Kidney function, kidney failure and dialysis: HSC Biology Module 8

Syllabus dot point

“Explain a disorder by investigating the structure and function of the affected organ, and investigate technologies that are used to assist people who experience that disorder, including the loss of kidney function and the use of dialysis”

13 min readUpdated 28 Jun 2026

Quick answer

The kidney's functional unit, the nephron, makes urine by filtration at the glomerulus, reabsorption of glucose, water and ions along the tubule, and secretion of extra wastes - regulating blood water, ions, pH and urea (homeostasis). Loss of kidney function (from diabetes, hypertension or polycystic kidney disease) lets wastes and water build up, which is fatal untreated. Haemodialysis and peritoneal dialysis remove wastes by diffusion across a semi-permeable membrane against waste-free dialysate, while a kidney transplant restores full, regulated function but needs a scarce donor and lifelong immunosuppressants.

Jump to a section

What this dot point is asking
The answer
Practice questions

What this dot point is asking

NESA wants you to explain a disorder by linking it to the structure and function of the affected organ, then investigate the technologies that assist people who have it. Here the organ is the kidney, the disorder is loss of kidney function, and the technologies are dialysis (haemodialysis and peritoneal) and kidney transplant.

The command words matter: "explain" needs the biology of how the kidney works and how failure disrupts it; "investigate technologies" needs the mechanism of each device; and exam questions almost always ask you to "compare" or "evaluate" the technologies, which needs a judgement, not a list.

The answer

Structure and function of the kidney

The two kidneys filter the entire blood supply many times a day. Blood enters through the renal artery, is processed, and leaves cleaned through the renal vein; waste fluid (urine) drains via the ureter to the bladder. Inside, each kidney has an outer cortex and inner medulla packed with about a million nephrons - the functional units that actually make urine.

The kidney is central to homeostasis. It regulates:

Water balance (osmoregulation) - how much water is reabsorbed, controlling blood volume and pressure (under ADH).
Solute balance - the concentration of sodium, potassium and hydrogen ions, and therefore blood pH.
Waste removal - excretion of urea (the main nitrogenous waste, made in the liver from excess protein), creatinine and excess water.

How a nephron makes urine: filtration, reabsorption, secretion

Each nephron forms urine in three steps:

Filtration. At the glomerulus (a knot of capillaries inside Bowman's capsule), high blood pressure forces water and small solutes (glucose, ions, urea) out of the blood into the tubule. Blood cells and large plasma proteins are too big and stay in the blood.
Reabsorption. As the filtrate passes along the proximal convoluted tubule (PCT), loop of Henle and distal convoluted tubule (DCT), useful substances - all the glucose, most water, and needed ions - are reabsorbed back into the surrounding capillaries. The loop of Henle builds a salt gradient in the medulla so water can be drawn out of the collecting duct, concentrating the urine.
Secretion. Extra wastes (hydrogen and potassium ions, some drugs) are actively added from the blood into the filtrate, fine-tuning pH and ion levels.

What remains - water, urea and excess ions - drains from the collecting duct as urine.

When the kidney fails

In chronic kidney failure, nephrons are progressively destroyed and filtration falls. The main causes are:

Diabetes mellitus - chronically high blood glucose damages the glomerular capillaries (diabetic nephropathy). The leading cause in Australia.
Hypertension (high blood pressure) - sustained high pressure damages and narrows the renal vessels and glomeruli.
Polycystic kidney disease (PKD) - an inherited disorder in which fluid-filled cysts grow and destroy functional kidney tissue.

As function is lost, homeostasis breaks down: urea and creatinine build up (uraemia), water and sodium are retained (oedema, raised blood pressure), potassium rises (dangerous for the heart), pH falls (acidosis), and reduced erythropoietin causes anaemia. End-stage kidney failure is fatal within weeks unless treated by dialysis or transplant.

Technology 1: haemodialysis

Haemodialysis uses a machine as an external kidney. Blood is pumped from the patient (usually from a surgically made arteriovenous fistula in the forearm) through a dialyser containing many thin tubes of semi-permeable membrane. On the other side flows dialysate - a fluid matched to normal blood plasma in glucose and essential ions, but containing no urea or wastes.

Because of the concentration gradients across the membrane:

Wastes diffuse out of the blood (urea, creatinine, excess potassium) into the dialysate.
Needed solutes stay put - glucose and required ions are at the same level on both sides, so there is no net loss of them.
Excess water is removed (by a pressure gradient, ultrafiltration).

Blood and dialysate flow in opposite directions (counter-current) so the gradient stays steep along the whole membrane. Typical treatment is 3 sessions a week, 4 to 5 hours each, at a clinic.

Technology 2: peritoneal dialysis

Peritoneal dialysis uses the patient's own peritoneum (the membrane lining the abdomen) as the semi-permeable membrane. Dialysate is run through a catheter into the abdominal cavity; wastes diffuse from the blood vessels of the peritoneum into the dialysate, which is then drained and replaced. It can be done at home, daily (often overnight), giving more independence, but carries a risk of peritonitis (infection of the peritoneum).

Technology 3: kidney transplant

A kidney transplant replaces the failed organ with a working donor kidney (from a living or deceased donor). It restores continuous, fully regulated kidney function - including the hormone roles (erythropoietin, vitamin D activation) that dialysis cannot provide. It needs a tissue-matched donor (scarce, long waiting lists), major surgery, and lifelong immunosuppressant drugs to prevent rejection.

Evaluating the technologies

Technology	How it works	Strengths	Limitations
Haemodialysis	Blood across artificial membrane vs dialysate (clinic)	Effective clearance; widely available; no surgery to start	12 to 15 h/week; no hormone roles; diet/fluid limits; access needed
Peritoneal dialysis	Dialysate in abdomen, peritoneum is the membrane (home)	Home-based, more independence; gentler, daily	Risk of peritonitis; less efficient per hour; ongoing
Kidney transplant	Working donor kidney replaces the organ	Restores full, continuous, regulated function; best QoL	Donor shortage; major surgery; lifelong immunosuppression

Key fact

The nephron is the kidney's functional unit and makes urine in three steps: filtration at the glomerulus, reabsorption of glucose/water/ions along the tubule, and secretion of extra wastes into the filtrate - keeping blood water, ions, pH and urea in balance (homeostasis). When kidneys fail, dialysis removes wastes by diffusion across a semi-permeable membrane between blood and waste-free dialysate (counter-current, no net loss of needed solutes), while a transplant restores full kidney function but needs a donor and lifelong immunosuppression.

Worked exam answers

Worked example 1 - explain filtration, reabsorption and secretion (4 marks)

Question. Explain how the structure of the nephron allows it to carry out filtration, reabsorption and secretion.

Model answer. At the glomerulus, a knot of capillaries inside Bowman's capsule, high blood pressure forces water and small solutes (glucose, ions, urea) through the filtration membrane into the tubule, while blood cells and large proteins are retained - this is filtration. As the filtrate flows along the proximal convoluted tubule, loop of Henle and distal convoluted tubule, these tubules are lined with cells rich in mitochondria and surrounded by capillaries, so useful substances (all glucose, most water and needed ions) are reabsorbed back into the blood. Cells of the tubule also actively transport extra wastes such as hydrogen and potassium ions from the blood into the filtrate - secretion. The structure therefore matches each function: a high-pressure filter, a long absorptive tubule, and an exchange surface with the blood.

Marker's note: full marks need each of the three processes named, located in the right part of the nephron, AND linked to a structural feature. Naming the processes without the structures caps at 2.

Worked example 2 - how dialysis replaces the kidney (4 marks)

Question. Explain how haemodialysis replaces the waste-removal function of a failed kidney.

Model answer. Blood is pumped through a dialyser containing a semi-permeable membrane, with dialysate flowing on the other side. The dialysate matches normal plasma in glucose and ions but contains no urea or other wastes, so wastes (urea, creatinine, excess potassium) diffuse down their concentration gradient out of the blood into the dialysate, while glucose and required ions, being equal on both sides, are not lost. Blood and dialysate flow counter-current to keep the gradient steep. The machine therefore artificially performs the kidney's filtration and waste-clearance role, keeping blood composition within homeostatic limits.

Marker's note: the marks hinge on naming the membrane, the waste-free dialysate, diffusion down a gradient, and "no net loss of needed solutes". "It cleans the blood" alone scores poorly.

Worked example 3 - evaluate dialysis vs transplant (6 marks)

Question. Evaluate dialysis and kidney transplantation as technologies to assist a patient with end-stage kidney failure.

Model answer. Both technologies keep the patient alive when the kidneys can no longer maintain homeostasis. Dialysis removes wastes and excess water by diffusion across a membrane, is immediately available to almost anyone and needs no donor, but it works only intermittently, imposes a heavy time burden (12 to 15 hours a week) and dietary limits, and does not restore the kidney's hormone functions, so anaemia and bone problems persist. A transplant replaces the organ with a working donor kidney, restoring continuous, fully regulated function and giving better long-term survival and quality of life, but it depends on a scarce matched donor, requires major surgery, and needs lifelong immunosuppressants that raise infection and cancer risk. Judgement: for an otherwise healthy patient, transplantation is the more effective long-term treatment, with dialysis as the essential bridge keeping the patient alive while awaiting a donor.

Marker's note: "evaluate" needs a clear, supported judgement. Top responses weigh effectiveness, quality of life and limitations for both, and do not just list pros and cons.

Common traps

Confusing reabsorption and secretion: Reabsorption moves useful substances out of the filtrate back into the blood; secretion adds extra wastes from the blood into the filtrate. The directions are opposite.
Saying dialysis "filters" the blood like the glomerulus: Dialysis works by diffusion across a membrane down concentration gradients, not by pressure filtration. Use the word diffusion.
Forgetting "no net loss of needed solutes": Markers reward the point that the dialysate already contains glucose and ions at normal levels, so only wastes leave the blood.
Treating dialysis as a cure: Dialysis is an ongoing, partial substitute that does not replace the kidney's hormone functions; only a transplant restores full function.
Listing pros and cons with no judgement on an "evaluate" question: You must reach and justify a conclusion.

Exam technique

Link structure to function: When asked to "explain the disorder via the organ", always tie a process (filtration, reabsorption, secretion) to a named structure (glomerulus, PCT, loop of Henle, collecting duct). That link is where the marks sit.
Name the mechanism of the technology: For dialysis, state semi-permeable membrane, waste-free dialysate, diffusion down a gradient, counter-current flow, and no net loss of solutes. For transplant, state donor matching and immunosuppression.
Match the command word: "Compare" needs the same feature for both technologies; "evaluate" needs a justified judgement; "explain" needs cause and mechanism, not just description.
Use precise vocabulary: Nephron, glomerulus, Bowman's capsule, reabsorption, secretion, osmoregulation, urea, dialysate, peritoneum, immunosuppression. Precise terms separate Band 6 from Band 4.

Note

Think of your kidneys as the body's water-treatment plant. Blood flows in dirty, full of dissolved waste from using up food, and the kidneys clean it and send it back, sending the rubbish out as urine. They do this in a tiny filter unit called a nephron - first squeezing fluid out of the blood, then carefully grabbing back the useful stuff (sugar, water, salts) and letting only the rubbish leave. If the plant breaks (often from diabetes or high blood pressure), waste piles up in the blood and that is deadly. A dialysis machine is a stand-in plant: it runs your blood past a thin barrier with clean fluid on the other side, so the rubbish leaks across but the good stuff stays. The best fix, though, is a brand-new plant - a transplanted kidney - but you have to wait for a matching donor and take drugs forever so your body does not reject it.

Practice questions

Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.

foundation2 marks

Name the structure that is the functional unit of the kidney, and state the THREE processes by which it forms urine.

Show worked solution →

1 mark - functional unit. The nephron is the functional unit of the kidney (each human kidney contains about a million).

1 mark - the three processes. Urine is formed by filtration (blood is filtered at the glomerulus into Bowman's capsule), reabsorption (useful substances such as glucose, water and ions are returned to the blood) and secretion (additional wastes such as hydrogen and potassium ions are added to the filtrate).

Naming the nephron earns the first mark; the second mark requires all three processes named in the correct sense, not just "it filters blood".

foundation3 marks

Describe the role of the kidney in homeostasis.

Show worked solution →

1 mark - water balance (osmoregulation): The kidney adjusts how much water is reabsorbed (under the control of ADH), so it regulates blood water content and blood pressure.
1 mark - solute balance: It regulates the concentration of ions (sodium, potassium, hydrogen) and so maintains blood pH and electrolyte balance.
1 mark - waste removal: It removes nitrogenous wastes, chiefly urea (from protein breakdown), plus creatinine and excess water, as urine.

Each mark needs a distinct homeostatic role; listing "removes waste" three different ways caps the answer at 1 mark.

foundation3 marks

Identify THREE causes of chronic kidney failure and, for each, state briefly how it damages the kidney.

Show worked solution →

1 mark each (up to 3)
Diabetes mellitus: Chronically high blood glucose damages the small blood vessels (capillaries) of the glomeruli, reducing filtration (diabetic nephropathy).
Hypertension (high blood pressure): Sustained high pressure damages and narrows the renal blood vessels and the delicate glomerular capillaries, lowering filtration over time.
Polycystic kidney disease (PKD): An inherited (genetic) disorder in which fluid-filled cysts grow in the kidney, enlarging it and progressively destroying functional nephron tissue.

A cause with no mechanism earns half credit at best; the mark is for linking the cause to nephron/vessel damage.

core4 marks

Explain how a haemodialysis machine replaces the function of a failed kidney, referring to the membrane, the dialysate and the direction of diffusion.

Show worked solution →

Award up to 4 marks for an explanation that links the machine's design to the kidney processes it replaces.

The membrane (1 mark): Blood is pumped from the patient through a dialyser containing a semi-permeable (partially permeable) membrane, which lets small molecules (urea, ions, water) pass but holds back blood cells and large plasma proteins.
The dialysate (1 mark): On the other side of the membrane flows dialysate, a fluid made to match normal blood plasma in glucose and essential ions but containing no urea or wastes.
Direction of diffusion (1 mark): Wastes (urea, creatinine) and excess ions diffuse down their concentration gradient out of the blood into the dialysate; because the dialysate already holds glucose and ions at correct levels, there is no net loss of needed solutes.
Counter-current / link to function (1 mark): Blood and dialysate flow in opposite directions (counter-current) to keep the gradient steep along the whole membrane, so the machine performs the kidney's filtration and waste-removal role artificially. An answer that says "it filters the blood" without the membrane/dialysate/diffusion detail caps at 2 marks.

core4 marks

Compare haemodialysis with peritoneal dialysis as technologies for treating loss of kidney function.

Show worked solution →

A compare question needs similarities AND differences, addressed for both.

Similarity (1 mark): Both remove wastes and excess water by diffusion across a semi-permeable membrane between blood and a dialysate, replacing the failed kidney's clearance role.
The membrane used (1 mark): Haemodialysis uses an artificial membrane in an external dialyser; peritoneal dialysis uses the patient's own peritoneum (the membrane lining the abdomen) as the exchange surface, with dialysate run into the abdominal cavity.
Setting and schedule (1 mark): Haemodialysis is usually done at a clinic, 3 times a week for 4 to 5 hours each, needing a machine and trained staff; peritoneal dialysis can be done at home, daily (often overnight), giving more independence.
Risk / evaluation point (1 mark): Haemodialysis needs vascular access (a fistula) and carries bleeding/clotting risk; peritoneal dialysis avoids needling but carries a real risk of peritonitis (infection of the peritoneum). Full marks need a feature stated for BOTH technologies, not one described in isolation.

exam6 marks

A 45-year-old patient has end-stage kidney failure. Evaluate the use of dialysis compared with a kidney transplant as the technology to assist this patient. In your response, refer to effectiveness, quality of life and limitations.

Show worked solution →

"Evaluate" needs a supported judgement, not just a list. Target a sequenced response weighing both technologies.

How each assists (1 mark): Dialysis (haemo or peritoneal) artificially removes wastes and excess water by diffusion across a membrane, but only intermittently and without the kidney's hormonal/homeostatic functions. A transplant replaces the failed organ with a working donor kidney that restores continuous, fully regulated function.
Effectiveness (1-2 marks): A successful transplant gives better waste clearance, better long-term survival and near-normal homeostasis (including erythropoietin and vitamin D activation that dialysis cannot provide). Dialysis keeps the patient alive but clears wastes less completely and does not restore hormone functions.
Quality of life (1 mark): A transplant frees the patient from the 12 to 15 hours a week time burden of dialysis and dietary/fluid restrictions; dialysis is ongoing, tiring and lifestyle-limiting.
Limitations (1-2 marks): A transplant requires a matched donor (scarce, long waiting lists), major surgery, and lifelong immunosuppressant drugs to prevent rejection (raising infection and cancer risk). Dialysis is immediately available to almost anyone and avoids surgery and immunosuppression, but is a permanent, costly, partial substitute.
Judgement (1 mark): A supported conclusion: for an otherwise healthy 45-year-old, transplant is the superior long-term treatment (better survival, quality of life and homeostasis), with dialysis as the essential bridge keeping the patient alive while awaiting a donor. An answer with no explicit judgement caps below full marks.

exam7 marks

Explain how loss of kidney function disrupts homeostasis, and assess how effectively the available technologies (dialysis and transplant) restore it.

Show worked solution →

"Explain ... assess" needs the biology of the disruption PLUS a judged appraisal of the technologies.

Normal homeostatic role (1 mark): Healthy kidneys regulate blood water and solute (ion) balance, blood pH, and remove nitrogenous waste (urea); they also secrete hormones (erythropoietin, renin) and activate vitamin D.
Disruption when function is lost (2 marks): As nephrons fail, urea and creatinine build up (uraemia, toxic), water and sodium are retained (oedema, raised blood pressure), potassium rises (dangerous for the heart), pH falls (acidosis), and reduced erythropoietin causes anaemia. Homeostatic set points are no longer maintained, which is fatal untreated.
How effectively dialysis restores it (1-2 marks): Dialysis clears urea and excess water and corrects ion/pH balance intermittently, keeping the patient alive, but it works only during sessions (levels rise again between them), and it does NOT replace hormone functions, so anaemia and bone problems persist. It is a partial, ongoing substitute.
How effectively a transplant restores it (1-2 marks): A working donor kidney restores continuous, fully regulated filtration, reabsorption, secretion AND the hormone roles, so homeostasis is much more completely restored - the most effective option when a matched donor is available and rejection is controlled.
Judgement (1 mark): Conclusion: transplant restores homeostasis most completely; dialysis restores it partially but is vital and universally available. Naming uraemia, hyperkalaemia and acidosis as specific disruptions lifts the response into the top band.

Practise this

Sources & how we know this

Biology Stage 6 Syllabus (2017) — NESA
Biology HSC exam papers — NESA