13.1 Excretion
Waste products are made by the chemical reactions in cells. If they built up, they would damage the body.
| Waste product | Where it is made | How it is excreted |
|---|---|---|
| carbon dioxide | a waste product of respiration | through the lungs (breathed out) |
| urea | made in the liver from the breakdown of excess amino acids | by the kidneys, in urine |
| water and salts (excess) | from the diet and from metabolism | by the kidneys in urine (and some in sweat) |
Urea is toxic, which is why it must be excreted: if it built up, it would be poisonous to cells.
13.2 Urinary system
| Organ | Function |
|---|---|
| kidneys | remove urea, excess salts and excess water from the blood, to form urine |
| ureters | tubes that carry urine from the kidneys to the bladder |
| bladder | stores urine |
| urethra | carries urine out of the body |
The nephron
Each kidney contains about a million tiny filtering units called nephrons.
| Part | Function |
|---|---|
| glomerulus | a knot of capillaries; blood is under high pressure, so small molecules are filtered out |
| Bowman's capsule | a cup around the glomerulus that collects the filtrate |
| tubule (and loop of Henle) | where useful substances are reabsorbed into the blood |
| collecting duct | collects urine from the tubules and carries it to the ureter |
What the nephron does
- Filtration. In the glomerulus, water, glucose, urea and ions are filtered out of the blood into the Bowman's capsule. Blood cells and large proteins are too big and stay in the blood.
- Reabsorption. As the filtrate passes along the tubule, all of the glucose, some of the ions and most of the water are taken back into the blood.
- Urine formation. What is left is urine: urea, excess water and excess ions. It flows down the collecting duct to the ureter.
(The details of how these processes happen are not needed.) The amount of water and ions in the urine varies, so the kidneys help keep the blood's composition constant.
All the glucose is reabsorbed into the blood from the tubule. (If glucose appears in the urine, the person may have diabetes.)
The liver and the breakdown of amino acids
The liver is involved in the assimilation of amino acids: it converts amino acids into proteins, such as plasma proteins.
Amino acids cannot be stored, so any that are in excess are broken down in the liver by deamination.
The urea is carried in the blood plasma to the kidneys, where it is excreted in urine.
Practise this chapter
Every past-paper question on these topics, with the mark scheme.