1.5 Homeostasis: Keeping Internal Conditions StableHave you ever noticed that your body maintains an average internal temperature of about 37°C (98.6°F), regardless of the outside temperature? Perhaps you also have noticed that the size of your pupil is altered in response to light intensity entering your eye, or that your breathing returns to normal shortly after exercise. Likewise, your heart rate, blood pressure, and blood levels of sugar (glucose) and O2 are also regulated and maintained within certain parameters. In fact, there are hundreds of anatomic structures and physiologic processes that are continuously monitored and adjusted within your body so that they are kept within normal limits.Homeostasis refers to the body's ability to maintain a relatively stable internal environment in response to changing internal or external conditions. Homeostasis is a central theme throughout this text, and you will be learning the appropriate details about homeostasis in each chapter. In this section, we introduce you to the general concept of homeostasis. We describe the general components of homeostatic systems, provide specific examples of these regulatory processes, and then describe the relationship of homeostasis, health, and disease.1.5a Components of Homeostatic SystemsLEARNING OBJECTIVES1. Define the components of a homeostatic system.2. Be able to recognize each of the components in representative systems.The body maintains homeostasis by utilizing homeostatic control systems. Three components areassociated with each homeostatic system: receptor, control center, and effector (figure 1.11).Figure 1.11Components of a Homeostatic Control Mechanism.A homeostatic control mechanism consists of a receptor (detects a stimulus), a control center (integrates input and initiates change through the effector), and an effector (brings about a change in response to the stimulus).Positive and Negative FeedbackReceptorReceptor The receptor is the body structure that detects changes in a variable, which is either thesubstance or process that is regulated. A receptor typically consists of sensory nerves. These nerves may be in the skin, internal organs of the body, or specialized organs such as the eye, ear, tongue, or nose. The change in the variable is the stimulus. The stimulus might be a change in temperature, chemicals, or stretch in muscle.Control CenterControl Center The control center is the structure that interprets input from the receptor and initiates changes through the effector. You can think of it as the “go between” for the other two components of a homeostatic system. The control center is generally a portion of the nervous system (brain or spinal cord) or an endocrine organ (such as the thyroid gland). A homeostatic system involving the nervous system provides a relatively quick means of responding to change. An example is regulating blood pressure when you rise from bed in the morning. In contrast, the endocrine system usually provides a means of a more sustained response over several hours or days through the release of hormones. An example is when the parathyroid hormone continuously regulates blood calcium levels, a process that is essential for the normal function of both muscles and nerves. Note that the control center is sometimes the same structure as the receptor because it both detects the stimulus and causes a response to regulate it. For example, the pancreas acts as a receptor because it detects an increase in blood glucose and also acts as a control center because it releases the hormone insulin in response.Page 19EffectorEffector The effector is the structure that brings about the change to alter the stimulus. Most body structures can serve as effectors, including muscles—such as smooth muscles in the walls of air passageways (bronchioles) to regulate airflow into and out of the lungs—or glands, such asinsulin-secreting cells of the pancreas.As you view figure 1.11, notice that the response of a homeostatic system occurs through a feedback loop that includes the following:- A stimulus- The detection of the stimulus by a receptor- Input information relayed to the control center (if a separate structure)- Integration of the input by the control center and initiation of a change through effectors- Return of homeostasis by the actions of effectorsHomeostatic control systems are separated into two broad categories based on whether the system maintains the variable within a normal range by moving the stimulus in the opposite direction, or amplifies the stimulus in the same direction. These two types of feedback control are called negative feedback and positive feedback, respectively.WHAT DID YOU LEARN?List and describe the three components of a homeostatic system, and give examples of eachin the human body.1.5b Homeostatic Systems Regulated by Negative FeedbackLEARNING OBJECTIVES3. Define negative feedback.4. Explain how homeostatic mechanisms regulated by negative feedback detect and respond to environmental changes.Most processes in the body are controlled by negative feedback. If a homeostatic system is controlled by negative feedback, the resulting action will always be in the opposite direction of the stimulus. In this way, the variable is maintained with a normal level, or what is called its set point.How a variable that is regulated by negative feedback fluctuates over time can be viewed in figure 1.12. Notice that the variable does not remain constant over time but rather it fluctuates, and its fluctuation occurs around the set point. If the stimulus increases, the homeostatic system is activated to cause a decrease in the stimulus until it returns to the set point. In contrast, if the stimulus decreases, the homeostatic system causes an increase in the stimulus until it returns to normal. This idea is generally better understood by describing a specific example, such as temperature regulation.Figure 1.12Negative Feedback.Note that when a variable is regulated by negative feedback, the variable fluctuates around a set point (rather than being a constant).Temperature RegulationTemperature Regulation We begin by first explaining how a negative feedback mechanism worksto maintain the temperature of your home at a set point of 70°F. On a very cold day, the indoor temperature drops. This drop in temperature is detected by the thermostat. The drop in temperature is relayed through the electrical wiring of your home to the furnace, which is then