Homeostasis and drinking
Negative feedback systems 39
Homeostatic behaviour 40
Fluid regulation 41
Thirst and drinking 42
Hypovolaemic thirst 43
Osmotic thirst 43
Control mechanisms 44
Satiety mechanisms 46
Non-deficit drinking in humans 46
Further reading 47
Animal cells and organs will only work optimally when their operating environment is maintained within a very narrow range of conditions. We have physiological mechanisms for controlling many aspects of the internal environment to provide optimal conditions for temperature,electrolyte concentrations, pH (acidity) of body fluids, oxygen level, carbohydrate concentrations of tissues and so on. The physiological process that produces this stability in the face of fluctuations in the demands that the environment makes on the body is known as homeostasis, a term introduced by Walter Cannon in 1929.
Homeostasis operates through negative feedback.The operation of such a system is often likened to the operation of thermostatically controlled heating systems. As we shall see the analogy is a poor one, although it does serve to illustrate the basic principles of negative feedback control.
Negative feedback systems
The essential features of such a control system are as shown in Figure 4.1. First, there is a system variable, which is the property that is to be controlled. In the case of a thermostat this is room temperature. Second, there is a set point, which is the target value of the property, in this case a temperature which the system tries to maintain.Third, the system needs a sensor, in this example, this would be some form of thermometer, to detect and report the current state of the system to, fourth, a comparator, which tests if the system variable is different from the set point. Fifth, the system needs a control, a mechanism to start and stop the sixth feature, which is a correctional process. In a thermostatic system, these are respectively a switch and a heater. Frequently, the sensor, set-point comparator and the control will be combined. For example, in a room thermostat, the sensor might be a bimetallic strip which bends when the temperature rises and falls, switching a heater on and off. Such a control
FIGURE 4.1 A simple negative feedback control system system is described as a negative feedback system because increases in the system variable feed back to the control to switch off the correctional process. In the case of a room heater, when the sensor detects that the temperature has fallen below the set point, it causes a switch to start a heater, which causes the temperature to rise. When the thermometer detects that the temperature has risen above the set point, it causes the switch to stop the heater.
Now, a simple system like a room thermostat has a number of disadvantages which might be merely inconvenient in controlling room temperature, but which could be fatal in a homeostatic mechanism. First, the physical nature of thermostats determines that they cannot maintain an exact temperature. Instead, the temperature at which they switch the heater on is always lower than the temperature at which they switch it off. Usually, this difference is quite large, as you can tell for yourself by turning the control on a room thermostat up and then down. You will hear the click of the switch turning on and off at different positions on the thermostat. In a physiological system this could lead to an unacceptably wide variation in metabolic efficiency. It is possible to improve on this by having a heater that is capable of a continuously variable output, which is controlled by detected variations in temperature. Such a system is a servo system, but it retains the essential characteristic of working on the basis of negative feedback. Physiological control systems are more like servos.
A second limitation of a simple thermostatically controlled heating system is that it permits correction only in one direction. That is, if the temperature rises above the set point, there is no way to lower it. This can be overcome by adding another correctional process that would cool the room if it gets too hot, and we can do this with air conditioning, which again would operate as a negative feedback system. This, too, is a feature of physiological control. A third difficulty with the room heater analogy is that it is vulnerable to failure of components or of the connections between them. If any one of these fails, the whole system fails. The solution to this is to build redundancy into the control system, by having more than one of each component, preferably with more than one connection between components. It would also be an advantage if the correctional processes were of different types, which would permit the system to cope if, for example, one source of energy failed. For similar reasons we might expect to find that there are satiety mechanisms, separate from the mechanism that starts the correction, to stop the correctional process. Both of these are characteristic of homeostatic systems.
Mammals and other endotherms, mostly birds, are able to maintain temperature within a very narrow range that is optimal for biochemical activity, and hence physiological processes. Other groups of animals, ectotherms, such as reptiles and amphibians, cannot control their own body temperature by internal mechanisms. The metabolism of ectotherms slows down when the environment gets colder, and speeds up when it gets warmer. The only ways that they have of controlling temperature are behavioural.To increase their body temperature they will seek sunlight and orient their bodies to maximise their absorption of heat. To cool themselves they will seek shade. Mammals also engage in such behaviour (for example, usually preferring to rest in warmer parts of the environment). In general, we shall refer to behaviours that promote homeostasis as homeostatic behaviour. We will see that, as in the case of temperature control, the control of other physiological states is accomplished by a combination of physiological homeostatic mechanisms and homeostatic behaviour.