By the end of this lesson
- Name the excretory products of lungs, kidneys and skin.
- Distinguish excretion from egestion and secretion.
- Relate each organ to the origin of its wastes.
BIG QUESTION
Why does the body need three different organs to remove metabolic wastes?
Different wastes are produced in different places and require gas exchange, filtration or secretion for removal.
Excretion removes metabolic waste
excretionRemoval from the body of waste products of metabolism, such as carbon dioxide and urea, and of substances in excess of requirements. is removal of toxic materials, excess substances and waste products of metabolism. It is not the same as egestionRemoval of undigested food from the gut as faeces; it is not excretion because the material was never part of metabolism in cells., which removes undigested food from the gut.
A substance can be useful at one concentration and excreted when present in excess.
Lungs remove respiratory products
Carbon dioxide made by respiring cells travels in blood to the lungs and diffuses into alveoli. Exhaled air also removes water vapour.
Carbon dioxide must be regulated because its accumulation alters blood pH.
Kidneys remove dissolved wastes
Kidneys excrete ureaA nitrogen-containing waste product made in the liver from excess amino acids and removed by the kidneys in urine., excess water and excess mineral ions in urine. Urea is made in the liver after deamination of excess amino acids.
Kidneys also regulate water and ion balance, so they combine excretion with homeostasis.
Skin contributes through sweat
Sweat contains water and mineral ions, with small amounts of urea. Its major physiological role is cooling, but these substances are still lost from the body.
The skin is not the principal route for urea removal.
Human excretory organs
Match each organ to its products and route.
| Organ | Main excretory products | Route |
|---|---|---|
| Lungs | Carbon dioxide and water vapour. | Exhaled air. |
| Kidneys | Urea, excess water and ions. | Urine. |
| Skin | Water, ions and a little urea. | Sweat. |
| Liver | Makes urea and detoxifies substances. | Products enter blood for kidney removal. |
Language that earns precision
- excretion
- Removal of metabolic wastes, toxic materials and excess substances.
- egestion
- Removal of undigested material from the alimentary canal.
- deamination
- Removal of the amino group from excess amino acids in the liver.
- urea
- Nitrogenous waste made in the liver and excreted mainly by kidneys.
Common misconceptions
Claim: Faeces are an excretory product.
Correction: Most faecal material was never absorbed; its removal is egestion.
Claim: Kidneys make urea.
Correction: The liver makes urea; kidneys remove it from blood.
Claim: Sweating is mainly for removing urea.
Correction: Its main role is thermoregulation; only a small amount of urea is lost.
Match the answer to the command word
Give the organ and its correctly matched product.
Use metabolism in the definition of excretion.
Recall, apply, then justify
State the excretory products of the lungs, kidneys and skin.
Hint
One matched set per organ.
Suggested marking points
- Lungs remove carbon dioxide and water vapour.
- Kidneys remove urea, excess water and excess ions.
- Skin loses water, ions and small amounts of urea in sweat.
Explain why removing faeces is not usually described as excretion.
Hint
Was the material made by cells?
Suggested marking points
- Most faecal material is undigested/unabsorbed food rather than metabolic waste.
- Its removal is egestion.
Explain how failure of the liver can alter the excretory load presented to the kidneys.
Planning hint
The organs act in sequence.
What a strong design includes
- Excess amino acids are normally deaminated in the liver.
- Nitrogen is converted to urea.
- Urea enters blood and is filtered by kidneys.
- Liver failure can reduce normal urea formation and allow toxic nitrogen compounds to accumulate.
- Kidneys cannot replace all liver detoxification functions.
- Excretory-organ functions are therefore interdependent.