5.2.4 Maintaining Water & Nitrogen Balance in the Body🧬

Maintaining Water and Nitrogen Balance in the Body (Biology Only)

• If the water concentration of the blood increases then cells in the body take up water.
• This is 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.
• Cells then expand as they take up more water, and may eventually burst.
• If the water concentration of the blood decreases then cells in the body lose water.
• This is 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.
• Cells shrink as they lose water.

The Kidneys

• Filtering out the waste products
• Selectively reabsorbing useful substances such as glucose, ions and water

Anti-diuretic hormone (ADH) is a hormone involved in the control of the loss of water as urine.

It is released into the pancreas by the pituitary gland when a receptor in the brain detects that the blood is too concentrated.

• It travels in the bloodstream to the kidney tubules
• An increased amount of ADH reaching the tubules increases their permeability to water, so more moves out of the tubule and back into the bloodstream
• This results in a smaller volume of more concentrated (yellow) urine and the blood becoming less concentrated as more water moves into it. This is an example of a negative feedback loop, because if the concentration of the blood increases/decreases, more/less ADH is secreted to reverse this change.

Kidney failure is when your kidneys stop working.

This means waste products build up which can be harmful to the body, and eventually results in death.

There are two ways to treat people with kidney failure.

1. Dialysis: the function of the kidneys is carried out using an artificial membrane

• Blood moves between partially permeable membranes surrounded by dialysis fluid (has the same concentrations of ions and glucose as healthy blood).
• Useful ions and glucose are not lost from the blood but urea, excess ions and water diffuse across the membrane.
• It has to be done three times a week, with the process itself taking three to four hours.
• It acts as an artificial kidney and keeps people alive while they wait for a transplant.
• The problems of dialysis are the possibility of blood clots forming, the amount of time it takes and the fact you have to follow a strict diet. It is also an unpleasant experience, and the build up of waste products between sessions can leave you feeling ill.

2. Kidney transplants: providing the individual with a healthy kidney

• This is currently the only cure.
• They are transplanted from people who have died suddenly, or from people still alive (as we have two and only need one to survive).
• They mean that the person does not need to visit the hospital as regularly as they did when undergoing dialysis, and it is cheaper than the overall cost of dialysis.
• The main problem is that they could be rejected from the body as a result of the immune system recognising the antigens on the donor organ as foreign.
• The chance of this happening can be reduced by using immunosuppressant drugs.
• However, this means that the person can be left vulnerable to other diseases. In addition, transplanted kidneys do not last forever.