Kidney: Excretion & Osmoregulation (Edexcel IGCSE Biology)

The Kidney

• The kidneys are located in the back of the abdomen and have two important functions in the body:

  • They regulate the water content of the blood (vital for maintaining blood pressure and osmoregulation)

  • They excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts)

Waste substances removed by the kidney

Osmoregulation

• Osmoregulation is the process of maintaining water and salt concentrations (osmotic balance) across membranes within the body

• It is an example of homeostasis in the human body 

The importance of osmoregulation

• The cytoplasm of all cells is largely composed of water, as is the blood plasma

• Maintaining water levels in the body is vital to prevent harmful changes occurring to cells of the body as a result of osmosis

• If body cells lose or gain too much water by osmosis they do not function efficiently:

  • Too much water in the blood results in cells swelling as water moves into them, this has a diluting effect and can lead to cell lysis (bursting)

  • Too little water in the blood (or too high an ion concentration) and the cells lose water by osmosis, this has a dehydrating effect and can lead to cell death

The effect of water concentration on body cells due to osmosis

Water content of the body

• There are two sources of water in the body:

  • Water produced as a result of aerobic respiration

  • Water in the diet

• Water is lost from the body in the following ways:

  • Via the lungs during exhalation (breathing out)

  • Lost from the skin as sweat (along side mineral ions and urea)

• Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys