Homeostasis and Feedback

Homeostasis

Homeostasis is a process essential for survival. It can be defined as the maintenance of an internal “set point” in response to a fluctuating external environment. Another way to define it is the maintaining of equilibrium by regulating the internal environment. Without homeostasis, pathology, disease, or even death can result.

Examples of “set points” that must be maintained by homeostatic mechanisms are core body temperature of 98.6°F (average), blood glucose level of 100mg/100ml of blood (average), and blood pressure of 120/70 mm/Hg (average).

Homeostasis is a process essential for survival. It can be defined as the maintenance of an internal _________ in response to a fluctuating external environment.

Answer: setpoint

Why is homeostasis important to survival?      Without it, disease and even death can occur.

Negative Feedback Systems and Components

Homeostasis is maintained through feedback control systems. The most common of these is the negative feedback mechanism.

Negative feedback mechanisms, or loops, are inhibitory in nature, meaning they oppose the change from the “set point” causing the opposite effect. Body temperature control is a negative feedback system that occurs in the body. If the body temperature rises or falls, the response of shivering or sweating will reverse the change that has occurred. Something that is important to note about a negative feedback mechanism is that it is has 3 basic components:

• receptor (or sensor) – provides information about internal conditions,

• control center (set point or integrator) – tells what a particular value should be, and

• effector – cause responses that alter conditions in the internal environment.

Let’s use an example in the body to understand the 3 basic components within a negative feedback system. We will use homeostasis of body temperature control (also called thermoregulation) to illustrate this.

If the body is exposed to cold external temperatures and the internal temperature drops below the set point, the brain sends a message to the muscles. Muscles then shiver in order to produce heat. A message is also sent to the peripheral blood vessels to constrict. This shunts blood to the core to raise the body temperature. If the reverse happens and the temperature falls below the “set point,” the brain sends signals to the glands so that sweating will occur, and the body will be cooled. 

The control of blood sugar (glucose) by insulin is a good example of a negative feedback mechanism. When blood sugar rises, receptors in the body sense a change. In turn, the control center (pancreas) secretes insulin into the blood effectively lowering blood sugar levels. Once blood sugar levels reach homeostasis, the pancreas stops releasing insulin. Examine the graphic below to understand how this feedback loop works.

Positive feedback loops

Homeostatic circuits usually involve negative feedback loops. The hallmark of a negative feedback loop is that it counteracts a change, bringing the value of a parameter—such as temperature or blood sugar—back towards it set point.

Some biological systems, however, use positive feedback loops. Unlike negative feedback loops, positive feedback loops amplify the starting signal. Positive feedback loops are usually found in processes that need to be pushed to completion, not when the status quo needs to be maintained.

A positive feedback loop comes into play during childbirth. In childbirth, the baby's head presses on the cervix—the bottom of the uterus, through which the baby must emerge—and activates neurons to the brain. The neurons send a signal that leads to release of the hormone oxytocin from the pituitary gland.

Oxytocin increases uterine contractions, and thus pressure on the cervix. This causes the release of even more oxytocin and produces even stronger contractions. This positive feedback loop continues until the baby is born.

Significant patterns in thyroid function

Consecutively raised TSH levels, with paired levels of T4 below the normal range indicate primary hypothyroidism.

Conversely, levels of TSH below the normal range with paired levels of T4 above the upper end of the normal range indicate an overactive thyroid.

If the situation were that clear-cut, then this would be a very short blog. Needless to say, there are some situations requiring careful scrutiny.