Human Physiology
“Imagine driving along in your car when you hear an awful clatter under the hood. Your first thought, “What’s wrong?” Most people do not think much about how a car works until something goes wrong with it. In this situation, it would be beneficial to know something about the car’s design and the function of its parts in order to narrow down the possible problems. In the same way, most people do not think about how their body works as long as everything is working properly. However, as soon as they feel ill or have an injury, parts of the body and how they work become first and foremost in their minds.
A human body is something we all live with, and increasing your knowledge about its parts can help you understand and appreciate ways to keep it in working condition. However, just like a finetuned car that can sometimes break down, even the most health-conscious individual can become sick or experience “body” damage. This book can teach you about the human body in much the same way that a service manual can teach a mechanic about an automobile. For those seeking a career in the healthcare profession, this in-depth knowledge of the body is essential to understanding how drugs and medical procedures can help correct problems brought about by disease and injury.”
This first chapter provides a foundation on which to build your knowledge of the human body. First, we introduce the scientific disciplines that study human form and function. Then we describe the body’s levels of organization so you can understand how both large and small parts of the body operate. Next, we describe what makes your body “living” and what the body needs to stay alive. Finally, we introduce vocabulary that will help you locate various body parts. To help you learn this vocabulary, we provide a pronunciation key next to most scientific terms when we first define them. We may also show the term’s literal meaning alongside an italicized root word or surrounded by quotation marks.
(et-a-MOL-ō-jē; etymo, true meaning) is the study of word origins, and it can help you remember difficult terms. Knowing the etymology of a structure’s name may reveal something about the structure’s shape, location, or function. Another word, etiology (et-ē-OL-ō-jē; etio, cause) sounds like etymology but is the study of the causes of diseases.
(fiz-ē-OL-ō-jē; physio, nature of) is the study of how structures function, but it also refers to the actual function of a body part. After describing the anatomy of the heart, someone may ask you to explain the physiology of the heart.
Before we can explain how a structure performs a particular function, we must first describe the structure’s anatomy (structure). Imagine how vaguely a person would understand vision or hearing if he or she knew nothing about the anatomy of the eyeball or the ear.
THE SCIENCE OF PHYSIOLOGY
Physiology has a number of subdivisions ranging from studies of microscopic structures such as individual cells to studies of entire systems in the body. Three examples are listed below.
- Cell physiology is the study of how individual cells carry out their activities. Obviously, this is the study of physiology at the cellular level.
- Renal physiology is the study of how the kidneys filter the blood and produce urine; this is an example of physiology at the “organ” level.
- Neurophysiology is the study of how different parts of the nervous system (brain, spinal cord, and nerves) work; this is an example of physiology at the “system” level.
In addition to studies that deal with the normal functioning of different parts of the body, some studies deal with what can go wrong with body parts. Below are two examples.
- Pathophysiology (patho, suffering) is the study of how disease disrupts body parts.
- Pathology deals with all aspects of disease, including its cause and the anatomical and physiological changes that occur in the affected structures. Pathophysiology is a subdivision of pathology.
ORGANIZATION OF THE BODY
Thus far, we have hinted that the human body is not as simple as it might seem when viewed from a surface anatomy perspective. There are many levels of organization in the body, much like there are different levels of organization in this book, from letters to words, to sentences, and so on. We will use this analogy to gain a better understanding of the body’s organization. Like this book, the human body consists of many smaller components that function together as a whole. From microscopic to macroscopic, the human body includes chemical, cellular, tissue, organ, organ system, and organismal levels of organization. See Figure 1-1 and read the description of each level that follows.
Figure 1-1. Levels of organization in the body.
- Chemical level: The chemical level of organization deals with matter, which is anything in the universe that occupies space; therefore, the body is made of matter. The building blocks of matter are elements, so-named because they represent the “elementary” (lowest or simplest) form of matter. Examples of elements in the body are carbon (C), hydro-gen (H), and oxygen (O). The smallest stable form of an element is an atom, which can bind with other atoms to form molecules (two or more atoms) and compounds (two or more different elements). Atoms, molecules, and compounds, which we collectively call chemicals, represent the lowest level of organization in the body. In our book analogy, the chemical level of organization would be the ink used for letters in words.
- Cellular level: The cellular level of organization deals with cells. A cell is the basic unit of life; that is, it is the smallest thing in the body that can be “living.” The word “cell” literally means “small room,” and was applied because the first cells viewed under a microscope looked like tiny rooms occupied by monks in a monastery. Molecules come together to form organelles, which are specialized structures that perform specific functions inside cells. Examples of cells include liver cells, skin cells, and pancreas cells. An average adult body may contain 100 trillion cells and there are about 200 different kinds of cells in the body. Cells would be like the letters on this page, with each letter consisting of ink (“chemicals”).
- Tissue level: A tissue (TISH-ū; “woven”) is a group of cells working together to perform a similar function. A tissue would be like a word in this paragraph, since most words contain two or more letters (“cells”) functioning together to form a meaningful term. The body contains four general types of tissues: epithelial, connective, muscle, and nervous. A specific type of epithelial tissue functions as a membrane to cover a surface. The visible part of your skin is a type of epithelium.
- Organ level: An organ (“instrument”) is a well-defined, anatomical structure consisting of two or more tissues working together to perform one or more functions for the body. We could compare an organ to this sentence, which has multiple words (“tissues”) functioning together to make a meaningful statement. An example of an organ is the heart, which contains muscle tissue, epithelial tissue, connective tissue, and nervous tissue.
- System level: An organ system (“organized whole”) is a group of organs working together to perform a specific task. In our analogy, this paragraph is like a system because it contains multiple sentences (“organs”) that function together to elaborate on a particular thought. The stomach, intestines, pancreas, and liver are examples of organs that are part of the digestive system. Some organs play important roles in several systems. For example, the pancreas makes chemicals that break down food in the intestine, making it part of the digestive system. It also produces hormones, chemicals that affect other cells in the body, making it part of the endocrine system. The body’s eleven organ systems include the following and are shown in Figure 1-2:
- Integumentary: skin, hair and nails.
- Skeletal: bones, cartilage, ligaments,tendons
- Muscular: muscles attached to skeletalcomponents
- Nervous: brain, spinal cord, nerves,sensory organs
- Endocrine: endocrine glands
- Cardiovascular: heart, blood vessels
- Lymphatic: lymphatic vessels, lymphnodes, spleen, thymus gland
- Respiratory: nasal cavity, larynx (voicebox), trachea (windpipe), lungs
- Digestive: mouth, teeth, tongue, salivary glands, throat, esophagus, stomach, small intestines, large intestines, liver, gall bladder, pancreas
- Urinary: kidneys, ureters, urinary bladder, urethra
- Reproductive (Female): ovaries, oviducts, uterus, vagina, accessory glands; (Male): testes, vas deferens, seminal vesicles, prostate and bulbourethral glands, and penis
- Organismal level: The organismal level includes all organ systems working together to form the human body, which we call an organism. An organism is any living thing, and as the name might imply, living things are highly organized. A human body is a multicellular organism, meaning it has numerous cells (multi, many). Some organisms, such as bacteria and yeast, consist of only one cell, which makes them unicellular (uni, one). To finish our analogy, we could say this page is like an organism because it consists of multiple paragraphs (“organ systems”). There are still more subdivisions in biology, and the next two would include a population, two or more similar organisms living in the same area, and a community, two or more populations living in the same area. This chapter would be like a population and the entire book would be like a community. But how can you say that your body is “alive” and this book is not? You will find the answer in the next section.
THE CHARACTERISTICS OF LIFE
The fact that you are reading this sentence means you are alive, but what is life? Answering this question requires more than a simple one-line response; instead, it requires describing a number of “life” characteristics. In this section, we describe the characteristics that distinguish humans from nonliving objects. Humans exhibit cellularity, metabolism, excretion, growth, reproduction, organization, adaptability, irritability, movement, homeostasis, and inheritance.
Note:
The following mnemonic (ne-MON ik; “memory”) may help you remember the traits of life; each letter represents one trait: C-ME-GRO-AIM-HI (Pronounced “See Me Grow Aim High”).
Earlier, we stated that a cell is the smallest thing that can be alive, but in your body, a single cell does not function independently of other cells. Amazingly, the trillions of cells that comprise your body work together as a single, living unit. In a biological sense, the following eleven characteristics help validate that your body is alive:
- Cellularity implies that the body is made of tiny functional units called cells (“little room”). The cell theory in biology states that the cell is the basic unit of life, so all organisms exhibit cellularity and contain at least one cell. You have trillions of cells in your body. The “C” could also stand for “carbon.” All living things consisting of matter contain organic compounds, which are those that contain carbon. Examples of carbon compounds include proteins, carbohydrates, and lipids.
- Metabolism (“change”) refers to the countless chemical reactions, or interactions between two or more chemicals, which occur in the body continuously. Simply, metabolism includes all the chemical reactions in the body and includes anabolism and catabolism. Anabolism (“raise up”) includes reactions that produce larger, more complex molecules. Catabolism (“cast down) includes reactions that break down large, complex molecules into smaller, simpler ones.
- Excretion (“to separate”) refers to the elimination of wastes from the body. Catabolism always generates certain waste molecules, and allowing these wastes to accumulate would poison the body and eventually cause death. Four organ systems perform excretion or have excretory functions. The urinary system excretes liquid waste in the form of urine; the digestive system excretes solid waste called feces (FĒ-sēz; “dregs”); the respiratory system excretes gaseous waste—primarily carbon dioxide; and the integumentary system excretes certain wastes in sweat and oil.
- Growth, the process of getting larger, is a feature of the body as a whole and a characteristic of its cells at some time in their life. You began life as a single cell that was smaller than the head of a pin but look at you now! Your body grew as that original cell divided repeatedly to form additional cells. However, simply dividing one cell again and again does not cause growth, since that process might simply produce smaller and smaller cells. The body can grow only if its newly formed cells grow too. Seeing yourself in the mirror without the aid of a microscope validates the notion that your cells grew after they formed, although they remain microscopic
- Reproduction, or producing “copies” of oneself, can occur at different levels of organization. It occurs at the cellular level, which is how you became multicellular, and it occurs at the organismal level when a person produces a child. Many of your cells reproduce throughout your lifetime, with the newly formed cells replacing worn-out or damaged ones. In addition, special cells called gametes (GAM-ēts, “seeds”) from a male and female can unite to form a single-celled zygote (ZĪ-gōt; “yoked”), which divides repeatedly to form a new human body.
- Organization, or the ordered arrangement of structures, implies that the body is not simply a collection of matter arranged in a haphazard fashion. Recall that organization is the basis for why we call living things organisms. This is also the basis of the term organic, which refers to the carbon-based compounds made within organisms.
- Adaptability is the ability to change over time in response to a change in lifestyle or a change in the external environment; adapt means “to adjust.” If an inactive, sedentary person began a vigorous exercise regimen that included lifting weights and running, the body would adapt to this new lifestyle. Changes would include stronger and larger skeletal muscles, thicker bones, and a stronger heart able to pump blood through blood vessels more effectively. Acclimation, also called acclimatization (a-clī-ma-ti-ZĀ-shun), refers to the adaptation of an individual to a different climate or altitude, and this type of adjustment may take several days or longer.
- Irritability is the ability to change quickly in response to a change that occurred inside or outside the body. To be irritable means to be excitable. Whereas adaptability is a relatively slow change, irritability is a more immediate reaction to a sudden environmental change. For instance, you demonstrate irritability when you flinch at the sound of a firecracker or jerk your hand away from a hot stove.
- Movement may not seem applicable to all living organisms, such as a tree, but movement occurs, albeit not necessarily at all levels of organization. Fluids and various microscopic structures move around inside cells, blood moves within blood vessels, and food moves through your digestive system. Moreover, most people can move their body from one place to another and can move items in their surrounding environment.
- Homeostasis (homeo, same; stasis, stand still) refers to the maintenance of relatively stable internal conditions. For example, you can maintain a core body temperature (that is, deep inside the body) near 37oC (98.6oF) even if the room temperature is 0oC (32oF). The roles that various cells, tissues, organs, and organ systems play in maintaining homeostasis will be a recurring theme in this book.
- Inheritance refers to passing on chemical information from one generation to the next, whether at the cellular or the organismal level of organization. You inherited chemicals from your parents that influenced your anatomy and physiology. Inheritance comes from a word meaning “heir.” Even for people who never produce a child, each time one of their cells divides, it passes chemical information to the newly formed cells.
Nonliving things may exhibit some of the “life” characteristics described above, but only in a limited way. For instance, mineral crystals grow and exhibit organization, while a virus reproduces and demonstrates inheritance, but neither crystals nor viruses exhibit cellularity, metabolism, irritability, or any other “life” characteristic. Although they are nonliving, viruses can cause certain diseases, including the cold, the flu, and acquired immune deficiency syndrome (AIDS).
REQUIREMENTS FOR LIFE
In this final section, we point out certain factors that humans need from their external environment in order to stay alive. While some factors in our external environment could harm us, we can identify four factors that are necessary for our survival: water, nutrients, oxygen, and a suitable temperature.
Water is the most vital substance that you obtain from the external environment. It makes up about 70% of your body’s weight and is important to your survival in the following ways:
(1) it provides the medium in which all metabolic reactions occur; (2) it transports nutrients and cellular wastes in the blood; and (3) it is an effective heat absorber, preventing dramatic fluctuation of body temperature.
Nutrients are chemicals found in food that cells can use for energy, building cellular components, or maintaining normal metabolism. Nutrients include carbohydrates, lipids, proteins, vitamins, and minerals. Carbohydrates are the body’s major energy source and lipids provide energy and form the boundaries of cells. Proteins are the major building blocks for cells and their products, while vitamins help maintain normal chemical reactions within cells. Minerals are common in the earth’s crust and examples include calcium, sodium, and phosphorus. Minerals provide structural support to bones, function in transmitting nerve signals, play a role in certain chemical reactions, and serve in many other capacities.
Oxygen is a gaseous element that enables most cells in the body to obtain more useable energy from certain nutrients. Some cells, such as many of those in the brain, are so dependent on oxygen that experiencing an oxygen-deficit for more than a few minutes can cause irreversible cell damage and even death. Oxygen from the atmosphere enters the blood in the lungs and then the blood transports it to cells throughout the body.
Temperature around the body must remain within tolerable limits to allow the body to maintain normal metabolism. If a high outside temperature causes the body’s core temperature to rise to 41oC (106oF), certain proteins and other molecules begin to lose their ability to function properly. A core temperature of 42oC (109oF) is usually fatal. If a low outside temperature causes the body’s core temperature to drop to near 27oC (82oF), most metabolic reactions cease. In the next section, we will describe several mechanisms that enable the body to maintain its core temperature at or near optimum (37oC).
HOMEOSTASIS
Now that you know about certain factors in the external environment that are necessary for life, we will elaborate on mechanisms the body uses to deal with changes in its internal environment. Rephrasing from the previous section, homeostasis (homeo, same; stasis, standing still) is a relatively stable internal environment in which cells can live. The internal environment is the fluid surrounding the body’s cells. This fluid has several general names: extracellular fluid (ECF; extra, outside), intercellular fluid (inter, between). Interstitial fluid (in-ter-STI-shul; “between spaces”) is ECF out side of blood vessels. The ECF that flows within blood vessels is called plasma (PLAZ-ma), while the ECF that flows within lymph vessels is called lymph. Intracellular fluid (intra, within) is fluid the within cells.
Although homeostasis translates “to stay the same,” many aspects of the ECF fluctuate continuously above or below a desired value. For example, the temperature and the amounts of water, nutrients, and oxygen in the ECF are always changing. However, the key to homeostasis, or the key to maintaining good health, is to make sure these fluctuations stay within tolerable limits. Any aspect of the environment that can change is a variable (“to vary”), but a variable that the body has some control over is a regulated variable.
Figure 1-3. Example of negative feedback. See text for explanation.
Minor fluctuations in regulated variables are normal and unpreventable, but sometimes these minor fluctuations can quickly lead to major disruptions to homeostasis and pose a threat to life if not corrected quickly. Fortunately, the body has a number of regulatory mechanisms that can counter minor disruptions in homeostasis before they develop into life-threatening situations. One of these mechanisms involves homeostatic control systems.
HOMEOSTATIC CONTROL SYSTEMS
Control systems monitor variables and help regulate homeostatic activities of other systems. The nervous system and endocrine system play important roles as homeostatic control systems, because they direct many of the homeostatic activities of other systems.
We can compare a control system to a person riding a bicycle along a thin line in the middle of a sidewalk (see Figure 1-3). The thin line running along the center of the sidewalk represents a desired value for some regulated variable, while the bike’s tire path denotes the actual value for the variable. The edges of the sidewalk symbolize the tolerable limits within which values of the variable can fluctuate before the body “crashes” (suffers ill effects). Finally, because ferocious dogs and strong winds cause the bike to veer off the line, they correspond to factors that disrupt homeostasis. In the case of the dog’s effect, the rider may actually cause the bike to veer off course, and this is a temporary disruption in the control system itself.
While it would be virtually impossible to prevent the bike from veering off the line from time to time, even without dogs or strong winds present, the rider’s corrective steering may at least keep the bike near the middle of the sidewalk. Similarly, a homeostatic control system cannot prevent regulated variables from fluctuating but can help keep them within tolerable limits. To elaborate further, the rider integrates the actions of three anatomical features to keep the bike on the line: the eyes, which inform the brain about the bike’s position; the brain, which decides when the arms should move; and the arms, which turn the handlebars to guide the bike back on course. In a similar way, a homeostatic control system integrates the actions of three components: sensors, integrating center, and effectors.
A sensor (also called a receptor) is a structure that monitors a particular regulated variable and sends information about it to an integrating center. Examples of sensors in the body include chemoreceptors, which detect changes in the amounts of various chemicals in the blood, and thermoreceptors (thermo, heat)) which detect changes in core temperature (in the brain and deep in the torso). Any factor that causes a sensor to respond in some way is a stimulus (“to provoke”), and exposing a sensor or other structure to a stimulus is stimulation.
The integrating (or control) center receives information about a regulated variable from a sensor, compares it to a desired value, called the set point, and then sends information to a structure called the effector that helps return the variable to its set point. In other words, the integrating center “knows” what to do when the variable’s value moves away from the set point. Since information sent from a receptor comes into the control center, we call it input, or afferent information (AF-er-ent, “bring in”). Information that leaves the control center is output, or efferent information (EF-er-ent, “bring out”). The brain and certain endocrine glands determine the set point for various chemicals in the blood, while the brain alone determines the set point for body temperature.
An effector (ē-FEK-tor) is a structure that receives efferent information from a control center and produces a reaction (an effect) that brings the value of a regulated variable back to its set point. Hence, we say the control center stimulates the effector to produce a certain effect. Most organs in the body act as effectors for either the nervous system or endocrine system, and some organs are effectors for both systems. To avoid confusion over the words “affect” and “effect,” you could say, “The control center’s output affects the effector,” or “The control center’s output has an effect on the effector.”
FEEDBACK MECHANISMS
Feedback involves reacting to a stimulus in a way that either counteracts or intensifies the stimulus. After stimulating an effector that helps adjust the value of a regulated variable, the control center must re-evaluate the variable’s status to determine whether stimulation of the effector must continue or cease. For re-evaluation to occur, a homeostatic control system relies on a process called feedback, in which the sensor “feeds back” information about the variable to the control center. In some situations, feedback does not involve a homeostatic control system; however, feedback always includes a stimulus and a reaction that either counteracts or intensifies the stimulus.
Negative Feedback
If the body’s reaction to a stimulus negates (opposes or counteracts) the stimulus, the process is negative feedback. In other words, when a stimulus changes the value of a regulated variable in one direction, negative feedback moves the value in the opposite direction. In this sense, being “negative” is a good thing because negative feedback prevents dramatic fluctuations in the values of regulated variables. If we plot the actual values for a regulated variable over time, negative feedback produces an oscillating pattern that might look something like the tire path in our bicycle analogy. In that figure, the rider demonstrates negative feedback control when trying to keep the bike on the line. When the bike veers left, the rider negates that movement by turning the handlebars to the right; when the bike veers right, the rider negates that movement by turning the handlebars to the left.
An example of negative feedback is thermoregulation, in which a homeostatic control system counteracts fluctuations in body temperature. First, thermoreceptors in the skin and brain respond to changes in body temperature by sending afferent signals to the thermoregulatory center (TRC) located in the brain. The TRC then compares the afferent signals to the brain’s temperature set point. If the temperature is higher than the set point, the TRC initiates heat-loss mechanisms. This includes sending efferent signals to sweat glands, causing them to release sweat onto the skin’s surface. As the sweat evaporates, the body cools. In addition, the brain causes blood vessels in the skin to dilate (DĪ-lāt, “open up”), allowing more warm blood to come near the body’s surface where it can radiate heat into the surrounding environment.
If body temperature is lower than the set point, the TRC initiates heat-producing mechanisms. This includes causing efferent signals to be sent to skeletal muscles, which “shiver” in response. Shivering generates heat and causes the body temperature to rise. Additionally, the brain causes blood vessels in the skin to constrict (become narrower) so that less warm blood is brought near the body’s surface, thus, preventing excessive heat loss. While the TRC cannot prevent fluctuations in body temperature, it attempts to minimize those fluctuations. See Figure 1-4 for a depiction of thermoregulation.
Positive Feedback
If the body’s reaction to a stimulus intensifies the stimulus, the process is positive feedback. Defined another way, when a stimulus moves the value of a regulated variable in one direction, positive feedback moves the value farther along in the same direction. Positive feedback has been called “a vicious cycle” because it involves a cycle of recurring events that reinforce each other. When we plot these events on a circle, each complete cycle or positive feedback loop, occurs faster than the previous cycle. Although it seems that positive feedback could quickly cause the variable’s value to get out of control, several events prevent that from happening: (1) some factor eliminates the stimulus; or (2) the body lacks the resources or energy to continue reacting to the stimulus.
Since positive feedback causes the value of a regulated variable to move quickly away from homeostatic conditions, how can this process ever be beneficial to one’s health? The answer is that sometimes the body must amplify or intensify the effect of a stimulus in order to remove the stimulus as quickly as possible. It might help to think of positive feedback as the body’s way of “fighting fire with fire.” Firefighters often fight forest fires by starting fires ahead of the main fire in order to burn up potential fuel; as a result, the forest fire burns out more quickly. Childbirth and blood clotting are two examples that incorporate positive feedback loops as a way of reducing the amount of time that the body is under stress. We will describe childbirth in a later chapter but will provide a brief overview of blood clotting here.
Blood clotting (formation of a blood clot) is an example of how a self-amplifying positive feedback loop can counter a potentially life-threatening loss of blood. A blood clot is a jelly-like mass that plugs a tear or cut in a blood vessel’s wall. First, tearing a blood vessel disrupts homeostasis by causing bleeding, which reduces the amount of cell-nourishing blood circulating through the body. The tear in the vessel acts as a stimulus, causing platelets (tiny particles in the blood) to attach to the damaged tissue and then release a variety of chemicals. Some of these chemicals cause the platelets at the damaged site to become “sticky,” and some of the chemicals initiate reactions necessary for forming a blood clot. A positive feedback loop begins as more platelets arrive on the scene, stick to the platelets already there, and release their chemicals like the platelets that arrived earlier. As a result, the newly arrived platelets become sticky and their added chemicals speed up the clotting process. This positive feedback loop continues until the clot seals the tear in the vessel wall and bleeding stops.
Homeostasis and Negative/Positive Feedback
Figure 1-4. Thermoregulation is an example of negative feedback
RELATIONSHIPS AMONG VARIABLES
When discussing feedback and describing how a change in one variable affects a change in some other variable, it is often helpful to show the relationship between the two variables on a graph. The variable we place on the X-axis (abscissa, or horizontal line at the bottom of the graph) is the independent variable, so-named because we assume that its value is not dependent on the value of the other variable of interest. In contrast, the variable we place on the Y-axis (ordinate, or vertical line forming the left side of the graph) is the dependent variable, so-named because we assume its value depends on the value of the independent variable. We sometimes call the dependent variable the response variable because its value changes in response to changes in the independent variable. See Figure 1-5 for a depiction of a relational graph.
Figure 1-5. Relational graph.
We can illustrate the usefulness of a graph by showing the relationship between several regulated variables: body temperature, sweating. and shivering. Using these variables, we can illustrate two types of relationships: positive and negative.
Creating Data Tables and Graphs from Independent and Dependent Variables
Positive Feedback
First, we might ask, “How does body temperature affect sweating,” or “How does sweating respond to a change in body temperature?” In this case, body temperature is the independent variable, and the amount of sweating is the dependent variable. If body temperature rises only slightly above the set point, the body responds by producing a small amount of sweat in a given time. To show this, we first find the measured temperature on the X-axis then move upward to plot a point at a level corresponding to a “small” amount of sweat on the Y-axis. At higher temperatures, the body sweats more, and at lower temperatures the body sweats less. For each measured temperature value, there is a corresponding point that represents the amount of sweat produced at that temperature.
So, what do the points on our graph tell us about the relationship between body temperature and sweating? If we draw a straight line among the dots, we see that it slants upward and to the right. (The procedure for determining the exact slope of this line is based on a statistical formula and is beyond the scope of this book, so the line we will draw here is simply an educated guess.) A line with this type of slope indicates a positive relationship between the variables (see Figure 1-6a).
Recall in positive feedback, the body’s response to a stimulus moves in the same direction as the stimulus. In a positive relationship, the response variable (Y) changes in the same direction as the X variable. In our example of thermoregulation, if body temperature increases, then the amount of sweating increases too, whereas if body temperature decreases, then the amount of sweating decreases. Now you can say, “The amount of sweating is positively related to body temperature,” or “Body temperature has a positive effect on the amount of sweating.”
Negative Feedback
A negative (or inverse) relationship exists if the value of variable Y changes in the opposite direction to that of variable X. That is, if the value of X increases, then the value of Y decreases, or if the value of X decreases, then the value of Y increases. This would be the case if we ask, “How does body temperature affect the amount of shivering?” Since shivering generates body heat, it tends to increase when body temperature decreases. In contrast, shivering decreases as body temperature increases. Thus, we can say, “The amount of shivering is negatively related to body temperature,” or “Body temperature has a negative effect on the amount of shivering” (see Figure 1-6b).
HOMEOSTASIS AND DISEASE
While the body may be able to prevent dramatic fluctuations in regulated variables through negative feedback, and it may be able to cope quickly with certain stimuli through positive feedback, there are limitations to its ability to maintain homeostasis. Sometimes the body may face stimuli that push the value of a regulated variable beyond tolerable limits, at which time we say the body experiences a homeostatic imbalance. We can compare a homeostatic imbalance to our bike rider losing his/her balance and suffering injury because the bike swerves too far and gets off the sidewalk. The body’s reaction to any stimulus that causes a homeostatic imbalance is stress. If the body’s homeostatic control systems are able to reestablish homeostasis through negative feedback, we can now define negative feedback in the body simply as “reaction negates stress.”
When a homeostatic imbalance causes an interruption or cessation of some bodily function, we say the person is ill or experiences illness. A disease (dis-, not + ease) is an illness that produces certain recognizable signs and symptoms that usually have a known cause. A sign is some aspect of a disease that is visible or measurable, such as redness, swelling, fever, vomiting, bleeding, etc. A symptom is a “feeling” or subjective description of the way a person feels and includes nausea, fatigue, headache, pain, etc. A syndrome (sym-, together; drom-, running) is a combination of signs and symptoms for a particular disease; two examples are respiratory distress syndrome (RDS) and acquired immune deficiency syndrome (AIDS).
REVIEW
Before moving on to the next chapter, take some time to review the Topics to Know in Chapter 1 that follow.
Figure 1-6. Variable relationships (a) Positive (b) Negative
Flashcards
Explore the fundamental concepts of how the human body functions.
Glossary
A comprehensive glossary of terms for the "Introduction to Human Physiology" chapter. Click on any term to see its definition.
