Excretion — removing metabolic waste
Excretion is the removal of the waste products of metabolism (the chemical reactions in cells that release energy) and of substances in excess of requirements. Two organs are named in this topic: the lungs, which excrete carbon dioxide () from aerobic respiration, and the kidneys, which excrete urea dissolved in urine together with excess water and ions. The defining test is that the substance was made inside the body's own cells.
Urea — made in the liver, excreted by the kidneys
Urea is produced in the liver and removed by the kidneys — two organs, two different jobs. Amino acids cannot be stored (unlike glucose as glycogen, or fat in adipose tissue), so any eaten in excess of the body's needs for growth and repair are broken down by the liver into urea. The kidneys then filter that urea out of the blood and pass it out dissolved in urine. Urea is toxic at high concentration, so this removal runs continuously.
The nephron — filter first, then reabsorb (Extended)
Ultrafiltration at the glomerulus forces small molecules — water, glucose, urea, ions — from the blood into the Bowman's capsule as the filtrate; proteins and blood cells are too big and stay behind. Selective reabsorption in the tubule then returns ALL the glucose, most water and some ions to the blood. What is left — urea, excess water, excess ions — becomes urine. Urea comes from deamination in the liver: amino group removed → ammonia → urea.
Drawn from real examiner reports.
Urea is made in the liver, not the kidneys
The kidneys excrete urea; they never make it. Urea is produced in the liver, by deamination of the amino acids the body cannot store. Treat "where is urea made?" and "which organ excretes urea?" as two separate questions with two different answers — examiners record candidates giving "the kidneys" to both in every recent sitting.
Persistent across all three sittings: s23 Paper 23 Q23; w22 Papers 13/32/33 Q25; s22 Papers 23/33/42 Q23; s22 Paper 33 Q4b i/ii.
Excretion ≠ egestion
Excretion removes waste that the body's own metabolism produced — from respiration, urea from broken-down amino acids. Egestion removes undigested food as faeces at the anus; that food was never absorbed into cells, so it is not a metabolic waste. Faeces is egested, never excreted, and the anus is not an excretory organ.
Flagged s22 P31 Q6a; P32 Q6a · s23 P11/12 Q15; P32 Q4c ii
Ureter ≠ urethra — know which is which
The ureter (one per kidney) carries urine from the kidney down to the bladder. The urethra is the single tube carrying urine from the bladder to outside the body. Examiners report the two being swapped on labelling questions, and correct spelling is required — a spelling that could be read as the other word earns nothing.
Ureter/urethra confusion: s23 Paper 33 Key messages; w22 Paper 32 Q5d.
Glomerulus must be spelled correctly
Phonetic attempts such as "glomerulous" or "glomerulis" are sometimes too ambiguous for examiners to credit on a labelling question. Learn the exact spelling — glomerulus — and the same for Bowman's capsule. A label the examiner cannot read unambiguously scores nothing, however well the biology behind it is understood.
Glomerulus spelling: s22 Paper 42 Q2a i.
Filtration ≠ reabsorption
Filtration happens at the glomerulus, into the Bowman's capsule: small molecules are forced out of the blood non-selectively, under pressure. Reabsorption happens later, in the tubule: useful substances are taken back into the blood selectively. Placing reabsorption at the glomerulus, or filtration in the tubule, reverses the pair examiners flag most.
Filtration/reabsorption muddled: s22 Paper 42 Q2b.
Urine is not urea alone
Asked what urine contains, candidates name urea and stop. Examiners expect urea, excess water and excess ions (salts) — everything filtered out of the blood but not reabsorbed in the tubule. Healthy urine contains no glucose and no protein: the glucose is all reabsorbed, and protein was too large to be filtered at all.
Urine composition: candidates named only urea; s22 Paper 42 Q2b.
Answer nephron questions in two stages
Stage 1 — filtration at the glomerulus/Bowman's capsule: say what leaves the blood (water, glucose, urea, ions), what stays (proteins, cells), and name it the filtrate. Stage 2 — selective reabsorption in the tubule: say what returns to the blood and what flows on as urine.
Watch the verb: produces vs excretes
The liver produces urea; the kidneys excrete it. Swapping the verbs loses the mark even when both organs are named correctly. The same discipline applies to filtration and reabsorption — never use one term for the other process.
Name deamination, do not just describe it
Examiners record candidates describing the process correctly but omitting the term itself. Write "deamination", then give the sequence in order: amino group removed from excess amino acids → ammonia (toxic) → converted to urea, all in the liver.
Glucose in urine is a diagnostic clue
A healthy tubule reabsorbs 100% of the filtered glucose, so healthy urine contains none. If data in a question shows glucose in the urine, the expected answer is that reabsorption is incomplete — a sign of a condition such as diabetes, not of normal kidney function.
Excretion is the removal of the waste products of metabolism (chemical reactions in cells that release energy) and of substances in excess of requirements, from the body.
Excretion vs egestion — do not confuse these:
| Excretion | Egestion | |
|---|---|---|
| What is removed | Waste products of metabolism (e.g. , urea) | Undigested food (faeces) |
| Where from | Made inside the body's own cells | Food that was never absorbed into the body |
| Organs involved | Lungs, kidneys | Anus (end of the gut) |
The two named excretory organs (Core):
| Organ | Waste excreted | Where the waste comes from |
|---|---|---|
| Lungs | Carbon dioxide () | Product of aerobic respiration in all body cells |
| Kidneys | Urea (in urine, with excess water and ions) | Made in the liver from excess amino acids |
State the definition of excretion.
(a) State the definition of excretion. [2]
(b) Name TWO organs of excretion in the human body and state the waste product removed by each. [4]
(c) State ONE difference between excretion and egestion. [1]
