Kidney structure and function

The kidneys filter the blood, removing urea and excess substances to form urine, while also regulating the water and salt balance of the body.

Parts and functions

How structure suits function

Each kidney contains about a million nephrons working side by side, giving a huge total surface area for filtering blood and processing urine efficiently. In each nephron, the glomerulus is a tightly coiled knot of capillaries under high pressure, which forces small molecules such as water, glucose, salts and urea out of the blood into the Bowman's capsule, while larger molecules like proteins and blood cells stay in the blood.

As the filtrate flows along the convoluted tubules, useful substances such as glucose are selectively reabsorbed back into the blood, and the long loop of Henle running deep into the medulla sets up a concentration gradient that allows the kidney to reabsorb variable amounts of water, fine-tuning how dilute or concentrated the urine becomes for osmoregulation.

• About a million nephrons per kidney provide an enormous total surface area for filtering blood and reabsorbing useful substances.
• The glomerulus is fed by a wide afferent arteriole and drained by a narrow efferent arteriole, which raises the blood pressure inside it and drives ultrafiltration.
• The proximal tubule is lined with cells whose surface is folded into microvilli and densely packed with mitochondria, giving both a large surface area and the ATP needed for selective reabsorption by active transport.
• The loop of Henle dips deep into the medulla and builds up a high salt concentration there, so water can be drawn out of the filtrate by osmosis.
• The collecting duct runs back through this salty medulla, so the amount of water reabsorbed can be adjusted under the control of the hormone ADH to make urine more dilute or more concentrated.

Related processes

Three processes work in sequence inside the kidney. Ultrafiltration at the glomerulus and Bowman's capsule uses pressure to separate small molecules, water, glucose, salts and urea, from the blood.

Selective reabsorption along the tubules returns glucose, useful salts and much of the water to the blood so they are not lost. Osmoregulation then adjusts the final water content of the urine to match how much water the body has to spare.

Excretion is completed when the remaining fluid, mainly urea and surplus water, leaves the kidney as urine and passes down the ureter.

Common labelling errors

Worked exam-style question

Question. The diagram shows a vertical section through a human kidney. Region X forms the outer layer, region Y lies beneath it, and Z is the funnel-shaped space where urine collects.

(a) Name regions X, Y and Z. (b) State where most Bowman's capsules are located.

(c) Blood entering the kidney contains both urea and glucose, yet normal urine contains urea but no glucose. Explain this difference.

(d) On a hot day a person sweats heavily and drinks little water. Explain how the kidney responds.

Model answer. (a) X is the cortex, Y is the medulla and Z is the renal pelvis. (b) Most Bowman's capsules are found in the cortex.

(c) At the glomerulus both urea and glucose are small enough to be forced out of the blood during ultrafiltration, so both enter the filtrate; as the filtrate flows along the tubule all the glucose is reabsorbed back into the blood by selective reabsorption using active transport, while urea is not reabsorbed and stays in the filtrate, so urine contains urea but no glucose. (d) Heavy sweating removes water, so the blood becomes more concentrated and its water potential falls; the collecting duct becomes more permeable to water under the control of ADH, so more water is reabsorbed by osmosis and a smaller volume of concentrated urine is produced, conserving water.

Source:SRC-DSKP-EN

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