AQA GCSE Regulating water and nitrogen levels in the body (Biology)

Water losses from the body

Water and mineral ions are lost from the body.

Excretory organ	Substances lost	Control
Lungs	Water (in exhaled air)	No control
Skin	Water, ions, urea (in sweat)	No control
Kidney	Water, ions, urea (in urine)	Controlled (regulated by the body)

Osmotic changes and cells

Water can enter or leave animal cells by osmosis.

If body cells lose or gain too much water by osmosis they do not function efficiently.

If body cells gain too much water, they can swell and burst

If body cells lose to much water they can shrivel up.

Therefore, it is important to maintain water levels in the body.

Function of kidneys

The kidneys have two main functions:

1. Filter the blood to produce urine

2. Maintain water balance in the body.

Filtering the blood by the kidneys to produce urine.

Each kidney contains about 1 million nephrons which act as filtering units.

As the blood passes through the kidney its filtered to remove excess substances.

Substances filtered out of the blood by the kidneys	Not filtered out of the blood by the kidneys
Glucose	Blood cells
Amino acids	Protein
Mineral ions
Urea
Water

After filtration, selective reabsorption occurs to reabsorb useful substances such as glucose, some ions and water. Substances that are not reabsorbed are excreted from the body in the urine.

Data analysis

This is a summary based on the data in the table below

1.Glucose is completely reabsorbed (present before, absent after).

2.Some ions are reabsorbed (amount decreases).

3.Urea is not reabsorbed and is excreted (higher concentration in urine). Concentration increases because water is reabsorbed, so urea concentration will increase.

Substance	Concentration in blood before filtration (arbitrary units)	Concentration in urine after filtration (arbitrary units)
Glucose	90	0
Ions	60	20
Urea	30	45

The data may also be presented using a bar chart or other data format. You will also need to draw conclusions on the data.

Nitrogen balance and deamination

When we eat foods containing protein, the proteins are digested using protease enzymes to form amino acids.

The amino acids are absorbed into the blood from the small intestine.

Excess amino acids undergo a process called deamination in the liver to form ammonia. Ammonia is toxic, so it is converted to urea for excretion.

Deamination is the removal of the amino group from an amino acid. See the image below.

Hormonal control of water

Receptors in the hypothalamus of the brain detect the water content of the blood.

If the blood is too concentrated the pituitary gland will secrete Anti diuretic hormone (ADH) into the blood.

ADH will travel in the blood to the kidney tubules where it makes them more permeable to water, so more water is reabsorbed.

This means a smaller volume of more concentrated urine is produced.

This feedback mechanism is controlled by negative feedback.

If the blood is too dilute, the pituitary gland secretes less ADH, so less water is reabsorbed in the kidney tubules, so a larger volume of more dilute urine is produced.

Kidney failure treatments

When the kidneys fail, urea builds up in the blood, which is toxic. Water and ion balance becomes uncontrolled. There are two main treatment options:

1.Kidney dialysis

2.Kidney transplant

Kidney Dialysis.

Dialysis is used when the kidneys fail to remove:

Urea
Excess ions
Excess water

It works by diffusion across a partially permeable membrane.

Dialysis fluid contains:

Dialysis fluid:

Contains normal concentrations of glucose and ions
Contains no urea

This ensures:

Urea diffuses out of blood
Glucose does NOT diffuse out of the blood (no concentration gradient)
Correct ion balance maintained

How dialysis works

1.Blood flows from patient into dialysis machine

2.Blood flows past dialysis fluid

3.Blood and dialysis fluid are separated by partially permeable membrane

4.Urea diffuses from blood into the dialysis fluid

5.Useful substances (glucose) remain in blood

6.Clean blood returned to patient

See the image below

In the diagram below the dialysis machine has been simplified to make it easier to see.

Concentration of glucose in the dialysis fluid is equal to the blood, so glucose will not diffuse out of the blood.

Concentration of urea is greater in the blood than the dialysis fluid, so the urea will diffuse from the blood into the dialysis fluid, where it is taken away.

Advantages to dialysis	Disadvantages to dialysis
No major surgery	Time consuming, each session is approx 4 hours and several sessions per week are needed
Can be used immediately	Expensive long term
No donor needed	Strict diet and fluid limits

Kidney Transplants

A kidney transplant is a treatment for organ failure. This is where a failed organ is replaced with a donor organ.

For kidneys, the new kidney is surgically inserted into the body.

There is a risk that the recipient’s immune system may reject the new donor organ because it will have different antigens to the receipient’s own cells.

The risk of rejection can be reduced by:

1.Tissue typing (finding a close match)

2.Using immunosupressant drugs which suppress the immune system. However, this can increase the risk of the receipient developing an infection.

Advantages of a kidney transplant	Disadvantages of kidney transplant
No more dialysis sessions needed	There is a risk of organ rejection
More normal lifestyle	Risk of surgery
More cost effective long term	Need immunosuppressant drugs which increase the risk of infections