The excretory system

What is waste?

The chemical reactions in your cells, together called metabolism, make substances your body doesn't need and some that are poisonous if they build up. Removing these wastes made by cells is called excretion.

The excretory system is made up of the kidneys and urinary system, the liver, the lungs and the skin.

Faeces don't count as excretion. Most of what's in faeces is undigested food that never got inside your cells, so getting rid of it is called egestion.

The liver: the body's chemical factory

The liver is the largest organ inside your body, weighing about 1.5 kg. It carries out hundreds of jobs, many to do with processing what comes in from the gut.

Your body can't store protein. When you eat more amino acids than you need to build proteins, the liver breaks the spare ones down and uses part of each to make glucose for energy. The leftover part, containing nitrogen, becomes ammonia, which is very toxic. The liver quickly uses energy to convert ammonia into urea, which is much less toxic. Urea dissolves in the blood plasma and is carried to the kidneys.

The liver also breaks down alcohol, many medicines and worn-out red blood cells. Heavy drinking over years can scar the liver so badly it can no longer work, a disease called cirrhosis.

The kidneys and urinary system

You have two kidneys, each about 10 to 12 cm long and shaped like a bean, on either side of your spine just below your lowest ribs, towards your back.

• Blood full of urea arrives in the renal artery ("renal" means to do with the kidneys).
• The kidney filters the blood, and clean blood leaves in the renal vein.
• Urine flows down a ureter from each kidney to the bladder, a stretchy muscular bag that holds about 400 to 600 mL.
• When you go to the toilet, a ring of muscle relaxes and urine leaves through the urethra.

The filtering is done by about a million microscopic tubes in each kidney called nephrons. Each nephron works in two steps:

1. Filtering. At the start of each nephron, blood is squeezed under pressure through a tiny knot of capillaries. Water and small molecules (glucose, salts, urea) are forced out into the nephron. Blood cells and proteins are too big and stay in the blood.
2. Taking back what's useful. As the liquid flows along the nephron, all the glucose, most of the salts and most of the water are taken back into the blood. What's left (water, urea and extra salts) is urine.

Keeping the water balance

The kidneys don't just remove waste; they keep the amount of water and salt in your blood steady. Keeping internal conditions steady is called homeostasis. Your brain constantly checks how concentrated your blood is and releases a hormone that tells the kidneys how much water to take back.

• Drink lots of water: the kidneys take back less, so you make lots of pale, dilute urine.
• Sweat a lot or drink little: the kidneys take back more water, so you make a small amount of dark, concentrated urine, and you feel thirsty.
• Eat a salty meal: the kidneys need water to flush the extra salt out, so you feel thirsty.

Urine colour is a simple check you can do yourself: pale straw colour means you're well hydrated; dark yellow or brown means you need to drink more. Some foods and vitamins can change the colour too.

The lungs and skin

The lungs excrete carbon dioxide made by respiration, along with some water vapour, every time you breathe out.

The skin releases sweat from about 2 to 4 million sweat glands. Sweat is mostly water with some salt and a little urea, which is why it tastes salty. As sweat evaporates, it carries heat away from the body. On a hot day or during hard exercise you can lose more than a litre of sweat an hour, and you need to replace both the water and the salt.