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Chapter 1. The Human Body: An Orientation

The Human Body:An Orientation

Your Goals

After completing this chapter, you will have mastered the objectives listed below.

Objective Checklist

An Overview of Anatomy and Physiology  (p. 2)

°      Define anatomy and physiology.

°      Explain how anatomy and physiology are related.

Levels of Structural Organization  (pp. 2-7)

°      Name the levels of structural organization that make up the human body and explain how they are related.

°      Name the organ systems of the body and briefly state the major functions of each system.

°      Classify by organ system all organs discussed.

°      Identify the organs shown on a diagram or a dissectible torso.

Maintaining Life  (pp. 7)

°      List functions that humans must perform to maintain life.

°      List the survival needs of the human body.

Homeostasis  (pp. 11)

°      Define homeostasis and explain its importance.

°      Define negative feedback and describe its role in maintaining homeostasis and normal body function.

The Language of Anatomy  (pp. 11=17)

°      Describe the anatomical position verbally or demonstrate it.

°      Use proper anatomical terminology to describe body directions, surfaces, and body planes.

°      Locate the major body cavities and list the chief organs in each cavity.

1An Overview of Anatomy and Physiology

Most of us are naturally curious about our bodies; we want to know what makes us tick. This curiosity is seen in infants, who can keep themselves happy for a long time staring at their own hands or pulling their mother’s nose. Older children wonder where food goes when they swallow it, and some believe that they will grow a watermelon in their belly if they swallow the seeds. They scream loudly when approached by medical personnel (fearing shots that sting), but they like to play doctor. Adults become upset when their hearts pound, when they have uncontrollable hot flashes, or when they cannot keep their weight down.

Anatomy and physiology, subdivisions of biology, explore many of these topics as they describe how our bodies are put together and how they work.

Anatomy

Anatomy (ah-nat-o-me) is the study of the structure and shape of the body and body parts and their relationships to one another. Whenever we look at our own body or study large body structures such as the heart or bones, we are observing gross anatomy; that is, we are studying large, easily observable structures. Indeed, the term anatomy, derived from the Greek words meaning to cut (tomy) apart (ana), is related most closely to gross anatomy studies, because in such studies preserved animals or their organs are dissected (cut up) to be examined. On the other hand, if a microscope or magnifying instrument is used to see very small structures in the body, we are studying microscopic anatomy. The cells and tissues of the body can only be seen through a microscope.

Physiology

Physiology (fiz0e-ol-o-je) is the study of how the body and its parts work or function (physio 5 nature; ology 5 the study of). Like anatomy, physiology has many subdivisions. For example, neurophysiology explains the workings of the nervous system, and cardiac physiology studies the function of the heart, which acts as a muscular pump to keep blood flowing throughout the body.

Relationship between
Anatomy and Physiology

In the real world, anatomy and physiology are always related. The parts of your body form a well-organized unit, and each of those parts has a job to do to make the body operate as a whole. Structure determines what functions can take place. For example, the lungs are not muscular chambers like the heart and cannot pump blood through the body, but because the walls of their air sacs are very thin, they can exchange gases and provide oxygen to the body. The intimate relationship between anatomy and physiology is stressed throughout this book to make your learning meaningful.

Levels of Structural Organization

From Atoms to Organisms

The human body exhibits many levels of structural complexity (Figure 1.1). The simplest level of the structural ladder is the chemical level, which we will study in Chapter 2. At this level, atoms, tiny building blocks of matter, combine to form molecules such as water, sugar, and proteins. Molecules, in turn, associate in specific ways to form microscopic cells, the smallest units of all living things. The cellular level is examined in Chapter 3. Individual cells vary widely in size and shape, reflecting their particular functions in the body.

The simplest living creatures are composed of single cells, but in complex organisms like human beings, the structural ladder continues on to the tissue level. Tissues consist of groups of similar cells that have a common function. As we will discuss in Chapter 3, each of the four basic tissue types plays a definite but different role in the body.

An organ is a structure, composed of two or more tissue types, that performs a specific function for the body. At the organ level of organization, extremely complex functions become possible. For example, the small intestine, which digests and absorbs food, is composed of all four tissue types. All the body’s organs are grouped so that a number of organ systems are formed. An organ system is a group of organs that cooperate to accomplish a common purpose. For example, the digestive system includes the esophagus, the stomach, and the small and large intestines (to name a few of its organs). Each organ has its own job to do, and working together, they keep food moving through the digestive system so that it is properly broken down and absorbed into the blood, providing fuel for all the body’s cells. In all, 11 organ systems make up the living body, or the organism, which represents the highest level of structural organization, the organismal level. The major organs of each of the systems are shown in Figure 1.2. Refer to the figure as you read through the following descriptions of the organ systems.

Organ System Overview

Integumentary System

The integumentary (in-teg0u-men-tar-e) system is the external covering of the body, or the skin. It waterproofs the body and cushions and protects the deeper tissues from injury. It also excretes salts and urea in perspiration and helps regulate body temperature. Temperature, pressure, and pain receptors located in the skin alert us to what is happening at the body surface.

Skeletal System

The skeletal system consists of bones, cartilages, ligaments, and joints. It supports the body and provides a framework that the skeletal muscles use to cause movement. It also has a protective function (for example, the skull encloses and protects the brain). Hematopoiesis (hem0ah-to-poi-e-sis), or formation of blood cells, goes on within the cavities of the skeleton. The hard substance of bones acts as a storehouse for minerals.

Muscular System

The muscles of the body have only one function—to contract, or shorten. When this happens, movement occurs. Hence, muscles can be viewed as the “machines” of the body. The mobility of the body as a whole reflects the activity of skeletal muscles, the large, fleshy muscles attached to bones. When these contract, you are able to walk, leap, grasp, throw a ball, or smile. The skeletal muscles form the muscular system. These muscles are distinct from the muscles of the heart and of other hollow organs, which move fluids (blood, urine) or other substances (such as food) along definite pathways within the body.

Nervous System

The nervous system is the body’s fast-acting control system. It consists of the brain, spinal cord, nerves, and sensory receptors. The body must be able to respond to irritants or stimuli coming from outside the body (such as light, sound, or changes in temperature) and from inside the body (such as decreases in oxygen or stretching of tissue). The sensory receptors detect these changes and send messages (via electrical signals called nerve impulses) to the central nervous system (brain and spinal cord) so that it is constantly informed about what is going on. The central nervous system then assesses this information and responds by activating the appropriate body muscles or glands.

Endocrine System

Like the nervous system, the endocrine (en-do-krin) system controls body activities, but it acts much more slowly. The endocrine glands produce chemical molecules called hormones and release them into the blood to travel to relatively distant target organs.

The endocrine glands include the pituitary, thyroid, parathyroids, adrenals, thymus, pancreas, pineal, ovaries (in the female), and testes (in the male). The endocrine glands are not connected anatomically in the same way that parts of the other organ systems are. What they have in common is that they all secrete hormones, which regulate other structures. The body functions controlled by hormones are many and varied, involving every cell in the body. Growth, reproduction, and food use by cells are all controlled (at least in part) by hormones.

Cardiovascular System

The primary organs of the cardiovascular system are the heart and blood vessels. Using blood as the transporting fluid, the cardiovascular system carries oxygen, nutrients, hormones, and other substances to and from the tissue cells where exchanges are made. White blood cells and chemicals in the blood help to protect the body from such “foreign invaders” as bacteria, toxins, and tumor cells. The heart acts as the blood pump, propelling blood through the blood vessels to all body tissues.

Lymphatic System

The role of the lymphatic system is complementary to that of the cardiovascular system. Its organs include lymphatic vessels, lymph nodes, and other lymphoid organs such as the spleen and tonsils. The lymphatic vessels return fluid leaked from the blood to the blood vessels so that blood can be kept continuously circulating through the body. The lymph nodes (and other lymphoid organs) help to cleanse the blood and house the cells involved in immunity.

Respiratory System

The jobs of the respiratory system are to keep the body constantly supplied with oxygen and to remove carbon dioxide. The respiratory system consists of the nasal passages, pharynx, larynx, trachea, bronchi, and lungs. Within the lungs are tiny air sacs. It is through the thin walls of these air sacs that gas exchanges are made to and from the blood.

Digestive System

The digestive system is basically a tube running through the body from mouth to anus. The organs of the digestive system include the oral cavity (mouth), esophagus, stomach, small and large intestines, and rectum. Their role is to break down food and deliver the products to the blood for dispersal to the body cells. The undigested food that remains in the tract leaves the body through the anus as feces. The breakdown activities that begin in the mouth are completed in the small intestine. From that point on, the major function of the digestive system is to reclaim water. The liver is considered to be a digestive organ, because the bile it produces helps to break down fats. The pancreas, which delivers digestive enzymes to the small intestine, also is functionally a digestive organ.

Urinary System

As it functions, the body produces wastes, which must be disposed of. One type of waste is nitrogen-containing waste (such as urea and uric acid), which results from the breakdown of proteins and nucleic acids by the body cells. The urinary system removes the nitrogen-containing wastes from the blood and flushes them from the body in urine. This system, often called the excretory system, is composed of the kidneys, ureters, bladder, and urethra. Other important functions of this system include maintaining the body’s water and salt balance and regulating the acid-base balance of the blood.

Reproductive System

The reproductive system exists primarily to produce offspring. Sperm are produced by the testes of the male. Other male reproductive system structures are the scrotum, penis, accessory glands, and the duct system, which carries sperm to the outside of the body. The ovary of the female produces the eggs, or ova; the female duct system consists of the uterine tubes, uterus, and vagina. The uterus provides the site for the development of the fetus (immature infant) once fertilization has occurred.

Maintaining Life

Necessary Life Functions

Now that we have introduced the structural levels composing the human body, the question that naturally follows is: What does this highly organized human body do? Like all complex animals, human beings maintain their boundaries, move, respond to environmental changes, take in and digest nutrients, carry out metabolism, dispose of wastes, reproduce themselves, and grow. We will discuss each of these necessary life functions briefly here and in more detail in later chapters.

Organ systems do not work in isolation; instead, they work together to promote the well-being of the entire body. Because this theme will be emphasized throughout this book, it is worthwhile to identify the most important organ systems contributing to each of the necessary life functions (Figure 1.3). Also, as you read through this material, you may want to refer back to the more detailed descriptions of the organ systems provided on pp. 3 through 6 and in Figure 1.2.

Maintaining Boundaries

Every living organism must be able to maintain its boundaries so that its “inside” remains distinct from its “outside.” Every cell of the human body is surrounded by an external membrane that contains its contents and allows needed substances in while restricting the entry of potentially damaging or unnecessary substances. Additionally, the body as a whole is enclosed by the integumentary system, or skin. The integumentary system protects internal organs from drying out (which would be fatal), from bacteria, and from the damaging effects of heat, sunlight, and an unbelievable number of chemical substances in the external environment.

Movement

Movement includes all the activities promoted by the muscular system, such as propelling ourselves from one place to another by walking, swimming, and so forth, and manipulating the external environment with our fingers. The muscular system is aided by the skeletal system, which provides the bones that the muscles pull on as they work. Movement also occurs when substances such as blood, foodstuffs, and urine are propelled through the internal organs of the cardiovascular, digestive, and urinary systems, respectively.

Responsiveness

Responsiveness, or irritability, is the ability to sense changes (stimuli) in the environment and then to react to them. For example, if you cut your hand on broken glass, you involuntarily pull your hand away from the painful stimulus (the glass). It is not necessary to think about it—it just happens! Likewise, when the amount of carbon dioxide in your blood rises to dangerously high levels, the response is an increase in your breathing rate to blow off the excess carbon dioxide.

Because nerve cells are highly irritable and can communicate rapidly with each other by conducting electrical impulses, the nervous system bears the major responsibility for responsiveness. However, all body cells exhibit responsiveness to some extent.

Digestion

Digestion is the process of breaking down ingested food into simple molecules that can then be absorbed into the blood for delivery to all body cells by the cardiovascular system. In a simple, one-celled organism like an amoeba, the cell itself is the “digestion factory,” but in the complex, multicellular human body, the digestive system performs this function for the entire body.

Metabolism

Metabolism is a broad term that refers to all chemical reactions that occur within body cells. It includes breaking down complex substances into simpler building blocks, making larger structures from smaller ones, and using nutrients and oxygen to produce ATP molecules, the energy-rich molecules that power cellular activities. Metabolism depends on the digestive and respiratory systems to make nutrients and oxygen available to the blood, and on the cardiovascular system to distribute these substances throughout the body. Metabolism is regulated chiefly by hormones secreted by the glands of the endocrine system.

Excretion

Excretion is the process of removing excreta (ek-skre-tah), or wastes, from the body. If the body is to continue to operate as we expect it to, it must get rid of the nonuseful substances produced during digestion and metabolism. Several organ systems participate in excretion. For example, the digestive system rids the body of indigestible food residues in feces, and the urinary system disposes of nitrogen-containing metabolic wastes in urine.

Reproduction

Reproduction, the production of offspring, can occur on the cellular or organismal level. In cellular reproduction, the original cell divides, producing two identical daughter cells that may then be used for body growth or repair. Reproduction of the human organism, or making a whole new person, is the task of the organs of the reproductive system, which produce sperm and eggs. When a sperm unites with an egg, a fertilized egg forms, which then develops into a bouncing baby within the mother’s body. The function of the reproductive system is exquisitely regulated by hormones of the endocrine system.

Growth

Growth is an increase in size, usually accomplished by an increase in the number of cells. For growth to occur, cell-constructing activities must occur at a faster rate than cell-destroying ones.

Survival Needs

The goal of nearly all body systems is to maintain life. However, life is extraordinarily fragile and requires that several factors be available. These factors, which we will call survival needs, include nutrients (food), oxygen, water, and appropriate temperature and atmospheric pressure.

Nutrients, taken in via the diet, contain the chemicals used for energy and cell building. Carbohydrates are the major energy-providing fuel for body cells. Proteins and to a lesser extent fats are essential for building cell structures. Fats also cushion body organs and provide reserve fuel. Minerals and vitamins are required for the chemical reactions that go on in cells and for oxygen transport in the blood.

All the nutrients in the world are useless unless oxygen is also available, because the chemical reactions that release energy from foods require oxygen. Approximately 20 percent of the air we breathe is oxygen. It is made available to the blood and body cells by the cooperative efforts of the respiratory and cardiovascular systems.

Water accounts for 60 to 80 percent of body weight. It is the single most abundant chemical substance in the body and provides the fluid base for body secretions and excretions. Water is obtained chiefly from ingested foods or liquids and is lost from the body by evaporation from the lungs and skin and in body excretions.

For good health, body temperature must be maintained at around 37°C (-8°F). As body temperature drops below this point, metabolic reactions become slower and slower and finally stop. When body temperature is too high, chemical reactions proceed too rapidly, and body proteins begin to break down. At either extreme, death occurs. Most body heat is generated by the activity of the skeletal muscles.

The force exerted on the surface of the body by the weight of air is referred to as atmospheric pressure. Breathing and the exchange of oxygen and carbon dioxide in the lungs depend on appropriate atmospheric pressure. At high altitudes, where the air is thin and atmospheric pressure is lower, gas exchange may be too low to support cellular metabolism.

The mere presence of these survival factors is not sufficient to maintain life. They must be present in appropriate amounts as well; excesses and deficits may be equally harmful. For example, the food ingested must be of high quality and in proper amounts; otherwise, nutritional disease, obesity, or starvation is likely.

Homeostasis

When you really think about the fact that your body contains trillions of cells in nearly constant activity, and that remarkably little usually goes wrong with it, you begin to appreciate what a marvelous machine your body really is. The word homeostasis (ho0me-o-sta-sis) describes the body’s ability to maintain relatively stable internal conditions even though the outside world is continuously changing. Although the literal translation of homeostasis is “unchanging” (homeo 5 the same; stasis 5 standing still), the term does not really mean an unchanging state. Instead, it indicates a dynamic state of equilibrium, or a balance in which internal conditions change and vary, but always within relatively narrow limits.

In general, the body is in homeostasis when its needs are being adequately met and it is functioning smoothly. Virtually every organ system plays a role in maintaining the constancy of the internal environment. Adequate blood levels of vital nutrients must be continuously present, and heart activity and blood pressure must be constantly monitored and adjusted so that the blood is propelled with adequate force to reach all body tissues. Additionally, wastes must not be allowed to accumulate, and body temperature must be precisely controlled.

Homeostatic Control Mechanisms

Communication within the body is essential for homeostasis and is accomplished chiefly by the nervous and endocrine systems, which use electrical signals delivered by nerves or bloodborne hormones, respectively, as information carriers. The details of how these two great regulating systems operate are the subjects of later chapters, but the basic characteristics of the neural and hormonal control systems that promote homeostasis will be explained here.

Regardless of the factor or event being regulated (this is called the variable), all homeostatic control mechanisms have at least three components (Figure 1.4). The first component is a receptor. Essentially, it is some type of sensor that monitors and responds to changes in the environment. It responds to such changes, called stimuli, by sending information (input) to the second element, the control center. Information flows from the receptor to the control center along the afferent pathway.

The control center, which determines the level (set point) at which a variable is to be maintained, analyzes the information it receives and then determines the appropriate response or course of action.

The third component is the effector, which provides the means for the control center’s response (output) to the stimulus. Information flows from the control center to the effector along the efferent pathway. The results of the response then feed back to influence the stimulus, either depressing it (negative feedback) so that the whole control mechanism is shut off, or enhancing it (positive feedback) so that the reaction continues at an even faster rate.

Most homeostatic control mechanisms are negative feedback mechanisms. In such systems, the net effect of the response to the stimulus is to shut off the original stimulus or reduce its intensity. A frequently used example of a negative feedback system is a home heating system connected to a thermostat. In this situation, the thermostat contains both the receptor and the control center. If the thermostat is set at 20°C (68°F), the heating system (effector) will be triggered ON when the house temperature drops below that setting. As the furnace produces heat, the air is warmed. When the temperature reaches 20°C or slightly higher, the thermostat sends a signal to shut off the furnace. Your body “thermostat,” located in a part of your brain called the hypothalamus, operates in a similar way to regulate body temperature. Other negative feedback mechanisms regulate heart rate, blood pressure, breathing rate, and blood levels of glucose, oxygen, carbon dioxide, and minerals.

Because they tend to increase the original disturbance (stimulus) and to push the variable farther from its original value, positive feedback mechanisms are much more rare in the body. Typically these mechanisms control infrequent events that occur explosively and do not require continuous adjustments. Blood clotting and the birth of a baby are the most familiar examples of positive feedback mechanisms.

Homeostatic Imbalance

Homeostasis is so important that most disease is regarded as a result of its disturbance, a condition called homeostatic imbalance. As we age, our body organs become less efficient, and our internal conditions become less and less stable. These events place us at an ever greater risk for illness and produce the changes we associate with aging.

Examples of homeostatic imbalance will be provided throughout this book to enhance your understanding of normal physiological mechanisms. These homeostatic imbalance sections are preceded by the symbol  to alert you that an abnormal condition is being described.

The Language
of Anatomy

Learning about the body is exciting, but our interest sometimes dwindles when we are confronted with the terminology of anatomy and physiology. Let’s face it. You can’t just pick up an anatomy and physiology book and read it as though it were anovel. Unfortunately, confusion is inevitable without specialized terminology. For example, if you are looking at a ball, “above” always means the area over the top of the ball. Other directional terms can also be used consistently because the ball is a sphere. All sides and surfaces are equal. The human body, of course, has many protrusions and bends. Thus, the question becomes: Above what? To prevent misunderstanding, anatomists have accepted a set of terms that allow body structures to be located and identified clearly with just a few words. This language of anatomy is presented and explained next.

Anatomical Position and
Directional Terms

To accurately describe body parts and position, we must have an initial reference point and use directional terms. To avoid confusion, it is always assumed that the body is in a standard position called the anatomical position. It is important to understand this position because most body terminology used in this book refers to this body positioning regardless of the position the body happens to be in. The face-front diagrams in Table 1.1 and Figure 1.5 illustrate the anatomical position. As you can see, the body is erect with the feet parallel and the arms hanging at the sides with the palms facing forward.

Stand up and assume the anatomical position. Notice that it is similar to “standing at attention” but is less comfortable because the palms are held unnaturally forward with thumbs pointing away from the body rather than hanging cupped toward the thighs.

Directional terms used by medical personnel and anatomists allow them to explain exactly where one body structure is in relation to another. For example, we could describe the relationship between the ears and the nose informally by saying, “The ears are located on each side of the head to the right and left of the nose.” Using anatomical terminology, this condenses to, “The ears are lateral to the nose.” Thus, using anatomical terminology saves a good deal of description and, once learned, is much clearer. Commonly used directional terms are defined and illustrated in Table 1.1. Although most of these terms are also used in everyday conversation, keep in mind that their anatomical meanings are very precise.

Before continuing, take a minute to check your understanding of what you have read in Table 1.1. Give the relationship between the following body parts using the correct anatomical terms.

The wrist is  to the hand.

The breastbone is  to the spine.

The brain is  to the spinal cord.

The lungs are  to the stomach.

The thumb is  to the fingers. (Be careful here. Remember the anatomical position.)

Regional Terms

There are many visible landmarks on the surface of the body. Once you know their proper anatomical names, you can be specific in referring to different regions of the body.

Anterior Body Landmarks

Look at Figure 1.5a to find the following body regions. Once you have identified all the anterior body landmarks, cover the labels that describe what the structures are, and again go through the list, pointing out these areas on your own body.

abdominal (ab-dom-˘ı-nal): anterior body trunk inferior to ribs

acromial (ah-kro-me-ul): point of shoulder

antecubital (an0te-ku-b˘ı-tal): anterior surface of elbow

axillary (ak-s˘ı-lar0e): armpit

brachial (bra-ke-al): arm

buccal (buk-al): cheek area

carpal (kar-pal): wrist

cervical (ser-v˘ı-kal): neck region

coxal (kox-al): hip

crural (kroo-ral): leg

digital (dij-˘ı-tal): fingers, toes

femoral (fem-or-al): thigh

fibular (fib-u-lar): lateral part of leg

inguinal (in-gw˘ı-nal): area where thigh meets body trunk; groin

nasal (na-zul): nose area

oral (o-ral): mouth

orbital (or-b˘ı-tal): eye area

patellar (pah-tel-er): anterior knee

pelvic (pel-vik): area overlying the pelvis anteriorly

pubic (pu-bik): genital region

sternal (ster-nul): breastbone area

tarsal (tar-sal): ankle region

thoracic (tho-ras-ik): chest

umbilical (um-bil-˘ı-kal): navel

Posterior Body Landmarks

Identify the following body regions in Figure 1.5b, and then locate them on yourself without referring to this book.

cephalic (seh-f˘a-lik): head

deltoid (del-toyd): curve of shoulder formed by large deltoid muscle

gluteal (gloo-te-al): buttock

lumbar (lumbar): area of back between ribs and hips

occipital (ok-sip-˘ı-tal): posterior surface of head

popliteal (pop-lit-e-al): posterior knee area

sacral (sa-krul): area between hips

scapular (skap-u-lar): shoulder blade region

sural (soo-ral): the posterior surface of lower leg; the calf

vertebral (ver-t˘e-bral): area of spine

Body Planes and Sections

When preparing to look at the internal structures of the body, medical students find it necessary to make a section, or cut. When the section is made through the body wall or through an organ, it is made along an imaginary line called a plane. Since the body is three-dimensional, we can refer to three types of planes or sections that lie at right angles to one another (Figure 1.6).

A sagittal (saj-˘ı-tal) section is a cut made along the lengthwise, or longitudinal, plane of the body, dividing the body into right and left parts. If the cut is made down the median plane of the body and the right and left parts are equal in size, it is called a midsagittal, or median, section.

A frontal section is a cut made along a lengthwise plane that divides the body (or an organ) into anterior and posterior parts. It is also called a coronal (ko-ro-nal) section.

A transverse section is a cut made along a horizontal plane, dividing the body or organ into superior and inferior parts. It is also called a cross section.

Sectioning a body or organ along different planes often results in very different views. For example, a transverse section of the body trunk at the level of the kidneys would show kidney structure in cross section very nicely; a frontal section of the body trunk would show a different view of kidney anatomy; and a midsagittal section would miss the kidneys completely. Information on body organ positioning that can be gained by taking MRI scans along different body planes is illustrated in Figure 1.6. (MRI scans are described in the “Closer Look” box on pp. 28–2- in Chapter 2.)

Body Cavities

The body has two sets of internal cavities that provide different degrees of protection to the organs within them (Figure 1.7).

Dorsal Body Cavity

The dorsal body cavity has two subdivisions, which are continuous with each other. The cranial cavity is the space inside the bony skull. The brain is well protected because it occupies the cranial cavity. The spinal cavity extends from the cranial cavity nearly to the end of the vertebral column. The spinal cord, which is a continuation of the brain, is protected by the vertebrae, which surround the spinal cavity.

Ventral Body Cavity

The ventral body cavity is much larger than the dorsal cavity. It contains all the structures within the chest and abdomen. Like the dorsal cavity, the ventral body cavity is subdivided. The superior thoracic cavity is separated from the rest of the ventral cavity by a dome-shaped muscle, the diaphragm (di-ah-fram). The organs in the thoracic cavity (lungs, heart, and others) are somewhat protected by the rib cage. The cavity inferior to the diaphragm is the abdominopelvic (ab-dom0˘ı-no-pel-vik) cavity. Some prefer to subdivide it into a superior abdominal cavity, containing the stomach, liver, intestines, and other organs, and an inferior pelvic cavity, with the reproductive organs, bladder, and rectum. However, there is no actual physical structure dividing the abdominopelvic cavity. If you look carefully at Figure 1.7, you will see that the pelvic cavity is not continuous with the abdominal cavity in a straight plane, but that it tips away from it in the posterior direction.

Homeostatic ImbalanceWhen the body is subjected to physical trauma (as often happens in an automobile accident, for example), the most vulnerable abdominopelvic organs are those within the abdominal cavity, because the cavity walls of that portion are formed only of trunk muscles and are not reinforced by bone. The pelvic organs receive a somewhat greater degree of protection from the bony pelvis in which they reside. s

Because the abdominopelvic cavity is quite large and contains many organs, it is helpful to divide it up into smaller areas for study. A scheme commonly used by medical personnel divides the abdominopelvic cavity into four more or less equal regions called quadrants. The quadrants are then simply named according to their relative positions—that is, right upper quadrant, right lower quadrant, left upper quadrant, and left lower quadrant (Figure 1.8a).

Another system, used mainly by anatomists, divides the abdominopelvic cavity into nine separate regions by four planes, as shown in Figure 1.8b. Although the names of the nine regions are unfamiliar to you now, with a little patience and study they will become easier to remember. As you locate these regions in the figure, notice the organs they contain by referring to Figure 1.8c.

The umbilical region is the centermost region, deep to and surrounding the umbilicus (navel).

The epigastric (ep0˘ı-gas-trik) region is located superior to the umbilical region (epi =upon, above; gastric= stomach).

The hypogastric (pubic) region is inferior to the umbilical region (hypo =below).

The right and left iliac, or inguinal, regions are lateral to the hypogastric region (iliac = superior part of the hip bone).

The right and left lumbar regions lie lateral to the umbilical region (lumbus =loin).

The right and left hypochondriac (hi0po-kon-dre-ak) regions flank the epigastric region and contain the lower ribs (chondro= cartilage).

 

Table 1.1 Orientation and Directional Terms

Term            Definition            Illustration            Example

Superior (cranial            Toward the head end or upper                         The forehead is superior
or cephalad)            part of a structure or the body;                         to the nose.
            above

 

 

Inferior (caudal)            Away from the head end or                         The navel is inferior to the
            toward the lower part of a                         breastbone.
            structure or the body; below

 

 

Anterior (ventral)*            Toward or at the front of                         The breastbone is anterior to
            the body; in front of                        the spine.

Posterior (dorsal)*            Toward or at the backside of                         The heart is posterior to the
            the body; behind                        breastbone.

Medial            Toward or at the midline of the                         The heart is medial to the
            body; on the inner side of                        arm.

 

Lateral            Away from the midline of the                         The arms are lateral to the
            body; on the outer side of                        chest.

 

Intermediate            Between a more medial and a                         The armpit is intermediate
            more lateral structure                        between the breastbone and                                     shoulder.

Proximal            Close to the origin of the body                         The elbow is proximal to the
            part or the point of attachment                         wrist (meaning that the                         of a limb to the body trunk                        elbow is closer to the shoulder or attachment point of the arm than the wrist is).

 

Distal            Farther from the origin of a                         The knee is distal to the                         body part or the point of                         thigh.
            attachment of a limb to the
            body trunk

 

Superficial            Toward or at the body surface                        The skin is superficial                                                             to the skeleton.

Deep            Away from the body surface;                         The lungs are deep to the                         more internal                        rib cage.

*Ventral and anterior are synonymous in humans; this is not the case in four-legged animals. Ventral refers to the “belly” of an animal and thus is the inferior surface of four-legged animals. Likewise, although the dorsal and posterior surfaces are the same in humans, the term dorsal refers to an animal’s back. Thus, the dorsal surface of four-legged animals is their superior surface.

Focus on Careers

Dental Hygienist

 “We learned every nerve and blood vessel above the shoulders.”

Dental hygienists need thorough training in human anatomy and physiology in order to properly care for their patients.

Feel a bit nervous when getting your teeth cleaned? Many people do. But probably you’ll feel better after you hear how well trained your dental hygienist is.

Dental hygienists are licensed preventive oral health professionals who provide educational, clinical, and therapeutic services to the public. Take Robin Mendica, a hygienist for 15 years, who holds an associate degree in dental hygiene from a community college. Mendica’s course work included extensive training in whole-body anatomy and physiology, plus specialized classes that focused on head and neck anatomy. “We learned every nerve and blood vessel above the shoulders,” she recalls. “This is important because dental hygienists can give injections of anesthetic for deep cleaning and root cleaning procedures. You have to know where the nerves are to minimize pain and avoid injuries that could lead to facial paralysis.”

When patients come into the office, Mendica conducts a preliminary examination: “I look at their dental X rays and charts to remind myself of any problems or concerns they may have. Then I examine the mouth inside and look at their teeth, tongue, crowns, and fillings. I check for any signs of tooth decay or decay under the gingiva.” Then Mendica polishes patients’ teeth, removing stains, tartar, and plaque (a soft, sticky deposit of bacteria that leads to periodontal problems) in preparation for the dentist’s examination.

Mendica plays an important role in alerting the dentist to symptoms of dental problems and other health conditions. “Untreated dental conditions can lead to trouble in other parts of the body; I’ve seen tooth abscesses spread into the sinuses and cause bad infections. If people have lost a lot of teeth, they can’t chew food properly and that can cause digestive problems. In school we also learned to recognize nodules, pre-cancerous conditions, and oral cancer.”

What does Mendica enjoy most about her work? “I love dealing with people. Every patient is different, and I love teaching them about good oral hygiene and motivating them to take care of their teeth.” She tells of many patients she has helped, such as the man who tried to “glue” a rotten molar back in his mouth with bubble gum, or the hapless woman who attempted to brighten stained bridge work with white nail polish. “I consider patient education an important part of my job, because I’m a big advocate of regular dental exams. Getting your teeth cleaned every six months is an investment in your lifelong health.”

Dental hygiene education requires a high school diploma or GED, and generally lasts two to four years. Two-year programs offer a diploma, certificate, or associate degree; four-year programs offer a bachelor’s degree. Master’s degrees are available for those interested in education, research, or administration.

An accredited dental hygiene program requires an average of 1,-48 clock hours of curriculum. This includes 585 clock hours of supervised clinical instruction and courses in anatomy, physiology, chemistry, and microbiology. In addition, dental hygienists must be licensed by the state in which they practice. Licensing requirements vary from state to state but usually include

n Graduation from an accredited dental hygiene program

n Successful completion of the written National Board Dental Hygiene Examination

n Successful completion of a regional or state clinical board examination

Please note that accreditation procedures vary from state to state. To learn more, contact the American Dental Hygienists Association (ADHA) at

444 N. Michigan Avenue,
Suite 3400

Chicago, IL 60611

(312)440-8-00or(800) 735-5121

http://www.adha.org

An Overview of Anatomy and Physiology  (p. 2)

1. Anatomy is the study of structure. Observation is used to see the sizes and relationships of body parts.

2. Physiology is the study of how a structure (which may be a cell, an organ, or an organ system) functions or works.

3. Structure determines what functions can occur; therefore, if the structure changes, the function must also change.

Levels of Structural Organization  (pp. 2=7)

1. There are six levels of structural organization. Atoms (at the chemical level) combine, forming the unit of life, the cell. Cells are grouped into tissues, which in turn are arranged in specific ways to form organs. A number of organs form an organ system, which performs a specific function for the body (which no other organ system can do). Together, all of the organ systems form the organism, or living body.

 Exercise: Chapter 1, Levels of Biological Organization.

2. For a description of organ systems naming the major organs and functions, see pp. 3–7.

Maintaining Life  (pp.7=-)

1. To sustain life, an organism must be able to maintain its boundaries, move, respond to stimuli, digest nutrients and excrete wastes, carry on metabolism, reproduce itself, and grow.

2. Survival needs include food, oxygen, water, appropriate temperature, and normal atmospheric pressure. Extremes of any of these factors can be harmful.

Homeostasis  (pp. -=11)

1. Body functions interact to maintain homeostasis, or a relatively stable internal environment within the body. Homeostasis is necessary for survival and good health; its loss results in illness or disease.

2. All homeostatic control mechanisms have a receptor that responds to environmental changes, and a control center that assesses those changes and produces a response by activating a third element, the effector.

3. Most homeostatic control systems are negative feedback systems, which act to reduce or stop the initial stimulus.

The Language of Anatomy  (pp. 11=17)

1. Anatomical terminology is relative and assumes that the body is in the anatomical position (erect, palms facing forward).

2. Directional terms

a. Superior (cranial, cephalad): above something else, toward the head.

b. Inferior (caudal): below something else, toward the tail.

c. Anterior (ventral): toward the front of the body or structure.

d. Posterior (dorsal): toward the rear or back of the body or structure.

e. Medial: toward the midline of the body.

f. Lateral: away from the midline of the body.

g Intermediate: between a more medial and a more lateral structure.

h. Proximal: closer to the point of attachment.

i. Distal: farther from the point of attachment.

j. Superficial (external): at or close to the body surface.

k. Deep (internal): below or away from the body surface.

3. Body planes and sections

a. Sagittal section: separates the body longitudinally into right and left parts.

b. Frontal (coronal) section: separates the body on a longitudinal plane into anterior and posterior parts.

c. Transverse (cross) section: separates the body on a horizontal plane into superior and inferior parts.

 Exercise: Chapter 1, Body Planes.

4. Regional terms. Visible landmarks on the body surface may be used to specifically refer to a body part or area. See p. 13 for terms referring to anterior and posterior surface anatomy.

 Exercise: Chapter 1, Anatomical Terminology: Orientation and Directional Terms.

5. Body cavities

a. Dorsal: well protected by bone; has two subdivisions.

(1) Cranial: contains the brain.

(2) Spinal: contains the spinal cord.

b. Ventral: less protected than dorsal cavity; has two subdivisions.

(1) Thoracic: The superior cavity that extends inferiorly to the diaphragm; contains heart and lungs, which are protected by the rib cage.

(2) Abdominopelvic: The cavity inferior to the diaphragm that contains the digestive, urinary, and reproductive organs. The abdominal portion is vulnerable because it is protected only by the trunk muscles. There is some protection of the pelvic portion by the bony pelvis. The abdominopelvic cavity is often divided into four quadrants or nine regions (see Figure 1.8).

 Exercise: Chapter 1, Dorsal and Ventral Cavities.

Review Questions

Multiple Choice*

1. Consider the following levels: (1) chemical; (2) tissue; (3) organ; (4) cellular; (5) organismal; (6) systemic. Which of the following choices has the levels listed in order of increasing complexity?

a. 1, 2, 3, 4, 5, 6 d. 1, 4, 2, 3, 6, 5

b. 1, 4, 2, 5, 3, 6 e. 4, 1, 3, 2, 6, 5

c. 3, 1, 2, 4, 6, 5

2. Which of the following is (are) involved in maintaining homeostasis?

a. Effector d. Feedback

b. Control center e. Lack of change

c. Receptor

3. Which is not essential to survival?

a. Water d. Atmospheric pressure

b. Oxygen e. Nutrients

c. Gravity

4. A neurosurgeon orders a spinal tap for a patient. Into what body cavity will the needle be inserted?

a. Ventral d. Cranial

b. Thoracic e. Pelvic

c. Dorsal

5. Which of the following groupings of the abdominopelvic regions is medial?

a. Hypochondriac, hypogastric, umbilical

b. Hypochondriac, lumbar, inguinal

c. Hypogastric, umbilical, epigastric

d. Lumbar, umbilical, iliac

e. Iliac, umbilical, hypochondriac

Short Answer Essay

1. Define anatomy and physiology.

2. Why would you have a hard time trying to learn and understand physiology if you did not also understand anatomy?

3. List the 11 organ systems of the body, briefly describe the function of each, and then name two organs in each system.

4. In addition to being able to metabolize, grow, digest food, and excrete wastes, what functions must an organism perform if it is to survive?

5. Define homeostasis.

6. What is the consequence of loss of homeostasis, or homeostatic imbalance?

7. Describe the anatomical position.

8. On what body surface is each of the following located: nose, calf of leg, ears, umbilicus, fingernails?

-. Several pairs of structures are given next. In each case, choose the one that meets the condition given first.

a. Distal—the knee/the foot

b. Lateral—the cheekbone/the nose

c. Superior—the neck/the chin

d. Anterior—the heel/the toenails

e. External—the skin/the skeletal muscles

10. What kind of section would have to be made to cut the brain into anterior and posterior parts?

11. Which of the following organ systems—digestive, respiratory, reproductive, circulatory, urinary, or muscular—are found in both subdivisions of the ventral body cavity? Which are found in the thoracic cavity only? In the abdominopelvic cavity only?

At the Clinic

1. A nurse informed John that she was about to take blood from his antecubital region. What part of his body was she referring to? Later, she came back and said that she was going to give him an antibiotic shot in the deltoid region. Did he take off his shirt or drop his pants to get the shot? Before John left the office, the nurse noticed that his left sural region was badly bruised. What part of his body was bruised?

2. How is the concept of homeostasis (or its loss) related to disease and aging? Provide examples to support your reasoning.

3. When we begin to become dehydrated, we usually become thirsty, which causes us to drink fluids. On the basis of what you now know about control systems, decide whether the thirst sensation is part of a negative or positive feedback control system and defend your choice.

4. Jennie Dip fell off her motorcycle and tore a nerve in her axillary region. She also tore ligaments in her cervical and scapular regions and broke the only bone of her right brachial region. Explain where each of her injuries is located.

 

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