Disease and response · GCSE Biology
The kidney
Explain kidney function for GCSE Biology: filtration in the glomerulus, selective reabsorption of glucose and water, ADH and urine concentration, and dialysis versus transplant as kidney-failure treatments.
Filter everything small, then take back what you need. Glucose is reabsorbed; urea is not. ADH makes the collecting duct more permeable to water when you are dehydrated. That is osmoregulation.
The important bits
What you need to know
- 1
The kidneys maintain water and ion balance (osmoregulation) and remove urea, a toxic waste from amino-acid breakdown in the liver.
- 2
Ultrafiltration: blood at high pressure in the glomerulus forces water, urea, ions and glucose into the Bowman’s capsule. Cells and large proteins stay in the blood.
- 3
Selective reabsorption: all glucose is reabsorbed in the first coiled tubule (active transport), plus some ions and much water. Urea remains in the tubule to be excreted in urine.
- 4
Healthy urine contains water, urea and ions, but not glucose or protein. Glucose in urine can indicate diabetes (blood glucose so high that not all is reabsorbed). Protein in urine can indicate damaged filters.
- 5
ADH (anti-diuretic hormone) from the pituitary: if blood is too concentrated (dehydrated), more ADH, collecting ducts more permeable, more water reabsorbed, small volume of concentrated urine. If hydrated, less ADH, large volume of dilute urine.
- 6
This is negative feedback for water content. Drinking a lot of water reduces ADH; sweating on a hot day increases ADH.
- 7
Kidney failure: dialysis regularly filters the blood using a partially permeable membrane and a fluid matched so urea diffuses out but useful solutes are balanced. A transplant replaces the kidney but needs a match and immunosuppressant drugs to reduce rejection.
- 8
Nephron is the functional unit (Triple diagrams). Combined still needs filtration, reabsorption, urea, and ADH in outline.
Quotations worth analysing
Short evidence. Real method.
“Glucose is reabsorbed; urea is excreted in urine.”
Selective reabsorption is the word. If glucose appears in urine, name diabetes as a possible cause.
“ADH increases the permeability of the collecting duct to water.”
More ADH → more water reabsorbed → concentrated urine. Do not say ADH is insulin, or that it is made in the kidney.
“Urea is made in the liver from excess amino acids and removed by the kidneys.”
Liver makes it; kidney filters it. Do not say the kidney makes urea.
Go deeper
What should I label on a nephron if the diagram is almost blank?
Glomerulus (knot of capillaries) inside Bowman’s capsule: filtration. First (proximal) tubule: all glucose reabsorbed by active transport, lots of mitochondria in those cells. Loop of Henle: water and ion adjustments (Triple). Distal tubule and collecting duct: ADH-controlled water reabsorption. Blood vessel leaving should have less urea and the same cells and proteins. Urine in the ureter has urea, water, ions, no glucose. If the paper is Combined and shows only a kidney outline, label cortex, medulla, ureter, renal artery (dirty blood in), renal vein (cleaned blood out). Artery has more urea than the vein. That comparison is a favourite table.
Go deeper
How does ADH actually change the urine on a hot day?
Sweating loses water. Blood plasma becomes more concentrated (higher solute concentration, lower water potential). Receptors in the brain detect this. Pituitary releases more ADH. Collecting-duct walls become more permeable (more water channels). More water is reabsorbed into the blood by osmosis. Urine volume falls and urine is darker and more concentrated. Urea is still removed. The opposite happens after a large drink: less ADH, ducts less permeable, large pale urine. This is negative feedback for water, not for glucose (that is insulin). Mixing ADH with insulin is the classic hormone-swap. ADH does not “make you stop sweating”; it saves water at the kidney.
Go deeper
How do I evaluate dialysis versus a transplant?
Dialysis: available, no waiting for a donor, but hours in hospital several times a week, diet limits (protein, salt, fluid), infection risk, does not cure, expensive long-term. Transplant: closer to normal kidney function, better quality of life if successful, but shortage of donors, need for tissue match, immunosuppressants (infection risk for life), risk of surgery and rejection. Conclusion should use the patient’s situation. Both are treatments for kidney failure when homeostasis of water, ions and urea has collapsed. Mention the partially permeable dialysis membrane and diffusion of urea as the science of dialysis, not just “a machine cleans the blood”.
See the idea in action
A student is dehydrated after sport. Blood is too concentrated, so the pituitary releases more ADH. Collecting ducts become more permeable, more water is reabsorbed by osmosis, and a small volume of concentrated urine is produced. Urea is still excreted. Glucose should still be absent from urine because it was reabsorbed in the first tubule by active transport. If glucose were present, diabetes would be a possible explanation because filtered glucose exceeded reabsorption capacity.
Exam technique
Turn knowledge into marks
Learn filtration then selective reabsorption. Urine: urea + water + ions, no glucose or protein. ADH from pituitary, permeability of collecting duct. For failure, compare dialysis (diffusion through a membrane) with transplant (match + immune suppression).
Common mistakes
Do not give these marks away
- 01
Saying the kidney makes urea, or that ADH is produced by the kidney.
- 02
Writing that glucose is supposed to be in urine, or that insulin controls urine volume.
- 03
Describing dialysis as osmosis of urea only, without a partially permeable membrane and diffusion of waste.
What does ADH do when you are dehydrated?
AIt lowers blood glucose by storing glycogen
BIt makes collecting ducts more permeable so more water is reabsorbed and urine is concentrated
CIt causes glucose to appear in the urine
DIt stops the glomerulus filtering blood
Show the answer
It makes collecting ducts more permeable so more water is reabsorbed and urine is concentrated. ADH is the water hormone from the pituitary. Insulin, not ADH, controls glucose. Filtration still happens; the amount of water taken back changes.
Quick questions
If this is the bit you searched
What does the kidney do in GCSE Biology?
It filters blood, reabsorbs useful substances such as glucose and the right amount of water, and excretes urea, excess water and ions in urine.
Why is there no glucose in healthy urine?
Glucose is filtered then all reabsorbed by active transport in the kidney tubule. Glucose in urine can be a sign of diabetes.
What is ADH?
Anti-diuretic hormone from the pituitary. It increases water reabsorption in the collecting ducts when you are dehydrated, producing a small volume of concentrated urine.
How does dialysis work?
Blood flows beside a dialysis fluid separated by a partially permeable membrane. Urea diffuses out. The fluid is designed so useful substances are not lost. It has to be repeated regularly.