Written by tutor Karen M.
As a human being, it is not hard to recognize that a body’s internal environment is maintained separately from its external environment. Simply imagine yourself on a cold winter’s day where the outside temperature has plummeted to -5°C. Your body does not simply succumb to this temperature but, in fact, does its very best to otherwise maintain your body temperature near its optimal 37°C. The idea of the balance between the internal and external environments of a living system was originally explained by the French physiologist Claude Bernard in the mid-1800’s. Bernard described this idea as “milieu intérieur,” the process of maintaining the stable internal environment despite the conditions of the external environment (Gross, 1998). The concept was later expanded upon by the American physiologist Walter Bradford Cannon who termed it “homeostasis” which translates to “steady state” or “unchanging.”
Many variables within the human or animal body are maintained with little room for fluctuation. Temperature control is one that often comes to mind, and will be discussed below, but there are many other aspects of animal life that are very tightly controlled. We control the pH of blood to a pH of 7.4, only allowing it to deviate by a tenth of a unit (7.3-7.5.) and changes beyond this range can render some enzymes nonfunctional and disrupt cellular activities. Other events or variables that are controlled by homeostatic processes include blood pressure, heart rate and blood oxygen levels, among many others.
The basics of biology teach that homeostatic control systems contain three individual components that work together to control a variable within the organism. The three parts of the system are the receptor, the control center and the effector. The receptor serves to sense a change in a certain condition of the organism’s (i.e., animal’s) internal environment. The control center interprets this information from the receptor and directs a response to the situation that is enacted through the effector. The simplest non-biological illustration of this is temperature regulation inside of a house. The thermostat is set at 22°C, but the temperature in the house drops to 19°C. The thermometer (i.e., the receptor) within the thermostat measures the temperature and recognizes the change away from 22°C, the control center within the thermostat determines whether how to correct the deviation. The heater (i.e., the effector) is turned on and raises the temperature back to 22°C until the thermometer recognizes the temperature is correct and turns off the heater.
This same type of course correction regarding temperature also occurs within the human body. When the temperature of the human body deviates from 37°C, a modification is made to heat/conserve heat or cool the body. For example, upon a drop in temperature, one response of the human body is to begin to “shiver”. Shivering is caused by increased activity of skeletal muscles that uses energy and therefore creates heat to help protect internal core temperature of the vital organs. Alternatively, when the temperature rises much above 37°C, sweat glands begin releasing moisture that increases evaporation and therefore cools the surface of the skin. Temperature regulation is a type of negative feedback where the direction of the change in the variable, in this case temperature, is counteracted by a physiological response. In other words, if the temperature is too high, the body responds by attempting to lower the temperature.
Another negative feedback mechanism is involved in regulating the amount of glucose in the blood, a process that involves a balance of pancreatic hormones that store or release glucose when needed. Blood sugar levels are typically maintained at ~90 mg of glucose/100 ml of blood (Marieb, 1995). Should the level of glucose in the blood rise, cells within the pancreas are stimulated to produce and secrete insulin into the blood. Insulin is responsible for increasing cellular uptake of glucose as well as storing glucose as glycogen in the liver; therefore the surplus of glucose is not “wasted” but instead is reserved for a time when it will be needed. In an occasion where there is too little glucose in the blood, glucagon, a separate hormone is secreted by the pancreas. Glucagon causes the liver to convert glycogen back into glucose and releases it into the blood thereby raising the blood glucose level. Hence, when blood glucose levels are too high, insulin decreases the amount of glucose and when blood glucose levels are too low, glucagon increases it. This negative feedback mechanism counteracts inappropriately high or low blood sugar levels and allows for a relatively stable level of glucose to be available to the body for energy production.
The opposite of the negative feedback mechanism is termed positive feedback, where the physiological response to a change in the variable serves to enhance or encourage the response instead of working in the opposite direction. Given that positive feedback mechanisms tend to exacerbate the change in the variable, driving it further in the same direction, it is reasonable that these mechanisms are not typically used for maintaining balance. While the majority of mechanisms involved in homeostasis are regulated by negative feedback processes, there are examples of positive feedback playing a role. One such example is that of regulating blood clotting following damage to a blood vessel. Following a break or a tear in a vessel, platelets soon begin to attach to the site of injury and release chemicals that attract even more platelets. Therefore, an event that causes the recruitment of platelets ultimately causes the recruitment of even more platelets. The accumulation of all of these platelets begins the process of forming a clot in order to seal off the injury.
It is important to caveat the significance of maintaining a constant internal environment with the idea that there are special circumstances that call for deviations from the mean and are considered acceptable within a narrow range. For instance, human females experience a cycling of hormone levels and therefore these hormones are not held at a “constant” level. Additionally, one of the body’s responses to infection is to purposefully raise the body temperature (i.e., fever) in an attempt to help fend off the infection. However, it should be noted that even in the case of a fever, the body does not stray far from the set point of 37°C and serious consequences can occur should the proper mechanisms not keep the extent and duration of the fever under control.
Animal bodies are highly complex by nature. They are able to overcome many obstacles and various conditions. The ability of the body to regulate itself and maintain a comparatively stable internal environment despite outside conditions, traumatic actions, food supply or other ongoing events embodies the “wisdom of the body” that physiologist Walter Bradford Cannon referred to when he described its ability to reach homeostasis.
Gross, Charles, G. Claude Bernard and the Constancy of the Internal Environment. The Neuroscientist. 4: 380-385, 1998.
Marieb, Elaine, N (editor). Human Anatomy and Physiology, 3rd Edition. The Benjamin/Cummings Publishing Company, Inc., Redwood City, CA. 1995.