cell membrane called microvilli. Microtubules are thin cylinders, larger than the microfilaments. In addition to their role as part of the cytoskeleton, microtubules are also found in centrioles, cilia, and flagella.

Centrioles

A dense area called the centrosome (SEN-troh-sohm), located near the nucleus, contains a pair of centrioles (SEN-tree- ohlz) (see Figure 5-8). Each centriole is a nonmembranous rod-shaped structure composed of microtubules. The two members of the pair are at right angles to each other. Cen­trioles function in cell reproduction by aiding in the distri­bution of chromosomes to the new daughter cells.

Cilia

Cilia (SIL-ee-ah) are short, cylindric, hairlike processes that project outward from the cell membrane. Each cilium con­sists of specialized microtubules surrounded by a membrane and anchored under the cell membrane. Cilia have an orga­nized pattern of movement that creates a wavelike motion to move substances across the surface of the cell. They are found in large quantities on the surfaces of cells that line the respiratory tract. Their motion moves mucus, in which particles of dust are embedded, upward and away from the lungs.

Flagella

Similar in structure to cilia, flagella (fluh-JELL-ah) are much longer and fewer. In contrast to cilia, which move substances across the surface of the cell, flagella beat with a whiplike motion to move the cell itself. In the human, the tail of the spermatozoon, or sperm cell, is a single flagellum that causes the swimming motion of the cell.

Cell Functions

The structural and functional characteristics of different types of cells are determined by the nature of the proteins present. Cells of various types have different functions because cell structure and function are closely related. A very thin cell is not well suited for a protective function. Bone cells do not have an appropriate structure for nerve impulse conduction. Just as there are many cell types, there are varied cell functions. The specific functions of cells will become more apparent as the tissues, organs, and systems are studied. This section deals with the more generalized cell functions—the functions that relate to the sustained viability and continuation of the cell itself. These functions include movement of substances across the cell membrane, cell division to make new cells, and protein synthesis.

Movement of Substances across the Cell Membrane

The cell membrane provides a surface through which substances enter and leave the cell. The cell membrane controls the composition of the cell’s cytoplasm by regul­ating the passage of substances through the membrane. If the membrane breaks, this control is removed and the cell dies. The survival of the cell depends on maintaining the difference between extracellular and intracellular material.

Table 5-41 Summary of Membrane Transport Mechanisms
MechanismDescription
Passive
Simple diffusionMolecular movement down a concentration gradient
OsmosisMovement of solvent toward high solute (low solvent) concentration; requires membrane
FiltrationMovement of solvent using hydrostatic pressure; requires membrane filter
Active
Active transportMovement of ions or molecules against a concentration gradient; requires carrier molecule and ATP
PhagocytosisIngestion of solid particles by creating vesicles; requires ATP
PinocytosisIngestion of fluid by creating vesicles; requires ATP
ExocytosisSecretion of cellular products by creating vesicles, then liberating contents to outside of cell; requires ATP

From Applegate E: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders.

Mechanisms of movement across the cell membrane include diffusion, osmosis, filtration, active transport, endocytosis, and exocytosis. These are summarized in Table 5-4.

Diffusion

Simple diffusion is the movement of substances from a region of high concentration to a region of low concentra­tion. Odors permeate a room because the aromatic mole­cules diffuse through the air. A crystal of dye will color a whole beaker of water because the dye particles diffuse from the region of high concentration in the dye crystal to regions of low concentration in the water (Figure 5-9).

In the examples of diffusion cited, there has been no membrane involved. Diffusion can also occur across a mem­brane as long as the membrane is permeable to the sub­stances involved. For example, oxygen and carbon dioxide are able to diffuse through the cell membrane. When carbon dioxide builds up in the capillaries to a concentration that is higher than in the lungs, the carbon dioxide diffuses into the lungs to be exhaled. Similarly, when the level of oxygen in the capillaries is lower than the level of oxygen in the lungs, oxygen diffuses into the capillaries for distribution to the body cells. In this way, the gases are exchanged between the air and the blood in the lungs, and between the blood and the cells of the various tissues (Figure 5-10).

Osmosis

Osmosis (os-MOH-sis) involves the movement of solvent (water) molecules through a selectively permeable mem­brane from a region of higher concentration of water mol­ecules (where the solute concentration is lower) to a region of lower concentration of water molecules (where the solute

concentration is higher). Figure 5-11 illustrates osmosis. When equilibrium is reached, the solutions on both sides of the membrane have the same concentration but the solu­tion that was more concentrated at the start (had more solute) will now have a greater volume. Water molecules continue to pass through the membrane after equilibrium, but because they move in both directions at the same rate there is no change in concentration or volume.

If a red blood cell, which contains 5% glucose, is placed in a container of 5% glucose solution, water will move in both directions at the same rate because the glucose con­centrations inside and outside the cell are the same.

Figure 5-12 A, Isotonic solution. The extracellular concentration equals the intracellular concentration and there is no net movement of solvent. B, Hypertonic solution. The extracellular concentration is greater than the intracellular concentration and fluid moves from the cell into the surrounding fluid. The cell shrinks (crenates). C, Hypotonic solution. The extracellular concentration is less than the intracellular concentration and the solvent moves into the cell. The cell expands.

Solutions that have the same solute concentration are iso­tonic (Figure 5-12, A).

When a red blood cell is placed in a 10% glucose solu­tion, water will leave the cell (where there are more water molecules) and enter the surrounding fluid (where there are fewer water molecules). When fluid leaves the cells, they will shrink or crenate. The 10% glucose solution is hyper­tonic (greater solute concentration) in relation to the cell (Figure 5-12, B).

When a red blood cell is placed in distilled water, water will enter the cell because there are more water molecules outside the cell than inside. The distilled water is hypotonic (lower solute concentration) in relation to the cell. As water enters the cell, it will swell because of the increased volume. If enough water goes into the cell, it may rupture. This is called lysis. When this happens to a red blood cell, it is called hemolysis (hee-MAHL-ih-sis) (Figure 5-12, C).

The terms isotonic, hypotonic, and hypertonic are relative. They are used to compare two solutions. A 5% glucose solution is hypertonic in relation to distilled water but hypotonic in relation to a 10% glucose solution.

Filtration

In diffusion and osmosis, particles (whether solute, solvent, or both) pass through a membrane by virtue of their own random movement, which is directed by a difference in concentration. In filtration, however, pressure pushes the particles through a membrane. Drip coffee makers, for example, use this principle. Water drips first through the coffee and then the water, and small particles pass through a filter. The large granules of coffee are too big to go through the pores in the filter. The size of the pores determines the size of the particles that can pass through the filter. The pressure is created by the weight of the water on the paper filter.

Contraction of the heart creates pressure in the blood. This fluid pressure or hydrostatic pressure, which is greater inside the blood vessels, pushes fluid, dissolved nutrients, and small ions through the capillary walls to form tissue fluid. The large protein molecules and blood cells are unable to pass through the pores in the capillary membrane. Blood is filtered through specialized membranes in the kidney as the initial step in urine formation. Water and small mole­cules and ions pass through the filtration membrane while blood cells and protein molecules remain in the blood.

Active Transport

In the transport mechanisms discussed thus far, no cellular energy has been involved and the molecules and/or ions have moved from a region of high concentration to one of low concentration. Active transport differs from these processes in that it moves molecules and ions “uphill” from an area of lower concentration to an area of higher concen­tration. To accomplish this, cellular energy is required in the form of ATP. If ATP is not available, active transport ceases immediately. Active transport also uses a carrier mol­ecule. Amino acids and glucose are transported from the small intestine into the blood by active transport.

As a result of active transport, substances such as electro­lytes are present in significantly higher concentrations on one side of the cell membrane than on the other. For example, sodium ions are more concentrated outside the plasma membrane than inside the cell. Potassium is just the opposite; its concentration is higher inside the cell. Normal passive transport, such as diffusion and osmosis, tends to equalize the concentrations on the two sides of the mem­brane. Active transport, in this case known as the sodium- potassium pump, moves the electrolytes against concentration gradients to maintain a high sodium concentration outside the cell and a high potassium concentration inside the cell. The process requires ATP and a protein carrier molecule.

Endocytosis

Endocytosis (en-doh-sye-TOH-sis) refers to the formation of vesicles to transfer particles and droplets from outside to inside the cell. In this case, the material is too large to enter the cell by diffusion or active transport. The process requires energy in the form of ATP. Phagocytosis, which means “cell eating,” is a form of endocytosis that involves solid material. The cell membrane engulfs a particle to form a vesicle in the cytoplasm. Lysosomes fuse with the vesicle and the enzymes digest the particle. Certain white blood cells are called phagocytes because they engulf and destroy bacteria in this manner. Another form of endocytosis is pinocytosis or “cell drinking.” It differs from phagocytosis in that the vesicles that are formed are much smaller and their contents are fluids. Pinocytosis is important in cells that function in absorption.

Exocytosis

In certain cells, secretory products are packaged into vesicles by the Golgi apparatus and are then released from the cell by a process called exocytosis (eck-soh-sye-TOH-sis). The secretory vesicle moves to the cell membrane, where the vesicle membrane fuses with the cell membrane and the contents are discharged to the outside of the cell. Secretion of digestive enzymes from the pancreas and secretion of milk from the mammary glands are examples of exocytosis. Exocytosis and endocytosis are similar except for working in opposite directions. They are both active processes that require cellular energy (ATP). Exocytosis releases substances to the outside of the cell, and endocytosis transports sub­stances to the inside of the cell.

Cell Division

Cell division is the process by which new cells are formed for growth, repair, and replacement in the body. This process includes division of the nuclear material and divi­sion of the cytoplasm. Periods of growth and repair are special periods in the life of an individual when it is obvious that new cells are needed either to increase the number of cells or to repair tissues after an injury. General maintenance and replacement needs of the body may not be quite as obvious. More than 2 million red blood cells are worn out and replaced in the body every second of every day. Skin cells are continually sloughed off the body’s surface and must be replaced. The lining of the stomach is replaced every few days. All cells in the body (somatic cells), except those that give rise to the eggs and sperm (gametes), repro­duce by mitosis. Egg and sperm cells are produced by a special type of nuclear division called meiosis, in which the number of chromosomes is halved. Division of the cyto­plasm is called cytokinesis.

Mitosis

All somatic cells reproduce by mitosis, in which a single cell divides to form two new “daughter cells,” each identical to the parent cell. Humans have 23 pairs of chromosomes (or 46 chromosomes) in their cells. Each new cell that forms must also have that same number. For this to occur, events must proceed in an organized manner, chromosome mate­rial must replicate exactly, and then the chromosomes must separate precisely so that each new cell receives a set of chromosomes that is a carbon copy of the parent cells. For descriptive purposes, it is convenient to divide the events of mitosis into stages, as illustrated in Figure 5-13. It is impor­tant to remember that the process is a continual one and that there are no starting and stopping points along the way.

Interphase

The period between active cell divisions is called interphase. This is a time of growth and metabolism and is usually the longest period of the cell cycle. In cells that are rapidly dividing it may last for as little as a few hours, but in other cells it may take days, weeks, or even months. Some highly

Figure 5-13 Mitosis. Interphase is the period between active cell divisions. Prophase is the first stage of mitosis. The process continues through metaphase, anaphase, and telophase. Cytokinesis occurs in telophase. With the division of nuclear material and cytoplasm, two exact copies of the parent cell are produced.

specialized cells, such as nerve and muscle cells, may never divide and spend their whole life in interphase.

During interphase the cell increases in size and synthe­sizes an exact copy of the DNA in its nucleus so that when the cell begins to divide it has identical sets of genetic infor­mation. In addition, just before division the cell synthesizes an additional pair of centrioles and some new mitochon­dria. In addition to these synthetic activities, which are a preparation for division, normal cellular function takes place during interphase.

Prophase

After interphase, the cell begins mitosis. The first stage of mitosis is prophase. During prophase, the chromatin short­ens, thickens, and becomes tightly coiled to form chromo­somes. As a result of the replication in interphase, each chromosome has two identical parts, called chromatids, that are joined by a special region on each called the centromere. The two pairs of centrioles separate and go to opposite ends of the cytoplasm. Microtubules called spindle fibers form and extend from the centromeres to the centrioles. The nucleolus and nuclear membrane disappear during the latter part of prophase.

Metaphase

Prophase ends when the nuclear membrane disintegrates, and this signals the beginning of the next stage, metaphase. The chromosomes align themselves along the center of the cell during metaphase. This is the time when the chromo­somes are most clearly visible and distinguishable.

Anaphase

The third stage of mitosis is anaphase. After the chromo­somes are aligned along the center of the cell, the centro­meres separate so that each chromatid now becomes a chromosome. At this time, there are actually two sets of chromosomes in the cell. The two chromatids (now chro­mosomes) from each pair migrate to the centrioles at oppo­site ends of the cell. The microtubules that are attached to the centrioles and centromeres shorten and pull the chro­mosomes toward the centrioles. At the end of anaphase, the cytoplasm begins to divide.

Telophase

The final stage of mitosis is telophase. This stage is almost the reverse of prophase. After the chromosomes reach the centrioles at the ends of the cell, a new nuclear membrane forms around them. The spindle fibers disappear. The chro­mosomes start to uncoil to become long, slender strands of chromatin, and nucleoli appear in the newly formed nucleus. During this time the cell membrane constricts in the middle to divide the cytoplasm and organelles into two parts that are approximately equal. Division of the cyto­plasm is called cytokinesis (sye-toh-kih-NEE-sis). Except for size, the two newly formed daughter cells are exact copies of the parent cell. The two daughter cells now become interphase cells to carry out designated cellular functions and to undergo mitosis as needed.

Normally, body cells divide at a rate required to replace the dying ones. Normal cells are subject to control mecha­nisms that prevent overpopulation and competition for nutrients and space. Occasionally, a series of events occurs that alters some cells, so they lack the control mechanisms that tell them when to stop dividing. When the cells do not stop their mitotic activity, they form an abnormal growth called a tumor, or neoplasm.

Meiosis

Meiosis is a special type of cell division that occurs in the production of the gametes, or eggs and sperm. These cells have only 23 chromosomes, one half the number found in somatic cells, so that when fertilization takes place the resulting cell will again have 46 chromosomes, 23 from the egg and 23 from the sperm. Meiosis is discussed in greater detail in Chapter 16. In brief, meiosis consists of two divi­sions, but DNA is replicated only once. The result is four cells, but each one has only 23 chromosomes. Figure 5-14 compares mitosis and meiosis.

DNA Replication and Protein Synthesis

Proteins that are synthesized in the cytoplasm function as structural materials, enzymes that regulate chemical reac­tions, hormones, and other vital substances. Because DNA in the nucleus directs the synthesis of the proteins in the cytoplasm, it ultimately determines the structural and func­tional characteristics of an individual. Whether a person has blue or brown eyes, brown or blond hair, or light or dark skin is determined by the types of proteins synthesized in response to the genetic information contained in the DNA in the nucleus. The portion of a DNA molecule that con­tains the genetic information for making one particular protein molecule is called a gene. If a cell produced for replacement or repair is to function exactly as its predeces­sor, then it must have the same genes, a carbon copy of the DNA. This is the purpose of DNA replication in cell division.

TISSUES AND MEMBRANES

A tissue is a group of cells that have similar structure and that function together as a unit. The microscopic study of tissues is called histology. A nonliving material called the intercellular matrix fills the spaces between cells. This may be abundant in some tissues and scarce in others. The inter­cellular matrix may contain special substances, such as salts and fibers, that are unique to a specific tissue and give that tissue distinctive characteristics.

Body Tissues

Four main tissue types are found in the body: epithelial, connective, muscle, and nervous. Each is designed for spe­cific functions.

Epithelial Tissue

Epithelial (ep-ih-THEE-lee-al) tissues are widespread throughout the body. They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands. They perform a variety of functions that include protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception.

Figure 5-14 Comparison of mitosis and meiosis. The result of mitosis in humans is two cells, each with 46 chromosomes (23 pairs). Meiosis results in four cells, each with 23 chromosomes.

The cells in epithelial tissue are tightly packed with little intercellular matrix. Because the tissues form coverings and linings, the cells have one free surface that is not in contact with other cells. Opposite the free surface, the cells are attached to underlying connective tissue by a noncellular basement membrane. Because epithelial tissues are typically avascular (without blood vessels), they must receive their nutrients and oxygen supply by diffusion from the blood vessels in the underlying tissues. Another characteristic of epithelial tissues is that they regenerate, or reproduce, quickly. For example, the cells of the skin and stomach are continually damaged and replaced, and skin abrasions heal quite rapidly.

Epithelia are classified according to cell shape and the number of layers in the tissue. Classified according to shape, the cells are squamous, cuboidal, or columnar, and the shape of the nucleus corresponds to the cell shape. Squa­mous cells are flat and the nuclei are usually broad and thin. Cuboidal cells are cubelike, as tall as they are wide, and the nuclei are spheric and centrally located. Columnar cells are tall and narrow, resembling columns, and the nuclei are usually in the lower portion of the cell near the basement membrane. According to the number of layers, epithelia are simple if they have only one layer of cells and stratified if they have multiple layers. Stratified epithelia are named according to the type of cells at the free surface of the tissue.

Simple Squamous Epithelium

Simple squamous epithelium (Figure 5-15) consists of a single layer of thin, flat cells that fit closely together with little intercellular matrix. Because it is so thin, simple squamous epithelium is well suited for areas in which diffusion and filtration take place. The alveoli or air sacs of the lungs, where diffusion of oxygen and carbon dioxide gases occurs, are made of simple squamous epithelium. This tissue is also found in the kidney, where the blood is filtered. Capillary walls, where oxygen and carbon dioxide diffuse between the blood and tissues, are made of simple squamous epithelium. Because it is so thin and delicate, this tissue is damaged easily and offers little protective function.

Simple Cuboidal Epithelium

Simple cuboidal epithelium (Figure 5-16) consists of a single layer of cube-shaped cells. These cells have more volume than squamous cells and also have more organelles. Simple cuboidal epithelium is found as a covering of the ovary, as a lining of kidney tubules, and in many glands, such as the thyroid, pancreas, and salivary glands. In the kidney tubules, the tissue functions in absorption and secretion. In glands, simple cuboidal cells form the secretory portions and the ducts that deliver the products to their destination.

Simple Columnar Epithelium

A single layer of cells that are taller than they are wide makes up simple columnar epithelium (Figure 5-17). The nuclei are in the bottom portion of the cell near the basement mem­brane. Simple columnar epithelium is found lining the stomach and intestines, where it secretes digestive enzymes and absorbs nutrients. Because the cells are taller (or thicker) than either squamous or cuboidal cells, this tissue offers some protection to underlying tissues.

In regions where absorption is of primary importance, such as in parts of the digestive tract, the cell membrane on the free surface has numerous small projections called microvilli. Microvilli increase the surface area that is

Highlight on Anatomy and Physiology

Congenital Abnormalities

Often, congenital anatomic abnormalities must be surgically repaired so that disruptions in physiology are corrected. For example, a cleft palate (anatomy) is repaired so that food will enter (physiology) the pharynx instead of the nasal cavity. Broken bones (anatomy) are reset so that function (physiology) is restored.

Cystic Fibrosis

Cystic fibrosis is an inherited disorder that affects 1 in every 2000 Caucasian live births. In cystic fibrosis the cell membrane proteins that function as channels for transporting chloride ions out of the cell are defective. Because chloride ion transport is altered, secre­tions such as mucus, sweat, and pancreatic juice are salty and thick. Thick mucus in the lungs leads to impaired breathing and increased infections. The ducts in the pancreas become plugged, which stops the flow of digestive enzymes. Life expectancy, with therapy, is about 27 years.

Kidney Dialysis

Normally functioning kidneys remove waste products from the blood. When the kidneys do not function properly, waste molecules can be removed from the blood artificially by a process called dialysis. Dialysis is a form of diffusion in which the size of the pores in a selectively permeable membrane separates smaller solute particles from larger solutes. In kidney dialysis, small waste mol­ecules pass through the membrane and are removed from the blood. The protein molecules, which are needed in the blood, are too large to pass through the pores and thus are retained.

Neoplasms

A benign neoplasm consists of highly organized cells that closely resemble normal tissue. In contrast, a malignant neoplasm, or cancer, consists of unorganized and immature cells that are inca­pable of normal function. These cells may detach from the tumor site and travel in the blood or lymph to another site and establish a new tumor. This is called metastasis and is probably the most devastating property of malignant cells. A benign tumor of glan­dular epithelial cells is called an adenoma. An adenocarcinoma is a cancerous tumor arising from glandular cells. Carcinomas are solid cancerous tumors that are derived from epithelial tissue. Approximately 85% of all malignant neoplasms are carcinomas. Sarcomas are cancerous tumors that are derived from connective tissue. These account for approximately 10% of all malignant neoplasms.

Genetic Disorders

Genetic disorders are pathologic conditions caused by mistakes, or mutations, in a cell’s genetic code. Mutations may occur natu­rally, or they may be induced by mutagens, such as radiation and certain chemicals. If the mutations occur in the gametes, the faulty code is passed from one generation to the next. Errors in the genes (DNA) cause the production of abnormal proteins, which result in abnormal cellular function. For example, in sickle cell anemia, a genetic blood disorder, red blood cells have abnormal hemoglobin because there is an “error” in the gene that directs hemoglobin synthesis.

Peritonitis

An inflammation of the serous membranes in the abdominal cavity is called peritonitis. This is sometimes a serious complication of an infected appendix. ■

available for absorption of nutrients. Goblet cells are fre­quently interspersed among the simple columnar cells. Goblet cells are flask- or goblet-shaped cells that secrete mucus onto the free surface of the tissue. Cilia may be present to move secretions along the surface.

Pseudostratified Columnar Epithelium

Pseudostratified columnar epithelium (Figure 5-18) appears to have multiple layers (stratified), but it really does not. This is because the cells are not all the same height. Some cells are short and some are tall, and the nuclei are at dif­ferent levels. Close examination reveals that all the cells are attached to the basement membrane but that not all cells reach the free surface of the tissue. Cilia and goblet cells are often associated with pseudostratified columnar epithelium. This tissue lines portions of the respiratory tract in which the mucus, produced by the goblet cells, traps dust particles and is then moved upward by the cilia. Pseudostratified columnar epithelium also lines some of the tubes of the male reproductive system. Here the cilia help propel the sperm from one region to another.

Stratified Squamous Epithelium

Stratified squamous epithelium, the most widespread strati­fied epithelium, is thick because it consists of many layers of cells (Figure 5-19). The cells on the bottom layer, next to the basement membrane, are usually cuboidal or colum­nar, and these are the cells that undergo mitosis. As the cells are pushed toward the surface, they become thinner, so the surface cells are squamous. As the cells are pushed farther away from the basement membrane, it is more difficult for them to receive oxygen and nutrients from underlying con­nective tissue, and the cells die. As cells on the surface are damaged and die, they are sloughed off and replaced by cells from the deeper layers. Because this tissue is thick, it is found in areas in which protection is a primary function. Stratified squamous epithelium forms the outer layer of the skin and extends a short distance into every body opening that is continuous with the skin.

Transitional Epithelium

Transitional epithelium is a specialized type of tissue that has several layers but can be stretched in response to tension. The lining of the urinary bladder is a good example of this type of tissue. When the bladder is empty and contracted, the epithelial lining has several layers of cuboidal cells. As the bladder fills and is distended or stretched, the cells become thinner and the number of layers decreases.

Glandular Epithelium

Glandular epithelium consists of cells that are specialized to produce and secrete substances. Glandular epithelium nor­mally lies deep to the epithelia that cover and line parts of the body. If the gland secretes its product onto a free surface via a duct, it is called an exocrine gland. Examples of exo­crine glands include sebaceous glands, mammary glands, and salivary glands. If the gland secretes its product directly

into the blood, it is a ductless gland, or endocrine gland. Endocrine glands are discussed in Chapter 11.

Connective Tissue

Connective tissues bind structures together, form a frame­work and support for organs and the body as a whole, store fat, transport substances, protect against disease, and help repair tissue damage. They occur throughout the body. Connective tissues are characterized by an abundance of intercellular matrix with relatively few cells. Connective tissue cells are able to reproduce but not as rapidly as epi­thelial cells. Most connective tissues have a good blood supply, but some do not. Examples of connective tissue include adipose tissue, cartilage, and bone.

The intercellular matrix in connective tissue has a gel­like base of water, nonfibrous protein, and other molecules. Various mineral salts in the matrix of some connective tissues, such as bone, make them hard. Two types of fibers, collagenous and elastic, are frequently embedded in the matrix. Collagenous fibers, composed of the protein col­lagen, are strong and flexible but are only slightly elastic. They are able to withstand considerable pulling force and are found in areas in which this is important, such as in tendons and ligaments. When collagenous fibers are grouped together in parallel bundles, the tissue appears white, so they are sometimes called white fibers. Elastic fibers, com­posed of the protein elastin, are not very strong, but they are elastic. They can be stretched and will return to their original shape and length when released. Elastic fibers, also called yellow fibers, are located where structures are stretched and released, such as the vocal cords.

Numerous cell types are found in connective tissue. Three of the most common are the fibroblast, macro­phage, and mast cell. As the name implies, fibroblasts produce the fibers that are in the intercellular matrix. Mac­rophages are large phagocytic cells that are able to move about and clean up cellular debris and foreign particles from the tissues. Mast cells contain heparin, an anticoagulant, and histamine, a substance that promotes inflammation and that is active in allergies.

Loose Connective Tissue

Loose connective tissue, also called areolar (ah-REE-oh-lar) connective tissue, is one of the most widely distributed tissues in the body. It is the packing material in the body. It attaches the skin to the underlying tissues and fills the spaces between muscles. Most epithelial tissue is anchored to this tissue by the basement membrane, and the blood vessels in the loose connective tissue supply nutrients to the epithelium above. The matrix is characterized by a loose network of collage­nous and elastic fibers. The predominant cell is the fibro­blast, but other connective tissue cells are also present (Figure 5-20).

is little intercellular matrix. Some of the cells accumulate liquid triglyceride, or fat, droplets. When this happens, the cytoplasm and nucleus are pushed off to one side, and the cells swell and become closely packed together. Fat cells have the ability to take up fat and then release it at a later time. Adipose tissue forms a protective cushion around the kidneys, heart, eyeballs, and various joints. It also accumu­lates under the skin, where it provides insulation for heat. Adipose tissue is an efficient energy storage material for excess calories.

Dense Fibrous Connective Tissue

Dense fibrous connective tissue is characterized by closely packed parallel bundles of collagenous fibers in the intercel­lular matrix. There are relatively few cells, and the ones that are present are fibroblasts to produce the collagenous fibers. This is the tissue that makes up tendons, which connect muscles to bones, and ligaments, which connect bones to bones. Dense fibrous connective tissue has a poor blood supply, and this, along with the relatively few cells, accounts for the slow healing of this tissue.

Elastic Connective Tissue

Elastic connective tissue has closely packed elastic fibers in the intercellular matrix. This type of tissue yields easily to a pulling force and then returns to its original length as soon as the force is released. The vocal cords and the ligaments that connect adjacent vertebrae are composed of elastic con­nective tissue.

Cartilage

Cartilage has an abundant matrix that is solid, yet flexible, with fibers embedded in it. The matrix contains the protein chondrin (KON-drihn). Cartilage cells, or chondrocytes, are located in spaces called lacunae (lah-KOO-nee) that are scattered throughout the matrix. Typically, cartilage is sur­rounded by a dense fibrous connective tissue covering called the perichondrium. The perichondrium has blood vessels, but they do not penetrate the cartilage itself, and the cells obtain their nutrients by diffusion through the solid matrix. Cartilage heals slowly because there is no direct blood supply, and this also contributes to slow cellular reproduc­tion. Cartilage protects underlying tissues, supports other structures, and provides a framework for attachments.

Hyaline cartilage (Figure 5-21) is the most common type of cartilage. It has fine collagenous fibers in the matrix and a shiny, white, opaque appearance. It is found at the ends of long bones, in the costal cartilage that connects the ribs to the sternum, and in the supporting rings of the trachea. Most of the fetal skeleton is formed of hyaline cartilage before it is replaced by bone.

Fibrocartilage has an abundance of strong collagenous fibers embedded in the matrix. This allows it to withstand compression, act as a shock absorber, and resist pulling forces. It is found in the intervertebral discs, or pads between the vertebrae; in the symphysis pubis, or pad between the two pubic bones; and between the bones in the knee joint.

Elastic cartilage has numerous yellow elastic fibers embedded in the matrix, which makes it more flexible than hyaline cartilage or fibrocartilage. It is found in the frame­work of the external ear, the epiglottis, and the auditory tubes.

Bone

Osseous tissue or bone is the most rigid of all the connective tissues. Collagenous fibers in the matrix give strength to bone, and its hardness is derived from the mineral salts, particularly calcium, that are deposited around the fibers. Bones form the framework for the body and help protect underlying tissues. They serve as attachments for muscles and act as mechanical levers in producing movement. Bone also contributes to the formation of blood cells and func­tions as a storage area for mineral salts.

Cylindric structural units, called osteons or haversian (hah-VER-shun) systems, are packed together to form the substance of compact bone (Figure 5-22). The center or hub of the osteon is a tubular osteonic or haversian canal that contains a blood vessel. The matrix is deposited in concentric rings called lamellae (lah-MEL-ee) around the canal. Osteocytes, or bone cells, are located in lacunae between the lamellae so that they are also arranged in con­centric rings. Slender processes from the bone cells extend through tiny tubes in the matrix called canaliculi (kan-ah- LIK-yoo-lye) to other cells or to the osteonic canals. This provides a readily available blood supply for the bone cells, which allows a faster repair process for bone than for cartilage.

Blood

Blood is a unique connective tissue because it is the only one that has a liquid matrix. It is a vehicle for transport of substances throughout the body. Erythrocytes, or red blood cells, and leukocytes, or white blood cells, are suspended in a liquid matrix called plasma (Figure 5-23). The red blood cells transport oxygen from the lungs to the tissues. White blood cells are important in fighting disease. Another formed element in the blood is the platelet, or thrombo­cyte, which is not actually a cell but a fragment of a giant cell in the bone marrow. Platelets are important in initiating the blood clotting process. Blood is discussed in more detail in Chapter 12.

Muscle Tissue

Muscle tissue is composed of cells that have the special ability to shorten or contract in order to produce movement of body parts. The tissue is highly cellular and is well supplied with blood vessels. The cells are long and slender, so they are sometimes called muscle fibers, and these are usually arranged in bundles or layers that are surrounded by con­nective tissue. Muscle tissue is of three types: skeletal muscle, smooth muscle, and cardiac muscle.

Skeletal Muscle

Skeletal muscle tissue (Figure 5-24) is what is commonly thought of as “muscle.” It is the meat of animals, and it constitutes about 40% of an individual’s body weight. Skel­etal muscle cells (fibers) are long and cylindric with many nuclei (multinucleated) peripherally located next to the cell membrane. The cells have alternating light and dark bands

that are perpendicular to the long axis of the cell. These bands are a result of the organized arrangement of the con­tractile proteins in the cytoplasm and give the cell a striated appearance. Skeletal muscle fibers are collected into bundles and wrapped in connective tissue to form the muscles, which are attached to the skeleton and which cause body movements when they contract in response to nerve stimulation. Skeletal muscle action is under conscious or voluntary control. Chapter 8 describes skeletal muscles in more detail.

Smooth Muscle

Smooth muscle tissue (Figure 5-25) is found in the walls of hollow body organs, such as the stomach, intestines, urinary bladder, uterus, and blood vessels. It normally acts to propel substances through the organ by contracting and relaxing. It is called smooth muscle because it lacks the striations evident in skeletal muscle. Because it is found in the viscera or body organs, it is sometimes called visceral muscle. Smooth muscle cells are shorter than skeletal muscle cells, are spindle-shaped and tapered at the ends, and have a single, centrally located nucleus. Smooth muscle usually cannot be stimulated to contract by conscious or voluntary effort, so it is called involuntary muscle.

Cardiac Muscle

Cardiac muscle tissue (Figure 5-26) is found only in the wall of the heart. The cardiac muscle cells are cylindric and appear striated, similar to skeletal muscle cells. Cardiac muscle cells are shorter than skeletal muscle cells and have only one nucleus per cell. The cells branch and interconnect to form complex networks. At the point where one cell attaches to another, there is a specialized intercellular con­nection called an intercalated (in-TER-kuh-lay-ted) disc.

Cardiac muscle appears striated like skeletal muscle, but its contraction is involuntary. It is responsible for pumping the blood through the heart and into the blood vessels.

Nervous Tissue

Nervous tissue is found in the brain, spinal cord, and nerves. It is responsible for coordinating and controlling many body activities. It stimulates muscle contraction, creates an awareness of the environment, and plays a major role in emotions, memory, and reasoning. To do all of these things, cells in nervous tissue need to be able to communicate with each other by way of electrical nerve impulses.

The cells in nervous tissue that generate and conduct impulses are called neurons or nerve cells. These cells have three principal parts: the dendrites, the cell body, and one axon (Figure 5-27). The main part of the cell, the part that carries on the general functions, is the neuron cell body. Dendrites are extensions, or processes, of the cytoplasm that carry impulses to the cell body. An extension or process called an axon carries impulses away from the cell body.

Nervous tissue also includes cells that do not transmit impulses but instead support the activities of the neurons. These are the glial (GLEE-al) cells (or neuroglial cells),

Highlight on Conditions Affecting Tissues

Adhesion (add-HEE-shun) Abnormal joining of tissues by fibrous scar tissue

Carcinoma (kar-sih-NOH-mah) A malignant growth derived from epithelial cells

Lipoma (lih-POH-mah) Benign tumor derived from fat cells Marfan syndrome (mahr-FAHN SIN-drohm) A congenital disorder of connective tissue characterized by abnormal length of the extremities and cardiovascular abnormalities Myoma (mye-OH-mah) Benign tumor formed of muscle tissue Papilloma (pap-ih-LOH-mah) Benign epithelial tumor; may occur on any epithelial surface or lining

Sarcoma (sar-KOH-mah) A malignant growth derived from con­nective tissue cells

Scurvy (SKUR-vee) A condition caused by a deficiency of vitamin C in the diet, which results in abnormal collagen synthesis Systemic lupus erythematosus (sih-STEM-ik LOO-pus air-ith- eh-mah-TOH-sis) Chronic autoimmune connective tissue disease characterized by injury to the skin, joints, kidneys, nervous system, and mucous membranes, but can affect any organ of the body ■

together termed the neuroglia. Supporting, or glial, cells bind neurons together and insulate the neurons. Some are phagocytic and protect against bacterial invasion, whereas others provide nutrients by binding blood vessels to the neurons. Further detail on nerve tissue is presented in Chapter 9.

Body Membranes

Body membranes are thin sheets of tissue that cover the body, line body cavities, cover organs within the cavities, and line the cavities in hollow organs. By this definition, the skin is a membrane because it covers the body, and indeed, the skin, or integument, is sometimes called the cutaneous mem­brane. This membrane is discussed in Chapter 6. This section examines two epithelial membranes and two con­nective tissue membranes. Epithelial membranes consist of epithelial tissue and the connective tissue to which it is attached. The two main types of epithelial membranes are the mucous membranes and serous membranes. Connective tissue membranes contain only connective tissue. Synovial membranes and meninges belong to this category.

Mucous Membranes

Mucous membranes are epithelial membranes that consist of epithelial tissue attached to underlying loose connective tissue. These membranes (sometimes called mucosae) line the body cavities that open to the outside. The entire diges­tive tract is lined with mucous membranes. Other examples include the respiratory, urinary, and reproductive tracts. The type of epithelium varies depending on its function. In the mouth, the epithelium is stratified squamous for its protection function, but the stomach and intestines are lined with simple columnar epithelium for absorption and secretion. The mucosa of the urinary bladder is transitional epithelium so that it can expand. Mucous membranes get their name from the fact that the epithelial cells secrete mucus for lubrication and protection.

Serous Membranes

Serous membranes line body cavities that do not open directly to the outside, and they cover the organs located in those

cavities. A serous membrane, or serosa, consists of a thin layer of loose connective tissue covered by a layer of simple squamous epithelium called mesothelium. These membranes always have two parts. The part that lines a cavity wall is the parietal layer, and the part that covers the organs in the cavity is the visceral layer (Figure 5-28). Serous membranes are covered by a thin layer of serous fluid that is secreted by the epithelium. Serous fluid lubricates the membrane and reduces friction and abrasion when organs in the thoracic or abdominopelvic cavity move against one another or the cavity wall.

Serous membranes have special names according to their location. The serous membrane that lines the thoracic cavity and covers the lungs is the pleura, with the parietal pleura lining the cavity and the visceral pleura covering the lungs. The pericardium (pair-ih-KAR-dee-um) lines the pericardial cavity and covers the heart. The serous mem­brane in the abdominopelvic cavity is the peritoneum (pair-ih-toh-NEE-um).

Synovial Membranes

Synovial (sih-NOH-vee-al) membranes are connective tissue membranes that line the cavities of the freely movable joints such as the shoulder, elbow, and knee. Similar to serous membranes, they line cavities that do not open to the outside. Unlike serous membranes, they do not have a layer of epithelium. Synovial membranes secrete synovial fluid into the joint cavity, and this lubricates the cartilage on the ends of the bones so that they can move freely and without friction. In certain types of arthritis, these membranes become inflamed and the fluid becomes viscous. This reduces lubrication and increases friction, and movement becomes difficult and painful.

Meninges The connective tissue coverings around the brain and spinal cord, within the dorsal cavity, are called meninges (meh-NIN-jeez). They provide protection for these vital structures. The outermost layer of the meninges is the toughest and is called the dura mater (DOO-rah MAY-ter). The middle layer, the arachnoid (ah-RAK-noyd), is quite fragile. The pia mater (PEE-ah MAY-ter), the innermost layer, is delicate and closely adherent to the surface of the brain and spinal cord.