Homeostasis and Negative Feedback Loops

Homeostasis is the maintenance of a relatively stable internal environment in the body, and negative feedback is the main control mechanism that achieves it by reversing any change away from a set point. In brief: if body temperature rises above about 37 °C, sweating and widening of skin blood vessels cool the body, and if it falls, shivering and narrowing of skin vessels warm it. The loop always works to bring the value back toward the set point.

Cells function properly only within narrow limits of temperature, pH, water content and glucose concentration. Homeostasis keeps the conditions of the internal environment within those limits even when the outside world changes. This guide explains the internal environment, the parts of a feedback loop, the example of temperature control, how positive feedback differs, a worked example with blood sugar and the typical mistakes.

What is homeostasis?

The word homeostasis comes from Greek words meaning “similar” and “standing still”, but it does not mean that conditions are fixed. Values fluctuate around a set point, within a normal range. Body temperature in a healthy person, for example, stays near 37 °C but varies a little during the day.

The failure of homeostasis causes illness. In diabetes, blood glucose control fails. In heat stroke, temperature control fails, and in dehydration water balance is disturbed.

What is the internal environment?

The internal environment is the fluid surrounding the body’s cells, the tissue fluid, together with the blood plasma. The French physiologist Claude Bernard called it the “milieu intérieur”. Cells take nutrients and oxygen from it and release waste into it.

The main conditions that are controlled include body temperature, blood glucose concentration, water content and solute concentration, blood pH, blood pressure and the levels of oxygen and carbon dioxide. Several organ systems take part: the nervous and endocrine systems for control, and organs such as the skin, kidneys, liver, lungs and heart as effectors.

How does negative feedback work?

A negative feedback loop has four basic components.

1. Receptor (sensor): detects a change in the controlled variable.
2. Control center (coordinator): compares the value with the set point and decides on a response. It is often the brain or an endocrine gland.
3. Effector: a muscle or gland that carries out the response.
4. Feedback: the response changes the variable back toward the set point, which is detected by the receptor, and the response is reduced or stopped.

It is called “negative” because the response is opposite to the change. A rise leads to a fall, and a fall leads to a rise. A household thermostat is a useful analogy: when the room is too warm, it switches the heater off, and when too cool, it switches it on.

How is body temperature controlled?

The control center for temperature is the hypothalamus in the brain. It receives information from temperature receptors in the skin and from blood flowing through the brain.

If the body is too hot

• Sweat glands release sweat, and evaporation takes heat from the skin.
• Vasodilation: arterioles in the skin widen, more blood flows near the surface and more heat is lost.
• Hair lies flat, and behavior changes, such as seeking shade.

If the body is too cold

• Shivering: rapid muscle contractions release heat.
• Vasoconstriction: skin arterioles narrow and less heat is lost.
• Sweating stops, and hair stands up to trap air, which has a small effect in humans. Behavior changes, such as putting on clothes.

Variables controlled by homeostasis

How does positive feedback differ?

In positive feedback, the response increases the original change instead of reducing it. It pushes a process further until an end point is reached, so it does not maintain stability. It is rare in the body and is used in short processes.

Childbirth is a classic example. Contractions of the uterus push the baby’s head against the cervix, which stimulates the release of the hormone oxytocin, which in turn causes stronger contractions. The loop continues until the baby is born. Blood clotting is another example: platelets release chemicals that attract more platelets to a wound.

Worked example: blood glucose control

After a meal, blood glucose rises. Follow the negative feedback loop.

1. Change: blood glucose rises above the set point, which is around 5 mmol/L.
2. Receptor and control center: beta cells in the pancreatic islets detect the rise.
3. Effector response: the pancreas releases insulin. Insulin causes liver and muscle cells to take up glucose and store it as glycogen, and it increases glucose use by cells.
4. Result: blood glucose falls back toward the set point.
5. Feedback: as glucose falls, the beta cells release less insulin.

If glucose falls too low, for example between meals, alpha cells release the hormone glucagon. Glucagon causes the liver to break down glycogen into glucose, and the level rises. Two opposing hormones therefore provide control in both directions. In type 1 diabetes the beta cells are damaged and insulin is not produced, so the loop fails and blood glucose stays high unless insulin is given.

Who developed the concept?

Claude Bernard proposed in the 1850s and 1860s that the constancy of the internal environment is a condition for a free and independent life. Walter Cannon, an American physiologist, introduced the word homeostasis in 1926 and described its features in his book The Wisdom of the Body (1932). Later, ideas from engineering about control systems and feedback contributed to the modern description.

Which mistakes are common?

Mistake 1: Thinking negative feedback means a bad outcome

“Negative” means that the response opposes the change. It is beneficial and keeps the body stable.

Mistake 2: Saying homeostasis keeps conditions constant

Values vary around a set point within a normal range. Absolute constancy does not occur.

Mistake 3: Forgetting the receptor and control center

A complete description names the stimulus, receptor, control center, effector and response, not only the response.

Mistake 4: Saying that sweating cools the body because sweat is cold

Sweat cools the skin because it takes heat from the body when it evaporates.