What is Homeostasis ?

Definition of homeostasis

Note: Some definitions state or imply that homeostasis is the stable condition in which the body is maintained by the action of feedback systems making appropriate adjustments as necessary. Other definitions* of homeostasis state that homeostasis is the (active) maintenance of stable conditions within the body.

* E.g. according to the Oxford concise Colour Medical Dictionary, "homeostasis the physiological process by which the internal systems of the body ... are maintained at equilibrium, despite variations in the external conditions."

1. How does the body control Homeostasis?

Short Answer: Feedback Mechanisms (see 2. below).

The following are notes about the body systems involved.

All the body's structures are kept in balance by one or more homeostatic mechanisms. The body's homeostatic mechanisms are controlled mainly by the Nervous System, and the Endocrine System. The role of parts of these and other tissues varies according to the specific homeostatic mechanism. However, in general:

• Structures within the nervous system detect variation from the balanced state, i.e. parameters such as heat or pH being within the range of acceptable values, and communicate that information by sending signals in the form of nerve impulses to the glands, organs or tissues in the body responsible for taking action to restore the balanced state.
• In many cases the glands of the endocrine system (endocrine glands) take action to restore the body (or a part or system thereof) to a balanced state by producing and/or secreting hormone molecules into the blood. This controls homeostasis because hormones are chemicals that can move around the body and are targeted to interact with specific cells that have receptors matching the specific hormone. Hormones are described as "chemical messengers" because by interacting with target cells they stimulate those cells to take specific action, e.g. antidiuretic hormone (ADH) directs the kidneys (to decrease the volume of urine they produce), whose overall effect is to maintain the stability of the body's internal environment, i.e. homeostasis.

As indicated above, the overall (general) process of homeostasis involves the body detecting some - usually small - variation from the ideal stable, balanced, state. It requires (unconscious) decisions to be taken about the adjustment(s) necessary and instructions to be conveyed to the structures of the body responsible for taking restorative action. Action must be taken to change the out-of-balance condition back within the acceptable range e.g. range of values of temperature. Throughout and beyond the process of adjustment, conditions within the body part/area/system must be continually monitored and any necessary adjustments made to maintain the stability of that aspect of the body's internal environment.

This is achieved via feedback mechanisms (see below).

A key difference between the contributions of the Nervous System and the Endocrine System in the feedback systems that control and maintain homeostasis is that nerve impulses are generally much faster than the effects of the hormones - which generally* travel via blood circulation and can remain active in the blood for much longer periods of time - from minutes up to as much as a few hours in some cases**.

* Most hormones are circulating hormones, as opposed to local hormones that only act locally, i.e. on the cell that secreted them or on adjacent cells, without entering the bloodstream.

** Hormone half-life and duration of activity differ from hormone to hormone. In general, peptide hormones very short of seconds-minutes, while steroids and thyroid hormone have longer half-lives of up to an hour or sometimes longer.

2. Explain "Feedback Mechanisms"

The basic principle of the feedback mechanisms (sometimes called "feedback systems") that maintain homeostasis is that a parameter called a controlled condition e.g. the concentration in the blood of a specific hormone is monitored by receptors that send ("input") information to a control centre e.g. the hypothalamus controls the concentration in the blood of certain hormones that responds by ("output") sending signals e.g. in the form of nerve impulses or chemical signals to effectors which are the cells or other structures within the body that cause the controlled condition to change as necessary to bring that particular aspect of the body back into a steady, stable, balanced state. While this is happening the receptors continue to send information to the control centre, which than therefore continue to adjust its output as appropriate to the current state of the controlled condition.

There are two types of feedback mechanisms:

• Negative Feedback Systems (Negative feedback mechanisms)
  Produce negative feedback which means that negative feedback systems reverse changes in the controlled condition, hence negative feedback tends to bring conditions within the body back into balance.
• Positive Feedback Systems (Positive feedback mechanisms)
  Produce positive feedback so positive feedback systems reinforce (increase) changes in the controlled condition.
  Therefore positive feedback systems operate when an event will occur to discontinue the feedback system when appropriate - e.g. positive feedback systems control infrequent conditions such as ovulation, childbirth and blood clotting.

See hormone regulation feedback mechanisms for more about positive vs. negative feedback systems.

3. Examples of Homeostasis

There are many possible examples of homeostasis mechanisms in the human body.
The following are included in some courses in biology, human biology and anatomy & physiology.

• Temperature Homeostasis (Thermoregulation)
• Blood Glucose Homeostasis - the cycle of regulation of blood sugar levels is described on the page about diabetes.
• Blood Water Homeostasis (Osmoregulation)

4. Homeostasis, Disorders and Disease

Question: Why is homeostasis important ?
Answer: Homeostasis is necessary for good health. Without it, disorders, disease and death may result.

Provided that the body's controlled conditions are kept within certain constraints e.g. of temperature, concentration of biochemicals, etc., the body stays healthy and can generally be expected to thrive - including growing and developing during childhood and supporting developing fetus(es) during pregnancy. However, if homeostasis is not maintained, the balance of the body's processes may be disturbed, leading to disorder(s), disease or even death.

• A disorder is an abnormality or malfunction of some part of the body or a system of the body.
  E.g. heat-stress can take many forms depending on the severity and if the person is too hot or too cold. In general most severe abnormalities have become recognized diseases - see below.
• A disease is a disorder characterized by specific recognizable signs and symptoms - except forms of ill-health that are (only) a direct result of physical injury e.g. due to an accident.
  E.g. Diabetes mellitus is a disease due to failure of glucose homeostasis.
• Death may result from some (though not all) untreated diseases - including those due to the failure of homeostasis.
  E.g. if untreated, a person with diabetic ketoacidosis - which involves the accumulation of ketone bodies in the blood together with increased blood acidity - can quickly experience shock then fall into a coma, from which death might occur.

The above notes are part of a general description of the human endocrine system likely to be appropriate for advanced or enthusiastic students of first-level courses (e.g. AS and A-Level Human Biology, ITEC Anatomy and Physiology and other Health Science subjects) and for students returning to this topic for more advanced courses.