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 initiates a signal in response to the disruption. This signal leads to the activation of an effector, which may be a muscle (that contracts or relaxes) or a gland (which secretes substances). In mammals, many of these responses are coordinated by the hypothalamus, a region of the brain that plays a primary role in regulating homeostasis. The hypothalamus works closely with the nervous system, which uses rapid electrical signals, and the endocrine system, which relies on slower but longer-lasting hormonal signals.

Figure 1. When conditions deviate from the set point, receptors detect the imbalance and trigger a negative feedback loop. Effectors then act to restore levels toward equilibrium, helping maintain homeostasis over time.

Negative Feedback Mechanisms

In a negative feedback loop, a homeostatic process functions to reverse the direction of the initial change in a regulated variable. When a stimulus (a deviation from the set point) is detected by a receptor, effectors are signaled to produce a response that directly counteracts the original stimulus, bringing the regulated variable back towards its set point. In other words, if a level is too high, the body acts to lower it; if a level is too low, the body acts to raise it. This reversal of the initial change is what we refer to as negative feedback.

A classic example of a negative feedback loop is the regulation of body temperature in mammals. When temperature rises above the set point, receptors in the hypothalamus detect this increase and trigger effectors such as sweat glands and blood vessels. Sweating and vasodilation (widening of blood vessels) promote heat loss, helping lower body temperature.

Conversely, when temperature drops below the set point, the hypothalamus activates effectors like skeletal muscles and blood vessels to conserve and generate heat. Shivering (rapid muscle contractions) and vasoconstriction (narrowing of skin blood vessels) work to raise body temperature back toward normal. In both cases, the system’s response opposes the initial change, demonstrating how negative feedback maintains stable internal conditions.

BIOL 1C Example:

A key example of a negative feedback loop is the maintenance of blood glucose levels in animals. Once an animal has eaten, blood glucose levels rise. This increase is sensed by specialized receptor cells within the pancreas (specifically, beta cells). In response, 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.

Conversely, if an animal has not eaten and blood glucose levels fall, this decrease is sensed by another group of cells within the pancreas (specifically, alpha cells). These cells then release hormone glucagon, triggering the release of stored glucose into the blood and causing glucose levels to increase. It is critical to note that this is still a negative feedback loop, even if the result is an increase in blood glucose; the “negative” refers to the system’s action opposing the initial change (i.e., counteracting the decrease).

Another example of an increase as a result of a negative 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). PTH causes an increased absorption of calcium through the intestines and kidneys, and may also stimulate bone breakdown to release calcium into the blood. The collective effects of PTH are to raise blood levels of the element, counteracting the initial fall.

Negative feedback loops are the central 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) – Keep for BIOL 1C

Figure 2. When body temperature deviates from the set point, receptors in the hypothalamus detect the change and trigger effectors that help restore balance. If temperature rises, sweat glands increase sweating and peripheral capillaries dilate, promoting heat loss. If temperature drops, shivering generates heat and peripheral capillaries constrict to conserve it. In both cases, the response opposes the initial change, maintaining homeostasis.

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 occurs, the feedback loop continues to function but now works to maintain the new set point.

An example of this is blood pressure: over time, the normal set point for blood pressure can increase due to sustained elevated blood pressure levels. The body then perceives this higher pressure as normal and no longer attempts to return to the initial, lower set point. This results in the maintenance of elevated blood pressure, which can have harmful effects on the body. Medication can help lower blood pressure and reset the system’s set point to a healthier level. This process is known as alteration of the set point in a feedback loop.

Changes can also occur across multiple organ systems to maintain a set point in another system. This process, called acclimatization, is seen when animals migrate to higher altitudes with lower oxygen saturation. To adjust, the body increases red blood cell production to ensure adequate oxygen delivery to tissues. Another example is seasonal changes in animal coats: a heavier coat in winter helps retain heat, while a lighter coat in summer aids in preventing body temperature from rising to harmful levels.

Figure Descriptions

Figure 1. The image illustrates a homeostasis feedback loop. At the center, a purple seesaw labeled “Homeostasis (Set point)” is shown with one side tilted to indicate “Imbalance.” On the left, an orange upward arrow and a blue downward arrow represent a stimulus triggering levels to increase or decrease, moving away from the set point. This leads to the label “Receptor detects imbalance,” which points to “Effector released to stimulate change” at the top. An arrow leads to “Levels return to homeostasis.” The cycle continues with an arrow pointing back to “Receptor detects imbalance,” completing the loop.

Figure 2 (BIOL 1A). The image illustrates the human body’s thermoregulation process, focusing on maintaining body temperature. At the center is a balance labeled “Body Temperature set point,” with arrows indicating imbalance when temperature deviates. On the left, an arrow points upwards from a thermometer icon showing a rise in temperature. This leads to a depiction of the hypothalamus, labeled “Receptor in hypothalamus detects high temperature,” which releases effectors that activate sweat glands, promoting blood flow to the skin’s surface. A cross-section of skin with sweat glands is shown, and a note mentions that water secretion aids cooling. On the right, arrows indicate the body temperature decrease, showing another hypothalamus detection triggering physiological responses to conserve heat. A muscle icon signifies heat generation from muscle contractions. The image outlines both vasodilation and vasoconstriction processes performed by peripheral capillaries, contributing to temperature regulation.

Figure 2. (BIOL 1C). 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.