Homeostasis basically means the state of balance in internal conditions maintained by living organisms. 

Living cells need their environment to be ideal for them in order to survive. 

The nervous system and hormones are responsible for this. 

Here are some things that need to be in balance in living things:

  • Temperature
  • Blood glucose
  • Water content

These three conditions are the most common things that can be asked of you. 


Temperature

The hypothalamus is the part of the brain which monitors body temperature. It receives information from temperature-sensitive receptors present in the skin and circulatory system.

When it is too hot, we want maximise heat loss from our body:

  • Sweat glands release more sweat. Sweat evaporates, creating a ‘cooling effect’.
  • Blood vessels dilate (become wider), allowing more blood to flow and more heat to be lost. 
  • Hair on the skin become flat, trapping less air, preventing insulation

When it is too cold, we want to minimise heat loss from our body: :

  • Blood vessels constrict (become narrower), allowing less blood to flow and less heat to be lost. 
  • Hair on the skin rise, trapping more air, creating some insulation and preventing heat loss
  • Shivering occurs, causing rapid muscle contractions, releasing heat. 

Blood Glucose

Insulin is the main hormone which helps to control blood glucose concentration. It is produced by the pancreas
Insulin causes the liver to convert glucose to glycogen. Glycogen is primarily stored in the liver. 

When blood glucose is low, insulin is not secreted into the blood. Hence, glucose is not converted into glycogen. Therefore, blood glucose concentration increases. 

When blood glucose is high, insulin is secreted into the blood. Hence, glucose is converted into glycogen. Therefore, blood glucose concentration decreases. 


Water Content

How do we lose water?

  • Sweat
  • Urine
  • Faeces

It is important to keep a constant water content in the body as our cells require water in order to function efficiently. 

A hormone that plays a crucial role in maintaining body water content is the anti-diuretic hormone (ADH). 

When blood water content is low, the hypothalamus sends a signal to the pituitary gland.
The pituitary gland releases more ADH into the blood stream and it travels to the kidneys.
ADH causes the kidneys to reabsorb more water back into the body.
This produces more concentrated (yellowish) urine.

When blood water content is high, the hypothalamus sends a signal to the pituitary gland.
The pituitary gland releases less ADH into the blood stream. 
This causes the kidneys to reabsorb less water back into the body. More water leaves the body.
This produces more dilute (clear) urine.