Lesson 1 of 5 / Excretion in humans
Excretion: removing metabolic waste
Why is removing urea excretion, while removing undigested food is egestion?
In this lesson: Define excretion and explain why metabolic wastes and excess substances must be removed.
About 4 min
The key ideaExcretion removes metabolic wastes, toxic materials and substances in excess of the body's requirements.
A route model. Ask where the material came from before classifying its removal.
Explanation
Metabolism means the chemical reactions occurring in living cells. These reactions produce wastes, and normal body fluids can also contain more water or ions than required. Excretion removes metabolic waste, toxic materials and excess substances from the body.
Excess amino acids cannot be stored as a separate reserve in the same way as glycogen. In the liver, their nitrogen-containing groups are removed during deamination. Toxic ammonia is converted into the less toxic compound urea, which travels dissolved in blood plasma to the kidneys.
Urea must not be allowed to accumulate indefinitely. Nitrogenous waste accumulation and disturbances to ion and water levels interfere with normal cell function. Removing excess water and ions helps keep the conditions around cells suitable for life.
Carbon dioxide is produced during aerobic respiration in cells. Blood carries it to the lungs, where it diffuses into alveoli and is exhaled. Excess carbon dioxide would change the chemical conditions of body fluids, including their acidity.
Egestion is different: undigested material passes out through the anus without being a metabolic waste produced by body cells. The fact that a material leaves the body does not automatically make its removal excretion.
Step by step
- 1
Find the source
Was the substance produced by cell metabolism, present in excess, or simply left undigested in the gut?
- 2
Name the removal route
For example, urea leaves mainly in urine; carbon dioxide leaves through the lungs.
- 3
Explain the need
Link removal to preventing harmful accumulation or disruption of internal conditions.
Worked example
Liver and kidney: different jobs
Explain why both the liver and kidneys are involved in removing nitrogen from excess amino acids.
One way to explain it
The liver removes the nitrogen-containing part of excess amino acids and converts toxic ammonia into urea. Blood plasma transports the urea to the kidneys, which remove it from the body in urine.
Why this answer works
- Formation of urea and removal of urea happen at different organs.
- Include transport through plasma between the two organs.
- Do not describe the kidneys as the main site of urea production.
Is this true? "The kidney makes urea from undigested food."
The liver forms urea during amino-acid metabolism. Kidneys remove it from blood. Undigested food is removed by egestion.