The basic functional unit of the body is the cell. A stable environment is essential for cells to carry out their normal functions. However, few living cells are in contact with the external environment, rather, they live in a liquid environment comprised of extracellular fluid (ECF).
For optimum cell function several factors must be kept within narrow limits in the extracellular fluid. Examples of these factors are: (1) temperature; (2) concentration of metabolites (glucose); (3) osmotic pressure; (4) oxygen and carbon dioxide; (5) ion concentrations (Ca++, Na+, etc.).
This maintenance of static or constant conditions in the internal environment (ECF) is termed HOMEOSTASIS. The organs and tissues which act to maintain homeostasis are termed control systems. Some controls are built into the organ and generate a built in response to change. This type of mechanism is termed intrinsic control. However, the majority of the control systems are initiated by factors external to the organ and are mediated either by the nervous system or the endocrine system. This type of control system is referred to as extrinsic control.
In order for a control system to function there must be receptors to monitor changes (stimuli) in the internal environment. These receptors send information to a control center which monitors the change and integrates the information. The control center then sends information to effectors which generate a response to the change in the internal environment.
Control systems work primarily through two types of mechanisms:
1) Negative Feedback 2) Positive feedback
Negative feedback is the most common type of control system. In negative feedback a change in the internal environment (stimulus), is monitored by the receptor and transmitted to the control center. The control center then activates the effector system generating a response which cancels the effect of the stimulus (change) and reestablishes the homeostatic condition. Thus, negative feedback acts by negating or eliminating the stimulus (change).
EXAMPLES OF NEGATIVE FEEDBACK
1) Endocrine System
The concentration of calcium in the blood is maintained at 9-11 mg/dl. This is the homeostatic condition for Ca++. This homeostatic condition is maintained as follows: A) Increased Ca++ -- Response when concentration in the blood rises above 9-11 mg/dl
B) decreased Ca++ -- Response when concentration of blood calcium drops below 9-11 mg/dl
2) Nervous System
The normal temperature of the body is 37 C. This is the homeostatic condition for body temperature. If the body temperature varies from this homeostatic condition control system come into play to return the temperature to normal
Positive Feedback
In contrast to negative feedback, positive feedback is relatively uncommon in the human body. In positive feedback a change in the homeostatic condition is detected by receptors and the information is transmitted to the control center. The control center activates effectors which generate a response which increases the stimulus further reinforcing a the initial change. Thus, positive feedback acts to reinforce or strengthen the stimulus or change.
An example of positive feedback is the action of the hormone oxytoxin on the uterus during birth. During normal conditions the uterine muscle is passive and not contracting. Dilation of the cervix triggers stretch receptors which transmit nerve impulses to the Brain (hypothalamus). Stimulation of the hypothalamus results in oxytoxin (OT) being released from the posterior pituitary. Oxytocin is carried by the blood to the uterus where it causes uterine contractions. During the birth process dilation of the cervix initiates the release of OT which causes uterine muscle contractions. The uterine contractions begin to force the fetus through the cervix. As the fetus is pushed through the cervix this further stretches the cervix which results in more oxytocin being released. This positive feedback will continue until the baby has cleared the birth canal and the cervix is no longer stretched.
There are many more examples of homeostatic mechanisms. In some ways
it can be said that the study of physiology is a study of homeostatic mechanisms.