Excretion vs egestion: the definition that recurs

Excretion is the removal of the waste products of metabolism and substances in excess of requirements. The key examples: carbon dioxide (from respiration, excreted by the lungs) and urea (from the breakdown of excess amino acids, excreted by the kidneys in urine). Egestion, the removal of undigested food (faeces) from the gut, is NOT excretion, because faeces were never made by the body's metabolism. Calling faeces 'excretion' is a guaranteed lost mark and signals a basic misunderstanding to the examiner.

Where urea comes from: the liver's role

The chain examiners want: excess amino acids cannot be stored; in the liver they are broken down by deamination; this removes the amino group and forms urea; urea is carried in the blood to the kidneys and removed in urine. The key terms are 'liver', 'deamination', 'excess amino acids' and 'urea'. A common slip is saying the kidney makes urea. The kidney removes urea; the liver makes it.

The kidney and nephron: structure and the two processes

Blood enters the kidney and is processed in tiny tubules called nephrons. Two processes carry nearly all the marks:

  • Ultrafiltration: at the glomerulus, high blood pressure forces small molecules (water, glucose, ions, urea) out of the blood into the capsule. Large molecules (proteins) and blood cells stay in the blood because they are too big to pass through. The driving force is pressure.
  • Selective reabsorption: as the filtrate flows along the tubule, useful substances are reabsorbed back into the blood: all the glucose (by active transport), much of the water and some ions. Urea is not reabsorbed and continues to the bladder as urine.

The word 'selective' is the point: the kidney chooses what to take back. 'All the glucose is reabsorbed' is a standing mark, which is why glucose in urine indicates a problem (diabetes), a favourite application question.

Water balance and the bigger picture

The kidney also adjusts how much water is reabsorbed to control the water content of the blood. The link to homeostasis. When you are dehydrated, more water is reabsorbed, so urine is more concentrated and smaller in volume; when you drink a lot, less is reabsorbed, so urine is dilute and larger in volume. Questions phrase this as 'explain why urine is more concentrated on a hot day'. Answer via water lost in sweat, less water in blood, more water reabsorbed in the kidney. Excretion ties to several topics: CO2 excretion connects to gas exchange, and urea connects to nutrition (excess protein), so cross-topic questions are common.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

  • Bowman's capsule: A cup-shaped structure at the start of a nephron that surrounds the glomerulus. It collects the filtrate produced by ultrafiltration: a liquid containing water, glucose, amino acids, urea and mineral ions, but not blood cells or large proteins.
  • Collecting duct: The final part of the nephron where the permeability of the walls is controlled by ADH (antidiuretic hormone). When the body is dehydrated, ADH increases the permeability of the collecting duct walls, allowing more water to be reabsorbed back into the blood, producing concentrated urine.
  • Deamination: The removal of the nitrogen-containing part of amino acids to form urea, which takes place in the liver.
  • Excretion: The removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements.
  • Glomerulus: A tight knot of blood capillaries inside the Bowman's capsule of a nephron in the kidney. Blood enters under high pressure through the afferent arteriole. Small molecules (water, glucose, urea, ions) are forced out of the blood into the Bowman's capsule by ultrafiltration.
  • Nephron: The functional unit of the kidney responsible for filtering blood and forming urine. Each nephron consists of a Bowman's capsule containing a glomerulus, followed by the proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.
  • Selective reabsorption: The process in the kidney tubules where useful substances from the filtrate (all glucose, most water, some ions and amino acids) are reabsorbed back into the blood. Glucose and amino acids are reabsorbed by active transport in the proximal convoluted tubule, while water is reabsorbed by osmosis.
  • Ultrafiltration: The process in the kidney where blood is filtered under high pressure in the glomerulus. Small molecules (water, glucose, urea, amino acids, ions) are forced through the capillary walls and Bowman's capsule into the nephron, while large molecules (proteins) and blood cells are retained in the blood.
  • Urea: A nitrogen-containing waste substance produced in the liver by the deamination of excess amino acids. Urea is transported in the blood plasma to the kidneys, where it is filtered out and excreted in urine.

Browse the full IGCSE Biology glossary when a related term needs checking.