4.5.3.3 Maintaining water and nitrogen balance in the body

What happens if water concentration of the blood increases?

Cells in the body take up water because the concentration of water in the bloodstream is higher than the concentration of water in the cells so water moves into the cells by osmosis. The cells then expand as they take up water and may burst

What happens if water concentration of the blood decreases?

Cells in the body lose water because the concentration of water in the bloodstream is lower than the concentration of water in the cells so water moves out of the cells by osmosis and cells shrink as they lose water

What do the kidneys do?

Make urine by filtering out the waste products and selectively reabsorbing useful substances such as glucose, ions and water

What waste products are processed at the kidney?

Water, ions, urea

How is water processed at the kidney?

Depending on the concentration of water in the blood, a certain amount of water is lost as urine but water can also leave the body at the lungs as a result of exhalation or from the skin in sweat

How are ions processed at the kidney?

If the ion concentration is incorrect then too little or too much water may enter body cells because the water potential of the blood would be altered. In the kidneys, certain amounts of ions are reabsorbed into the blood after it has been filtered so that the concentration in the blood is maintained

How is urea processed at the kidney?

Amino acids are made from digesting proteins. In the liver amino acids are deaminated to form ammonia which is toxic so it’s converted to urea which is filtered out of the blood in the kidneys

What is ADH?

A hormone involved in the control of the loss of water as urine

When is ADH released?

When a receptor in the brain detects that the blood is too concentrated

What is ADH released by?

The pituitary gland into the blood and travels to receptors in the kidney

What happens if the water concentration in the blood increases or decreases?

When there’s low water concentration in the blood, the receptors in the hypothalamus recognise this and signal to the pituitary gland to secrete ADH into the blood causing the kidney to reabsorb more water and more concentrated urine is produced. When there’s high water concentration, ADH isn’t secreted so less water is reabsorbed by the kidney so more dilute urine is produced

Why is blood concentration an example of a negative feedback loop?

If the concentration of the blood increases/decreases, more/less ADH is secreted to reverse it

What is kidney failure and why does it happen?

The kidneys stop working because waste products build up which can be harmful to the body and eventually lead to death

What are the two ways to treat people with kidney failure?

Dialysis, kidney transplants

How does dialysis help to treat people with kidney failure?

Blood moves between partially permeable membranes surrounded by dialysis fluid which has the same concentrations of ions and glucose as healthy blood

How often would an individual do dialysis?

Three times a week, taking three to four hours

What are the problems of dialysis?

The possibility of blood clots forming, the amount of time it takes, you have to follow a strict diet, it is an unpleasant experience, the build up of waste products between sessions can leave you feeling ill

How do kidney transplants help to treat people with kidney failure?

The individual is provided with a healthy kidney which means the person doesn’t need to visit the hospital regularly as they did when going through dialysis and is cheaper than the overall cost of dialysis

What is the main problem of kidney transplants?

They could be rejected from the body as a result of the immune system recognising the antigens on the donor organ as foreign

How can chances of rejection be reduced in kidney transplants?

Using immunosuppressant drugs but this means the person can be left vulnerable to other diseases