Excretion is the expulsion from the body of the wastes produced by the chemical activity occurring within the living cells of our body.
- The major wastes are carbon dioxide, water and urea.
- Water and carbon dioxide are produced by aerobic respiration by every living cell.
- Urea made in the liver during the conversion of excess amino acids to carbohydrate and fat.
- The three major excretory organs are the lungs, kidneys and skin.
- The lungs excrete carbon dioxide and water in the form of gases.
- The kidneys excrete water and urea in a liquid called urine.
- The skin excretes water, and a small amount of urea, in a liquid called sweat.
- The kidneys also function in keeping the blood at the correct concentration.
- Excess salt and water taken in the diet is expelled from the body in sweat and urine.
The Urinary System
a) The Structure and Function of the Urinary System
Two kidneys are in the abdominal cavity below the diaphragm – the kidneys produce urine, the excretory liquid of excess water, excess salts and urea. Renal artery to each kidney from the aorta supplying blood that is oxygenated but carrying urea, excess water and excess salts. A renal vein passes from each kidney carrying blood that is lower in oxygen but is not carrying urea, excess water or excess salts.
Urine is carried from each kidney to the bladder by a muscular tube called a ureter.
The bladder is a muscular bag that temporarily stores the urine
forcing it to the outside by way of the urethra at a time when we choose it is suitable to urinate.
A special circular muscle, or sphincter muscle, at the base of the bladder controls the release of urine by relaxing; when this muscle is contracted the opening from the bladder is closed and urine cannot escape.
b) The Formation of Urine by the Kidneys
The renal arteries deliver a quarter of the blood pumped by the heart to the kidneys. This huge volume is not for the supply of oxygen or nutrients but for cleaning i.e. the removal of wastes. This blood is filtered as it passes through the blood capillaries in the kidneys. The red blood cells, white blood cells, platelets and plasma proteins remain in the blood as they are too big to pass through the tiny holes in the blood capillaries. The filtrate, the liquid that passes through the walls of the blood capillaries, is composed of water, glucose, amino acids and urea.
(ii) Selective Reabsorption
The kidneys then select from the filtrate what is to be taken back into the blood. The urea waste is not selected. Glucose and amino acids are completely taken back. Only the amount of water and salt needed to keep the blood at the correct concentration is returned to the blood. The deselected material (excess water, excess salts and the urea) becomes the urine that passes out of the kidneys for expulsion from the body.
The Excretory Role of the Skin
At rest about 300g of water is lost per day due to evaporation of water from the skin. If the body becomes overheated the sweat glands in the skin are stimulated to secrete sweat onto the surface of the skin for cooling by evaporation – up to a litre and a half of water per hour can be lost due to sweating. The skin has about 2,500,000 sweat glands. Evaporation of water leaves the solutes behind and that is why the skin has a fine layer of salt on it after sweating.
N.B. The expulsion of undigested food from the gut is not excretion, it is called egestion.