Water and Ion Balance (Kidneys)AQA GCSE Biology: Revision notes
Section 1
Why and How Does the Body Lose Water?
Water levels are one of the things homeostasis controls in the body. Water and ions must be kept in balance because too much or too little affects cells and enzyme function. Water leaves the body by three routes: the lungs (as water vapour in exhaled air), the skin (as sweat), and the kidneys (as urine). Excess water, excess ions and urea are removed from the blood via the kidneys.
Section 2
How Do the Kidneys Make Urine?
The kidneys produce urine by two processes:
- Filtration of the blood – small molecules including water, ions, glucose and urea pass out of the blood into the kidney tubules; blood cells and large proteins are too big to be filtered and stay in the blood
- Selective reabsorption – useful substances that the body needs are reabsorbed back into the blood from the tubules, including all the glucose, some ions and some water; whatever is not reabsorbed leaves the body as urine
Section 3
Removing Excess Protein: Deamination (Higher Tier)
Digestion of proteins produces amino acids, and the body only needs a certain amount for making new proteins. Excess amino acids cannot be stored, so they are broken down in the liver in a process called deamination.
- Deamination converts excess amino acids into ammonia
- Ammonia is toxic, so it is immediately converted into urea in the liver for safe excretion
- Urea travels in the blood to the kidneys, where it is removed and excreted in urine
Urea is made in the liver, not the kidneys — the kidneys only remove it from the blood.
Section 4
Controlling Water Reabsorption with ADH (Higher Tier)
The amount of water reabsorbed by the kidney tubules is controlled by the hormone ADH (antidiuretic hormone).
- ADH is released by the pituitary gland when the blood is too concentrated (too little water)
- ADH acts on the kidney tubules, increasing their permeability to water, so more water is reabsorbed back into the blood
- This is controlled by negative feedback: as blood becomes more dilute, less ADH is released, so less water is reabsorbed and more dilute urine is produced
ADH works like a thermostat for blood water content: too concentrated triggers more ADH, which pulls water back in, and the level self-corrects.
Section 5
Kidney Failure and Its Treatment
If the kidneys fail, urea, excess ions and excess water are not removed from the blood, which can be fatal without treatment. Two main treatments are used:
- Dialysis – blood is passed through a machine containing a partially permeable membrane and dialysis fluid, which removes urea and excess substances by diffusion; must be repeated regularly (e.g. several times a week)
- Kidney (organ) transplant – a healthy donor kidney is transplanted to replace kidney function
| Treatment | Advantage | Disadvantage |
|---|---|---|
| Dialysis | No surgery needed; widely available | Time-consuming, must be repeated regularly, restricts diet |
| Transplant | Restores normal kidney function long-term | Requires a donor match, surgery risk, lifelong immunosuppressant drugs |
Must Know
- Water leaves the body via the lungs, skin (sweat) and kidneys (urine)
- Kidneys make urine by filtration of the blood followed by selective reabsorption of glucose, some ions and water
- Excess amino acids are deaminated in the liver, forming ammonia which is converted to urea (HT)
- ADH from the pituitary gland increases kidney tubule permeability so more water is reabsorbed, controlled by negative feedback (HT)
- Kidney failure can be treated by dialysis or by a kidney transplant
That's the notes covered.
Carry on to the next subtopic.