Chapter1 An Introduction to the Human Body

The Levels of Organization

To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, known as atoms and molecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as protons, electrons, and neutrons. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.

A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small and independently living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells. A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid, together with a variety of tiny functioning units called organelles.

A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function.

An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions.

An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body.

This book covers eleven distinct organ systems in the human body (Figure 1.2 and Figure 1.3). Assigning organs to organ systems can be imprecise, since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system.

In this book and throughout your studies of biological sciences, you will often read descriptions of similarities and differences among biological structures, processes, and health related to a person’s biological sex. People often use the words female and male to describe two different concepts: our sense of gender identity and our biological sex as determined by our chromosomes, hormones, organs, and other physical characteristics. For some people, gender identity is different from biological sex or their sex assigned at birth. Throughout this book, female and male refer to sex only, and the typical anatomy and physiology of XX and XY individuals are discussed.

The organism level is the highest level of organization; it is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.

Organization

A human body consists of trillions of cells organized in a way that maintains distinct internal compartments. These compartments keep body cells separated from external environmental threats and keep the cells moist and nourished. They also separate internal body fluids from the countless microorganisms that grow on body surfaces, including the linings of certain passageways that connect to the outer surface of the body. The intestinal tract, for example, is home to more bacterial cells than the total of all human cells in the body, yet these bacteria are outside the body and cannot be allowed to circulate freely inside the body.

Example: Cells have a cell membrane (also referred to as the plasma membrane) that keeps the intracellular environment—the fluids and organelles—separate from the extracellular environment. Blood vessels keep blood inside a closed circulatory system, and nerves and muscles are wrapped in connective tissue sheaths that separate them from surrounding structures. In the chest and abdomen, a variety of internal membranes keep major organs such as the lungs, heart, and kidneys separate from one another.

The body’s largest organ system is the integumentary system, which includes the skin and its associated structures, such as hair and nails. The surface tissue of skin is a barrier that protects internal structures and fluids from potentially harmful microorganisms and other toxins.

Metabolism

The first law of thermodynamics holds that energy can be neither created nor destroyed—it can only change form. Your basic function as an organism is to consume (ingest) energy and molecules in the foods you eat, convert some of them into fuel for movement, sustaining your body functions, and building and maintaining your body structures. There are two types of reactions that accomplish this: anabolic and catabolic.

• • Anabolism is the process whereby smaller, simpler molecules are combined into larger, more complex substances. Your body can assemble, utilizing energy, the complex chemicals it needs by combining small molecules derived from the foods you eat.
• • Catabolism is the process by which larger, more complex substances are broken down into smaller, simpler molecules. Catabolism releases energy. The complex molecules found in foods are broken down so the body can use their parts to assemble the structures and substances needed for life.

Taken together, these two processes are called metabolism. Metabolism is the sum of all anabolic and catabolic reactions that take place in the body (Figure 1.4). Both anabolism and catabolism occur simultaneously and continuously to keep you alive.

Every cell in your body makes use of a chemical compound, adenosine triphosphate (ATP), to store and release energy. The cell stores energy in the synthesis (anabolism) of ATP, then moves the ATP molecules to the location where energy is needed to fuel cellular activities. Then the ATP is broken down (catabolism), and a controlled amount of energy is released, which is used by the cell to perform a particular job.

Responsiveness

Responsiveness is the ability of an organism to adjust to changes in its internal and external environments.

An example of responsiveness to external stimuli could include moving toward sources of food and water and away from perceived dangers. Changes in an organism’s internal environment, such as increased body temperature, can cause the responses of sweating and the dilation of blood vessels in the skin in order to decrease body temperature.

Movement

Human movement includes not only actions at the joints of the body but also the motion of individual organs and even individual cells.

Example: Red and white blood cells are moving throughout your body, muscle cells are contracting and relaxing to maintain your posture and to focus your vision, and glands are secreting chemicals to regulate body functions. Your body is coordinating the action of entire muscle groups to enable you to move air into and out of your lungs, to push blood throughout your body, and to propel the food you have eaten through your digestive tract. Consciously, of course, you contract your skeletal muscles to move the bones of your skeleton to get from one place to another and to carry out all the activities of your daily life.

Development, Growth, and Reproduction

Development is all the changes the body goes through in life. Development includes the process of differentiation, in which unspecialized cells become specialized in structure and function to perform certain tasks in the body. Development also includes the processes of growth and repair, both of which involve cell differentiation.

Growth is the increase in body size. Humans, like all multicellular organisms, grow by increasing the number of existing cells, increasing the amount of noncellular material around cells (such as mineral deposits in bone), and, within very narrow limits, increasing the size of existing cells.

Reproduction is the formation of a new organism from parent organisms. In humans, reproduction is carried out by the male and female reproductive systems. Because death will come to all complex organisms, without reproduction, the line of organisms would end.

Negative Feedback

Negative feedback is a mechanism that reverses a deviation from the set point, maintaining body parameters within their normal range. The maintenance of homeostasis by negative feedback goes on throughout the body at all times, and an understanding of negative feedback is thus fundamental to an understanding of human physiology. A negative feedback system has three basic components (Figure 1.5a). A sensor, also referred to as a receptor, is a component of a feedback system that monitors a physiological value. This value is reported to the control centre. The control centre is the component in a feedback system that compares the value to the normal range. If the value deviates too much from the set point, then the control centre activates an effector. An effector is the component in a feedback system that causes a change to reverse the situation and return the value to the normal range.

In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (i.e., beyond homeostasis). This stimulus is “heard” by a specific sensor.

Example: In the control of blood glucose, specific endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream. These pancreatic beta cells respond to the increased level of blood glucose by releasing the hormone insulin into the bloodstream. The insulin signals skeletal muscle fibres, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream. As glucose concentration in the bloodstream drops, the decrease in concentration—the actual negative feedback—is detected by pancreatic alpha cells, and insulin release stops. This prevents blood sugar levels from continuing to drop below the normal range.

Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain (Figure 1.5b).

Positive Feedback

Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point. Childbirth and the body’s response to blood loss are two examples of positive feedback loops that are normal but are activated only when needed.

Example: Childbirth at full term is a situation in which the maintenance of the existing body state is not desired. Enormous changes in a person’s body are required to expel the baby at the end of pregnancy. And the events of childbirth, once begun, must progress rapidly to a conclusion, or the life of the person giving birth and the baby will be at risk. The extreme muscular work of labour and delivery is the result of a positive feedback system (Figure 1.6).

The first contractions of labour (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors). These nerve cells send messages to the brain, which in turn causes the pituitary gland at the base of the brain to release the hormone oxytocin into the bloodstream. Oxytocin causes stronger contractions of the smooth muscles of the uterus (the effectors), pushing the baby farther down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby is born. At this point, the stretching of the cervix halts, stopping the release of oxytocin.

Example: A second example of positive feedback centres on reversing extreme damage to the body. Following a penetrating wound, the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced perfusion (penetration of blood) to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut down and the person will die. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. As each step of clotting occurs, it stimulates the release of more clotting substances. This accelerates the processes of clotting and sealing off the damaged area. Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. This is an adaptive, lifesaving cascade of events.

Anatomical Position

To further increase precision, anatomists standardize the way in which they view the body. Just as maps are normally oriented with north at the top, the standard body “map,” or anatomical position, is that of the body standing upright, with the feet at shoulder width and parallel, toes forward. The upper limbs are held out to each side, and the palms of the hands face forward as illustrated in Figure 1.7. Using this standard position reduces confusion. It does not matter how the body being described is oriented; the terms are used as if it is in anatomical position.

Example: A scar in the “anterior (front) carpal (wrist) region” would be present on the palm side of the wrist.

The term anterior would be used even if the hand were palm-down on a table. A body that is lying down is described as either prone or supine. Prone describes a face-down orientation, and supine describes a face-up orientation. These terms are sometimes used in describing the position of the body during specific physical examinations or surgical procedures.

Regional Terms

The human body’s numerous regions have specific terms to help increase precision (see Figure 1.7). Notice that the term brachium or arm is reserved for the “upper arm,” and antebrachium or forearm is used rather than “lower arm.” Similarly, femur or thigh is correct, and leg or crus is reserved for the portion of the lower limb between the knee and the ankle. You will be able to describe the body’s regions using the terms from the figure.

Directional Terms

Certain directional anatomical terms appear throughout this and any other anatomy textbook (Figure 1.8). These terms are essential for describing the relative locations of different body structures.

Example: An anatomist might describe one band of tissue as “inferior to” another, or a physician might describe a tumor as “superficial to” a deeper body structure.

Commit these terms to memory to avoid confusion when you are studying or describing the locations of particular body parts.

• • Anterior (or ventral) describes the front or direction toward the front of the body. The toes are anterior to the foot.
• • Posterior (or dorsal) describes the back or direction toward the back of the body. The popliteus muscle is posterior to the patella.
• • Superior (or cranial) describes a position above or higher than another part of the body proper. The orbits are superior to the oris.
• • Inferior (or caudal) describes a position below or lower than another part of the body proper, near or toward the tail (in humans, the coccyx, or lowest part of the spinal column). The pelvis is inferior to the abdomen.
• • Lateral describes the side or direction toward the side of the body. The thumb (pollex) is lateral to the digits.
• • Medial describes the middle or direction toward the middle of the body. The hallux is the medial toe.
• • Proximal describes a position in a limb that is nearer to the point of attachment or the trunk of the body. The brachium is proximal to the antebrachium.
• • Distal describes a position in a limb that is farther from the point of attachment or the trunk of the body. The crus is distal to the femur.
• • Superficial describes a position closer to the surface of the body. The skin is superficial to the bones.
• • Deep describes a position farther from the surface of the body. The brain is deep relative to the skull.

Body Planes

A section is a two-dimensional surface of a three-dimensional structure that has been cut. Modern medical imaging devices enable clinicians to obtain “virtual sections” of living bodies. We call these scans. Body sections and scans can only be correctly interpreted, however, if the viewer understands the plane along which the section was made. A plane is an imaginary two-dimensional surface that passes through the body. There are three planes commonly referred to in anatomy and medicine, as illustrated in Figure 1.9. Transverse planes produce images referred to as cross sections.

• • The sagittal plane is the plane that divides the body or an organ vertically into right and left sides. If this vertical plane runs directly down the middle of the body, it is called the midsagittal or median plane. If it divides the body into unequal right and left sides, it is called a parasagittal plane or, less commonly, a longitudinal section.
• • The frontal plane is the plane that divides the body or an organ into an anterior (front) portion and a posterior (rear) portion. The frontal plane is often referred to as a coronal plane.
• • The transverse plane is the plane that divides the body or an organ horizontally into upper and lower portions.

Body Cavities and Serous Membranes

The body maintains its internal organization by means of membranes, sheaths, and other structures that separate compartments. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments (Figure 1.10). These cavities contain and protect delicate internal organs, and the ventral cavity allows for significant changes in the size and shape of the organs as they perform their functions. The lungs, heart, stomach, and intestines can expand and contract without distorting other tissues or disrupting the activity of nearby organs.

1.1 Overview of Anatomy and Physiology

Human anatomy is the scientific study of the body’s structures. In the past, anatomy has primarily been studied via observing injuries and later by the dissection of anatomical structures of cadavers, but in the past century, computer-assisted imaging techniques have allowed clinicians to look inside the living body. Human physiology is the scientific study of the chemistry and physics of the structures of the body. Physiology explains how the structures of the body work together to maintain life. It is difficult to study structure (anatomy) without knowledge of function (physiology). The two disciplines are typically studied together because form and function are closely related in all living things.

1.2 Structural Organization of the Human Body

Life processes of the human body are maintained at several levels of structural organization. These include the chemical, cellular, tissue, organ, organ system, and organism levels. Higher levels of organization are built from lower levels. Therefore, molecules combine to form cells, cells combine to form tissues, tissues combine to form organs, organs combine to form organ systems, and organ systems combine to form organisms.

1.3 Functions of Human Life

Most processes that occur in the human body are not consciously controlled. They occur continuously to build, maintain, and sustain life. These processes include organization (in terms of the maintenance of essential body boundaries), metabolism (including energy transfer via anabolic and catabolic reactions), responsiveness, movement, differentiation (including growth and renewal), and reproduction.

1.4 Requirements for Human Life

Humans cannot survive for more than a few minutes without oxygen, for more than several days without water, and for more than several weeks without carbohydrates, lipids, proteins, vitamins, and minerals. Although the body can respond to high temperatures by sweating and to low temperatures by shivering and increased fuel consumption, long-term exposure to extreme heat and cold is not compatible with survival. The body requires a precise atmospheric pressure to maintain its gases in solution and to facilitate respiration—the intake of oxygen and the release of carbon dioxide. Humans also require blood pressure high enough to ensure that blood reaches all body tissues but low enough to avoid damage to blood vessels.

1.5 Homeostasis

Homeostasis is the activity of cells throughout the body to maintain the physiological state within a narrow range that is compatible with life. Homeostasis is regulated by negative feedback loops and, much less frequently, by positive feedback loops. Both have the same components of a stimulus, sensor, control centre, and effector; however, negative feedback loops work to prevent an excessive response to the stimulus, whereas positive feedback loops intensify the response until an end point is reached.

1.6 Anatomical Terminology

Ancient Greek and Latin words are used to build anatomical terms. A standard reference position for mapping the body’s structures is the normal anatomical position. Regions of the body are identified using terms such as occipital that are more precise than common words and phrases such as “the back of the head.” Directional terms such as anterior and posterior are essential for accurately describing the relative locations of body structures. Images of the body’s interior commonly align along one of three planes: the sagittal, frontal, or transverse. The body’s organs are organized in one of two main cavities—dorsal (also referred to as posterior) and ventral (also referred to as anterior)—which are further subdivided according to the structures present in each area. The serous membranes have two layers—parietal and visceral—surrounding a fluid-filled space. Serous membranes cover the lungs (pleural serosa), heart (pericardial serosa), and some abdominopelvic organs (peritoneal serosa).