Homeostasis and Systems Control

The human body contains about 15 liters of fluid. The trillions of cells in our bodies must draw nutrients and dump wastes into the same fluid. Fluid outside of cells is extracellular fluid. The extracellular fluid consists of interstitial fluid (between the cells and tissues) and plasma (blood fluid).

Changes in extracellular fluid cause changes in cells. The component parts of every animal work to maintain a stable fluid environment for living cells.

Homeostatic mechanisms operate to maintain chemical and physical environments within tolerable limits.

Homeostatic control mechanisms require three components:

• Sensory receptor cells detect specific changes (stimuli) in the environment.
• Integrators (brain and spinal cord) act to direct impulses to the place where a response can be made.
• Effectors (muscles and glands) perform the appropriate response.

A common homeostatic mechanism is negative feedback. It works by detecting a change in the internal environment that brings about a response that tends to return conditions to the original state. It is similar to the functioning of a thermostat in a heating/cooling system. Some activity alters a condition in the internal environment. The alteration triggers a response. The response reverses the altered condition.

In positive feedback, some activity alters the internal environment. The alteration triggers a response. The response intensifies the change in the internal condition. Positive feedback mechanisms may intensify the original signal. Sexual arousal is an example.

This animation (Audio - Important) describes feedback mechanisms.

This animation (Audio - Important) provides an example of how homeostasis functions in a dog.

This animation (Audio - Important) describes the structure of how the body regulates itself.

REVIEW: If your body is too cold, it will
Answer cause muscles to shiver.