Excretion in Humans
Senior 2 · Biology
Excretion in Humans
Lesson 1 of 4
20 min
Let's begin
Every second, the chemical reactions in your cells produce waste. Some of it, such as carbon dioxide and urea, is poisonous if it builds up. A person whose kidneys stop working will die within days or weeks unless a machine cleans their blood. Getting rid of these wastes is called excretion, and several organs share the job.
Worked example
Is passing faeces an example of excretion? Is passing urine?
Excretion is the removal from the body of the waste products of metabolism (all the chemical reactions in cells), together with substances present in excess, such as extra water and salts.
| Organ | What it excretes |
|---|---|
| Lungs | carbon dioxide and water vapour from respiration, in exhaled air |
| Kidneys | urea, excess water and excess salts, as urine |
| Skin | sweat: water, salts and a little urea (its main job is cooling the body) |
| Liver | makes urea; excretes bile pigments from broken-down red blood cells, which leave the body in the faeces and give them their colour |
Proteins are digested into amino acids. The body uses them to build its own proteins, but it cannot store extra amino acids. The liver deals with the excess by deamination:
The liver also:
Worked example
After several days of meals rich in beans, eggs and meat, the amount of urea in a student's urine rises. Explain. More protein is digested, so more amino acids are absorbed than the body needs. The liver deaminates the excess, producing more urea, which the kidneys excrete.
Fish excrete ammonia straight into the water around them, which dilutes it. Mammals make urea, which is safer to carry in the blood. Birds, reptiles and insects make uric acid, a white paste that needs very little water to remove, a big advantage for animals that must save water.
Tip
Excretion = waste made by cells. Egestion = food that was never absorbed.
The liver makes urea; the kidneys remove it. "Urea is made in the kidneys" is a common wrong answer.
Define excretion.
Solution: The removal from the body of the waste products of metabolism and of substances in excess of requirements.
Name the waste excreted by the lungs and the process that produces it.
Solution: Carbon dioxide (with water vapour), produced by respiration in the body's cells.
Why does the liver convert ammonia to urea?
Solution: Ammonia is very toxic. Urea is much less toxic, so it can be carried safely in the blood to the kidneys.
Explain why removing bile pigments counts as excretion even though they leave the body in the faeces.
Solution: Bile pigments are made by the body, when the liver breaks down haemoglobin from old red blood cells. They are metabolic waste, unlike the undigested food in faeces.
Remember
Sources and credits
