Kidney Failure and Dialysis

HSC Biology | Study Notes

Kidney failure and dialysis are an important part of NSW Biology Stage 6, Module 8, Non-infectious Disease and Disorders. This topic matters because the syllabus specifically includes loss of kidney function as a disorder and dialysis as a technology used to assist with its effects. HSC materials also directly assess how dialysis removes urea from the blood and compensates for a lost kidney function.  

In this lesson

• the normal function of the kidneys

• why urea removal is important

• how dialysis works

• how dialysis helps when kidneys fail

• the main limitations of dialysis

Why the kidneys are important

The kidneys help maintain the internal environment by filtering the blood.

Main functions of the kidneys

The kidneys:

• remove wastes from the blood

• help regulate water balance

• help regulate ion concentrations

• produce urine

In Module 8, the key focus for dialysis is the kidney’s role in removing waste such as urea from the blood. 

Kidney function

A normal kidney filters blood continuously.

Glomerular filtration

One indicator of kidney function is the volume of filtrate formed at the glomerulus each minute, called the glomerular filtration rate, or GFR.

A 2020 HSC question gives:

• healthy adult GFR > 100 mL min⁻¹

• GFR needing dialysis < 15 mL min⁻¹ 

Why this matters

If kidney function falls too low:

• wastes build up in the blood

• the internal environment is disrupted

• treatment may be needed to replace part of the kidney’s function

Urea removal

Urea is a nitrogen-containing waste produced by the body.

Why urea must be removed

If the kidneys cannot remove urea effectively:

• urea concentration in the blood rises

• the blood no longer has its normal composition

• this can become harmful

HSC marking guidance states directly that loss of kidney function may result in a failure to remove urea from blood. 

HSC-style data link

A 2025 HSC multiple-choice question on dialysis identifies the correct blood pattern as:

• before dialysis: high urea and high glucose

• after dialysis: low urea and high glucose 

This shows that the main waste being removed is urea, while useful substances such as glucose should remain in the blood.

What is dialysis?

Dialysis is a treatment that helps remove waste from the blood when the kidneys can no longer do this properly.

Why dialysis is needed

Dialysis is used when kidney function has fallen so much that waste removal is no longer effective enough.

The Module 8 syllabus specifically names dialysis as a technology used to assist with the effects of loss of kidney function. 

Dialysis process

The key process in dialysis is the movement of urea out of the blood.

What happens in dialysis

HSC marking guidance describes the process as:

• blood from the patient passes through selectively permeable dialysis tubing

• urea moves from the blood into the dialysate

• this happens because urea diffuses from a high concentration in the blood to a low concentration in the dialysate 

Key idea

Dialysis works by using a concentration gradient.

Because the blood has a higher concentration of urea than the dialysate:

• urea diffuses out of the blood

• the blood leaves with less urea

Why the membrane matters

The membrane is selectively permeable, which means:

• some substances can move across it

• larger components of the blood stay in the bloodstream

How dialysis helps

Dialysis helps by replacing one important lost kidney function.

What function it replaces

The main function being replaced is:

• removal of urea and other wastes from the blood

HSC-style explanation

The 2020 HSC marking guidelines state that dialysis compensates for kidney failure because urea diffuses from blood to dialysate, so the urea is removed from the blood. 

Important point

Dialysis does not create a healthy kidney. It helps manage the disorder by taking over part of the kidney’s filtering role.

Before and after dialysis

A useful exam idea is the blood composition before and after treatment.

Before dialysis

Blood is likely to contain:

• high urea

After dialysis

Blood should contain:

• lower urea

The 2025 HSC question specifically tests this pattern and shows that after dialysis, urea should be lower while glucose can remain high. 

Limitations

Dialysis is useful, but it also has limitations.

It does not restore full kidney function

Dialysis only replaces some kidney functions, especially waste removal. It does not fully return the kidneys to normal.

Ongoing treatment is needed

A 2020 HSC question about falling GFR shows that dialysis becomes necessary once kidney function drops below a critical level, which suggests continuing management rather than a one-off cure. 

It manages rather than cures

Dialysis helps control the effects of kidney failure, but it does not cure the underlying damage to the kidneys.

Comparing normal kidney function and dialysis

Feature	Healthy kidneys	Dialysis
Waste removal	Happens continuously in the body	Assisted treatment removes waste from blood
Urea removal	Done naturally by kidneys	Urea diffuses into dialysate
Outcome	Maintains blood composition continuously	Helps compensate when kidney function is too low

Why this topic matters in Module 8

This topic is important because it shows how knowledge of organ structure and function can be linked to:

• a non-infectious disorder

• a treatment technology

• the maintenance of the internal environment

That is exactly the focus of the Module 8 technologies and disorders section. 

Worked example

Exam-style question

Explain how dialysis compensates for the loss of a function of the kidneys.

Worked answer

Loss of kidney function may result in a failure to remove urea from the blood. In dialysis, blood passes through selectively permeable dialysis tubing, and urea diffuses from its high concentration in the blood to its lower concentration in the dialysate. This removes urea from the blood and helps replace the kidney’s waste-removal function. 

Why this works

This answer:

• identifies the lost kidney function

• explains the role of the selectively permeable membrane

• links diffusion to urea removal

Common mistakes

• Saying dialysis cures kidney failure.

• Forgetting that the main waste discussed here is urea.

• Saying urea moves into the blood during dialysis instead of out of it.

• Ignoring the role of the concentration gradient.

• Describing dialysis without explaining how it replaces a kidney function.

Quick quiz

1. What is one important function of the kidneys?

2. Why must urea be removed from the blood?

3. What is dialysis?

4. How is urea removed during dialysis?

5. Why is dialysis a treatment rather than a cure?

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