Chapter 1: Introduction to the Human Body PDF

CHAPTER 1 Introduction to the Human Body Randy Faris/Corbis Super RF/Alamy Stock Photo CHAPTER OUTLINE Careers I...

CHAPTER 1 Introduction to the Human Body Randy Faris/Corbis Super RF/Alamy Stock Photo CHAPTER OUTLINE Careers In 1.1 Anatomy and Physiology 1.2 Levels of Students take a course in anatomy and physiology for a variety of reasons. Organization Some are simply fulfilling a core requirement. Some are taking the course just Chemical Level because they find the human body interesting. Others are taking the course Cellular Level because they seek a career in health care. Tissue Level Organ Level The health care field offers many opportunities for individuals with an inter- Organ System Level est and a passion for helping others. However, many students are not aware Organismal Level of the diversity of options available. The “Careers In...” section at the start of 1.3 Directional Terms each chapter describes health-related careers related to the chapter’s content. 1.4 Body Regions Reading about the “Careers In...” will not just expose students to a broader 1.5 Body Planes and range of health-related careers but will also show students how chapter con- Sections tent supports professionals within those fields. 1.6 Body Cavities Membranes of Body Cavities 1.7 Abdominopelvic Subdivisions 1.8 Maintenance of Life Survival Needs Homeostasis Chapter Summary Module 1 Body Orientation 2 Chapter 1 Introduction to the Human Body YOU ARE BEGINNING a fascinating and challenging in figure"1.1. The levels of organization from simplest to study—the study of the human body. As you progress most complex are chemical, cellular, tissue, organ, organ through this text, you will begin to understand the com- system, and organismal (the body as a whole). plex structures and functions of the human organism. This first chapter provides an overview of the human Chemical Level body to build a foundation of knowledge that is necessary The chemical level consists of atoms, molecules, and for your continued study. Like the chapters that follow, macromolecules. At the simplest level, the body is com- this chapter introduces a number of new terms for you to posed of chemical substances that are formed of atoms learn. It is important that you start to build a vocabulary and molecules. Atoms are the fundamental building of technical terms and continue to develop it throughout blocks of chemicals, and atoms combine in specific ways your study. This vocabulary will help you reach your goal to form molecules. Some molecules are very small, such of understanding human anatomy and physiology. as water molecules, but others may be very large, such as the macromolecules of proteins. Various small and large 1.1 Anatomy and Physiology molecules are grouped together to form organelles. An organelle (or"-ga-nel! ) is a complex of macromolecules Learning Objective acting like a “mini-organ” that carries out specific func- 1. Define anatomy and physiology. tions within a cell. Nuclei, mitochondria, and ribosomes are examples. Knowledge of the human body is obtained primarily from two scientific disciplines—anatomy and physiology—and Cellular Level each consists of a number of subdisciplines. !-me !) is the study of the Cells are the basic structural and functional units of the Human anatomy (ah-nat!-o body because all of the processes of life occur within cells. structure and organization of the body and the study A cell is the lowest level of organization that is alive. The of the relationships of body parts to one another. There human body is composed of trillions of cells and many are two major subdivisions of anatomy. Gross anatomy different types of cells, such as muscle cells, blood cells, involves the dissection and examination of various parts and nerve cells. Each type of cell has a unique structure of the body without magnifying lenses. Microanatomy, that enables it to perform specific functions. also known as histology, consists of the examination of tissues and cells with various magnification techniques. !-ol!-o !-je !) is the study of the Tissue Level Human physiology (fiz-e function of the body and its parts. Physiology involves Similar types of cells are usually grouped together in the observation and experimentation, and usually requires body to form a tissue. Each body tissue consists of an the use of specialized equipment and materials. aggregation of similar cells that perform similar functions. In your study of the human body, you will see There are four major classes of tissues in the body: epithe- that there is always a definite relationship between the lial, connective, muscle, and nerve tissues. anatomy and physiology of the body and body parts. Just as the structure of a knife is well suited for cutting, the Organ Level structure (anatomy) of a body part enables it to perform Each organ of the body is composed of two or more specific functions (physiology). For example, the arrange- tissues that work together, enabling the organ to per- ment of bones, muscles, and nerves in your hands enables form its specific functions. The body contains numerous the grasping of large objects with considerable force and organs, and each has a definite structure and function. also the delicate manipulation of small objects. Correlat- The stomach, heart, brain, and even bones are examples of ing the relationship between structure and function will organs. Later in this chapter, we will focus our attention make your study of the human body much easier. on the body cavities. Body cavities are spaces within the body that contain organs, collectively referred to as 1.2 Levels of Organization internal organs. Learning Objectives Organ System Level 2. Describe the levels of organization in the human body. The organs of the body are arranged in functional groups 3. List the major organs and functions for each organ so that their independent functions are coordinated to system. perform specific system functions. These coordinated, functional groups are called organ systems. The diges- The human body is complex, so it is not surprising that tive and nervous systems are examples of organ systems. there are several levels of structural organization, as shown Most organs belong to a single organ system, but a few  Part 1 Organization of the Body 3 Macromolecule Molecules Organelle Cells 2 Cellular level A cell is composed of molecules 1 Chemical level Molecules are formed from atoms Tissue Organism Organ system 3 Tissue level Atoms A tissue is made Organ up of similar cells 6 Organismal level 5 Organ system level 4 Organ level An organism is Organ systems include An organ contains several formed from organs with coordinated types of tissues combined organ functions systems Figure 1.1 Levels of Organization. Six levels of organization in the human body range from chemical (simplest) to organismal (most complex). ONOKY - Fabrice LEROUGE/Brand X Pictures/Getty Images CheckMyUnderstanding organs are assigned to more than one organ system. For example, the pancreas belongs to both the digestive and endocrine systems. 1. What are the organizational levels of the human Figure" 1.2 illustrates the 11 organ systems of the body? human body and lists the major organs and functions for 2. What are the major organs and general functions each system. Although each organ system has its own of each organ system? unique functions, all organ systems support one another. For example, all organ systems rely on the cardiovascu- lar system to transport materials to and from their cells. Organ systems work together to enable the functioning 1.3 Directional Terms of the human body. Learning Objective Organismal Level 4. Use directional terms to describe the locations of body parts. The highest organizational level dealing with an indi- vidual is the organismal level, the human organism as Directional terms are used to describe the relative a whole. It is composed of all of the interacting organ position of a body part in relationship to another body systems. All of the organizational levels from chemicals part. The use of these terms conveys a precise meaning to organ systems contribute to the functioning of the enabling the listener or reader to locate the body part of entire body. interest. It is always assumed that the body is in a standard 4 Chapter 1 Introduction to the Human Body Integumentary system Skeletal system Muscular system Organs: skin, hair, nails, and Organs: bones, ligaments, Organs: skeletal muscles and tendons associated glands and associated cartilages Functions: moves the body and Functions: protects underlying tissues Functions: supports the body, body parts and produces heat and helps regulate body temperature protects vital organs, stores min- erals, and is the site of blood cell production Figure 1.2 The 11 Organ Systems of the Body. position, the anatomical position, in which the body is standing upright with upper limbs at the sides and palms of the hands facing forward, as in figure" 1.3. Directional terms occur in pairs, and the members of each pair have opposite meanings, as noted in table"1.1. 1.4 Body Regions Learning Objective 5. Locate the major body regions on a diagram or anatomical model. The human body consists of an axial (ak!-se !-al) portion, the head, neck, and trunk, and an appendicular (ap-pen- !-lar) portion, the upper and lower limbs and their dik!-u girdles. Each of these major portions of the body is divided into regions with special names to facilitate com- munication and to aid in locating body components. The major body regions are listed in tables"1.2 and Respiratory system Cardiovascular system 1.3 to allow easy correlation with figure"1.4, which shows Organs: blood, heart, Organs: nose, pharynx, larynx, the locations of the major regions of the body. Take time trachea, bronchi, and lungs arteries, veins, and capillaries to learn the names, pronunciations, and locations of the Functions: exchanges O2 and CO2 Functions: transports heat and between air and blood in the lungs, materials to and from the body cells body regions. pH regulation, and sound production  Part 1 Organization of the Body 5 Lymphoid system Urinary system Endocrine system Organs: lymph, lymphatic vessels, Organs: kidneys, ureters, Organs: hormone-producing glands, and lymphoid organs and tissues urinary bladder, and urethra such as the pituitary and thyroid glands Functions: collects and cleanses Functions: regulates volume and Functions: secretes hormones that regulate interstitial fluid, and returns it to composition of blood by forming body functions the blood; provides immunity and excreting urine Nervous system Digestive system Male reproductive system Female reproductive system Organs: brain, spinal cord, Organs: mouth, pharynx, Organs: testes, epididymides, Organs: ovaries, uterine tubes, nerves, and sensory receptors esophagus, stomach, intestines, vasa deferentia, prostate, uterus, vagina, and vulva Functions: rapidly coordinates liver, pancreas, gallbladder, and bulbo-urethral glands, seminal Functions: produces oocytes, receives body functions and enables learning associated structures vesicles, and penis sperm, provides intrauterine and memory Functions: digests food and absorbs Functions: produces sperm and development of offspring, and nutrients transmits them into the female enables birth of an infant vagina during sexual intercourse Figure 1.2 6 Chapter 1 Introduction to the Human Body Superior Right Left Superior Midline Proximal Anterior Posterior Inferior Medial (Ventral) (Dorsal) Distal Lateral Inferior Proximal Distal Distal Proximal Figure 1.3 Anatomical Position and Directional Terms. (a) Eric Wise; (b) Eric Wise Table 1.1 Directional Terms Term Meaning Example Anterior (ventral) Toward the front or abdominal surface The abdomen is anterior to the back. of the body Posterior (dorsal) Toward the back of the body The spine is posterior to the face. Superior (cephalic) Toward the top/head The nose is superior to the mouth. Inferior (caudal) Away from the top/head The navel is inferior to the nipples. Medial Toward the midline of the body The breastbone is medial to the nipples. Lateral Away from the midline of the body The ears are lateral to the cheeks. Parietal Pertaining to the outer boundary of The parietal pleura lines the pleural body cavities cavity. Visceral Pertaining to the internal organs The visceral pleura covers the lung. Superficial (external) Toward or on the body surface The skin is superficial to the muscles. Deep (internal) Away from the body surface The intestines are deep to the abdominal muscles. Proximal Closer to the beginning The elbow is proximal to the wrist. Distal Farther from the beginning The hand is distal to the wrist. Central At or near the center of the body or The central nervous system is in the organ middle of the body. Peripheral External to or away from the center The peripheral nervous system extends of the body or organ away from the central nervous system.  Part 1 Organization of the Body 7 Table 1.2 Major Regions of the Axial Portion Region Head and Neck Anterior Trunk Posterior Trunk Lateral Trunk Auricular (aw-rik!-yuh-ler) Abdominal (ab-dom!-i-nal) Dorsum (dor!-sum) Axillary (ak!-sil-lary) Buccal (bu!-kal) Inguinal (ing!-gwi-nal) Gluteal (glu!-t!-al) Coxal (kok!-sal) Cephalic (se-fal!-ik) Pectoral (pek!-t"r-al) Lumbar (lum!-bar) Inferior Trunk Cervical (ser!-vi-kal) Presternal (pre-ster!-nal) Sacral (s#k!-ral) Genital ($$ jen!-i-tal) Cranial (kr#!-n!-al) Pubic (pyoo!-bik) Scapular (skap'-yuh-ler) Perineal (per-i-n!!-al) Facial (f#!-shal) Sternal (ster!-nal) Vertebral (ver-t!!-bral) Nasal (n#!-zel) Umbilical (um-bil!-i-kal) Oral (or!-al) Orbital (or!-bit-al) Otic (o!-tic) Table 1.3 Major Regions of the Appendicular Portion Region Upper Limb Digital (di!-ji-tal) Patellar (pa-tel!-lar) Antebrachial (an-t!-br#!-k!-al) Palmar (pal!-mar) Pedal (pe!-dal) Brachial (br#!-k!-al) Lower Limb Plantar (plan!-tar) Carpal (kar!-pal) Crural (kr%!-ral) Popliteal (pop-li-t!!-al) Cubital (k%!-bi-tal) Digital (di!-ji-tal) Sural (s%!-ral) Deltoid (del!-tȯid) Femoral (fem!-"r-al) Tarsal (tahr!-sul) 1.5 Body Planes and Sections halves. A paramedian plane does not pass through the midline of the body. Frontal (coronal) planes divide the body into Learning Objective anterior and posterior portions. These planes are perpen- 6. Describe the three planes used in making sections dicular to sagittal planes and parallel to the longitudinal of the body or body parts. axis of the body. In studying the body or organs, you often will be observ- Cuts made through sagittal and frontal planes, which ing the flat surface of a section that has been produced by are parallel to the longitudinal axis of the body, produce a cut through the body or a body part. Such sections are longitudinal sections. However, the term longitudinal section made along specific planes. These well-defined planes— also refers to a section made through the longitudinal axis transverse, sagittal, and frontal planes—lie at right angles of an individual organ, tissue, or other structure. Similarly, to each other, as shown in figure" 1.5. It is important to cuts made through the transverse plane produce cross understand the nature of the plane along which a section sections of the body and can also be produced in organs and was made in order to understand the three-dimensional tissues when cutting at a 90° angle to the longitudinal axis. structure of an object being observed. Oblique sections are created when cuts are made in between Transverse, or horizontal, planes divide the body the longitudinal and cross-sectional axes. into superior and inferior portions and are perpendicular to the longitudinal axis of the body. Sagittal planes divide the body into right and left CheckMyUnderstanding portions and are parallel to the longitudinal axis of the 3. How do sagittal, transverse, and frontal planes body. A median plane passes through the midline of differ from one another? the body and divides the body into equal left and right 8 Chapter 1 Introduction to the Human Body Orbital (eye) Auricular; otic (ear) Cranial Nasal (nose) (skull) Cephalic Buccal (cheek) Facial (head) (face) Oral (mouth) Vertebral Cervical (spinal Deltoid (neck) column) (shoulder) Pectoral Scapular (chest) (shoulder blade) Axillary (armpit) Brachial (arm) Brachial Sternal; Dorsum (back) (arm) presternal (breastbone) Cubital Cubital (elbow) (elbow) Umbilical Lumbar Abdominal (navel) (lower back) (abdomen) Sacral Antebrachial (between (forearm) hips) Carpal (wrist) Palmar (palm) Gluteal Digital (finger) (buttocks) Coxal (hip) Perineal (between Genital genitals and anus) (reproductive Inguinal (groin) organs) Femoral Pubic (above (thigh) genitalia) Patellar Popliteal (front of knee) (back of knee) Sural (calf) Crural (leg) Tarsal (ankle) Pedal (foot) Digital (toe) Plantar (sole) Figure 1.4 Major Regions of the Body. 1.6 Body Cavities The cranial cavity is enclosed in the bones of the skull and contains the brain. Within the vertebral column Learning Objectives is the vertebral canal, which contains the spinal cord. 7. Locate the body cavities and their subdivisions and Note in figure 1.6 how the cranial bones and the vertebral membranes on a diagram. column form the walls of these cavities and provide pro- 8. Name the organs located in each body cavity. tection for these delicate organs. The two large body cavities in front of the vertebral The body cavities protect and cushion the contained column are divided by the diaphragm, a thin dome-shaped internal organs and permit changes in their size sheet of muscle. Above the diaphragm is the thoracic and shape without impacting surrounding tissues. cavity and below it is the abdominopelvic cavity. Note the locations and subdivisions of these cavities in The thoracic cavity is protected by the thoracic cage figure"1.6. and contains the heart and lungs. The abdominopelvic  Part 1 Organization of the Body 9 Median plane Paramedian plane Transverse (horizontal) plane Oblique plane Frontal (coronal) plane Figure 1.5 Anatomical Planes of Reference. cavity is subdivided into a superior abdominal cavity Meninges and an inferior pelvic cavity, but there is no structural The cranial cavity and vertebral canal are lined by three separation between them. To visualize the separation, layers of protective membranes that are collectively imagine a transverse plane passing through the body called the meninges (me-nin!-je!z; singular, meninx). The just above the pelvis. The abdominal cavity contains the outer membrane is attached to the wall of the cavity, and stomach, intestines, liver, gallbladder, pancreas, spleen, the inner membrane tightly envelops the brain and spinal and kidneys. The pelvic cavity contains the urinary blad- cord. The meninges will be covered in chapter 8. der, sigmoid colon, rectum, and internal reproductive organs. Serous Membranes The thoracic and abdominopelvic cavity organs are sup- CheckMyUnderstanding ported and protected by serous membranes, or serosae (singular, serosa). The serous membranes are thin layers 4. What organs are located in the cranial cavity and of tissue that line the body cavity and cover the inter- vertebral canal? nal organs. Serous membranes have an outer parietal (pah- 5. What organs are located in the thoracic and rı!!-e-tal) layer that lines the cavity and an inner visceral abdominopelvic cavities? (vis!-er-al) layer that covers the organ. The parietal and visceral layers secrete a watery lubricating fluid, generi- cally called serous fluid, into the cavity formed between the layers. This arrangement is similar to that of a fist Membranes of Body Cavities pushed into a balloon (figure" 1.7). The serous mem- The membranes lining body cavities support and protect branes of the body are the pleura, serous pericardium, and the internal organs in the cavities. peritoneum. 10 Chapter 1 Introduction to the Human Body Cranial Cranial cavity cavity Vertebral Vertebral canal canal Mediastinum Pleural cavity Pleural Thoracic cavity Pericardial cavity cavity Pericardial cavity Diaphragm Diaphragm Abdominal Abdominal cavity cavity Abdominopelvic Pelvic cavity Pelvic cavity cavity (a) (b) Pericardial cavity Pleural cavity Pleura Mediastinum Spinal cord in (c) vertebral canal Figure 1.6 Body Cavities and Their Subdivisions. (a) Sagittal section. (b) Frontal section. (c) Transverse section through the thoracic cavity. The serous membranes lining the thoracic cavity The left and right portions of the thoracic cavity are are called pleurae (singular, pleura). The walls of the divided by a membranous partition, the mediastinum (me !- ! ! left and right portions of the thoracic cavity are lined by de-a-sti !-num). Organs located within the mediastinum the parietal pleurae. The surfaces of the lungs are covered include the heart, thymus, esophagus, and trachea. by the visceral pleurae. The space between the parietal The heart is enveloped by the serous pericar- and visceral pleurae is called the pleural cavity. The dium (per-i-kar!-de !-um), which is formed by membranes pleural cavity contains a thin film of serous fluid called of the mediastinum. The thin visceral layer of the serous pleural fluid, which reduces friction as the pleurae rub pericardium is tightly adhered to the surface of the heart. against each other as the lungs expand and contract dur- The parietal layer of the serous pericardium lines the inside ing breathing. surface of a loosely fitting sac around the heart. The space  Part 1 Organization of the Body 11 Clinical Insight Physicians use certain types of diagnostic imaging by the detector are then processed by a computer to systems—for example, computerized tomography produce sectional images on a screen for viewing by a (CT), magnetic resonance imaging (MRI), and posi- radiologist. A good understanding of sectional anatomy tron emission tomography (PET)—to produce images is required to interpret CT scans. Transverse sections, of sections of the body to help them diagnose dis- such as the image on the left, are always shown in the orders. In computerized tomography, an X-ray emit- same way. Convention is to use supine (face up), infe- ter and an X-ray detector rotate around the patient rior views as if looking up at the section from the foot so that the X-ray beam passes through the body of the patient’s bed. What structures can you identify in from hundreds of different angles. X-rays collected the CT image shown on the right? Scott Camazine Du Cane Medical Imaging Ltd./Science Photo Library/Getty Images Outer balloon wall Outer balloon wall (parietal serous membrane) Inner balloon wall Inner balloon wall (visceral serous membrane) Cavity Cavity Fist Fist Figure 1.7 Model of a Serous Membrane. Illustration of a fist pushed into a balloon as an analogy to a serous membrane. 12 Chapter 1 Introduction to the Human Body Parietal layer of the Parietal serous peritoneum pericardium Serous pericardium Visceral Liver layer of the surrounded serous by visceral pericardium peritoneum Pericardial cavity containing Visceral pericardial fluid peritoneum Peritoneal Heart cavity containing (a) peritoneal fluid Mesenteries Parietal pleura Visceral Retroperitoneal pleura organs Pleural cavity containing pleural fluid Lung (b) Diaphragm (c) Figure 1.8 Serous Membranes. (a) Anterior view of serous pericardium. (b) Anterior view of pleurae. (c) Sagittal view of peritoneum. between the visceral and parietal layers of the serous peri- cardia is the pericardial cavity, and it contains serous 1.7 Abdominopelvic Subdivisions fluid, called pericardial fluid, that reduces friction as the Learning Objectives heart contracts and relaxes. The walls of the abdominal cavity and the surfaces of 9. Name the abdominopelvic quadrants and regions. abdominal organs are covered with the peritoneum (per- 10. Locate the abdominopelvic quadrants and regions on !-ne i-to !!-um). The parietal peritoneum lines the walls of the a diagram. abdominal cavity but not the pelvic cavity. It descends The abdominopelvic cavity is subdivided into either only to cover the top of the urinary bladder. The kidneys, four quadrants or nine regions to aid health-care provid- pancreas, and parts of the intestines are located behind the ers in locating underlying organs. Physicians may feel parietal peritoneum in a space known as the retroperitoneal (palpate) or listen to (auscultate) the abdominopelvic space. The visceral peritoneum, an extension of the parietal region to examine it. Changes in firmness or sounds peritoneum, covers the surface of the abdominal organs. may indicate abnormalities in the structures of a quad- Double-layered folds of the visceral peritoneum, the rant or region. !s), extend between the abdomi- mesenteries (mes!-en-ter!!-e The four quadrants are formed by two planes that nal organs and provide support for them (see figure"1.8c). intersect just above the umbilicus (navel), as shown in The space between the parietal and visceral peritoneum is figure"1.9a. Note the organs within each quadrant. called the peritoneal cavity and contains a small amount The nine regions are formed by the intersection of serous fluid called peritoneal fluid (figure"1.8). of two sagittal and two transverse planes, as shown in CheckMyUnderstanding figure"1.9c. The sagittal planes extend inferiorly from the midpoints of the collarbones. The superior transverse 6. What membranes line the various body cavities? plane lies just below the borders of the 10th costal carti- 7. What is the function of serous fluid? lages, and the inferior transverse plane lies just below the top of the hip bones.  Part 1 Organization of the Body 13 Quadrants Sternum Lung Right Left Stomach upper upper quadrant quadrant 10th costal cartilage Right Left lower lower quadrant quadrant (a) (b) Regions Sternum Liver Epigastric Gallbladder region Right Left hypochondriac hypochondriac 10th costal cartilage region region Umbilical Large intestine region Right flank Left flank region region Small intestine Pubic region Right inguinal Left inguinal region region (c) (d) Figure 1.9 Abdominopelvic Subdivisions. The four quadrants and nine regions of the abdominopelvic cavity. Study figures"1.8 and 1.9 to increase your understand- stages of dissection that reveals major muscles, blood ves- ing of the locations of the internal organs and associated sels, and internal organs. Study these plates to learn the membranes. locations of the organs. Also, check your understanding Now examine the colorplates that follow this chapter. of the organs within each abdominopelvic quadrant and They show an anterior view of the body in progressive region. 14 Chapter 1 Introduction to the Human Body CheckMyUnderstanding Healthy range 8. What are the four quadrants and nine regions of the abdominopelvic region? Set point 1.8 Maintenance of Life 0.1 0.2 0.3 0.4 0.5 Time (sec) Learning Objectives 11. Define metabolism, anabolism, and catabolism. 12. List the five basic needs essential for human life. 13. Define homeostasis. Figure 1.10 Healthy Range and Set Point. 14. Explain how homeostasis relates to both healthy body functions and disorders. range. This relatively stable internal environment is main- 15. Describe the general mechanisms of negative- tained in spite of the fact that internal and external factors feedback and positive-feedback. tend to alter body temperature, and materials are continu- ously entering and exiting the blood and interstitial fluid. Humans, like all living organisms, exhibit the fundamental All of the organ systems work in an interdependent processes of life. Metabolism (me-tab!-o !-lizm) is the term manner to maintain homeostasis. For example, changes that collectively refers to the sum of all of the chemical in one system tend to affect one or more other organ sys- reactions that occur in the body. tems. Therefore, any disruption in one organ system tends There are two phases of metabolism: anabolism and to be corrected but may disrupt another organ system. catabolism. Anabolism (ah-nab!-o !-lizm) refers to pro- The internal environment is maintained via a dynamic cesses that use energy and nutrients to build the complex equilibrium where there is constant fluctuation taking organic molecules that compose the body. Catabolism place in order to maintain homeostasis. Malfunctioning or (kah-tab!-o!-lizm) refers to processes that release energy and overcompensation in a homeostatic mechanism can lead break down complex molecules into simpler molecules. to disorders and diseases. Life is fragile. It depends upon the healthy function- The dynamic equilibrium of homeostasis is primarily ing of trillions of body cells, which, in turn, depends upon maintained by physiologic processes called negative- factors needed for survival and the ability of the body to feedback mechanisms. Body fluid composition and maintain relatively stable internal conditions. other physiological variables fluctuate near a healthy value, called a set point, and negative-feedback mecha- Survival Needs nisms are used to keep these variables within their healthy range (figure" 1.10). For a negative-feedback There are five basic needs that are essential to human life: mechanism to work, it needs to be able to monitor 1. Food provides chemicals that serve as a source of and respond to any changes in homeostasis. The struc- energy and raw materials to grow and to maintain ture of the negative-feedback mechanism allows it to cells of the body. function in exactly this manner and is a great example 2. Water provides the environment in which the of how anatomical structure complements function. To chemical reactions of life occur. monitor a physiological variable, a negative-feedback 3. Oxygen is required to release the energy in mechanism utilizes a receptor to detect deviation organic nutrients, which powers life processes. from the set point and send a signal notifying the inte- 4. Body temperature must be maintained close to grating center about the deviation. The integrating 36.8°C (98.2°F) to allow the chemical reactions of center, which is the body region that knows the set human metabolism to occur. point for the variable, processes the information from 5. Atmospheric pressure is required for breathing a receptor and determines the course of action that is to occur. needed. It then sends a signal that activates an effector. The effector will carry out the necessary response accord- ing to the directions of the integrating center and return Homeostasis the variable back toward the set point. In a negative-feed- Homeostasis is the maintenance of a relatively stable back mechanism, the response of the effector will always internal environment by self-regulating physiological pro- be the opposite of the change detected by the receptor cesses. Homeostasis keeps body temperature and the com- (figure 1.11). Once the set point is reached, the negative- position of blood and interstitial fluids within their healthy feedback mechanism will automatically turn off.  Part 1 Organization of the Body 15 Our body’s ability to maintain a relatively constant Stimulus Signal blood glucose level relies on negative-feedback mecha- RECEPTOR Begin nisms. When the blood glucose level begins to rise, as it does after a meal, there are receptors in the pancreas PHYSIOLOGICAL INTEGRATING that can detect this stimulus (change). The beta cells of VARIABLE CENTER the pancreas act as an integrating center and release the hormone insulin in response to this change. Insulin travels through the blood to several effectors, one of which is the EFFECTOR liver. Insulin causes the liver cells to take excess glucose Response Signal out of the blood and thus decrease the blood glucose level back toward the set point. The pancreas possesses other Figure 1.11 A Typical Negative-Feedback Mechanism. receptors that can detect decreases in blood glucose, such as occurs between meals. The alpha cells of the pancreas, acting as the integrating center, release the hormone glucagon. Glucagon causes the liver to release glucose into the blood, which will increase blood glucose back toward the set point (figure"1.12). INTEGRATING CENTER RECEPTORS Information affects Beta cells of the pancreas Beta cells of the pancreas Blood glucose STIMULUS: Rising blood Release insulin level disturbed glucose level into blood RESPONSE: Decreased blood glucose EFFECTORS Begin VARIABLE level through entry of Liver and other Blood glucose glucose into cells cells of the body level take in glucose Blood glucose from the blood level restored Figure 1.12 An Example of a Negative-Feedback Mechanism. The negative-feedback mechanism that regulates blood glucose level. 16 Chapter 1 Introduction to the Human Body Insulin Levels After Meals Begin PHYSIOLOGICAL VARIABLE Breakfast Lunch Dinner Snack 8:30am 12:00pm 5:00pm 8:30pm Sugars enter the mouth 8 Plasma glucose 7 mmol/l 6 RECEPTORS 5 Sugar-sensitive cells 4 in the mouth detect Plasma insulin 60 sugar molecules m U/l 40 Input 20 0 INTEGRATING CENTER Brain interprets Positive-feedback: input and triggers Digestion of carbohydrates effector Figure 1.13 The Graded Response. to sugars by amylase results in more sugars The graded response of insulin release is based on the amount of blood glucose elevation. Output Source: The FASEB Journal C Kendall, vol. 28. no. 1 Supplement 1039.6. It is important to note that the response of the inte- EFFECTORS grating center will be stronger if the original stimulus Salivary glands release is further from the set point. For example, if the blood more saliva to digest glucose level rises sharply out of the healthy range, caus- carbohydrates ing hyperglycemia (blood glucose level above the healthy range), the amount of insulin the beta cells release will be more than the amount released if the blood glucose PHYSIOLOGICAL level is elevated but is still within the healthy range. This VARIABLE type of response is called a graded response because it can Production of more sugars respond on different levels (figure"1.13). Positive-feedback mechanisms utilize the same basic components as negative-feedback mechanisms. How- ever, the outcome of a positive-feedback mechanism is RECEPTORS very different from that of a negative-feedback mechanism. Swallowing of food A positive-feedback mechanism is used when the originat- and/or stopping eating ing stimulus needs to be amplified and continued in order decreases sugar level for the desired result to occur. A few examples of positive- in the mouth, stopping feedback mechanisms include fever, activation of the the positive-feedback mechanism immune response, formation of blood clots, certain aspects of digestion, and uterine contractions of labor. If you think about blood clot formation, blood clots do not form “normally”; when they begin to form, it occurs quickly and Figure 1.14 A Positive-Feedback Mechanism. completely in order to stop blood loss. This is a necessary Saliva is produced as needed, and production increases mechanism for overall homeostasis. Figure 1.14 illustrates with continued stimulation. the specific steps of the positive-feedback mechanism of saliva production. positive-feedback mechanisms are used for rare events Positive-feedback mechanisms can be harmful within the body, rather than for the daily maintenance of because they lack the ability to stop on their own. homeostasis. They will continue to amplify the effect of the original stimulus, which can push the body dangerously out of homeostasis, until the cycle is interrupted by an outside CheckMyUnderstanding factor. For example, an uncontrolled fever can increase 9. What is homeostasis? How is homeostasis regulated? body temperature to a point that is fatal. For this reason,