Chapter 1: The Human Organism PDF

Page 9 CHAPTER 1 The Human Organism  The human body is a complex system. The structures in the body work in concert to maintain homeostasis, a balance in the body’s internal environment....

Page 9 CHAPTER 1 The Human Organism  The human body is a complex system. The structures in the body work in concert to maintain homeostasis, a balance in the body’s internal environment. Page 10 LEARN TO APPLY IT Renzo, a dancer, can move his body such that it is perfectly balanced, yet a slight movement in any direction would cause him to adjust his position. The human body  adjusts its balance among all its parts through a process called homeostasis. Let’s imagine that Renzo is suffering from a blood sugar disorder. Earlier, he’d eaten an energy bar. As an energy bar is digested, blood sugar rises. Normally, tiny collections of cells embedded in the pancreas respond to the rise in blood sugar by secreting the chemical insulin. Insulin increases the movement of sugar from the blood into the cells. However, Renzo did not feel satisfied from his energy bar. He felt dizzy and was still hungry, all symptoms he worried could be due to a family history of diabetes. Fortunately, the on-site trainer tested his blood sugar and noted that it was much higher than normal. After a visit to his regular physician, Renzo was outfitted with an insulin pump and his blood sugar levels are more consistent. What is a good explanation for Renzo’s blood sugar levels before and after his visit to the doctor? Answers to this question and the chapter’s odd-numbered Apply It questions can be found in Appendix E. Module 1 Body Orientation 1.1 ANATOMY AND PHYSIOLOGY Learning Outcomes After reading this section, you should be able to A. Define anatomy and describe the levels at which anatomy can be studied. B. Explain the importance of the relationship between structure and function. C. Define physiology and describe the levels at which physiology can be studied. What lies ahead is a fantastic adventure—learning about the structure and function of the human body and the intricate checks and balances that regulate it. Perhaps you have had the experience of oversleeping, rushing to your 8 a.m. class, and missing breakfast. Afterward, on the way to your Anatomy & Physiology class, you bought an energy bar from the vending machine. Eating the energy bar helped you feel better. The explanation for this experience is the process of homeostasis, the maintenance of a relatively constant internal environment despite fluctuations in the external environment. For you, homeostasis was maintained, but for Renzo, the dancer in this chapter’s Learn to Apply It feature, there was a disruption in homeostasis. Throughout this book, the major underlying theme is homeostasis. As you think about Renzo’s case, you will come to realize just how capable the human body is of an incredible coordination of thousands upon thousands of processes. Learning about human anatomy and physiology is important for understanding disease. The study of human anatomy and physiology is important for students who plan a career in the health sciences because health  professionals need a sound knowledge of structure and function in order to perform their duties. In addition, understanding anatomy and physiology prepares all of us to evaluate recommended treatments, critically review advertisements and reports in the popular literature, and rationally discuss the human body with health professionals and nonprofessionals. Anatomy (ah-NAT-oh-mee) is the scientific discipline that investigates the structure of the body—for example, the parts and chambers of the heart. The word anatomy means to dissect, or cut apart and separate, the parts of the body for study. In addition, anatomy examines the relationship between the structure of a body part and its function. For example, the structure of a hammer informs us of its primary use of driving nails into wood. Similarly, the fact that bone tissue is a hard, mineralized substance enables the bones to sostems provide strength and support. Understanding the relationship between structure and function makes it easier to understand and appreciate anatomy. There are two basic approaches to the study of anatomy: (1) systemic anatomy and (2) regional anatomy. - Systemic anatomy is the study of the Xbarea body by systems, such as the cardiovascular, nervous, skeletal, and muscular systems. It is the approach taken in this textbook. Regional anatomy is the study of the organization of the body by areas. Within each region, such as the head, abdomen, or arm, all systems are studied simultaneously. This is the approach taken in many medical and dental schools. external Anatomists have two general ways to examine the internal structures of a living person: (1) surface anatomy and (2) anatomical imaging.z Surface anatomy is the study of external features, such as bony projections, which serve as landmarks for locating deeper structures. For example, the sternum (breastbone) is used by health professionals as a landmark for listening to - heart sounds. Anatomical imaging involves the use of x-rays, ultrasound, magnetic resonance imaging (MRI), and other technologies to create pictures of internal structures, such as when determining if a bone is broken or a ligament is torn. Both surface anatomy and anatomical imaging provide important information for diagnosing disease. Physiology (fiz-ee-OL-oh-jee; the study of nature) is the scientific discipline that deals with the processes or functions of living things—it is important in physiology to recognize structures as dynamic. There are two major goals when studying physiology: (1) examining the body’s responses to stimuli and (2) examining the body’s maintenance of stable internal conditions. Human physiology is the study of humans. Like anatomy, physiology can be studied at multiple levels. For example, cellular physiology focuses on processes inside cells such as the manufacturing of substances, including proteins, whereas systemic physiology focuses on the functions of organ systems. Anatomy : the study of body parts. Page 11 Physiology : the study of body parts' functions 1.2 STRUCTURAL AND FUNCTIONAL ORGANIZATION OF THE HUMAN BODY Learning Outcomes After reading this section, you should be able to A. Describe the six levels of organization of the body, and describe the major characteristics of each level. B. List the eleven organ systems, identify their components, and describe the major functions of each system. The body can be studied at six structural levels: chemical, cell, tissue, organ, organ system, and organism (figure 1.1). PROCESS Figure PROCESS Figure 1.1 Levels of Organization for the Human Body  The simplest level of organization in the human body is the atom. Atoms combine to form molecules. Molecules aggregate into cells. Cells form tissues, which combine with other tissues to form organs. Organs work in groups called organ systems. All organ systems work together to form an organism. 1.1(6) BJI/Blue Jean Images/Getty Images Why is the skin considered an organ? What characterizes the integumentary system as an organ system? 1 Chemical Level The structural and functional characteristics of all organisms are determined by their chemical makeup. The chemical level of organization involves how atoms, such as hydrogen and carbon, interact and combine into molecules. This is important because a molecule’s structure determines its function. For example, collagen molecules are strong, ropelike fibers that give skin structural strength and flexibility. With aging, the structure of collagen changes, and the skin becomes fragile and more easily torn during everyday activities. We present a brief overview of chemistry in chapter 2. 2 Cell Level Cells are the basic structural and functional units of organisms, such as plants and animals. Most cells contain smaller structures inside them, called organelles (OR-gah-nellz; - little organs). Organelles carry out particular functions, such as digestion and movement, for the cell. For example, the nucleus contains the cell’s hereditary information, and mitochondria manufacture adenosine triphosphate (ATP), a molecule cells use for a source of energy. Although cell types differ in their structure and function, they have many characteristics in common. Knowledge of these characteristics as well as their variations is essential to understanding anatomy and physiology. We discuss the cell in chapter 3. Page 12 MICROBES IN YOUR BODY Getting to Know Your Bacteria Did you know that you have more microbial cells than human cells in your body? Astoundingly, for every cell in your body, there is one microbial cell. That’s as many as 40 trillion microbial cells, which collectively can account for 2 to 6 pounds of your body weight! A microbe is any life form that can only be seen with a microscope (for example, bacteria, fungi, and protozoa). All living organisms fit into one of three domains of living organisms: (1) Bacteria, (2) Archaea, and (3) Eukarya. The cells of organisms in each domain are unique. Bacterial cells’ genetic material is not separated from the rest of the cell by a barrier. In addition, bacterial cells have far fewer separate structures made of membrane for carrying out the cell’s metabolic processes than eukaryotic cells. Archaea cells are constructed similarly to bacteria; however, they share certain structures, called ribosomes, with eukaryotic cells. We will discuss cell structure in detail in chapter 3. Commonly, the term prokaryotic is used to describe bacterial and archaea cells. Eukarya cells, which include human cells, have the most structural complexity with many smaller structures, called organelles, surrounded by membranes. These smaller structures conduct the metabolic processes of the cell. In addition to structural differences, there are many other differences far too numerous to adequately describe here. However, size differences between bacteria and archaea and cells of eukaryotes is quite evident with most eukaryotic cells being significantly larger than most prokaryotic cells. The total population of microbial cells on the human body is referred to as the microbiota, while the collection of all the microbial cell genes is known as the microbiome. The microbiota includes so-called good bacteria, which do not cause disease and may even help us. It also includes pathogenic, or “bad,” bacteria. With that many microbes in and on our bodies, you might wonder how they affect our health. To answer that question, the National Institutes of Health (NIH) initiated the Human Microbiome Project. Five significant regions of the human body were examined: the airway, skin, mouth, gastrointestinal tract, and vagina. This project identified over 5000 species and sequenced over 20 million unique microbial genes. What did scientists learn from the Human Microbiome Project? Human health is dependent upon the health of our microbiota, especially the “good” bacteria. More specifically, the human microbiome is intimately involved in the development and maintenance of the immune system. And more evidence is mounting for a correlation between a host’s microbiota, digestion, and metabolism. Researchers have suggested that microbial genes are more responsible for our survival than human genes are. There are even a few consistent pathogens that are present without causing disease, suggesting that their presence may be good for us. However, there does not seem to be a universal healthy human microbiome. Rather, the human microbiome varies across life span, ethnicity, nationality, culture, and geographic location. Instead of being a detriment, this variation may actually be very useful for predicting disease. There seems to be a correlation between autoimmune and inflammatory diseases (Crohn’s disease, asthma, multiple sclerosis), which have become more prevalent, and a “characteristic microbiome community.” Early research seems to indicate that any significant change in the profile of the microbiome of the human gut may increase a person’s susceptibility to autoimmune diseases. It has been proposed that these changes may be associated with exposure to antibiotics, particularly in infancy. Fortunately, newer studies of microbial transplantations have shown that the protective and other functions of bacteria can be transferred from one person to the next. However, this work is all very new, and much research remains to be done. Throughout this text, we will highlight specific instances in which our microbes influence our body systems. In light of the importance of our bodies’ bacteria and other microbes, the prevalence of antibacterial soap and hand gel usage in everyday life may be something to think about. 3 Tissue Level A tissue (TISH-you) is a group of similar cells and the materials surrounding them. The characteristics of the cells and surrounding materials determine the functions of the tissue. The many tissues that make up the body are classified into four primary types: (1) epithelial, (2) connective, (3) muscle, and (4) nervous. We discuss tissues in chapter 4. 4 Organ Level An organ (OR-gan; a tool) is composed of two or more tissue types that together perform one or more common functions. For example, the heart, stomach, liver, and urinary bladder are all organs (figure 1.2). 5 Organ System Level An organ system is a group of organs that together perform a common function or set of functions. For example, the urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys produce urine, which is transported by the ureters to the urinary bladder, where it is stored until being eliminated from the body through the urethra. In this text, we consider eleven major organ systems: (1) integumentary, (2) skeletal, (3) muscular, (4) nervous, (5) endocrine, (6) cardiovascular, (7) lymphatic, (8) respiratory, (9) digestive, (10) urinary, and (11) reproductive. Figure 1.3 presents a brief summary of these organ systems and their functions. Page 13 Figure 1.2 Major Organs of the Body The body’s major organs include the brain, lungs, heart, liver, pancreas, spleen, stomach, gallbladder, kidneys, large intestine, small intestine, urinary bladder, and urethra. Page 14 midterm coverage B (proces. ( Testosterone Page 15  Lalveoli : gas exchange Figure 1.3 Organ Systems of the Body There are 11 body systems: integumentary, skeletal, muscular, lymphatic, respiratory, digestive, nervous, endocrine, cardiovascular, urinary, and reproductive.  CLINICAL IMPACT Cadavers and the Law The study of human bodies is the foundation of medical education, and for much of history, anatomists have used the bodies of people who have died, called cadavers, for these studies. However, historically, public sentiment has made it difficult for anatomists to obtain human bodies for dissection. In the early 1800s, the benefits of human dissection for training physicians had become very apparent, and the need for cadavers increased beyond the ability to acquire them legally. Thus arose the resurrectionists, or body snatchers. For a fee and no questions asked, they removed bodies from graves and provided them to medical schools. Because the bodies were not easy to obtain and were not always in the best condition, two enterprising men named William Burke and William Hare went one step further. Over a period of time, they murdered seventeen people in Scotland and sold their bodies to a medical school. When discovered, Hare testified against Burke and went free. Burke was convicted, hanged, and publicly dissected. Discovery of Burke’s activities so outraged the public that sensible laws regulating the acquisition of cadavers were soon passed, and this dark chapter in the history of anatomy was closed. Today, in the United States, it is quite simple to donate your body for scientific study. The Uniform Anatomical Gift Act allows individuals to donate their organs or entire cadaver by putting a notation on their driver’s license. You need only to contact a medical school or private agency to file the forms that give them the rights to your cadaver. Once the donor dies, the family of the deceased usually pays only the transportation costs for the remains. After dissection, the body is cremated, and the cremains can be returned to the family. Page 16 Throughout this text book, each Systems Pathology essay presents a specific disease state and considers how this affects the interactions of the organ systems. 6 Organism Level An organism is any living thing considered as a whole, whether composed of one cell, such as a bacterium, or of trillions of cells, such as a human. The human organism is a network of organ systems that are mutually dependent upon one another. 1.3 CHARACTERISTICS OF LIFE  Learning Outcome After reading this section, you should be able to A. List and define six characteristics of life. Humans are organisms sharing characteristics with other organisms. The most important common feature of all organisms is life. This textbook recognizes six essential characteristics of life: 1. Organization refers to the specific interrelationships among the individual parts of an organism, and how those parts interact to perform specific functions. Living things are highly organized. All organisms are composed of one or more cells. In turn, cellular function depends on the precise organization of large molecules. Disruption of this organized state can result in loss of functions. 2. Metabolism (meh-TAB-oh-lizm) is the ability to use energy to perform other vital functions, such as growth, movement, and reproduction. Human cells possess specialized proteins that can break down food molecules to use as a source of energy. 3. Responsiveness is an organism’s ability to sense changes in the external or internal environment and adjust to those changes. Responses include actions such as moving toward food or water and moving away from danger or poor environmental conditions such as extreme cold or heat. Organisms can also make adjustments that maintain their internal environment. For example, if our body temperature rises, sweat glands produce sweat, which can lower body temperature down to the normal range. 4. Growth refers to an increase in the size or number of cells, which produces an overall enlargement in all or part of an organism, cell size, or the amount of substance surrounding cells. For example, bones grow when the number of bone cells increases and the bone cells become surrounded by mineralized materials. 5. Development includes the changes an organism undergoes through time, beginning with fertilization and ending at death. Development usually involves growth, but it also involves differentiation. Differentiation involves changes in a cell’s structure and function from an immature, generalized state to a mature, specialized state. For example, following fertilization, immature cells differentiate to become specific types of cells, such as skin, bone, muscle, or nerve cells. These differentiated cells form tissues and organs. 6. Reproduction is the formation of new cells or new organisms. Reproduction of cells allows for growth and development. Formation of new organisms prevents extinction of species. 1.4 HOMEOSTASIS  Learning Outcomes After reading this section, you should be able to A. Define homeostasis, and explain why it is important for proper body function. B. Describe a negative-feedback mechanism and give an example. C. Describe a positive-feedback mechanism and give an example. Homeostasis (hoh-mee-oh-STAY-sis; homeo-, the same; -stasis, to stop) is the maintenance of a relatively constant environment within the body. To achieve homeostasis, the body must actively regulate body conditions that are constantly changing. As our bodies undergo their everyday processes, we are continuously exposed to new conditions, both internally and externally. Changes in our environmental conditions, such as moving into hot or cold outdoor temperatures, can result in a change in our body temperature. Body temperature is one of our body’s variables. These changes in body conditions are called variables because their values are not constant. For cells to function normally, the volume, temperature, and chemical content of the cells’ environment must be maintained within a narrow range. Body temperature is a variable that increases when you are too hot and decreases when you are too cold. The homeostatic mechanisms that maintain normal body temperature include sweating or shivering to maintain body temperature near an ideal normal value, or set point (figure 1.4). Most homeostatic mechanisms are regulated by the nervous system or the endocrine system. Note that homeostatic mechanisms are not able to maintain body temperature precisely at the set point. Instead, body temperature increases and decreases slightly around the set point, producing a normal range of values. As long as body temperatures remain within this normal range, homeostasis is maintained. Keep in mind that these fluctuations are minimal. Note in figure 1.4a that the normal body temperature range is no more than 1°F above or below normal. Our average body temperature is 98.6°F. Just as your home’s thermostat does not keep the air temperature at exactly 75°F at all times, your body’s temperature is not kept at exactly 98.6°F at all times. PROCESS Figure NORMAL BODY TEMP 36 5 - = 37 5. > Hypothermia Hyperthermia  PROCESS Figure 1.4 Negative- and Positive-Feedback Mechanisms (a) Homeostasis is the maintenance of a variable around an ideal normal value, or set point. The value of the variable fluctuates around the set point to establish a normal range of values. (b) Negative feedback is one of the mechanisms by which homeostasis is maintained. Receptors signal the control center, which regulates the action of the effectors. In the example, body temperature is too high, so sweating occurs. Negative feedback stops the sweating when the body temperature returns to normal. (c) Positive feedback is also a type of mechanism that works to maintain homeostasis. Receptors signal the control center that the cervix is being stretched, which results in the control center sending signals to increase the contractions of the uterus. This cycle continues, becoming stronger over time until the baby is born. Occasionally an individual will not be able to produce sweat and can overheat, potentially suffering a heat stroke. Within the context of the body temperature homeostatic mechanism, where might the disruption occur? Propose at least three ways sweat production might be inhibited when the body temperature rises above the set point. The organ systems help keep the body’s internal environment relatively constant. For example, the digestive, respiratory, cardiovascular, and urinary systems function together so that each cell in the body receives adequate oxygen and nutrients while also ensuring that waste products do not accumulate to a toxic level. If body fluids deviate from homeostasis, body cells do not function normally and can even die. Disease disrupts homeostasis and sometimes results in death. Modern medicine attempts to understand disturbances in homeostasis and works to reestablish a normal range of values. Negative Feedback Most systems of the body are regulated by negative-feedback mechanisms which maintain homeostasis. In everyday terms, the word negative is used to mean “bad” or “undesirable.” In this context, negative means “to decrease.” Negative feedback is when any deviation from the set point is made smaller or is resisted. Negative feedback does not prevent variation but maintains variation within a normal range.  The maintenance of normal body temperature is an example of a negative- feedback mechanism. Normal body temperature is critical to our health because it allows molecules and enzymes to keep their normal shape so they can function optimally. An optimal body temperature prevents molecules from being permanently destroyed. Picture the change in appearance of egg whites as they are cooked: the egg whites change from a transparent liquid to a white solid because the heat changes the shape of the egg white molecules. Similarly, if the body is exposed to extreme heat, the shape of the molecules in the body could change, which would eventually prevent them from functioning normally. Page 17 Most negative-feedback mechanisms have three components: (1) A receptor (ree-SEP-tor), which monitors the value of a variable, such as body temperature, by detecting stimuli; (2) a control center, such as part of the brain, which determines the set point for the variable and receives input from the receptor about the variable; and (3) an effector (ee-FEK-ter), such as the sweat glands, which can adjust the value of the variable when directed by the control center, usually back toward the set point. A changed variable is a stimulus because it initiates a homeostatic mechanism. Several negative- feedback mechanisms regulate body temperature, and they are described more fully in chapter 5. Figure 1.4b demonstrates the steps in the negative-feedback regulation of body temperature if it becomes too high. Normal body temperature depends on the coordination of multiple structures, which are regulated by the control center (the hypothalamus). ❶ Receptors in the skin monitor body temperature. If body temperature rises, the receptors send a message to the control center. ❷ The control center compares the value of the variable against the set point. ❸ If a response is necessary, the control center will stimulate the effectors, the sweat glands, to produce their response, which is secretion of sweat. ❹ Once the value of the variable has returned to the set point, the effectors do not receive any more information from the control center. For body temperature, this means that secretion of sweat stops. These same steps can be used to help you answer the Learn to Apply It question at the beginning of this chapter. Page 18 Often there is more than one effector for a particular homeostatic mechanism. In these cases the control center must coordinate the effectors’ responses. For example, cooling the body involves not only the production of sweat by the sweat glands, but also the action of the blood vessels to alter blood flow to the skin. Once body temperature has returned to normal, the effectors stop. This is the hallmark of negative feedback—effectors stop their response once the variable has returned to its set point. They do not produce an indefinite response (figure 1.5). releases thyroid gland Page 19 / pituitarstimulatea a hypothalamus & Homeostasis Figure 1.5 Negative-Feedback Control of Body Temperature  Throughout this book, all homeostasis figures have the same format as shown here. The changes caused by the increase of a variable outside the normal range are shown in the upper, green boxes, and the changes caused by a decrease are shown in the lower, red boxes. To help you learn how to interpret homeostasis figures, some of the steps in this figure are numbered. (1) Body temperature is within its normal range. (2) Body temperature increases outside the normal range, which causes homeostasis to be disturbed. (3) The body temperature control center in the brain responds to the change in body temperature. (4) The control center causes sweat glands to produce sweat and blood vessels in the skin to dilate. (5) These changes cause body temperature to decrease. (6) Body temperature returns to its normal range, and homeostasis is restored. Observe the responses to a decrease in body temperature outside its normal range by following the lower, red arrows. Apply It 1 What effect would swimming in cool water have on body temperature regulation mechanisms? What would happen if a negative-feedback mechanism did not return the value of a variable, such as body temperature, to its normal range? : child birth example used for homeostasis Page 20 not Positive Feedback ↑ Positive-feedback mechanisms occur when a response to the original stimulus results in the deviation from the set point becoming even greater. In other words, positive means “increase.” At times, this type of response is required to return to homeostasis. For example, during blood loss, a chemical responsible for blood clot formation, called thrombin, stimulates production of even more thrombin. In this way, a disruption in homeostasis is resolved through a positive-feedback mechanism. What prevents the entire vascular system from clotting? The clot formation process is self-limiting. Eventually, the components needed to form a clot will be depleted in the damaged area and more clot material cannot be formed. As shown in figure 1.4c, birth is another example of a normally occurring positive-feedback mechanism. ❶ Near the end of pregnancy, the baby’s large size stretches the uterus. ❷ This stretching, especially around the opening of the uterus, stimulates contractions of the uterine muscles. ❸ The uterine contractions push the baby against the opening of the uterus, which results in additional stretching. ❹ This positive-feedback sequence ends when the baby is delivered from the uterus and the stretching stimulus is eliminated. Two basic principles about homeostatic mechanisms to remember are that (1) many disease states result from the failure of negative-feedback mechanisms to maintain homeostasis, and (2) some positive-feedback mechanisms can be detrimental instead of helpful. One example of a detrimental positive-feedback mechanism is inadequate delivery of blood to cardiac (heart) muscle. Contraction of cardiac muscle generates blood pressure. The heart pumps blood to itself through a system of blood vessels on the outside of the heart. Just as with other tissues, blood pressure must be maintained to ensure adequate delivery of blood to the cardiac muscle. Following extreme blood loss, such as a severe cut on the body, not enough blood is delivered to cardiac muscle. As a result, the heart cannot function normally. The heart pumps less blood, which causes the blood pressure to drop even further, which causes even less blood to be delivered to the heart. The process continues until the heart stops beating, and death results. In this example, we see the deviation from the heart-rate set point becoming larger and larger—this is the hallmark of positive feedback. Thus, if blood loss is severe, negative-feedback mechanisms may not be able to maintain homeostasis and the positive feedback of ever-decreasing blood pressure can develop. On the other hand, following a moderate amount of blood loss (e.g., after donating a pint of blood), negative-feedback mechanisms result in an increase in heart rate that restores blood pressure. Apply It 2 Is the sensation of thirst associated with a negative- or a positive-feedback mechanism? Explain. (Hint: What is being regulated when you become thirsty?) 1.5 TERMINOLOGY AND THE BODY PLAN Learning Outcomes After reading this section, you should be able to A. Describe a person in anatomical position. B. Define the directional terms for the human body, and use them to locate specific body structures. C. Know the terms for the parts and regions of the body. D. Name and describe the three major planes of the body and the body organs. E. Name and describe the three major ways to cut an organ. F. Describe the major trunk cavities and their divisions. G. Describe the serous membranes, their locations, and their functions. As you study anatomy and physiology, you will be learning many new words. Knowing the etymology (ET-ee-MOL-oh-jee) of these words can make learning them easy and fun. Most anatomical terms are derived from Latin or Greek. For example, foramen is a Latin word for “hole,” and magnum means “large.” The foramen magnum is therefore a large hole in the skull (through which the spinal cord continues from the brain). Words are often modified by adding a prefix or suffix. For example, the suffix -itis means an inflammation, so appendicitis is an inflammation of the appendix. As new terms are introduced in this text, their meanings are often explained. The glossary and the list of word roots, prefixes, and suffixes also provide additional information about the new terms. Body Positions Anatomical position refers to a person standing upright with the face directed forward, the upper limbs hanging to the sides, and the palms of the hands facing forward (figure 1.6). A person is supine when lying face upward and prone when lying face downward. Page 21 Figure 1.6 Directional Terms All directional terms are in relation to the body in the anatomical position: a person standing erect with the face directed forward, the arms hanging to the sides, and the palms of the hands facing forward. ©Eric Wise In anatomical position, the head is above the feet, but if a person were to do a handstand, the head would be closer to the ground than the feet were. However, we would still refer to the head as being “above” the feet because the point of reference for anatomical structures is their position on the body, not the position of the body compared to the earth. Directional Terms Directional terms describe parts of the body relative to each other (figure 1.6 and table 1.1). It is important to become familiar with these directional terms as soon as possible because you will see them repeatedly throughout the text. Right and left are used as directional terms in anatomical terminology. In anatomy, the term superior means above, and the term inferior means below. Anterior is used for “in front of” and the term posterior is used for “behind.” TABLE 1.1 Directional Terms for the Human Body Term Etymology Definition* Example Right Toward the body’s right The right ear side Left Toward the body’s left The left ear side Inferior Lower Below The nose is inferior to the forehead. Superior Higher Above The mouth is superior to the chin. Anterior To go before Toward the front of the The teeth are anterior body to the throat. Posterior Posterus, Toward the back of the The brain is posterior to following body the eyes. Dorsal Dorsum, Toward the back The spine is dorsal to back (synonymous with the breastbone. posterior) Ventral Venter, belly Toward the belly The navel is ventral to (synonymous with the spine. anterior) Proximal Proximus, Closer to a point of The shoulder is nearest attachment proximal to the elbow. Distal di + sto, to Farther from a point of The ankle is distal to be distant attachment the hip. Lateral Latus, side Away from the midline of The nipple is lateral to the body the breastbone. Medial Medialis, Toward the middle or The bridge of the nose middle midline of the body is medial to the eye.  Term Etymology Definition* Example Superficial Superficialis, Toward or on the surface The skin is superficial to surface muscle. Deep Deop, deep Away from the surface, The lungs are deep to internal the ribs. *All directional terms refer to a human in the anatomical position. In human anatomy, two terms can be used interchangeably to refer to the front of the body (or an organ)—anterior and ventral (belly). Likewise, two terms can be used interchangeably to refer to the back of the body (or an organ)— posterior and dorsal (back). Page 22 Proximal means “close to,” whereas distal means “far from.” These terms are used to refer to relative positions of structures, such as on the limbs. Each limb is attached at its proximal end to the body, and the distal end, such as the hand, is farther away. “Proximal” and “distal” can also describe one structure’s position relative to another’s, such as specialized structures in the kidney called the proximal and distal convoluted tubules. Their position is described relative to another kidney structure. Medial means “toward the midline,” and lateral means “away from the midline.” The nose is located in a medial position on the face, and the ears are lateral to the nose. The term superficial refers to a structure close to the surface of the body, and deep is toward the interior of the body. For example, the skin is superficial to muscle and bone. Apply It 3 Provide the correct directional term for the following statement: When a boy is standing on his head, his nose is ______ to his mouth. Body Parts and Regions Health professionals use a number of terms when referring to different regions or parts of the body. Figure 1.7 (a, anterior, and b, posterior) shows the anatomical terms, with the common terms in parentheses. Figure 1.7 Body Parts and Regions The anatomical and common (in parentheses) names are indicated for some parts and regions of the body. (a) Anterior view. (b) Posterior view.  ©Eric Wise The central region of the body consists of the head, neck, and trunk. The trunk can be divided into three regions: (1) the thorax, (2) the abdomen, and (3) the pelvis. The thorax is the chest cavity where the heart and lungs are located. The abdomen contains organs such as (1) the liver, (2) the stomach, and (3) the intestines. The pelvis contains the bladder and reproductive organs. The upper limb is divided into (1) the arm, (2) the forearm, (3) the wrist, and (4) the hand. The arm extends from the shoulder to the elbow, and the forearm extends from the elbow to the wrist. The lower limb is divided into (1) the thigh, (2) the leg, (3) the ankle, and (4) the foot. The thigh extends from the hip to the knee, and the leg extends from the knee to the ankle. Note that, contrary to popular usage, the terms arm and leg refer to only a part of the limb. The abdomen is often subdivided superficially into four quadrants, by two imaginary lines—one horizontal and one vertical—that intersect at the navel (figure 1.8a). The quadrants formed are (1) the right-upper, (2) the left-upper, (3) the right-lower, and (4) the left-lower quadrants. In addition to these quadrants, the abdomen is sometimes subdivided into regions by four imaginary lines—two horizontal and two vertical. These four lines create a “virtual” tic-tac-toe grid on the abdomen, resulting in nine regions: (1) epigastric (ep-ee-GAS-trik), (2, 3) right and left hypochondriac (high-poh- KON-dree-ack), (4) umbilical (um-BIL-ih-kal), (5, 6) right and left lumbar (LUM-bar), (7) hypogastric (high-poh-GAS-trik), and (8, 9) right and left iliac (IL-ee-ack) (figure 1.8b). Health professionals use the quadrants or regions as reference points for locating the underlying organs. For example, the appendix is in the right-lower quadrant, and the pain of an acute appendicitis is usually felt there. Page 23  Figure 1.8 Subdivisions of the Abdomen Lines are superimposed over internal organs to demonstrate the relationship of the organs to the subdivisions. (a) Abdominal quadrants consist of four subdivisions. (b) Abdominal regions consist of nine subdivisions. Page 24 Apply It 4 Using figures 1.2 and 1.8a, determine in which quadrant each of the following organs is located: spleen, gallbladder, kidneys, most of the stomach, and most of the liver. Planes At times it is useful to describe the body as having imaginary flat surfaces, called planes, passing through it (figure 1.9). A plane divides, or sections, the body, making it possible to “look inside” and observe the body’s structures. Figure 1.9 Planes of Section of the Body (a) Planes of section through the body are indicated by “glass” sheets. Also shown are actual sections through (b) the head (viewed from the right), (c) the abdomen (inferior view; liver is on the right), and (d) the hip (anterior view). (a) ©Eric Wise; (b,c,d) ©R. T. Hutchings Page 25  1. A sagittal (SAJ-ih-tal) plane separates the body or a structure into right and left halves. The word sagittal means “the flight of an arrow” and refers to the way the body would be split by an arrow passing anteriorly to posteriorly. 2. A median plane is a sagittal plane that passes through the midline of the body, dividing it into equal right and left halves. 3. A transverse (horizontal) plane runs parallel to the ground, dividing the body into superior and inferior portions. 4. A frontal (coronal) (KOR-oh-nal; crown) plane divides the body into front (anterior) and back (posterior) halves. For example, the coronal suture on the skull is located across the top, where a person might wear a crown. Organs are often sectioned to reveal their internal structure (figure 1.10). A longitudinal section is a cut along the length of the organ, similar to the cut along a hot dog bun. A transverse section, or cross section, cuts completely through an organ, similar to cutting a hot dog or banana into round pieces. If a cut is made diagonally across the long axis, it is called an oblique section. Figure 1.10 Planes of Section Through an Organ Planes of section through the small intestine are indicated by “glass” sheets. The views of the small intestine after sectioning are also shown. Although the small intestine is basically a tube, the sections appear quite different in shape.  Body Cavities The body contains two types of internal cavities: (1) the dorsal body cavity and (2) the ventral body cavity (figure 1.11). These cavities, which are closed to the outside, contain our internal organs, providing protection for them. Some anatomy texts do not use the dorsal body cavity designation; however, in this textbook we have chosen to use the two internal body cavities model. encloses the nenour syst & Figure 1.11 Trunk Cavities (a) Anterior view showing the major trunk cavities. The diaphragm separates the thoracic cavity from the abdominal cavity. The mediastinum, which includes the heart, is a partition of organs dividing the thoracic cavity. (b) Sagittal section of the trunk cavities viewed from the left. The dashed line shows the division between the abdominal and pelvic cavities. The mediastinum has been removed to show the thoracic cavity. Dorsal Body Cavity The dorsal body encloses the organs of the nervous system, the brain and spinal cord. The two subdivisions of the dorsal body cavity are (1) the cranial cavity, which houses the brain, and (2) the vertebral canal, which houses the spinal cord. Both the brain and spinal cord are covered by membranes called meninges (figure 1.11a). We discuss the anatomy of the nervous system further in chapters 12 and 13. Ventral Body Cavity The ventral body cavity houses the vast majority of our internal organs, collectively referred to as the viscera (VIS-er-ah; internal organs) (figure 1.11b and c). The ventral body cavity also has two major subdivisions, which are (1) the thoracic cavity and (2) the abdominopelvic cavity. The Thoracic Cavity The thoracic cavity is more superior to the abdominopelvic cavity and houses primarily the heart and lungs, among other organs. This cavity is further subdivided into sections: (1) two lateral pleural cavities, each of which encloses a lung, and are surrounded by the ribs, and (2) a medial mediastinum (MEE-dee-ah-STIE-num; middle wall), which houses the heart and its major blood vessels, in addition to the thymus, the trachea, and the esophagus. The Abdominopelvic Cavity The abdominopelvic cavity is enclosed by abdominal muscles and consists of (1) the more superior abdominal cavity and (2) the more inferior pelvic cavity. The organs of the abdominopelvic cavity are housed within the peritoneal (per-ih-toh-NEE-al; to stretch over) cavity. The abdominal cavity contains the majority of the digestive organs, such as the stomach, the intestines, and the liver, in addition to the spleen. The pelvic cavity continues below the pelvis and contains the urinary bladder, urethra, rectum of the large intestine, and reproductive organs. Serous Membranes of the Ventral Body Cavity The walls of the body cavities and the surface of internal organs are in contact with membranes called serous (SEER-us) membranes. These membranes are double layered. The layer that lines the walls of the cavities is called the parietal (pah-RYE-eh-tal; wall) serous membrane. The layer covering the internal organs (the viscera) is the visceral serous membrane. To understand the relationship between the parietal and the visceral serous membranes, imagine pushing your fist (representing an organ) into a slightly deflated balloon (representing the membranes and the cavity) (figure 1.12). Since your fist represents the internal organs, the portion of the balloon in contact with your fist represents the visceral serous membrane, and the outer part of the balloon wall represents the parietal serous membrane. However, in the body, the parietal serous membrane is in close contact with the body cavity wall. Furthermore, in the body, there is no air between the visceral and parietal serous membranes as there is in the balloon; rather, the two membranes are separated by a thin film of serous fluid produced by the membranes. As organs move around in the cavities, the combination of serous fluid and smooth serous membranes reduces friction.  Page 26 Figure 1.12 Serous Membranes (a) Fist pushing into a balloon. A “glass” sheet indicates the location of a cross section through the balloon. (b) Interior view produced by the section in (a). The fist represents an organ, and the walls of the balloon represent the serous membranes. The inner wall of the balloon represents a visceral serous membrane in contact with the fist (organ). The outer wall of the balloon represents a parietal serous membrane. (c) View of the serous membranes surrounding the lungs. The membrane in contact with the lungs is the visceral pleura; the membrane lining the wall of the lung cavity is the parietal pleura. Page 27 Thoracic Cavity Membranes The serous membranes are named for the specific cavity and organs they are in contact with. They include: 1. Pericardial Cavity. The pericardial cavity (peri-, around; cardi-, heart), containing the heart, is housed in the mediastinum. The parietal serous membrane is called the parietal pericardium and the visceral serous membrane is called the visceral pericardium. The space between the two pericardial membranes is called the pericardial cavity and is filled with pericardial fluid (figure 1.13a). 2. Pleural Cavities. Each of the two pleural cavities (pleuron-, side of body, rib) houses a lung. The parietal serous membrane lining the pleural cavities is called the parietal pleura, while the visceral serous membrane covering the lungs is called the visceral pleura (figure 1.13b). The space between the two pleural membranes is called the pleural cavity and is filled with pleural fluid. 3. Peritoneal Cavity. The peritoneal cavity (peri-, around; -tonos-, stretched; stretched around) houses many internal organs, such as the liver, the digestive organs, and the reproductive organs. The parietal serous membrane in the peritoneal cavity is called the parietal peritoneum. The visceral serous membrane is called the visceral peritoneum. The space between the two serous  membranes is the specific location of the peritoneal cavity and is filled with peritoneal fluid (figure 1.13c). In addition to covering organs, a double-folded sheet of visceral peritoneum attaches the digestive organs at certain points to the posterior abdominopelvic cavity wall. These regions of double-folded visceral peritoneum are called mesenteries. The mesenteries also provide a pathway for nerves and blood vessels to reach the digestive organs (figure 1.13d). The most notable mesenteric structure is an enormous pouch containing adipose tissue that is suspended from the inferior border of the stomach. In some people, this pouch contributes to their “big belly” (see chapter 24). Figure 1.13 Location of Serous Membranes (a) Frontal section showing the parietal pericardium (blue), visceral pericardium (red), and pericardial cavity. (b) Frontal section showing the parietal pleura (blue), visceral pleura (red), and pleural cavities. (c) Sagittal section through the abdominopelvic cavity showing the parietal peritoneum (blue), visceral peritoneum (red), peritoneal cavity, mesenteries (purple), and retroperitoneal organs. (d) Photo of mesentery (green) in a cadaver. (d) MCOF Enterprises, Ltd./McGraw Hill Education Page 28 Some abdominal organs are tightly adhered to the posterior body wall and are covered by peritoneum only on their peritoneal cavity side. These organs have a retroperitoneal (RE-troh-PER-i-toh-NEE-uhl; behind the peritoneum) location and include the kidneys, ureters, adrenal glands, a large portion of the pancreas, parts of the large intestine, and the urinary bladder (see figure 1.13). Inflammation, often due to an infection, of the serous membranes in the ventral body cavities sometimes occurs. Serious consequences can arise if the inherent infection or problem cannot be resolved. The following is a list of the conditions caused by inflammation of the serous membranes: 1. Pericarditis (PER-i-kar-DIE-tis; -itis, inflammation) is inflammation of the pericardium. 2. Pleurisy (PLOOR-ih-see) is inflammation of the pleura. 3. Peritonitis (PER-ih-toe-NIGH-tis) is inflammation of the peritoneum. Apply It 5 Explain how an organ can be located within the abdominopelvic cavity but not be within the peritoneal cavity. SUMMAR Y Knowledge of anatomy and physiology can be used to predict the body’s responses to stimuli when healthy or diseased. 1.1 Anatomy and Physiology 1. Anatomy is the study of the structures of the body. 2. Systemic anatomy is the study of the body by organ systems. Regional anatomy is the study of the body by areas. 3. Surface anatomy uses superficial structures to locate deeper structures, and anatomical imaging is a noninvasive method for examining deep structures. 4. Physiology is the study of the processes and functions of the body. 1.2 Structural and Functional Organization of the Human Body  1. The human body can be organized into six levels: chemical, cell, tissue, organ, organ system, and organism. 2. The eleven organ systems are the integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, and reproductive systems (see figure 1.3). 1.3 Characteristics of Life The characteristics of life are organization, metabolism, responsiveness, growth, development, and reproduction. 1.4 Homeostasis Homeostasis is the condition in which body functions, body fluids, and other factors of the internal environment are maintained within a range of values suitable to support life. Negative Feedback Negative-feedback mechanisms maintain homeostasis. Positive Feedback Positive-feedback mechanisms make deviations from normal even greater. Although a few positive-feedback mechanisms normally exist in the body, most positive-feedback mechanisms are harmful. 1.5 Terminology and the Body Plan Body Positions 1. A human standing erect with the face directed forward, the arms hanging to the sides, and the palms facing forward is in the anatomical position. 2. A face-upward position is supine and a face-downward one is prone. Directional Terms Directional terms always refer to the anatomical position, regardless of the body’s actual position (see table 1.1). Body Parts and Regions 1. The body can be divided into the head, neck, trunk, upper limbs, and lower limbs. 2. The abdomen can be divided superficially into four quadrants or nine regions, which are useful for locating internal organs or describing the location of a pain. Planes 1. A sagittal plane divides the body into left and right parts, a transverse plane divides the body into superior and inferior parts, and a frontal plane divides the body into anterior and posterior parts. 2. A longitudinal section divides an organ along its long axis, a transverse section cuts an organ at a right angle to the long axis, and an oblique section cuts across the long axis at an angle other than a right angle. Body Cavities 1. There are two internal body cavities: the dorsal body cavity and the ventral body cavity. 2. The dorsal body cavity houses the brain and the spinal cord. 3. The mediastinum subdivides the thoracic cavity. 4. The diaphragm separates the thoracic and abdominal cavities. Page 29 5. Pelvic bones surround the pelvic cavity. 6. Serous membranes line the trunk cavities. The parietal portion of a serous membrane lines the wall of the cavity, and the visceral portion is in contact with the internal organs. The serous membranes secrete fluid, which fills the space between the visceral and parietal membranes. The serous membranes protect organs from friction. The pericardial cavity surrounds the heart, the pleural cavities surround the lungs, and the peritoneal cavity surrounds certain abdominal and pelvic organs. 7. Mesenteries are parts of the peritoneum that hold the abdominal organs in place and provide a passageway for blood vessels and nerves to the organs. 8. Retroperitoneal organs are located “behind” the parietal peritoneum. REMEMB ERING AND UNDERST ANDING  1. Define anatomy, surface anatomy, anatomical imaging, and physiology. 2. List six structural levels at which the body can be studied. 3. Define tissue. What are the four primary tissue types? 4. Define organ and organ system. What are the eleven organ systems of the body and their functions? 5. Name six characteristics of life. 6. What does the term homeostasis mean? If a deviation from homeostasis occurs, what kind of mechanism restores homeostasis? 7. Describe a negative-feedback mechanism in terms of receptor, control center, and effector. Give an example of a negative-feedback mechanism in the body. 8. Define positive feedback. Why are positive-feedback mechanisms generally harmful? Give one example each of a harmful and a beneficial positive-feedback mechanism in the body. 9. Why is knowledge of the etymology of anatomical and physiological terms useful? 10. Describe the anatomical position. Why is it important to remember the anatomical position when using directional terms? 11. Define and give an example of the following directional terms: inferior, superior, anterior, posterior, dorsal, ventral, proximal, distal, lateral, medial, superficial, and deep. 12. List the subdivisions of the trunk, the upper limbs, and the lower limbs. 13. Describe the four-quadrant and nine-region methods of subdividing the abdomen. What is the purpose of these methods? 14. Define the sagittal, median, transverse, and frontal planes of the body. 15. Define the longitudinal, transverse, and oblique sections of an organ. 16. Define the following cavities: thoracic, abdominal, pelvic, and abdominopelvic. What is the mediastinum? 17. What is the difference between the visceral and parietal layers of a serous membrane? What function do serous membranes perform? 18. Name the serous membranes associated with the heart, lungs, and abdominopelvic organs. 19. Define mesentery. What does the term retroperitoneal mean? Give an example of a retroperitoneal organ.  CRITICAL THINKING 1. A male has lost blood as a result of a gunshot wound. Even though the bleeding has been stopped, his blood pressure is low and dropping, and his heart rate is elevated. Following a blood transfusion, his blood pressure increases and his heart rate decreases. Propose a physiological explanation for these changes. 2. During physical exercise, the respiration rate increases. Two students are discussing the mechanisms involved. Student A claims they are positive-feedback mechanisms, and student B claims they are negative-feedback mechanisms. Do you agree with student A or student B, and why? 3. Of the six characteristics of life, why is organismal reproduction a characteristic of life? 4. Describe, using as many directional terms as you can, the relationship between your kneecap and your heel. 5. In some traditions, a wedding band is worn closest to the heart, and an engagement ring is worn as a “guard” on the outside. Should a person’s wedding band be worn proximal or distal to the engagement ring? 6. In which quadrant and region would a person experience discomfort in the event of a urinary bladder infection? 7. During pregnancy, which would increase more in size, the mother’s abdominal cavity or her pelvic cavity? Explain. 8. A bullet enters the left side of a male, passes through the left lung, and lodges in the heart. Name in order the serous membranes and the cavities through which the bullet passes. 9. Can a kidney be removed without cutting through the parietal peritoneum? Explain. Answers to odd-numbered questions from this chapter appear in Appendix D Design Elements: (Microbes in Your Body): Janice Haney Carr/CDC; (Clinical Impact): Comstock/Alamy Stock Photo