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Homeostasis and Negative Feedback Mechanisms

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PRESENTER: Keeping the body's internal environment in optimal condition in the face of a changing external environment is essential for survival. There is a name for this process. It's called homeostasis. What homeostasis really means is that many conditions in the body are held as close as possible to particular values. This allows body cells to survive. Think about a cell in the body. What conditions or levels of variables have to be regulated in order to keep that cell happy? Is it (a) temperature; (b) certain ions (such as sodium, potassium, calcium, and hydrogen); (c) nutrients (such as glucose); (d) oxygen and carbon dioxide; or (e) all of the above? The correct answer is (e). These and many other variables need to be controlled. Working together, the body's organ systems accomplish this feat using a mechanism called negative feedback. Negative feedback maintains homeostasis. Negative feedback mechanisms are so common in the body that we have a special symbol for them in the book-- a teeter-totter. Every negative feedback mechanism attempts to hold some variable-- for example, body temperature-- as close to a certain value as possible. This value is called the set point. When the variable changes in one direction, negative feedback brings the variable back toward the set point. Every negative feedback mechanism has three components; a receptor that senses the value of the variable, a control center that decides how to respond to a change in the variable, and an effector that can adjust the value of the variable back toward the set point. For example, temperature-sensitive cells in the skin and the brain acquire data about body temperature. A control center in the brain interprets this information and determines a response, and effectors, such as sweat glands and skeletal muscles, carry out the response. These three components are connected together by two pathways; an afferent pathway that carries sensory information from the receptor to the control center and an efferent pathway that carries information from the control center to the effector. To help you remember this, think about it this way-- afferent information approaches and efferent information exits the control center. Let's see how this works. One, a stimulus produces a change in the variable away from the set point. Two, the receptor detects the change. Three, information sent along the afferent pathway access input to the control center. Four, output from the control center is information sent along the efferent pathway to the effector. Five, the effector creates a response that reduces the effect of the stimulus and returns the variable to the set point-- its homeostatic value. Let's return to the example of body temperature. A stimulus-- heat, in this case-- increases body temperature, tipping the body out of homeostatic balance. Temperature-sensitive cells in the skin and brain act as receptors and detect this change. They transmit sensory information along afferent pathways to the control center-- in this case, the thermoregulatory center in the brain. The control center, having decided that the body is too hot, sends instructions along efferent pathways to the effectors. The effectors, in this case, are sweat glands that release water onto the surface of the skin. As the sweat evaporates, body temperature falls, eliminating the stimulus and restoring homeostatic balance. This feedback mechanism also works if the stimulus is cold-- body temperature falls, temperature-sensitive cells detect this change, and signal the control center in the brain. Faced with a different change, the control center makes a different choice to right the balance. It sends instructions to a different set of effectors-- not sweat glands that would cool the body further but skeletal muscles. Shivering generates heat, which raises body temperature, eliminates the stimulus, and restores homeostatic balance. You can see from these examples how negative feedback mechanisms maintain homeostasis. Given our example of the negative feedback that controls body temperature, you might wonder how it is that your body can raise its temperature when you have a fever in response to an infection. What happens is that the infection causes the set point in the control center in the brain to change. As a result, body temperature is now colder than the new set point, then the body generates and conserves heat so the temperature rises to match the new set point. It's just like turning up the thermostat in your house and having the heat turned on. Which of the following do you think will happen when your fever subsides? Is it (a) your set point returns to normal and then you shiver; (b) your set point returns to normal and then you sweat; (c) your set point remains elevated and you shiver; or (d) your set point remains elevated and you sweat? The correct answer is (b). Your set point returns to normal. And because your temperature is now higher than the set point, you sweat to bring your temperature down to the normal set point. As you've seen, negative feedback mechanisms play an important role in maintaining homeostasis. That is, they allow us to maintain the internal environment of the body within the limits that support life. You will encounter many more examples of homeostatic mechanisms as we investigate each of the body's systems. You will also see that the loss of homeostasis leads to most diseases.