The nephron

There are about a million nephrons in each kidney. Together they filter the blood, reabsorb what the body needs and form urine from the rest.

Parts and functions

How structure suits function

The glomerulus has high pressure so that ultrafiltration forces water, glucose, salts and urea out of the blood. As the filtrate flows along the tubule, glucose, useful salts and much of the water are reabsorbed, leaving urea and excess water as urine.

• The glomerulus is fed by a wide afferent arteriole and drained by a narrow efferent arteriole, which raises the blood pressure inside it so ultrafiltration can push small molecules out of the blood.
• The capillary wall of the glomerulus and the wall of the Bowman's capsule are both very thin with tiny pores, letting water, glucose, salts and urea through while holding back proteins and blood cells.
• The proximal convoluted tubule is lined with cells that have microvilli and are densely packed with mitochondria, giving a large surface area and the ATP needed for selective reabsorption by active transport.
• The loop of Henle dips deep into the medulla and sets up a high salt concentration there, so water can be reabsorbed from the tubule and collecting duct by osmosis.
• The tubules are wrapped in a dense network of capillaries, so reabsorbed glucose, salts and water pass quickly back into the blood, and the collecting duct's permeability to water is adjusted by the hormone ADH.

Related processes

Three processes turn blood into urine as it passes through the nephron. Ultrafiltration at the glomerulus and Bowman's capsule uses high pressure to push small molecules, water, glucose, salts and urea, out of the blood and into the tubule, while proteins and blood cells are too large to pass.

Selective reabsorption along the tubules returns all the glucose, useful salts and most of the water to the blood, much of it by active transport. Osmoregulation then adjusts how much water is reabsorbed from the collecting duct, so the body can make dilute or concentrated urine as needed.

What is left, mainly urea and surplus water, flows on as urine.

Common labelling errors

Worked exam-style question

Question. A table gives the concentration of three substances in three fluids of a healthy person: blood plasma, the glomerular filtrate in the Bowman's capsule, and urine. Protein is present in the plasma but absent from both the filtrate and the urine.

Glucose is present in the plasma and the filtrate but absent from the urine. Urea is present in all three and is most concentrated in the urine.

(a) Explain why protein is absent from the filtrate. (b) Explain why glucose is present in the filtrate but absent from normal urine.

(c) Explain why urea is more concentrated in the urine than in the filtrate. (d) Name the part of the nephron where ultrafiltration takes place.

Model answer. (a) Protein molecules are too large to pass through the pores of the glomerulus and Bowman's capsule during ultrafiltration, so they stay in the blood and do not enter the filtrate. (b) Glucose is small, so it is filtered into the Bowman's capsule, but as the filtrate flows along the tubule all the glucose is reabsorbed back into the blood by selective reabsorption using active transport, so none is left in normal urine.

(c) As the filtrate passes along the tubule and collecting duct, water is reabsorbed back into the blood by osmosis but urea is not reabsorbed, so the urea becomes more concentrated in the smaller volume of fluid that remains. (d) Ultrafiltration takes place at the glomerulus and Bowman's capsule.

Source:SRC-DSKP-EN

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