Water balance, kidneys and temperature control – GCSE Combined Science Biology

Start here

Your body makes waste all the time. If it built up, it would harm your cells. Your body also needs the right amount of water and the right temperature to work well.

This matters because your kidneys and your skin quietly keep these things steady. If the kidneys fail, a person needs dialysis or a transplant.

By the end you will be able to say where urea is made and how it leaves. You will explain how ADH controls water. You will describe dialysis and how the body keeps to 37 °C.

Urea and excretion

Proteins are built from amino acids. The body cannot store extra amino acids. The liver breaks them down and makes urea, which is a waste substance.

The urea goes into the blood plasma, the liquid part of blood. The blood carries it to the kidneys. The kidneys remove it, and it leaves in the urine. Removing waste like this is called excretion.

Other waste leaves too. Carbon dioxide leaves through the lungs. Water and salts are lost in sweat.

What the kidneys do

The kidneys filter the blood. Small molecules such as water, urea, glucose and ions are filtered out. Large molecules such as proteins and blood cells stay in the blood.

Then useful substances are reabsorbed, which means taken back into the blood. All the glucose is reabsorbed, with some ions and as much water as the body needs.

The urea, extra water and extra ions leave as urine. Urine is stored in the bladder. Think of tipping a bag of mixed items through a sieve, then picking out what you want to keep.

Water balance and ADH

The body must keep its water level steady. After sweating, the blood becomes too concentrated. The brain detects this. The pituitary gland releases a hormone called ADH into the blood.

ADH makes the kidney reabsorb more water. So a small amount of concentrated urine is made. If the blood is too dilute, less ADH is released. Less water is reabsorbed and more dilute urine is made.

This is negative feedback. A change triggers a response that reverses it, like a thermostat switching a heater off once the room is warm.

Kidney failure and dialysis

If the kidneys fail, urea and extra ions build up in the blood. The water balance is lost too. A dialysis machine can do the job of the kidneys.

The patient's blood flows between partially permeable membranes, with dialysis fluid on the other side. Urea and extra ions diffuse out of the blood into the fluid.

The fluid has the same level of glucose and ions as healthy blood, so useful substances are not lost. Dialysis takes several hours, and is done several times a week.

Kidney transplants

A kidney transplant gives the patient a healthy kidney from a donor. It is a long-term solution, and the patient is free from dialysis.

But there are problems. There are few donors, so patients wait a long time. The immune system may reject the new kidney. So the patient takes drugs to suppress the immune system, which can leave them open to infection.

Doctors try to match the tissue type of donor and patient, to reduce the chance of rejection.

Controlling body temperature

Enzymes work best at about 37 °C. The thermoregulatory centre in the brain monitors the blood temperature. Temperature receptors in the skin send it information.

If you are too hot, you sweat, and the sweat evaporates, which takes energy from the skin. The blood vessels near the skin widen. This is vasodilation, and more heat is lost.

If you are too cold, you shiver, and the muscles release heat. The vessels near the skin narrow. This is vasoconstriction, and less heat is lost. Sweating stops.

Recap

The liver makes urea. The kidneys remove it in urine.

Kidneys filter the blood, then reabsorb all the glucose and the water that is needed.

ADH increases water reabsorption. This is negative feedback.

Too hot: sweat and vasodilation. Too cold: shiver and vasoconstriction.