How do the kidneys clean the blood and control its water content, and how is kidney failure treated?
The role of the kidney in homeostasis and excretion, the structure of the kidney and nephron, ultrafiltration and selective reabsorption, the control of water balance by ADH, and the treatment of kidney failure by dialysis and transplant.
A focused answer to the WJEC GCSE Biology section 2.6 topic on the kidney, covering excretion of urea, the structure of the kidney and nephron, ultrafiltration and selective reabsorption, the control of water balance by ADH, and the treatment of kidney failure by dialysis and transplant.
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What this dot point is asking
WJEC wants you to know the role of the kidney in homeostasis and excretion, the structure of the kidney and nephron, how urine is made by ultrafiltration and selective reabsorption, how ADH controls water balance, and how kidney failure is treated.
The role of the kidney
The kidneys are excretory organs that help to maintain homeostasis. They:
- Remove urea, a toxic waste made in the liver from the breakdown of excess amino acids.
- Control the water content and ion (salt) content of the blood.
The cleaned blood leaves the kidney, and the waste liquid (urine) passes down the ureter to the bladder.
The structure of the kidney and nephron
A kidney has three regions: the outer cortex, the inner medulla, and the central pelvis, which collects urine and leads to the ureter.
The functional unit of the kidney is the nephron, a tiny tubule. Its key parts are:
- The glomerulus, a knot of capillaries where filtering happens.
- The Bowman's capsule, the cup that surrounds the glomerulus and collects the filtrate.
- The tubule, where useful substances are reabsorbed.
- The collecting duct, which carries the urine away, surrounded by a capillary network.
Making urine: ultrafiltration and selective reabsorption
Urine is formed in two stages.
- Ultrafiltration: blood enters the glomerulus at high pressure. Small molecules (water, glucose, ions, urea) are forced into the Bowman's capsule as filtrate. Large molecules (such as proteins) and blood cells are too big and stay in the blood.
- Selective reabsorption: as the filtrate passes along the tubule, useful substances are taken back into the blood: all the glucose, some ions and most of the water.
What remains, containing urea, excess water and excess ions, becomes urine.
Controlling water balance with ADH
The water content of the blood is controlled by the hormone ADH (antidiuretic hormone), released by the pituitary gland. ADH changes how much water the kidney tubules reabsorb.
| Blood water level | ADH released | Tubule permeability | Urine produced |
|---|---|---|---|
| Too low (dehydrated) | More | Higher | Small volume, concentrated |
| Too high | Less | Lower | Large volume, dilute |
When the blood has too little water, more ADH is released, the tubules become more permeable, and more water is reabsorbed, giving concentrated urine. When the blood has too much water, less ADH is released and dilute urine is produced. This keeps the water content of the blood steady.
Treating kidney failure
If the kidneys fail, toxic urea and excess water and ions build up in the blood, which can be fatal. Two treatments are used:
- Dialysis: the patient's blood is passed through a dialysis machine, where it flows next to a partially permeable membrane. Urea and excess substances diffuse out into the dialysis fluid, while useful substances at the right concentration stay in. It must be repeated regularly.
- Kidney transplant: a healthy kidney from a donor is placed into the patient. A transplant frees the patient from regular dialysis, but there is a shortage of donors and a risk that the body rejects the new kidney, so the patient takes drugs to prevent rejection.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe how urine is formed in the nephron by ultrafiltration and selective reabsorption.Show worked answer →
A 4-mark describe question on urine formation.
Blood at high pressure enters the glomerulus, and small molecules such as water, glucose, ions and urea are forced out into the Bowman's capsule. This is ultrafiltration, and the liquid is called the filtrate. As the filtrate passes along the tubule, useful substances are taken back into the blood by selective reabsorption: all the glucose, some ions and most of the water. The remaining liquid, containing urea, excess water and excess ions, becomes urine.
Markers reward: high pressure in the glomerulus forces small molecules into the capsule (ultrafiltration); large molecules and cells stay in the blood; useful substances reabsorbed (all glucose, some ions, most water); urine is urea plus excess water and ions. Saying glucose stays in the urine of a healthy person is wrong.
WJEC style4 marksExplain how the body responds when the blood contains too little water (the person is dehydrated).Show worked answer →
A 4-mark explain question on ADH.
When the blood has too little water, the pituitary gland releases more of the hormone ADH. ADH makes the kidney tubules more permeable, so more water is reabsorbed back into the blood. This means a small volume of concentrated urine is produced, and the water content of the blood returns to normal.
Markers reward: more ADH released by the pituitary; tubules become more permeable; more water reabsorbed into the blood; small volume of concentrated urine. The opposite (too much water) gives less ADH and dilute urine.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)