Excretion: definition, organs and products
Excretion = removal of the waste products of metabolism (not egestion, which removes undigested faeces). Three human excretory organs: lungs (carbon dioxide + water vapour), kidneys (urea, water, ions) and skin (water and ions in sweat). CO2 comes from respiration; urea is made in the liver by deamination of excess amino acids, then carried to the kidneys. In flowering plants, CO2 and O2 are metabolic wastes lost through the stomata.
The nephron: ultrafiltration and reabsorption
Nephron order: Bowman's capsule (glomerulus) → PCT → loop of Henle → distal tubule → collecting duct. Ultrafiltration: high blood pressure forces small molecules (water, urea, glucose, ions) into the capsule; proteins and blood cells are too large. Selective reabsorption: all glucose returns at the PCT by active transport; water is reabsorbed along the tubule and collecting duct. Urea and excess water/ions form urine.
ADH and osmoregulation by negative feedback
Osmoregulation controls the blood's water content by negative feedback. If blood water potential falls: the hypothalamus detects it → pituitary releases more ADH → collecting duct becomes more permeable → more water reabsorbed by osmosis → small volume of concentrated urine. If water potential rises: less ADH → duct less permeable → less water reabsorbed → large volume of dilute urine. ADH = antidiuretic hormone, so more ADH = less urine.
Drawn from real examiner reports.
Excretion is not egestion
Excretion removes the waste products of metabolism — made by chemical reactions inside cells (CO2, urea, water). Egestion removes undigested food (faeces) at the anus; it was never absorbed into cells, so it is not a metabolic waste. "Getting rid of waste" is too vague and blurs the two. Sweat, urine and exhaled CO2 are excreted; faeces is egested.
CO2 is excreted by the lungs, not kidneys
The kidneys excrete urea, water and ions (salts) in urine. Carbon dioxide is excreted by the lungs — it diffuses from the blood into the alveoli and is breathed out. Naming CO2 as a kidney product is a repeatedly flagged error. Remember: urea is a nitrogenous waste from the liver, carried to the kidneys; CO2 is made by respiration and leaves at the lungs.
Nov 2024 Paper 2B Q1(a): a common error was naming carbon dioxide as a kidney excretory product; the correct products are urea, water and salts.
ADH increases collecting-duct permeability
The commonest ADH error is writing that ADH reduces the duct's permeability — it does the opposite. More ADH increases permeability to water, so more water is reabsorbed by osmosis, giving smaller, more concentrated urine. Chain: low blood water potential → hypothalamus → more ADH → duct more permeable → more water reabsorbed → less urine.
June 2024 Paper 2B Q3(b)(i): a common error was saying ADH reduces collecting-duct permeability.
Rehydration drink lowers water potential
A rehydration/sports drink adds water and solutes, so afterwards blood water potential ends up lower than with plain water. That lower water potential drives more ADH → more water reabsorbed → less urine. Weak answers only describe the volume difference; you must explain it via the added solutes, blood water potential and the ADH response.
June 2024 Paper 2B Q3(b)(ii): weak answers described the volume difference instead of explaining that the added solutes lower blood water potential, raising ADH.
Explain the pathway, do not describe urine
Two wording traps on ADH questions: (1) writing vague "water levels in the body" instead of the precise term blood water potential; (2) describing the urine when the command word is explain. You must give the pathway: hypothalamus → pituitary → ADH → collecting duct. Name each step; describing the outcome alone misses the explain marks.
June 2024 Paper 2B Q3(b)(i): vague "water levels in the body" and describing urine instead of explaining the hormone pathway both lost marks.
Nephron order: PCT comes before the loop
The nephron sequence is Bowman's capsule → proximal convoluted tubule → loop of Henle → distal convoluted tubule → collecting duct. The commonest labelling trap is placing the loop of Henle before the PCT. Also mark ultrafiltration at the Bowman's capsule and glucose reabsorption at the PCT, not at the collecting duct.
Glucose is reabsorbed by active transport
All filtered glucose is reabsorbed at the proximal convoluted tubule by active transport, not diffusion — glucose is too valuable to lose and must move against its concentration gradient using energy from respiration. Glucose in urine is abnormal: it signals diabetes, where blood glucose exceeds the reabsorption threshold.
Link process, site and substance
For kidney questions, link three things: the process (ultrafiltration, selective reabsorption, active transport), the site (Bowman's capsule, PCT, collecting duct) and the substance (water, glucose, urea). A process named without its site and substance loses marks.
Write the whole ADH chain
Write the whole ADH chain, skipping no link: low blood water potential → hypothalamus detects → pituitary releases more ADH → collecting duct more permeable → more water reabsorbed by osmosis → small, concentrated urine. Each missing link costs a mark.
Match the command word; manipulate data
Watch the command word: describe wants what happens; explain wants the reason. With kidney data, manipulate the figures (give a change and its direction), don't just quote them. Answering the wrong verb — e.g. describing urine when asked to explain — loses marks.
Excretion is the removal of the waste products of metabolism from the body.
This is the precise, two-element mark-scheme definition. Do NOT say "getting rid of waste" — that is too vague and earns no credit.
| Term | Mark-scheme definition |
|---|---|
| Excretion | Removal of metabolic waste products produced by chemical reactions inside cells |
| Egestion | Removal of undigested food (faeces) from the gut — NOT a metabolic product; material was never absorbed |
These two are a key exam trap — see the "Don't mix these up" section below.
| Organ | Product(s) excreted |
|---|---|
| Lungs | Carbon dioxide () and water vapour |
| Kidneys | Urea, water, and ions (salts) |
| Skin | Water and ions (in sweat) |
Origin of urea: excess amino acids cannot be stored. In the liver, the amino group is removed from surplus amino acids by a process called deamination. This produces urea, which enters the blood and is carried to the kidneys for excretion in urine.
Plants: (from respiration) and (oxygen, from photosynthesis) are metabolic waste products lost through the stomata of leaves. Water vapour is also lost through stomata (transpiration).
Define excretion.
(a) Define excretion. (2 marks)
(b) Explain why the removal of faeces is not an example of excretion. (2 marks)
