http://www.biology4kids.com/files/systems_regulation.html

Homeostasis is a term that is used to both describe the survival of organisms in an ecosystem and to describe the successful survival of cells inside of an organism. Organisms and populations can maintain homeostasis in an environment when they have a steady level of births and deaths. It is similar to the idea of equilibrium.  When discussing the internal workings of an organism, homeostasis describes an environment that supports the survival of cells. All of your body’s systems work together maintain homeostasis inside of your body. Homeostasis is achieved by making sure the temperature, pH (acidity), and oxygen levels (and many other factors) are set just right for your cells to survive. Homeostasis levels are different for each species.

Negative Feedback

Negative feedback is a process that happens when your systems need to slow down or completely stop a process that is happening. When you eat, food travels into your stomach, and digestion begins. You don’t need your stomach working if you aren’t eating. The digestive system works with a series of hormones and nervous impulses to stop and start the secretion of acids in your stomach. Another example of negative feedback occurs when your body’s temperature begins to rise and a negative feedback response works to counteract and stop the rise in temperature. Sweating is a good example of negative feedback.

Positive Feedback

Positive feedback is the opposite of negative feedback in that encourages aphysiological process or amplifies the action of a system. Positive feedback is a cyclic process that can continue to amplify your body’s response to a stimulus until a negative feedback response takes over. An example of positive feedback also can happen in your stomach. Your stomach normally secretes a compound called pepsinogen that is an inactive enzyme. As your body converts pepsinogen to the enzyme pepsin, it triggers a process that helps convert other pepsinogen molecules to pepsin. This cascade effect occurs and soon your stomach has enough pepsin molecules to digest proteins.

Body Temperature Example

A good example of system regulation of your body can be found in the regulation of body temperatures. You are a homoeothermic organism, which means you regulate your own body temperature. Other species like reptiles are not homoeothermic. Anyway, if your body gets too cold, a series of actions are taken to warm your body. Sensors throughout your nervous system can recognize when the temperature drops and might trigger your muscular system to start shivering. The constant contractions of your muscles allow heat to be generated. Your nervous and endocrine systems may also contract the blood vessels of your circulatory system to keep blood in the core of your body and not the extremities (like fingers).