Excretion is the removal of waste products of metabolism and of toxic materials from the body. Egestion is the removal of undigested food from the gut. Both take something out of the body, which is why students mix them up.
This skill is tested as a definition, as a sorting task, and as a reason inside longer answers. It opens excretion and homeostasis and feeds every later lesson in the topic.
What decides which word to use?
Ask one question: was this substance ever made or used inside body cells?
If yes, it is a product of metabolism, and removing it is excretion. Carbon dioxide from respiration and urea from the breakdown of excess amino acids both come from cell chemistry.
If no, the substance only travelled through the gut. Undigested fibre was never absorbed into the blood, so pushing it out as faeces is egestion.
Which organs excrete what?
| Organ | Substance removed | Where it comes from |
|---|---|---|
| Lungs | carbon dioxide (and water vapour) | respiration in cells |
| Kidneys | urea, excess water and salts | urea is made in the liver from excess amino acids |
| Skin | water, salts and a little urea in sweat | blood plasma |
The large intestine egests faeces. It is not on the excretion list in this table because nothing here is a waste of cell metabolism.
Worked example
Sort these into excretion, egestion, or neither: carbon dioxide leaving the lungs, cellulose fibre leaving the anus, urea in urine, glucose absorbed in the small intestine.
Step 1: carbon dioxide leaving the lungs. It comes from respiration in cells. This is excretion.
Step 2: cellulose fibre leaving the anus. Humans cannot digest it, so it never entered cells. This is egestion.
Step 3: urea in urine. It is made in the liver from excess amino acids and removed by the kidneys. This is excretion.
Step 4: glucose absorbed in the small intestine. Nothing is leaving the body. This is neither. It is absorption.
The test question each time was: did it come from cell metabolism, and is it leaving the body?
The mistake to watch for
The common error is to write that faeces is excreted because it is also “waste”.
Mistaken answer: “Excretion is the removal of waste, for example faeces.”
The student noticed that faeces is waste but ignored where it came from.
The correction is to add the source. Write “waste products of metabolism” for excretion and “undigested food” for egestion. Those two phrases are the marks.
Check yourself
Try these, then open each answer.
1. Write a one-sentence definition of egestion.
Show answer
Egestion is the removal of undigested food from the gut, as faeces, through the anus.
2. Name two different organs that carry out excretion and one substance each removes.
Show answer
For example, the lungs remove carbon dioxide, and the kidneys remove urea. The skin, removing a little urea and salts in sweat, is also acceptable.
3. A student says “the kidneys make urea”. Correct the statement.
Show answer
The liver makes urea from excess amino acids. The kidneys filter the blood and remove the urea in urine.
Where this leads next
Now that the vocabulary is fixed, move to filtration and selective reabsorption in the kidney. Some students can recite the definitions but still mix them in full-sentence answers, and our teachers look for that pattern in online one-to-one Biology tuition.