13.1 Excretion in humans
- Syllabus
- 0610–2026–2027
- Topic
- 13.1
- Level
- —
Learning objectives
13.1.1Carbon dioxide is excreted through• State that carbon dioxide is excreted through the lungs13.1.2Kidneys excrete urea and excess water• State that the kidneys excrete urea and excess water and ions13.1.3Kidneys, ureters, bladder and urethra• Identify in diagrams and images the kidneys, ureters, bladder and urethra13.1.4Structure of the kidney• Identify in diagrams and images the structure of the kidney, limited to the cortex and medulla13.1.5Structure and function of a nephron• Outline the structure and function of a nephron and its associated blood vessels, limited to: (a) the role of the glomerulus in the filtration from the blood of water, glucose, urea and ions (b) the role of the nephron in the reabsorption of all of the glucose, some of the ions and most of the water back into the blood (c) the formation of urine containing urea, excess water and excess ions (details of these processes are not required)13.1.6Role of the liver in the assimilation• Describe the role of the liver in the assimilation of amino acids by converting them to proteins13.1.7Urea is formed in the liver• State that urea is formed in the liver from excess amino acids13.1.8Deamination as the removal• Describe deamination as the removal of the nitrogen-containing part of amino acids to form urea13.1.9Importance of excretion• Explain the importance of excretion, limited to toxicity of urea
State how carbon dioxide is excreted
Carbon dioxide made by respiration in cells is transported in the blood to the lungs and excreted in expired air.
cells → blood → lungs → exhaled air
Excretion removes a metabolic waste; carbon dioxide leaving the lungs is not egestion of undigested food.
State what the kidneys excrete
The kidneys excrete urea, excess water and excess ions from the blood as urine.
| Excreted substance | Why it is removed |
|---|---|
| urea | toxic nitrogen-containing metabolic waste |
| excess water | more than the body requires |
| excess ions | more mineral ions than required |
Kidneys excrete substances in urine; the bladder stores urine but does not make or filter it.
Identify the organs of the urinary system
Urine follows an ordered route through four named structures.
| Structure | Identification and role |
|---|---|
| kidneys | paired bean-shaped organs that form urine |
| ureters | two tubes carrying urine from kidneys to bladder |
| bladder | muscular sac that stores urine |
| urethra | single tube carrying urine from bladder out of the body |
A ureter links a kidney to the bladder; the urethra leaves the bladder and carries urine outside.
Identify the cortex and medulla of a kidney
A kidney section has an outer cortex surrounding an inner medulla.
| Region | Position in a section |
|---|---|
| cortex | outer layer beneath the kidney surface |
| medulla | inner region, often shown as darker pyramidal areas |
The ureter leaves the kidney but is not part of the cortex or medulla.
Outline how a nephron forms urine
A nephron filters small substances from blood, reabsorbs required substances and leaves wastes and excess substances in urine.
| Stage | Required substance movement |
|---|---|
| filtration at glomerulus | water, glucose, urea and ions pass from blood into the nephron |
| reabsorption to blood | all glucose, some ions and most water return to associated blood vessels |
| urine formation | urea, excess water and excess ions remain and flow towards the ureter |
Blood cells and large proteins remain in blood; detailed pressure, membrane and transport mechanisms are not required by this objective.
Describe amino acid assimilation in the liver
Assimilation of amino acids is their conversion into proteins that become part of the body.
The liver takes absorbed amino acids from the blood and uses them to synthesise proteins needed by the body.
Assimilation builds amino acids into proteins; deamination removes the nitrogen-containing part of excess amino acids.
State where urea is formed
Urea is formed in the liver from excess amino acids.
excess amino acids → liver forms urea → blood transports urea → kidneys excrete it in urine
The liver forms urea; the kidneys remove urea from blood and excrete it.
Describe deamination
Deamination is the removal of the nitrogen-containing part of excess amino acids to form urea.
Deamination occurs in the liver because excess amino acids cannot be stored as amino acids.
Deamination removes the nitrogen-containing part, not the carbon-containing part, and it is different from digestion of protein.
Explain why urea must be excreted
Urea is a toxic metabolic waste, so allowing it to accumulate would damage cells and disrupt body function.
The blood transports urea to the kidneys, which remove it and excrete it in urine, keeping its concentration below harmful levels.
Urea is harmful because it is toxic; it is not removed simply because it is undigested or because all nitrogen compounds are useless.