Saturday, May 2, 2009

Homeostasis and Feedback Mechanisms

Metabolic activities are carefully regulated to maintain homeostasis which is an appropriate internal environment, or steady state.

Homeostatic mechanisms are the self-regulating control systems that maintain homeostasis.  Homeostatic mechanisms are remarkably sensitive and efficient.  Homeostasis must be maintained even though conditions may continuously change in the external environment.

Stressors are the stimuli that disrupt homeostasis, cause stress that activate homeostatic mechanisms.  For example eating a bag of candy is a stressor that raises the blood sugar concentration above the homeostatic level.  This stress activates homeostatic mechanisms that bring the blood sugar concentration back to the normal range.  When homeostatic mechanisms are unable to manage stress, steady state is not restored.  The stress may then lead to a malfunction, which can cause disease or even death.

Many homeostatic mechanisms are negative feedback systems, in which the response of the regulator (control center) is opposite to the change.  Examples of negative feedback mechanisms are

  • Osmoregulation is the regulation of water concentrations in the bloodstream, effectively controlling the amount of water available for cells to absorb.
  • Osmoreceptors that are capable of detecting water concentration are situated on the hypothalamus next to the circulatory system.
  • The hypothalamus sends chemical messages to the pituitary gland next to it.
  • The pituitary gland secretes anti-diuretic hormone (ADH) which targets the kidney responsible for maintaining water levels.
  • When the hormone reaches its target tissue, it alters the tubules of the kidney to become more/less permeable to water.
  • If more water is required in the blood stream, high concentrations of ADH make the tubules more permeable.
  • If less water is required in the blood stream, less concentrations of ADH make the tubules less permeable.
Many homeostatic mechanisms are positive feedback systems, in which variation from the steady state sets off a series of events that intensify the change.  Examples of positive feedback mechanisms are

  • One example of a biological positive feedback loop is the onset of contraction in childbirth.  When a contraction occurs, the hormone oxytocin is released into the body, which stimulates further contractions.  This results in contractions increasing in amplitude and frequency.
  • Lactation involves positive feedback so that the more the baby suckles, the more milk is produced.
  • In most cases, once the purpose of the feedback loop is completed, counter-signals are released that suppress or break the loop.  Childbirth contractions stop when the baby is out of the mother's body.  Lactation stops when the baby no longer nurses.

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