Homeostasis
Homeostasis (hoh-mee-oh-STAY-sis) is the existence and maintenance of a relatively constant environment within the body. As our bodies undergo their everyday processes, we are continuously exposed to new conditions. Changes in our external environmental conditions can result in changes in our internal body conditions. Changes in internal body conditions are called variables because their values are not constant. To achieve and maintain homeostasis, the body must actively regulate responses to changes in variables. Variables include such conditions as body temperature, volume, chemical content and pH of body fluids, as well as many other variables. For our cells to function normally, all variables must be maintained within a narrow range.
This narrow range is referred to as a normal range. Homeostatic mechanisms normally maintain body conditions near an ideal normal value or set point (figure 1.4). Note that these mechanisms are not able to maintain body conditions precisely at the set point. Rather, body conditions increase and decrease slightly around the set point. Keep in mind that these fluctuations are minimal. For example, normal body temperature does not typically vary more than 1°F above or below normal. Our average body temperature is 98.6°F. Just as your home’s thermostat does not keep the air temperature exactly at 75°F at all times, your body conditions do not stay perfectly stable. As long as body conditions remain within the normal range, homeostasis is maintained.
Body temperature is an example of a variable maintained near an ideal value, or set point.
It is the body’s network of organ systems that helps keep the body’s internal environment relatively constant. For example, the digestive, respiratory, cardiovascular, and urinary systems work together to ensure that each cell in the body receives adequate oxygen and nutrients, while also ensuring that waste products do not accumulate to toxic levels. Disease states can disrupt these processes and disturb homeostasis in such a way that death could result. Modern medicine attempts to understand disturbances in homeostasis and works to reestablish a normal range of values.
Feedback Loops
Homeostasis is regulated by feedback loops. Here we will examine our fourth key concept of anatomy and physiology: feedback loops. A feedback loop allows for a process to be regulated by the outcome. In biological systems, there are two types of feedback loops: (1) negative feedback and (2) positive feedback. Note that a common misconception is that negative feedback is the decrease of a body parameter, while positive feedback is the increase of a body parameter. For example, students sometimes think a drop in blood glucose levels is negative feedback and an increase in blood glucose is positive feedback. Rather, blood glucose increases and decreases are both controlled by negative feedback. As you read about each type of feedback loop, keep in mind that both types of feedback loops regulate the body’s responses to either increased or decreased parameters.
Feedback loops have three components: (1) a receptor, which monitors the value of a variable by detecting stimuli; (2) a control center, such as a part of the brain, which determines the set point for the variable and receives input from the receptor about the variable; and (3) an effector, which generates the response that adjusts the value of a changed variable. A changed variable is a stimulus because it initiates a homeostatic mechanism.
Negative Feedback
Negative-feedback mechanisms are more commonly involved in maintenance of homeostasis than are positive-feedback mechanisms. In everyday terms, the word negative is used to mean “bad” or “undesirable.” In the context of homeostasis mechanisms, negative means “to decrease.” Negative feedback is when any deviation from the set point is made smaller or is resisted. In other words, the response by the effector is stopped once the variable returns to its set point (figure 1.5).
Feedback loops maintain homeostasis. Receptors signal the control center that a variable has deviated outside its normal range. The control center regulates the action of the effectors, which produce a response that returns the variable to the set point. In negative feedback, the return to set point stops the response.
One of the most familiar examples of a negative-feedback mechanism is maintenance of body temperature. Normal body temperature is critical to our health because it allows molecules and enzymes to keep their normal shape so they can function optimally. An optimal body temperature prevents molecules from being destroyed. For example, picture the change in appearance of egg whites as they are cooked; the egg whites change from a transparent fluid to a white solid because heat changes the shape of the egg white molecules. Similarly, if the body were to be exposed to extreme heat, the shape of the molecules in the body could change, thus preventing them from functioning normally. Figure 1.6a demonstrates the steps in the negative-feedback mechanism regulating body temperature if it becomes too high. Normal body temperature depends on the coordination of multiple structures, which are regulated by the control center (the hypothalamus in the brain).
Negative- and Positive Feedback Mechanism: Body Temperature
(a) Negative feedback is one of the mechanisms by which homeostasis is maintained. Receptors signal the control center, which regulates the action of the effectors. In the example, body temperature is too high, so sweating occurs. Negative feedback stops the sweating when the body temperature returns to normal. (b) Positive feedback is also a type of mechanism that works to maintain homeostasis. In this example, receptors signal the control center that the cervix is being stretched, which results in the control center sending signals to increase the contractions of the uterus. This cycle continues, becoming stronger over time until the baby is born.
Receptors in the skin (called thermoreceptors) monitor body temperature. If body temperature rises, the receptors send a message to the control center, the hypothalamus.
The control center compares the value of the variables against the set point.
If a response is necessary, the control center will stimulate the effectors to produce their response. Here, the sweat glands will secrete sweat.
Once the value of the variable has returned to the set point, the effectors do not receive any more information from the control center. For regulation of body temperature, this means that the secretion of sweat stops. These same steps can be used to help you answer the Learn to Predict question at the beginning of this chapter.
Often there is more than one effector for a particular homeostatic mechanism. In these cases, the control center must coordinate the effectors’ responses. For example, cooling the body involves not only the production of sweat by the sweat glands, but also the action of the blood vessels to alter blood flow to the skin (see chapter 5). Once body temperature has returned to normal, the effectors stop. This is the hallmark of negative feedback—effectors stop their response once the variable has returned to it set point. They do not produce an infinite response (figure 1.7).
Throughout this book, all homeostasis figures have the same format as shown here. The changes caused by the increase of a variable outside the normal range are shown in the top blue boxes, and the changes caused by a decrease are shown in the bottom pink boxes. To help you learn how to interpret homeostasis figures, some of the steps in this figure are numbered. (1) Body temperature is within its normal range. (2) Body temperature increases outside the normal range, which causes homeostasis to be disturbed. (3) The body temperature control center in the brain responds to the change in body temperature. (4) The control center causes sweat glands to produce sweat and blood vessels in the skin to dilate. (5) These changes cause body temperature to decrease. (6) Body temperature returns to its normal range, and homeostasis is restored. Observe the responses to a decrease in body temperature outside its normal range by following the bottom pink arrows.
What effect would swimming in cool water have on body temperature regulation? What would happen if a negative-feedback mechanism did not return the value of a variable, such as body temperature, to its normal range?
Positive Feedback
Positive-feedback mechanisms occur when a response to the original stimulus results in the deviation from the set point becoming even greater. In other words, positive means “to increase.” You may have experienced this: Perhaps you became embarrassed and realized your face was turning red, which caused you to become more embarrassed and your face turned even more red. Though not the typical physiological type of positive feedback, this example may help you understand the concept of positive feedback.
A physiological example of positive feedback occurs during blood loss. A chemical responsible for blood clot formation, called thrombin, stimulates production of even more thrombin. By continuing to produce thrombin, a disruption in homeostasis (blood loss) is resolved through a positive-feedback mechanism (blood clotting). But why doesn’t this continued production of thrombin lead to the entire vascular system forming a clot? Because the clot formation process is self-limiting. Eventually, the chemicals needed for clot formation will be depleted in the area of blood loss and no further clotting can occur.
As shown in figure 1.6b, birth is another example of a normally occurring positive-feedback mechanism.
Near the end of pregnancy, the baby’s larger size stretches the uterus, especially near its opening.
This stretching stimulates contractions of the uterine muscles.
The contractions push the baby against the opening and stretch it further. This stimulates additional contractions, which result in additional stretching.
This positive-feedback sequence ends only when the baby is delivered from the uterus and the stretching stimulus is eliminated.
This continued response is the hallmark of positive feedback—the effectors continue the response beyond the set point until the original stimulus is removed.
There are two basic principles about homeostatic mechanisms to remember: (1) many disease states result from the failure of negative-feedback mechanisms to maintain homeostasis and (2) some positive-feedback mechanisms can be detrimental instead of helpful. One example of a detrimental positive-feedback mechanism is inadequate delivery of blood to cardiac (heart) muscle. Contraction of cardiac muscle generates blood pressure and the heart pumps blood to itself through a system of blood vessels on the outside of the heart. Just as with other tissues, blood pressure must be maintained to ensure adequate delivery of blood to the cardiac muscle. Following extreme blood loss, blood pressure decreases to the point that the delivery of blood to cardiac muscle is inadequate. As a result, cardiac muscle does not function normally. The heart pumps less blood, which causes the blood pressure to drop even further—a deviation further from the set point. The additional decrease in blood pressure further reduces blood delivery to cardiac muscle, and the heart pumps even less blood, which again decreases the blood pressure. The process self-propagates until the blood pressure is too low to sustain the cardiac muscle, the heart stops beating, and death results. In this example, we see the deviation from the heart rate set point becoming larger and larger—this is a positive-feedback mechanism. Thus, if blood loss is severe, negative-feedback mechanisms may not be able to maintain homeostasis, and the postive feedback of ever-decreasing blood pressure can develop. On the other hand, following a moderate amount of blood loss (e.g., after donating a pint of blood), negative-feedback mechanisms result in an increase in heart rate, which restores blood pressure.
Although homeostasis is the maintenance of a normal range of values, this does not mean that all variables remain within the same narrow range of values at all times. Sometimes a deviation from the usual range of values can be beneficial. For example, during exercise the normal range for blood pressure increases above the resting range (figure 1.8). The increase in blood pressure helps supply muscle cells with the greater amount of oxygen and nutrients needed to support increased activity during exercise.
During exercise, muscle tissue demands more oxygen. To meet this demand, blood pressure (BP) increases, resulting in an increase in blood flow to the tissues. The increased blood pressure is not an abnormal or nonhomeostatic condition but a resetting of the normal homeostatic range to meet the increased demand. The reset range is higher and broader than the resting range. After exercise ceases, the range returns to that of the resting condition.
Ashley is on the track team and is running an 800-meter race. Throughout the race, her respiratory rate increases rapidly. Does this represent negative or positive feedback? Explain.