The kidneys

The kidneys

Our bodies need to get rid of waste products. Three waste products our bodies must excrete are CO2, urea and sweat. This is known as homeostasis (controlling conditions inside the body).



Urea (a waste product from the breakdown of amino acids) is produced in the liver. Urea is toxic in high concentrations, although the liver releases it into the blood stream to be filtered out by the kidneys.


We take in water from food and drink, and water is a waste product of respiration. We lose water in sweat, faeces, urine and breathing out.

For our cells to work properly their water content must be maintained at the correct level. Our kidneys help us to maintain that balance.

Stages of blood filtration in the kidneys:

Stage 1: Ultrafiltration. Blood is brought to the kidneys to be filtered – blood passes through tiny tubules and water, salt, glucose and urea are squeezed out.

Stage 2: Selective reabsorption. The kidneys send all of the glucose and as much water and salt as the body needs into the blood. Sugar and dissolved ions may be actively absorbed against a concentration gradient.

Stage 3: Waste. Water, salt and urea are left – this is urine. Urine is sent to the through the ureter to the bladder where it is stored before being excreted.


Kidney failure:

Sometimes the kidneys can fail due to infections, toxic substances or genetic reasons. A patient with kidney failure will soon die unless there is a way to rid the body of the urea and excess salt.

A kidney dialysis machine provides an artificial kidney for the sufferers of kidney failure. The patient must use a dialysis machine for 3-4 hours three times a week.

The patients’ blood flows alongside a partially permeable membrane, surrounded by dialysis fluid which contains the same concentration of dissolved ions and glucose as the blood (this ensures that glucose and useful mineral ions are not lost)

Ions and waste can pass through, but big molecules like blood cells and proteins can’t pass through (like in the kidneys).

Dialysis removes urea and maintains blood sodium and glucose levels.

Instead of dialysis a kidney could be transplanted into the patient. This option is cheaper than dialysis but it requires a donor (a normal person can still function with one kidney). The new kidney might be rejected by the body’s immune system.

To prevent rejection of the transplanted kidney a donor kidney with a ‘tissue-type’ similar to the recipient is used and the patient can take immunosuppressant drugs.


Transplanted kidneys only work for around 9 years, then the patient has to return to dialysis.

Mycoprotein is a low-fat, protein-rich food suitable for    vegetarians. It

is made from the fungus Fusarium.The fungus grows and reproduces rapidly on a cheap   energy supply

(sugar syrup) in a fermenter.It requires aerobic conditions to grow. Its mass doubles    every 5 hours or so.The biomass is harvested, purified and dried to leave mycoprotein –colours and flavours are added to enhance it.