THE LIVING WORLD
Unit Six. Animal Life
26. Maintaining the Internal Environment
The man above is sweating, something each of us has done when our bodies have become overheated from too much exercise or sun. The evaporation of the sweat cools our skin, a clever mechanism to remove heat. Your body, like that of all birds and mammals, tries to maintain a constant body temperature, no matter how hot or cold the surrounding air might be, and sweating is one of the ways it does this. If your body begins to heat above 37°C, you start to sweat and release heat; if you instead begin to cool below 37°C, you shiver and generate heat. Keeping your body temperature constant is only one example of a much broader physiological strategy: Vertebrates maintain relatively constant physiological conditions within their bodies. The pH of your blood, the rate at which you breathe, your blood pressure, the concentrations of water, salt, and glucose in your blood—all are monitored carefully by the brain, which acts continuously to keep each within narrow limits. Your body is constantly making dynamic adjustments in these parameters to counter changes caused by outside factors that would alter the body’s internal environment. This steady-state balance of internal conditions, known as homeostasis, is the subject of this chapter. A major objective of your study of biology will be to learn how animals maintain homeostasis.
26.1. How the Animal Body Maintains Homeostasis
As the animal body has evolved, specialization has increased. Each cell is a sophisticated machine, finely tuned to carry out a precise role within the body. Such specialization of cell function is possible only when extracellular conditions are kept within narrow limits. Temperature, pH, the concentration of glucose and oxygen, and many other factors must be held fairly constant for cells to function efficiently and interact properly with one another.
Homeostasis may be defined as the dynamic constancy of the internal environment. The term dynamic is used because conditions are never absolutely constant, but fluctuate continuously within narrow limits. Homeostasis is essential for life, and most of the regulatory mechanisms of the vertebrate body are concerned with maintaining homeostasis.
Negative Feedback Loops
To maintain internal constancy, the vertebrate body must have sensors that are able to measure each condition of the internal environment (indicated by the green box in figure 26.1). These constantly monitor the extracellular conditions and relay this information (usually via nerve signals) to an integrating center (the yellow triangle), which contains the set point, the proper value for that condition. This set point is analogous to the temperature setting on a house thermostat. In a similar manner, there are set points for body temperature, blood glucose concentration, the tension on a tendon, and so on. The integrating center is often a particular region of the brain or spinal cord, but in some cases it can also be cells of endocrine glands. It receives messages from several sensors, weighs the relative strengths of each sensor input, and then determines whether the value of the condition is deviating from the set point. When a deviation in a condition occurs (the “stimulus” indicated by the red oval), sensors detect it, and the integrating center sends a message to increase or decrease the activity of particular effectors. Effectors (the blue box) are generally muscle or glands, and can change the value of the condition in question back toward the set point value, which is “the response” (the purple oval).
To return to the idea of a home thermostat, suppose you set the thermostat at a set point of 70°F. If the temperature of the house rises sufficiently above the set point, the thermostat (equivalent to an integrating center) receives this input from a temperature sensor, like a thermometer within the wall unit. It compares the actual temperature to its set point. When these are different, it sends a signal to an effector. The effector in this case may be an air conditioner, which acts to reverse the deviation and bring it back to the set point.
In humans, if the body temperature exceeds the set point of 37°C, sensors in the brain detect this deviation. Acting via an integrating center (also in the brain), these sensors stimulate effectors (such as sweat glands) that lower the temperature. One can think of the effectors as “defending” the set points of the body against deviations. Because the activity of the effectors is influenced by the effects they produce, and because this regulation is in a negative, or reverse, direction, this type of control system is known as a negative feedback loop (figure 26.1).
Figure 26.1. A generalized diagram of a negative feedback loop.
Negative feedback loops maintain a state of homeostasis, or dynamic constancy of the internal environment, by correcting deviations from a set point.
The nature of the negative feedback loop becomes clear when we again refer to the analogy of the thermostat and air conditioner. After the air conditioner has been on for some time, the room temperature may fall significantly below the set point of the thermostat. When this occurs, the air conditioner will be turned off. The effector (air conditioner) is turned on by high temperature; and when activated, it produces a negative change (lowering of the temperature) that ultimately causes the effector to be turned off. In this way, constancy is maintained.
Regulating Body Temperature
Humans, together with other mammals and with birds, are endothermic; they can maintain relatively constant body temperatures independent of the environmental temperature. When the temperature of your blood exceeds 37°C (98.6°F), neurons in a part of the brain called the hypothalamus (discussed in chapters 28 and 30) detect the temperature change. Acting through the control of neurons, the hypothalamus responds by promoting the dissipation of heat through sweating, dilation of blood vessels in the skin, and other mechanisms. These responses tend to counteract the rise in body temperature. When body temperature falls, the hypothalamus coordinates a different set of responses, such as shivering and the constriction of blood vessels in the skin, which help to raise body temperature and correct the initial challenge to homeostasis.
Vertebrates other than mammals and birds are ectother- mic; their body temperatures are more or less dependent on the environmental temperature. However, to the extent that it is possible, many ectothermic vertebrates attempt to maintain some degree of temperature homeostasis. Certain large fish, including tuna, swordfish, and some sharks, for example, can maintain parts of their body at a significantly higher temperature than that of the water. Reptiles attempt to maintain a constant body temperature through behavioral means—by placing themselves in varying locations of sun and shade. That’s why you frequently see lizards basking in the sun. Sick lizards even give themselves a “fever” by seeking warmer locations!
Most invertebrates, like reptiles, modify behaviors to adjust their body temperature. Many butterflies, for example, must reach a certain body temperature before they can fly. In the cool of the morning, they orient their bodies to maximize their absorption of sunlight. Moths and other insects use a shivering reflex to warm their flight muscles.
Regulating Blood Glucose
When you digest a meal containing carbohydrates, you absorb glucose into your blood. This causes a temporary rise in the blood glucose concentration, which is brought back down in a few hours. What counteracts the rise in blood glucose following a meal?
Glucose levels within the blood are constantly monitored by a sensor, cells called the islets of Langerhans in the pancreas (discussed in chapters 25 and 30). When glucose levels increase (the condition of “high blood sugar” in figure 26.2a), the islets secrete the hormone insulin, which stimulates the uptake of blood glucose into muscles, liver, and adipose tissue. The muscles and liver can convert the glucose into the polysaccharide glycogen. Adipose cells can convert glucose into fat. These actions lower the blood glucose and help to store energy in forms that the body can use later. When enough blood glucose is absorbed, reaching the set point, the release of insulin stops. When blood glucose levels decrease below the set point, as they do between meals, during periods of fasting, and during exercise, the liver secretes glucose into the blood (the center arrow in figure 26.2b). This glucose is obtained in part from the breakdown of liver glycogen. The breakdown of liver glycogen is stimulated in two ways: by the hormone glucagon, which is also secreted by the islets of Langerhans, and by the hormone adrenaline, which is secreted from the adrenal gland (discussed in much more detail in chapter 30).
Figure 26.2. Control of blood glucose levels.
(a) When blood glucose levels are high, cells within the pancreas produce the hormone insulin, which stimulates the liver and muscles to convert blood glucose into glycogen. (b) When blood glucose levels are low, other cells within the pancreas release the hormone glucagon into the bloodstream; in addition, cells within the adrenal gland release the hormone adrenaline into the bloodstream. When they reach the liver, these two hormones act to increase the liver's breakdown of glycogen to glucose.
Key Learning Outcome 26.1. Negative feedback mechanisms correct deviations from a set point for different internal variables. In this way, body temperature and blood glucose, for example, are kept within a normal range.