Kidney Control

Kidney Control

Every day your body has to dump toxic waste and hold its water level inside a narrow band. Too little water and cells shrivel; too much and they burst. Water leaves you by three routes, and only one of them can be adjusted: • The lungs: water vapour in every breath out. Unavoidable, and not controllable. • The skin: sweat, which loses ions as well as water. Controlled for temperature, not for water balance, which is why sweating carries on even when you are dehydrated. • The kidneys: urine. The only route the body can turn up or down, which is why water balance is a kidney story. Urine carries off excess water, excess ions, and the waste product urea. (This topic is Biology-only, and the ADH and kidney-failure detail is Higher tier.)

Losing water

Select ALL THREE statements about water loss that are TRUE.

• Water is lost through the lungs every time you breathe out
• Sweating loses ions as well as water
• The kidneys are the only route whose water loss the body can adjust
• Sweating is controlled in order to regulate the body's water level
• The bladder decides how concentrated the urine will be
• Water loss through the lungs is reduced when you are dehydrated

Making urea

Your body cannot store excess protein or the amino acids it breaks down into, so the surplus has to go somewhere. It goes through the liver, in two steps. First the amino acids are deaminated in the liver, which strips off the nitrogen-containing part and produces ammonia. Ammonia is highly toxic, so it cannot be allowed to circulate. The liver immediately converts it into urea, which is far less toxic and can travel safely in the blood to the kidneys. Hold on to the division of labour: the liver makes urea, the kidney removes it. Getting that backwards is the commonest error in this topic.

When the liver fails

A patient's liver is failing, although their kidneys are working normally. Which substance would you expect to build up dangerously in their blood?

• Ammonia, because the conversion into urea is exactly what the liver has stopped doing
• Urea, because it can no longer be removed from the blood
• Glucose, because the liver stores it
• Water, because the liver controls the body's water balance

How the kidney makes urine

The kidney does not pick the waste out of the blood. It empties almost everything out and then takes back what is worth keeping, which sounds wasteful and is far easier to control:

Label the blood flow

Drag each label onto the right part of this kidney.

Filter, then reabsorb

The kidney first _____ the blood under pressure, so that small molecules pass out while _____ and blood cells stay behind. It then reabsorbs all of the _____, together with the water and ions the body needs. The waste product _____ is left behind and leaves the body as urine.

The water dial [Higher tier]

How much water gets reabsorbed is not fixed. It is tuned, continuously, by a hormone. Four terms, and the last one is the idea the rest hangs on:

Run the water loop [Higher tier]

It is a hot day, you have been sweating, and you have not drunk anything. Follow the loop.

• Your blood is now too concentrated. What does the pituitary do?
• That ADH reaches the kidney tubules. What happens to water there?
• What kind of urine do you produce as a result?
• Why does this count as negative feedback?

The opposite case [Higher tier]

You drink a litre of water quickly, so your blood becomes too dilute. What happens?

• Less ADH is released, less water is reabsorbed, and you produce a large volume of dilute urine
• More ADH is released, so you produce a small volume of concentrated urine
• ADH stays the same, and the extra water simply passes through
• The kidneys stop reabsorbing anything at all until the balance is restored

How dialysis works [Higher tier]

If the kidneys fail, urea and excess ions build up in the blood, and that is fatal untreated. A dialysis machine does the removing instead, and it does it with nothing but diffusion. The patient's blood flows on one side of a partially permeable membrane, and dialysis fluid flows on the other. The fluid is made to a recipe: • No urea at all, so there is a steep concentration gradient and urea diffuses out of the blood. • Glucose at the same concentration as healthy blood, so there is no gradient and none is lost. • Ions at normal blood concentrations, so only the patient's excess diffuses away. Every part of that recipe is the same principle: substances diffuse down a gradient, so you control what moves by controlling which gradients exist.

Break the recipe [Higher tier]

Test the recipe by getting it wrong. Suppose the fluid were mixed with no glucose in it. Glucose would then diffuse _____ of the blood and the patient would lose a nutrient they need, which is the loss the recipe exists to _____. Now suppose it were mixed with urea at the level already in the blood. There would be no _____ for urea, so none of it would leave, and the machine would fail at the one job it is there to do. Both faults have a single cause: the recipe works only by settling which way each substance is free to _____.

Transplant, not dialysis [Higher tier]

Select ALL THREE statements that describe a kidney TRANSPLANT rather than dialysis.

• It frees the patient from regular hospital sessions
• The body may reject it, so immunosuppressant drugs are needed
• Donors are scarce, so there is a waiting list
• It has to be repeated several times a week for the rest of the patient's life
• It can be started immediately, with no waiting
• The blood is filtered against a specially made fluid

Kidney summary

Excess amino acids are deaminated in the _____, producing toxic ammonia which is converted into urea. The kidney then _____ the blood under pressure and selectively reabsorbs all of the glucose along with the water and ions the body needs. Water balance is tuned by _____, released by the pituitary, in a _____ feedback loop: a change triggers the response that reverses it.