1. Homeostasis

2. Homeostasis is a term that is used to refer to the maintenance of a stable equilibrium inside an organism. This equilibrium however is not static, rather it is a dynamic one. Thus the organism and its internal environment will remain in a balance - a dynamic equilibrium. Levels will change but they will fluctuate around the desired levels.

3. Homeostasis is responsible for the maintenance of: Concentration of oxygen and carbon dioxide pH of the internal environment Concentration of nutrients and waste products Concentration of salt and other electrolytes Volume and pressure of extracellular fluid

4. Control Systems Depending on the type of organism, the system may be open or closed. An open system has no feedback. It may be compared to a light switch. The light does not influence whether or not the switch is being turned on or off.

5. A closed system uses a feedback mechanism and has two components: a sensor which senses a change and an effector which responds to the change. This type of system acts the same way as a thermostat (sensor) and a furnace (effector). When the thermostat is adjusted to an appropriate setting the furnace cuts in. When the desired temperature is reached, the furnace cuts out. Systems in the body use this type of feedback in homeostasis.

6. Extrinsic vs Intrinsic Homeostatic systems can be extrinsic; that is they are controlled externally. In higher order organisms like animals, the endocrine and the nervous system are the major feedback controls. Homeostatic systems can also be intrinsic; that is only one organ or tissue is controlled locally. For example, when muscles are depleted of oxygen and produce more carbon dioxide, there is a dilation of blood vessels that allow more blood, and thus more oxygen into the area. Eventually, the vessels constrict to normal size.

7. Feedback Systems in Homeostasis There are two types of feedback control mechanisms involved in homeostasis. Positive and Negative. Negative feedback control mechanisms is used by most body systems. It results in a reverse response. TSH production is an example of a negative feedback. As blood levels of TSH rise, the production of TSH is slowed and vice versa. Therefore, blood levels of TSH serve as a feedback for the production of TSH.

8. The second type of feedback mechanism is the positive feedback control mechanism. As input increases, the response is accelerated. For example, oxytocin is a hormone that is produced during labor as the uterus contracts. As levels of oxytocin increases, uterine contractions become stronger. As uterine contractions heighten, it signals more oxytocin to be produced.

9. Body Systems There are a number of systems in the human body. Our study will be limited to” –Circulatory –Digestive –Respiratory –Excretory –Immune