(3.7) Dialysis - an artificial means of purifying the blood

Dialysis (hemodialysis, haemodialysis)  is a treatment to filter wastes and water from your blood, when your kidneys do when they were healthy. Also, importantly, dialysis helps control blood pressure and balance important mineral ion concentrations in the blood e.g. potassium, sodium, and calcium, in your blood.

People who suffer from kidney failure can't filter their blood properly. to retain requirements or remove waste substances.

Kidney patients may be treated either using a kidney dialysis machine or having a healthy kidney transplanted.

 

If the kidneys don't function properly then waste products build up in the body's bloodstream and the body's ability to control the water content and ion concentrations is lost, a situation that can be fatal - kidney failure is a bad situation - a major lack of this aspect of homeostasis control.

Though uncommon, kidney failure can be caused by infection, severe poisoning, an injury with severe loss of blood or very high blood pressure

 

Treatment by haemodialysis restores the concentrations of dissolved substances in the blood to normal levels and has to be carried out on a regular intervals to keep them at the right 'normal' concentrations and remove waste that that the malfunctioning kidney cannot deal with.

In particular, keeping the ion concentrations at the right level and removing waste products is essential and a dialysis machine can perform some of the functions of a real kidney.

In a dialysis machine a person’s blood flows between partially permeable membranes (selectively permeable membranes) - shown in the dialysis system diagram below.

Haemodialysis

Diagram of kidney dialysis machine procedure - the dialyser connected to a patient's arm - blood is pumped from an artery to the dialysis machine and the 'cleaned' blood pumped back into a vein.

In a dialysis machine there is a selectively permeable membrane (partially permeable) where the patient's blood flows on one side and the dialysis fluid on the other.

The membrane allows substances like excess ions and urea waste to pass through the membrane from the blood to the dialysing fluid, but, does not allow larger molecules like proteins through or white or red blood cells.

This is a similar action to the membranes in the kidney - the membranes are indicated by the blue lines in the dialysis machine diagram above.

The dialysis fluid contains the same concentration of useful substances e.g. of ions and glucose as healthy blood to ensure that glucose and useful mineral ions are not lost from the blood in the dialysis process - minimises the formation of diffusion gradients, so minimises unwanted ion/molecule transfers.

Only waste substances such as urea and excess ions pass out from the blood through the membrane into the dialysis fluid.

This will naturally happen by diffusion because the dialysis fluid contains no urea, so, the diffusion gradient is from blood to dialysis solution.

However, large molecules like proteins cannot pass through the dialysis membrane and neither can any blood cells.

The levels of ions and glucose in the dialysis fluid are set at approximately those required in the patient's blood so that diffusion may occur in either direction to maintain the correct concentrations in the blood.

For people with poorly functioning kidneys, dialysis might be required several times a week with each session lasting several hours (3-4 hours).

Regular dialysis will keep the concentrations of dissolved substances in the blood at the normal levels and remove potentially toxic waste substances.

BUT, dialysis is not without risks:

Dialysis may cause blood clots.

There is an increased risk of blood infection.

It is also an expensive unpleasant experience, even if its free on the NHS.

These risks and unpleasantness must be balanced against giving the patient with kidney failure some time, and therefore a chance, getting a life-saving transplant of a donor organ.

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points about dialysis

What Is Haemodialysis?

Haemodialysis is a medical procedure used to artificially filter the blood when the kidneys are no longer able to do so effectively.

It is a form of dialysis that uses a machine to remove waste products, excess water, and ions from the blood.

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Why Is Haemodialysis Needed?

• The kidneys normally remove urea, excess salts, and water from the blood.

• If both kidneys fail (due to disease, injury, or genetic conditions), toxic substances build up in the body.

• This can lead to serious illness or death if not treated.

• Haemodialysis acts as an artificial kidney to keep the patient alive until a kidney transplant is possible or as a long-term treatment.

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How Does Haemodialysis Work?

1. Blood is removed from the patient’s arm and pumped into the dialysis machine.

2. Inside the machine, blood flows through a partially permeable membrane in the dialyser.

3. On the other side of the membrane is the dialysis fluid (dialysate), which:

   • Contains no urea → urea diffuses out of the blood.

   • Has normal levels of glucose and ions → only excess ions/glucose diffuse out.

4. The blood and dialysis fluid flow in opposite directions (a counter-current system) to maintain a concentration gradient.

5. Cleaned blood is returned to the patient’s body.

The process usually takes 3–5 hours, 3 times a week.

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What Does the Dialysis Machine Remove?

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Importance in Understanding Human Biology

• Shows how the excretory system maintains homeostasis.

• Demonstrates the role of diffusion and selective permeability in real-life medicine.

• Highlights the consequences of kidney failure and the importance of organ function.

• Reinforces understanding of concentration gradients, osmosis, and active transport.

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Exam Tips

• Use key terms: dialysis, urea, partially permeable membrane, diffusion, counter-current, homeostasis.

• Be able to label a diagram of a dialysis machine.

• Compare dialysis with kidney transplants in terms of pros and cons.

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Keywords, phrases and learning objectives for this part on dialysis

Be able to describe the process and function of dialysis (hemodialysis, haemodialysis).

Be familiar with a diagram of a dialysis machine and how it works in artificially purifying the blood.

Know a dialysis machine filters out waste helps control ion concentrations in blood and enables a better blood pressure balance.

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