The Kidneys: Water and Nitrogen Balance

Your body constantly takes in water and produces waste, yet the concentration of your blood and cells must stay roughly constant. Three substances need careful management:

• Water — too much or too little disrupts every cell.
• Ions — such as sodium ions, taken in with food.
• Urea — a waste product made when the body deals with excess protein.

These leave the body by three routes:

Route	What is lost	Controlled?
Lungs (breathing out)	Water (as vapour)	No
Skin (sweating)	Water, ions and urea	No (cooling, not balance)
Kidneys (urine)	Excess water, ions and urea	Yes

Sweating cools you down — it is not a way of controlling water balance. The body cannot choose how much water and ions it loses through the lungs or skin.

Because losses from the lungs and skin are uncontrolled, the kidneys do the precise work of keeping the blood at the right concentration by adjusting what ends up in the urine.

Body cells are surrounded by tissue fluid. Water moves between the cell and this fluid by osmosis — the diffusion of water across a partially permeable membrane, from a more dilute solution (high water concentration) to a more concentrated one (low water concentration).

If the blood and tissue fluid become too concentrated (too little water), water leaves the cells by osmosis. The cells shrink and cannot work efficiently.

If the blood becomes too dilute (too much water), water enters the cells by osmosis. The cells swell, and in extreme cases can burst.

If body cells lose or gain too much water by osmosis, they do not function efficiently. Keeping the blood at a steady concentration protects every cell in the body.

This is the core reason water balance matters. The chemical reactions inside cells — including those controlled by enzymes — depend on the right amount of water. The kidneys exist to hold that balance steady whatever you eat or drink.

(Higher Tier only — the deamination of amino acids and the formation of urea in the liver.)

You cannot store excess protein or the amino acids it is broken down into. When the diet contains more protein than the body needs, the surplus amino acids must be broken down and removed safely.

This happens in the liver in two steps:

1. Deamination — the amino group is removed from each excess amino acid, producing ammonia.
2. Ammonia is very toxic, so the liver immediately converts it into urea.

Urea is far less toxic than ammonia, so it can be carried safely in the blood to the kidneys, where it is removed in the urine.

Key chain to memorise (HT): excess amino acids → deaminated → ammonia → converted to urea (in the liver) → excreted by the kidneys. The liver makes urea; the kidneys remove it.

The kidneys clean the blood in two stages.

Stage 1 — Filtration. Blood is filtered under pressure. Small molecules are forced out of the blood: water, glucose, ions and urea all pass into the kidney tubules. Large items such as blood cells and proteins are too big to be filtered and stay in the blood.

Stage 2 — Selective reabsorption. The filtered liquid still contains useful substances the body needs to keep. As it flows through the tubules, these are taken back into the blood:

Substance	Filtered out of blood?	Reabsorbed back into blood?
Glucose	Yes	All of it (none should be lost)
Ions	Yes	Only as much as the body needs
Water	Yes	As much as the body needs
Urea	Yes	No — it stays in the urine as waste

Whatever is left after reabsorption — excess water, excess ions and the urea — leaves the body as urine.

"Selective" reabsorption means the body chooses what to take back. All glucose is reabsorbed; the amount of water and ions reabsorbed is adjusted to keep the blood balanced. Urea is left behind to be excreted.

(The spec does not require the structure of the kidney or the parts of a tubule, only that filtration and selective reabsorption take place.)

(Higher Tier only — the role of ADH and negative feedback control of water balance.)

The amount of water reabsorbed in the kidney tubules is controlled by a hormone called ADH (anti-diuretic hormone), released by the pituitary gland.

ADH changes how permeable the kidney tubules are to water. More ADH → tubules let more water back into the blood → less water in the urine, so the urine is smaller in volume and more concentrated. Less ADH has the opposite effect.

This is controlled by negative feedback — a change in one direction triggers a response that reverses it:

The detail for each arm:

• Blood too concentrated (e.g. dehydrated, sweated a lot) → pituitary releases more ADH → kidney tubules become more permeable and reabsorb more water → less water lost, urine small and concentrated → blood water level rises back to normal.
• Blood too dilute (e.g. drank a lot of water) → pituitary releases less ADH → tubules reabsorb less water → more water lost, urine large and dilute → blood water level falls back to normal.

Worked walk-through (HT) — you go for a long run on a hot day and sweat heavily. You lose water in sweat, so your blood becomes too concentrated. The pituitary detects this and releases more ADH. The kidney tubules become more permeable and reabsorb more water back into the blood. You produce a small volume of dark, concentrated urine, and your blood water level returns to normal. This is why dehydration makes urine darker.

If the kidneys fail, urea and excess ions build up in the blood and the water balance is lost — this is life-threatening. One treatment is kidney dialysis, which does the kidneys' job using a machine.

How dialysis works:

• The patient's blood flows out of the body and through a dialysis machine.
• Inside, the blood flows on one side of a partially permeable membrane; dialysis fluid flows on the other side.
• The dialysis fluid contains a normal concentration of glucose and ions, and no urea.
• Because of these concentration differences, substances move by diffusion across the membrane:
  • Urea diffuses out of the blood into the dialysis fluid (the fluid has no urea, so urea moves out down its concentration gradient).
  • Excess ions diffuse out of the blood, while ions at normal levels do not (the fluid matches normal blood, so there is no net movement of those).
  • Glucose does not leave the blood, because the fluid has the same glucose concentration as healthy blood — no concentration gradient, so no net loss.
• The cleaned blood is returned to the patient.

The dialysis fluid is carefully chosen: same glucose and ion levels as healthy blood (so useful substances are not lost), but no urea (so waste diffuses out). This mimics the selective work the kidneys normally do.

Dialysis must be repeated regularly — typically several sessions a week — because waste keeps building up between treatments.

The other treatment for kidney failure is a kidney transplant — a healthy kidney from a donor is placed into the patient. Each option has trade-offs, and the right choice depends on the patient.

Factor	Kidney dialysis	Kidney transplant
Lifestyle	Tied to a machine for regular sessions each week	Free to live normally once recovered
Diet	Must control diet (especially protein and salt)	Fewer dietary restrictions
Cost over time	Expensive long-term (ongoing treatment)	One-off operation is costly, but cheaper in the long run
Availability	Available when needed	Limited by a shortage of donor organs (long waits)
Rejection	No rejection risk	Body may reject the kidney; needs immunosuppressant drugs
Drugs / side effects	No anti-rejection drugs	Immunosuppressants for life, raising infection risk
Permanence	Ongoing, does not cure	Can last many years, but not forever

Worked walk-through — choosing for a young patient. A young, otherwise healthy patient is likely to benefit most from a transplant: it frees them from regular machine sessions, lets them eat more normally, and is cheaper over a lifetime. The downsides are the wait for a matching donor organ and a lifetime of immunosuppressant drugs with the risk of rejection. Dialysis is the option that keeps them alive while they wait for a suitable donor kidney.

There is no single "best" treatment — a good evaluation weighs cost, lifestyle, organ availability and rejection risk for the specific patient.

1. Saying the kidney removes urea by "filtering it out and keeping it"

Urea is filtered out of the blood and left in the tubule — it is not reabsorbed. It is glucose (all of it), some ions and water that are selectively reabsorbed. Urea leaves in the urine.

2. Confusing where ammonia and urea are made and removed

Deamination and the conversion to urea both happen in the liver, not the kidney. The kidney's job is to remove the urea that the liver has already made. (HT)

3. Getting the ADH effect backwards

More ADH means more water reabsorbed, so less urine that is more concentrated. A common error is to write that more ADH produces more urine — it is the opposite. (HT)

4. Forgetting that lung and skin losses are uncontrolled

Water leaves via the lungs and skin, but the body does not regulate these to balance water — only the kidneys do that. Sweating is for cooling, not water control.

5. Thinking dialysis fluid should contain no glucose or ions

Dialysis fluid contains glucose and ions at normal blood concentrations so these useful substances are not lost by diffusion. Only urea is absent from the fluid, so only waste diffuses out.

6. Treating "best treatment" as a fact rather than an evaluation

Dialysis vs transplant has no fixed answer. Marks come from weighing trade-offs — cost, lifestyle, donor availability and rejection — for the patient described.