HOMEOSTASIS

Definition

• Maintenance of a constant internal environment is called homeostasis.

Internal environment

• The internal environment is the immediate surrounding of the cell which provides the medium for the cell to survive.

• Tissue fluid around cells, plasma around blood cells and lymph are included in the internal environment.

• When the internal environment is constant, the conditions inside cells are also constant.

• If there is a small change in the internal environment, it affects cellular activities.

• Therefore, the internal environment should maintain stable conditions within a narrow range tolerated by cells.

• If not, automatic control systems act with feedback mechanisms.

Factors regulated in the internal environment

• Blood glucose level

• Body temperature

• Water balance

1. Regulation of Blood Glucose Level

   
   

Blood glucose level of a healthy adult is 80–120 mg/100 ml of blood.

When blood glucose level is higher than normal :

• Beta cells in the islets of Langerhans in the pancreas secrete insulin.

• Insulin converts glucose into glycogen, which is stored in the liver.

• Excess glucose is converted into fat and stored in adipose tissue.

When blood glucose level is lower than normal :

• Alpha cells in the islets of Langerhans in the pancreas secrete more glucagon.

• Glucagon acts on glycogen stored in the liver and converts it into glucose.

• Glucose is released into the blood and the blood glucose level increases to normal.

• Blood glucose level is regulated by the activities of insulin and glucagon.

• Absence of beta cells or secretion of insulin causes diabetes.

2. Regulation of Body Temperature

• Humans are homeothermic organisms, meaning they maintain a constant body temperature irrespective of fluctuations in environmental temperature.

• Normal body temperature is 37°C, but it can vary from 36°C to 37.5°C.

• The thermoregulatory centre is present in the hypothalamus of the brain.

When environmental temperature decreases :

• Blood supply to the skin is reduced by contracting blood capillaries in the skin, reducing heat loss.

• Sweat production is reduced, reducing heat loss.

• Hairs become erect and trap an air layer which acts as a heat-insulating layer.

• If heat loss is high, heat is generated by shivering.

When temperature of the internal environment increases :

• Blood vessels in the skin dilate, increasing blood supply to the skin and increasing heat loss.

• Sweat production increases. When sweat evaporates, heat is absorbed by the body and body

  temperature decreases.

• Regulation of body temperature is done by the hypothalamus.

3. Regulation of Water Balance

• When the water level of blood drops, the pituitary secretes ADH (Antidiuretic hormone).

• ADH acts on the kidney to increase reabsorption of water.

• Therefore, the amount of water released with urine decreases.

• When the water level in blood is high, reabsorption of water decreases.

• Therefore, the amount of water released with urine increases.

• Accordingly, water balance in the body is regulated.