Human Function

Objectives

When you have completed this section, you should be able to

• state the characteristics that distinguish living organisms from nonliving objects;

• explain the importance of defining a reference man and woman;

• define homeostasis and explain why this concept is central to physiology;

• define negative feedback, give an example of it, and explain its importance to homeostasis; and

• define positive feedback and give examples of its beneficial and harmful effects.

Characteristics of Life

Why do we consider a growing child to be alive, but not a growing crystal? Is abortion the taking of a human life? If so, what about a contraceptive foam that kills only sperm? As a patient is dying, at what point does it become ethical to disconnect life-support equipment and remove organs for donation? If these organs are alive, as they must be to serve someone else, then why isn't the donor considered alive? Such questions have no easy answers, but they demand a concept of what life is—a concept that may differ with one's biological, medical, or legal perspective.

From a biological viewpoint, life is not a single property. It is a collection of properties that help to distinguish living from nonliving things:

• Organization. Living things exhibit a far higher level of organization than the nonliving world around them. They expend a great deal of energy to maintain order,

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Chapter 1 Major Themes of Anatomy and Physiology 15

Commonest Position The Appendix

Figure 1.10 Variation in Human Anatomy. The left-hand figure in each case depicts the most common anatomy. (a) Variations in stomach shape correlated with body physique. (b) Variations in the position of the appendix. (c) Variations in the bile passages of the liver and gallbladder.

Figure 1.10 Variation in Human Anatomy. The left-hand figure in each case depicts the most common anatomy. (a) Variations in stomach shape correlated with body physique. (b) Variations in the position of the appendix. (c) Variations in the bile passages of the liver and gallbladder.

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16 Part One Organization of the Body and a breakdown in this order is accompanied by disease and often death.

• Cellular composition. Living matter is always compartmentalized into one or more cells.

• Metabolism and excretion. Living things take in molecules from the environment and chemically change them into molecules that form their own structures, control their physiology, or provide them with energy. Metabolism14 is the sum of all this internal chemical change. It consists of two classes of reactions: anabolism,15 in which relatively complex molecules are synthesized from simpler ones (for example, protein synthesis), and catabolism,16 in which relatively complex molecules are broken down into simpler ones (for example, protein digestion). Metabolism inevitably produces chemical wastes, some of which are toxic if they accumulate. Metabolism therefore requires excretion, the separation of wastes from the tissues and their elimination from the body. There is a constant turnover of molecules in the body; few of the molecules now in your body have been there for more than a year. It is food for thought that although you sense a continuity of personality and experience from your childhood to the present, nearly all of your body has been replaced within the past year.

• Responsiveness and movement. The ability of organisms to sense and react to stimuli (changes in their environment) is called responsiveness, irritability, or excitability. It occurs at all levels from the single cell to the entire body, and it characterizes all living things from bacteria to you. Responsiveness is especially obvious in animals because of nerve and muscle cells that exhibit high sensitivity to environmental stimuli, rapid transmission of information, and quick reactions. Most living organisms are capable of self-propelled movement from place to place, and all organisms and cells are at least capable of moving substances internally, such as moving food along the digestive tract or moving molecules and organelles from place to place within a cell.

• Homeostasis. While the environment around an organism changes, the organism maintains relatively stable internal conditions. This ability to maintain internal stability, called homeostasis, is explored in more depth shortly.

• Development. Development is any change in form or function over the lifetime of the organism. In most organisms, it involves two major processes:

(1) differentiation, the transformation of cells with no

14metabol = change + ism = process

15ana = up

16cata = down specialized function into cells that are committed to a particular task, and (2) growth, an increase in size. Some nonliving things grow, but not in the way your body does. If you let a saturated sugar solution evaporate, crystals will grow from it, but not through a change in the composition of the sugar. They merely add more sugar molecules from the solution to the crystal surface. The growth of the body, by contrast, occurs through chemical change (metabolism); for the most part, your body is not composed of the molecules you ate but of molecules made by chemically altering your food.

• Reproduction. All living organisms can produce copies of themselves, thus passing their genes on to new, younger containers—their offspring.

• Evolution. All living species exhibit genetic change from generation to generation and therefore evolve. This occurs because mutations (changes in DNA structure) are inevitable and because environmental selection pressures endow some individuals with greater reproductive success than others. Unlike the other characteristics of life, evolution is a characteristic seen only in the population as a whole. No single individual evolves over the course of its life.

Clinical and legal criteria of life differ from these biological criteria. A person who has shown no brain waves for 24 hours, and has no reflexes, respiration, or heartbeat other than what is provided by artificial life support, can be declared legally dead. At such time, however, most of the body is still biologically alive and its organs may be useful for transplant.

Physiological Variation

Earlier we considered the clinical importance of variations in human anatomy, but physiology is even more variable. Physiological variables differ with sex, age, weight, diet, degree of physical activity, and environment, among other things. Failure to consider such variation leads to medical mistakes such as overmedication of the elderly or medicating women on the basis of research that was done on men. If an introductory textbook states a typical human heart rate, blood pressure, red blood cell count, or body temperature, it is generally assumed that such values are for a healthy young adult unless otherwise stated. A point of reference for such general values is the reference man and reference woman. The reference man is defined as a healthy male 22 years old, weighing 70 kg (154 lb), living at a mean ambient (surrounding) temperature of 20°C, engaging in light physical activity, and consuming 2,800 kilocalories (kcal) per day. The reference woman is the same except for a weight of 58 kg (128 lb) and an intake of 2,000 kcal/day.

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Chapter 1 Major Themes of Anatomy and Physiology 17

Homeostasis and Negative Feedback

Homeostasis17 (ho-me-oh-STAY-sis) is one of the theories that will arise most frequently in this book as we study mechanisms of health and disease. The human body has a remarkable capacity for self-restoration. Hippocrates commented that it usually returns to a state of equilibrium by itself, and people recover from most illnesses even without the help of a physician. This tendency results from homeostasis, the ability to detect change and activate mechanisms that oppose it.

French physiologist Claude Bernard (1813-78) observed that the internal conditions of the body remain fairly stable even when external conditions vary greatly. For example, whether it is freezing cold or swelteringly hot outdoors, the internal temperature of your body stays within a range of about 36° to 37°C (97°-99°F). American physiologist Walter Cannon (1871-1945) coined the term homeostasis for this tendency to maintain internal stability. Homeostasis has been one of the most enlightening concepts in physiology. Physiology is largely a group of mechanisms for maintaining homeostasis, and the loss of homeostatic control tends to cause illness or death. Patho-physiology is essentially the study of unstable conditions that result when our homeostatic controls go awry.

Do not, however, overestimate the degree of internal stability. Internal conditions are not absolutely constant but fluctuate within a limited range, such as the range of body temperatures noted earlier. The internal state of the body is best described as a dynamic equilibrium (balanced change), in which there is a certain set point or average value for a given variable (such as 37°C for body temperature) and conditions fluctuate slightly around this point.

The fundamental mechanism that keeps a variable close to its set point is negative feedback—a process in which the body senses a change and activates mechanisms that negate or reverse it. By maintaining stability, negative feedback is the key mechanism for maintaining health.

These principles can be understood by comparison to a home heating system (fig. 1.11). Suppose it is a cold winter day and you have set your thermostat for 20°C (68°F)— the set point. If the room becomes too cold, a temperature-sensitive switch in the thermostat turns on the furnace. The temperature rises until it is slightly above the set point, and then the switch breaks the circuit and turns off the furnace. This is a negative feedback process that reverses the falling temperature and restores it to something close to the set point. When the furnace turns off, the temperature slowly drops again until the switch is reactivated—thus, the furnace cycles on and off all day. The room temperature does not stay at exactly 20°C but fluctuates a few degrees either way—the system maintains a state of dynamic equilibrium in which the temperature averages 20°C and deviates from the set point by only a few degrees. Because feed-

Room temperature falls to 66°F (19°C)

Room cools down

Thermostat activates furnace

Thermostat activates furnace

Thermostat shuts off furnace

Thermostat shuts off furnace

Feedback Home Heating System

Figure 1.11 Negative Feedback in a Home Heating System.

(a) The negative feedback loop that maintains room temperature.

(b) Fluctuation of room temperature around the thermostatic set point. What component of the heating system acts as the sensor? What component acts as the effector?

Figure 1.11 Negative Feedback in a Home Heating System.

(a) The negative feedback loop that maintains room temperature.

(b) Fluctuation of room temperature around the thermostatic set point. What component of the heating system acts as the sensor? What component acts as the effector?

back mechanisms alter the original changes that triggered them (temperature, for example), they are often called feedback loops.

Body temperature is also regulated by a "thermostat"— a group of nerve cells in the base of the brain that monitors the temperature of the blood. If you become overheated, the thermostat triggers heat-losing mechanisms (fig. 1.12). One of these is vasodilation (VAY-zo-dy-LAY-shun), the widening of blood vessels. When blood vessels of the skin dilate, warm blood flows closer to the body surface and loses heat to the surrounding air. If this is not enough to return your temperature to normal, sweating occurs; the evaporation of

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18 Part One Organization of the Body o O

18 Part One Organization of the Body

Human Body Sweating

Sweating

Set point

< Vasoconstriction -Shivering

Figure 1.12 Negative Feedback in Human Thermoregulation. Negative feedback keeps the human body temperature homeostatically regulated within about 0.5°C of a 37°C set point. Sweating and cutaneous vasodilation lower the body temperature; shivering and cutaneous vasoconstriction raise it. Why does vasodilation reduce the body temperature?

Sweating

Set point

< Vasoconstriction -Shivering

Figure 1.12 Negative Feedback in Human Thermoregulation. Negative feedback keeps the human body temperature homeostatically regulated within about 0.5°C of a 37°C set point. Sweating and cutaneous vasodilation lower the body temperature; shivering and cutaneous vasoconstriction raise it. Why does vasodilation reduce the body temperature?

water from the skin has a powerful cooling effect (see insight 1.3). Conversely, if it is cold outside and your body temperature drops much below 37°C, these nerve cells activate heat-conserving mechanisms. The first to be activated is vasoconstriction, a narrowing of the blood vessels in the skin, which serves to retain warm blood deeper in your body and reduce heat loss. If this is not enough, the brain activates shivering—muscle tremors that generate heat.

Insight 1.3 Medical History

Men in the Oven

English physician Charles Blagden (1748-1820) staged a rather theatrical demonstration of homeostasis long before Cannon coined the word. In 1775, Blagden spent 45 minutes in a chamber heated to 127°C (260°F)—along with a dog, a beefsteak, and some research associates. Being alive and capable of evaporative cooling, the dog panted and the men sweated. The beefsteak, being dead and unable to maintain homeostasis, was cooked.

To take another example, a rise in blood pressure is sensed by stretch receptors in the wall of the heart and the major arteries above it. These receptors send nerve signals to a cardiac center in the brainstem. The cardiac center integrates this input with other information and sends nerve signals back to the heart to slow it and lower the blood pressure. Thus we can see that homeostasis is maintained by self-correcting negative feedback loops. Many more examples are found throughout this book.

It is common, although not universal, for feedback loops to include three components: a receptor, an integrator, and an effector. The receptor is a structure that senses a change in the body, such as the stretch receptors that monitor blood pressure. The integrating (control) center, such as the cardiac center of the brain, is a mechanism that processes this information, relates it to other available information (for example, comparing what the blood pressure is with what it should be), and "makes a decision" about what the appropriate response should be. The effector, in this case the heart, is the structure that carries out the response that restores homeostasis. The response, such as a lowering of the blood pressure, is then sensed by the receptor, and the feedback loop is complete.

Positive Feedback and Rapid Change

Positive feedback is a self-amplifying cycle in which a physiological change leads to even greater change in the same direction, rather than producing the corrective effects of negative feedback. Positive feedback is often a normal way of producing rapid change. When a woman is giving birth, for example, the head of the baby pushes against her cervix (the neck of the uterus) and stimulates its nerve endings (fig. 1.13). Nerve signals travel to the brain, which, in turn, stimulates the pituitary gland to secrete the hormone oxytocin. Oxytocin travels in the blood and stimulates the uterus to contract. This pushes the baby downward, stimulating the cervix still more and causing the positive feedback loop to be repeated. Labor contractions therefore become more and more intense until the baby is expelled. Other cases of beneficial positive feedback are seen later in the book; for example, in blood clotting, protein digestion, and the generation of nerve signals.

Frequently, however, positive feedback is a harmful or even life-threatening process. This is because its self-amplifying nature can quickly change the internal state of the body to something far from its homeostatic set point. Consider a high fever, for example. A fever triggered by infection is beneficial up to a point, but if the body temperature rises much above 42°C (108°F), it may create a dangerous positive feedback loop (fig. 1.14). This high temperature raises the metabolic rate, which makes the body produce heat faster than it can get rid of it. Thus, temperature rises still further, increasing the metabolic rate and heat production still more. This "vicious circle" becomes fatal at approximately 45°C (113°F). Thus, positive feedback loops often create dangerously out-of-control situations that require emergency medical treatment.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

14. List four biological criteria of life and one clinical criterion. Explain how a person could be clinically dead but biologically alive.

15. What is meant by dynamic equilibrium? Why would it be wrong to say homeostasis prevents internal change?

16. Explain why stabilizing mechanisms are called negative feedback.

17. Explain why positive feedback is more likely than negative feedback to disturb homeostasis.

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Form and Function, Third Edition

Chapter 1 Major Themes of Anatomy and Physiology 19

Oxytocin carried in bloodstream to uterus

[^LOxytocin stimulates uterine contractions and pushes fetus toward cervix

(3) Brain stimulates pituitary gland to secrete oxytocin

(3) Brain stimulates pituitary gland to secrete oxytocin

(1 Head of fetus pushes against cervix

^ Nerve impulses from cervix transmitted to brain

^ Nerve impulses from cervix transmitted to brain

Figure 1.13 Positive Feedback in Childbirth. This is one of several cases in which positive feedback produces beneficial rapid change.

Positive Feedback Fever
Figure 1.14 Positive Feedback in Fever. In such cases as this, positive feedback can produce a life-threatening loss of homeostatic control.
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Patrick
    Why would it be wrong to say homeostasis prevents internal change?
    7 years ago
  • klaudia
    What support the importance of defining a "reference man and woman"?
    7 years ago
  • KRYSTAL
    Why would it be wrong to say homeostasis prevents internal change before you go own mcgraw hill?
    3 years ago

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