Excretion is the process whereby metabolic waste products and toxic materials are removed from the body of an organism. It takes place throughout our body. Without the excretory system, waste and toxic materials would gather up in our body and would be hazardous to our health.

Lungs

We may not know it, but our lungs also undergo excretion. When our cells convert oxygen into energy, they return carbon dioxide. This carbon dioxide has to be removed if not we might die. Thus, our lungs are important in removing carbon dioxide from our body.

Urinary system

Our urinary system consists of a pair of kidneys, a pair of ureters, a urinary bladder and the urethra. Our kidneys are concave in shape and face our backbone. At the centre of the concavity is a depression, known as the hilus. Here, the renal artery, the renal vein and the nerves are connected to the kidney. From the hilus a narrow tube, the ureter, emerges and travels downwards to join the dorsal surface of the urinary bladder. Urine from each kidney passes through the ureter to the urinary bladder. The bladder is an elastic, muscular bag, ventral to the rectum. It stores urine. At the bottom of the bladder is a sphincter muscle. When the bladder is full, the sphincter muscle relaxes and allows the urine to flow into the urethra and pass out of the body.

^ Our Bladder                   

Constituents of urine

In every 100g of urine there is:

• Water - 96.0g
• Mineral Salts (mainly salt) - 1.8g
• Urea - 2.0 g
• Other nitrogenous substances - 0.2 g

Skin

Our skin is actually an excretory organ too. When we exercise vigourously or feel hot, our skin excretes sweat. The sweat consists of mainly water and also nitrogenous substances present in minute quantities.