Regents Prep: Living Environment: Homeostasis:
Feedback Mechanisms
Dynamic Equilibrium and Feedback
is the maintenance of a stable internal state within an organism. Homeostasis is also known as steady state. Organisms must respond and maintain homeostasis in relation to many factors.

Stimulus and Response

Organisms detect changes in their environment and respond to these changes in a variety of ways.    These changes may occur at the cellular or organism level.

The graphic above shows the response of a human to being struck on the knee with a hammer.   A change in the environment is called a stimulus.   In this situation, the stimulus is the being struck with the hammer.   A response is the manner in which the organism reacts to the stimulus.  The knee jerk reflex which is pictured at the right is the response of this individual to being hit with this hammer.

Feedback Mechanism Examples
Feedback mechanisms have evolved in living things as a mechanism by which they maintain homeostasis or dynamic equilibrium.    

A feedback mechanism occurs when the level of one substance influences the level of another substance or activity of another organ.   

An example of a feedback mechanism in humans would be the increase in heart rate and respiratory rate which occurs in response to increased exercise or other increased muscle cell activity.   Some other examples of feedback mechanisms in living things appear below.

Temperature Homeostasis

Humans maintain a relatively constant body temperature of about 37 C.

  • when we "heat up" we sweat if possible
  • the evaporation of this perspiration returns the body to its original temperature


Blood Sugar Regulation

The pancreas is an endocrine gland which produces hormones which regulate blood glucose (sugar) levels

An increase in blood sugar level triggers the release of the hormone insulin by the pancreas

the hormone insulin lowers blood sugar level restoring the body to its original blood glucose level in two major ways:

  • it increases the ability of body cells to take in glucose from the blood
  • it converts blood glucose to the compound glycogen -- this compound is also called animal starch and is stored in our liver and muscles

Homeostasis by Plants

Maintenance of Water

  • plants need to regulate water loss and carbon dioxide intake for photosynthesis and other life activities
  • when plants do not keep enough water in their cells, they wilt and die

stomate: a microscopic hole in a plant leaf which allows gases to enter and leave and water vapor to leave as well. Stomata is the plural of stomate.

guard cells: open and close the stomate.

  • the ability of the guard cell to close during periods of limited water availability for the plant allows the plant to maintain water homeostasis

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