11.1 Introduction to Homeostasis

Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). This fundamental process is vital for an organism’s survival and appropriate functioning. These changes might concern the level of glucose or calcium in the blood, or shifts in external temperatures. Homeostasis refers to the maintenance of dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions stay within specific ranges. Even an animal that appears inactive is continuously maintaining this homeostatic equilibrium.

Homeostatic Processes

The aim of homeostasis is the maintenance of equilibrium around a point or value called a set point. While there are normal fluctuations from the set point, the body’s systems will typically attempt to return to this point. A change in the internal or external environment is called a stimulus, which is detected by a receptor. The response of the system is to adjust the altered condition toward the set point. For example, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after meal consumption, adjustments are made to lower blood glucose levels by transporting the nutrients into tissues or storing it for later use.

Control of Homeostasis

The adjustments required for homeostasis are managed through physiological control mechanisms, primarily feedback loops. Most homeostatic processes rely on negative feedback loops, which counteract changes to restore balance. In contrast, positive feedback loops amplify the initial change, actually pushing the system further away from the set point. The receptor senses the change in the environment and sends a signal to the control center (in most cases, the brain) which in turn produces a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a gland (which secretes substances). In mammals, the overall coordination of these homeostatic responses is primarily directed by the nervous system, which utilizes rapid electrical signals, and the endocrine system, which employs slower but longer-lasting hormonal signals.

Figure 1 – Homeostasis Process (Blood Glucose)

Negative Feedback Mechanisms

Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. This reversal of the initial change is the essence of negative feedback. An example is animal maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized receptor cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease by promoting its uptake into cells or storage as glycogen, as illustrated in Figure 2. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released, triggering the release of stored glucose into the blood and causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and may also stimulate bone breakdown to release calcium into the blood. The effects of PTH are to raise blood levels of the element. Negative feedback loops are the predominant mechanism used to maintain homeostasis in the body.

Figure 2. Blood sugar levels are controlled by a negative feedback loop. (credit: modification of work by Jon Sullivan)

Positive Feedback Loop

In contrast, a positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback.

Set Point

It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop.

Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than that to which it is accustomed. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels.

Figure Descriptions

Figure 1.

Figure 2. The image illustrates the cycle of glucose regulation in the body, centered around a photograph of a pizza. Surrounding the photo, four captions are connected by arrows to indicate the process flow. The top right caption reads, “Food is consumed and digested, causing blood level glucose to rise.” An arrow points to the text, “In response to higher glucose levels, the pancreas secretes insulin into the blood,” at the middle right. Below this, another arrow leads to, “In response to higher insulin levels, glucose is transported into cells and liver cells store glucose as glycogen. As a result, glucose levels drop.” The cycle completes with the lower left caption, “In response to the lower concentration of glucose, the pancreas stops secreting insulin,” which connects back to the top. The pizza itself is a round, golden-brown dish with visible toppings such as cheese, tomato sauce, and herbs.