Internal Environment and Homeostasis

The body is surrounded by the external environment that provides nutrients and oxygen that are necessary for life. The external environment also receives waste materials from the body. The body has an internal environment which is maintained more or less constant by certain biological mechanisms. The skin separates the internal environment from the external environment.

The internal environment is water-based in which the cells of the body exist. A fluid known as interstitial fluid or tissue fluid bathes the cells. Oxygen and nutrients from the internal transport systems reach the cells through the interstitial fluid. Waste products move through the interstitial fluid to transport systems which excrete them out of the body.

The cell membrane surrounds the cell. It acts as a barrier and regulatesthe substances entering and leaving the cell. The cell membrane is semi-permeable i.e. allows certain types of molecules to pass through but blocks others. Smaller molecules can pass through the cell membrane easily than larger molecules. Some molecules pass through more easily than others. Due to the selective nature of the cell membrane, the intracellular fluidhas a different chemical composition than the interstitial fluid.


The internal environment of the body is maintained more or less constant and within a narrow range of limits. This is called homeostasis. Homeostasis literally means ‘unchanging’. However, it should be noted that the internal environment is not exactly unchanging. It is dynamic but is maintained within particular limits. Many diseases involve a disturbance of homeostasis.

There are many factors in the internal environment that are maintained within narrow limits. Some of these factors include:

  • Temperature
  • Water and electrolyte concentration
  • pH of body fluids
  • Blood glucose level
  • Blood pressure
  • Blood and tissue oxygen and carbon dioxide levels

There are control systems in the body which detect and respond to changes in the internal environment and maintain homeostasis. There are three parts in any control system:

Detector or Sensor


The detector or sensor detects environmental stimuli and sends this information to the control centre.
Control centreThe control centre receives and processes the information supplied by the sensor. It signals the effector to respond to the stimuli.
EffectorThe effector responds to the commands of the control centre and opposes or enhances the stimulus.

Homeostatic imbalance

Homeostatic imbalance occurs when the body is unable to regulate a physiological variable within the normal range. If homeostasis is not maintained, an abnormal state results which can detrimental to the well-being of the body or can even be fatal.