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 paying words 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.