What you need to know
- 7.18–7.20Enrichment: replacing part of filtration when kidneys fail. Not a treatment protocol and not extra required recall.
No advice about who should have dialysis. No doses, schedules, or diet rules. The machine is not the loop of Henle and not ADH.
By the end, you should be able to…
- Describe haemodialysis as an external filter for wastes and extra fluid.
- State two nephronThe functional unit of the kidney, where blood is filtered and urine is formed. jobs the machine does not copy in full.
- Keep the account free of clinical instructions.
Build your understanding
A filter outside the body
NIDDK describes haemodialysis as a treatment that pumps blood through a dialyser, an artificial filter, to remove wastes and extra fluid and to balance some minerals. It is used when kidneys have failed. It replaces only part of kidney function, and it is not a cure. That is the enrichment fact. It is not a set of instructions.
ultrafiltrationFiltration of blood under high pressure in the glomerulus. Small molecules pass into the Bowman’s capsule; cells and large proteins stay behind. in a glomerulusA knot of capillaries inside the Bowman’s capsule, where ultrafiltration happens. is also a pressure-driven separation of small molecules from blood cells and proteins. The likeness stops at “small wastes leave the blood”. Your specification answer still needs the afferent arteriole, the basement membrane, and the nephronThe functional unit of the kidney, where blood is filtered and urine is formed. order. A machine does not give you those marks.
What is not copied
A healthy nephron then selectively reabsorbs glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. and amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group. in the proximal tubuleThe first coiled part of the nephron, where most useful substances are selectively reabsorbed., builds a medullary gradient in the loop of HenleThe hairpin loop of the nephron that makes the medulla salty, so water can be reabsorbed from the collecting duct., and responds to ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine. at the collecting ductThe tube that carries urine through the medulla. ADH controls how much water is reabsorbed from it.. A dialysis session is scheduled. It does not use osmoreceptorA receptor in the hypothalamus that detects changes in the water potential of the blood. minute by minute. Saying “dialysis is the loop of Henle” erases 7.20 and 7.21.
ureaThe main nitrogenous waste product made in the liver from ammonia, and excreted in urine. still comes from deaminationRemoving the amine group from excess amino acids in the liver, forming ammonia, which is converted to urea. in the liver. Dialysis does not move that chemistry into the arm. If kidneys cannot clear the urea, the machine can remove some of it from the blood. Production and removal remain different places.
Make your reasoning visible
Part, not all
The source says dialysis replaces part of kidney function. Write that limit.
No protocol
Do not add hours, diets, or access surgery to a specification answer.
Avoid this misconception
The claim: A dialysis machine performs the ornithine cycle and secretes ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine..
The correction: ureaThe main nitrogenous waste product made in the liver from ammonia, and excreted in urine. is made in the liver. ADH comes from the pituitary and acts on the collecting ductThe tube that carries urine through the medulla. ADH controls how much water is reabsorbed from it.. Dialysis filters blood outside the body. It replaces only part of excretion.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
According to the NIDDK account, what does a dialyser remove from the blood?
Show suggested answer
Wastes and extra fluid. It also helps balance minerals. It is not a cure for kidney failure.
Practice question
Retrieval and explanation · suggested answer
Name one nephron process that still has to be learned even if you can describe dialysis.
Show suggested answer
Any one of: ultrafiltration at the glomerulus, selective reabsorption in the proximal tubule, the countercurrent multiplier, ADH and the collecting duct.
Practice question
Retrieval and explanation · suggested answer
Where is urea produced, whether or not a person uses dialysis?
Show suggested answer
In the liver, from excess amino acids.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A paragraph says “the machine reabsorbs glucose like a proximal tubule and concentrates urine like ADH”. Using only the public description of haemodialysis as an external filter that removes wastes and fluid, rewrite what the machine can be said to do.
Show a hint
Do not give the machine the nephron’s selective steps unless the source says so.
Explore the reasoning
The machine takes blood outside the body and removes wastes and extra fluid. That is a partial replacement of clearance. The source used here does not say it selectively reabsorbs glucose the way a proximal tubule does, or that it responds to ADH. Those remain kidney processes in 7.20 and 7.21.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the functional unit of the kidney.
Kidney structure
Practice question
Retrieval and explanation · suggested answer
Name the functional unit of the kidney.
Lesson: 7C 2: Osmoregulation in Mammals: The Kidney
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Haemodialysis is not a second nephron
7C · Thinking BiggerNIDDK explains that haemodialysis filters wastes and extra fluid outside the body using a dialyser, and that this replaces only part of what healthy kidneys do. It is…Specification 7.20Read & use the scienceClose article
Why this matters
NIDDK explains that haemodialysis filters wastes and extra fluid outside the body using a dialyser, and that this replaces only part of what healthy kidneys do. It is not a cure. Use it to see the difference between clearing blood and the full specification nephron. Do not use it to advise anyone about treatment. 7.18–7.20: structure, ultrafiltration and selective reabsorption are still the required biology. A clinical observation reflects a working control system, but does not isolate one nephron, hormone or transcription factor without further evidence.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.20 Selective reabsorption and the loop of Henle.
Use the science
Why does a good dialysis summary not earn the countercurrent-multiplier mark by itself?
Explore a possible response
The multiplier is the loop of Henle building a medullary salt gradient. An external filter removing fluid is a different process. The source describes that filter, not the loop.
Explore the source
- NIDDK: haemodialysisUS National Institute of Diabetes and Digestive and Kidney Diseases
Filter and reabsorb, then the specification adds the mechanism
7C · Osmoregulation in Mammals: The KidneyNIDDK describes kidneys as filtering blood, removing wastes such as urea and extra water, and returning water and nutrients the body still needs. It names the nephron, the…Specification 7.20Read & use the scienceClose article
Why this matters
NIDDK describes kidneys as filtering blood, removing wastes such as urea and extra water, and returning water and nutrients the body still needs. It names the nephron, the glomerulus and the tubule. That is the right public outline. It does not teach the afferent arteriole, the basement membrane, or the loop as a countercurrent multiplier. Those mechanisms stay in 7.18–7.20. 7.18–7.20: structure, urea and ultrafiltration, selective reabsorption, and the loop of Henle.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.20 Selective reabsorption and the loop of Henle.
Use the science
Why is “the tubule filters like a sieve” less precise than the specification?
Explore a possible response
The sieve-like pressure filter is the glomerulus. The tubule then reabsorbs useful solutes. Calling the whole tubule a sieve misses selective reabsorption and the multiplier.
Explore the source
- NIDDK: your kidneys and how they workUS National Institute of Diabetes and Digestive and Kidney Diseases