The Nephron: Filtration, Selective Reabsorption and Urine
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
High pressure filters small molecules from the glomerulus into Bowman’s capsule. Useful substances are selectively reabsorbed into blood, all glucose normally returns, while excess water, salts and urea remain to form urine.
The nephron is difficult only when students memorise isolated labels. Follow substances through three questions: what is small enough to filter, what the body needs back, and what remains as waste or excess.
Filtration at the glomerulus
Blood enters the glomerulus at high pressure. Water and small dissolved substances, including glucose, mineral ions and urea, pass through the filtration barrier into Bowman’s capsule. Blood cells and large plasma proteins remain in the blood because they are too large.
Filtration is not the same as active transport and does not selectively choose only waste molecules.
Selective reabsorption
Useful substances move from the nephron back into surrounding capillaries. All glucose is normally reabsorbed, along with required mineral ions and much of the water. Active transport contributes to glucose and ion reabsorption; water follows by osmosis.
“Selective” means the amount reabsorbed depends on what the body needs, not that the original filtration included only selected substances.
What becomes urine
Urine contains urea, excess water and excess mineral ions. It should not normally contain blood cells, large proteins or glucose. The collecting ducts carry urine toward the renal pelvis and ureter.
Do not confuse urea with urine. Urea is a nitrogenous waste molecule made in the liver; urine is the liquid containing urea and other substances.
Dialysis as an application
Dialysis removes urea and balances water and ions when kidneys fail. Blood flows beside dialysis fluid across a partially permeable membrane. The fluid has normal glucose and ion concentrations so useful substances are not lost by a steep gradient, while urea diffuses out.
Try a focused question (4 marks)
Explain why glucose is present in the filtrate in Bowman’s capsule but is normally absent from urine.
Show the answer and marking guidance
Glucose molecules are small enough to pass from the glomerulus into Bowman’s capsule during filtration.
Glucose is useful for respiration.
It is selectively reabsorbed from the nephron into the blood.
Normally all filtered glucose is reabsorbed, so none remains in urine.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying only waste substances are filtered.
Claiming proteins pass into Bowman’s capsule in a healthy kidney.