What you need to know
- 7.17–7.22Original mixed practice. Not a clinical case.
No mark allocations. Ornithine-cycle steps, second-messenger names and renin stay out.
By the end, you should be able to…
- Place ureaThe main nitrogenous waste product made in the liver from ammonia, and excreted in urine. production in the liver and filtration in the glomerulusA knot of capillaries inside the Bowman’s capsule, where ultrafiltration happens..
- Run the ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine. loop in both directions.
- Give hot and cold effector responses without moving capillaryA tiny vessel whose wall is a single layer of endothelium, giving a short diffusion distance for exchange with tissues..
Build your understanding
Three organs, three jobs
Liver: deaminationRemoving the amine group from excess amino acids in the liver, forming ammonia, which is converted to urea. and ureaThe main nitrogenous waste product made in the liver from ammonia, and excreted in urine.. hypothalamusA region of the brain that controls homeostasis, including body temperature and water balance.: osmoreceptorA receptor in the hypothalamus that detects changes in the water potential of the blood. and the thermoregulatory centreThe region of the hypothalamus that detects blood temperature and controls responses to keep body temperature stable.. Posterior pituitary: releases ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine.. Kidney: ultrafiltrationFiltration of blood under high pressure in the glomerulus. Small molecules pass into the Bowman’s capsule; cells and large proteins stay behind., selective reabsorptionTaking useful substances, such as glucose, amino acids and much of the water, back into the blood from the filtrate., the loop, and the ADH-sensitive duct. Mixing these up is the usual way to lose the point.
Direction still matters
Concentrated plasmaThe liquid part of blood: water carrying dissolved substances such as glucose, ions, hormones and proteins. leads to more ADH and less urine. Dilute plasma leads to less ADH and more urine. High temperature leads to sweat and vasodilationWidening of arterioles when their smooth muscle relaxes, so more blood flows through.. Low temperature leads to vasoconstrictionNarrowing of arterioles when their smooth muscle contracts, so less blood flows through. and shivering. Each response opposes the change.
Make your reasoning visible
Proteins do not filter
If the question lists what is in the filtrate, leave proteins and cells out.
Peptide outside
ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine. binds on the cell surface. A steroid would enter.
Avoid this misconception
The claim: The collecting duct builds the salt gradient and the loop responds to ADH.
The correction: The loop of Henle is the countercurrent multiplier. ADH changes the water permeability of the distal tubule and collecting duct.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
State one difference in how a peptide hormone and a steroid hormone reach a transcription factor.
Show suggested answer
A peptide binds an extracellular receptor and the message is passed in. A steroid crosses the membrane and binds an intracellular receptor that affects DNA.
Practice question
Retrieval and explanation · suggested answer
What is removed from an amino acid in the liver before urea is formed?
Show suggested answer
The amino group, by deamination. The ammonia is converted to urea. Cycle details are not required.
Practice question
Retrieval and explanation · suggested answer
Name two solutes that are selectively reabsorbed in the proximal tubule.
Show suggested answer
Glucose and amino acids. Ions may also be reabsorbed.
Practice question
Retrieval and explanation · suggested answer
Give two heat-loss responses and the centre that coordinates them.
Show suggested answer
Sweating and vasodilation of skin arterioles, coordinated by the hypothalamus. Hairs lying flat is also valid.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
During a long run, core temperature rises slightly, plasma becomes more concentrated, and urine output later falls. Identify the two feedback loops. What would be inconsistent with both loops working?
Show a hint
Temperature effectors and ADH. Then pick a result that goes the wrong way.
Explore the reasoning
The hypothalamus is increasing heat loss, for example by sweating and vasodilation. Osmoreceptors are causing more ADH, so the collecting duct reabsorbs more water and urine volume falls. It would be inconsistent if temperature kept rising while sweating had stopped, or if urine volume rose while plasma was more concentrated and ADH was known to be acting. Those failures are limits, not extra diseases to name.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the part of the brain that coordinates thermoregulation.
Homeostasis in exercise
Practice question
Retrieval and explanation · suggested answer
Name the part of the brain that coordinates thermoregulation.
Lesson: 7C 4: Thermoregulation and Exercise
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The public filter is the start of the answer
7C · Exam PracticeNIDDK’s overview says kidneys filter wastes, including urea, and return substances the body needs. Use that sentence as a check that your longer answer has not put urea…Specification 7.18Read & use the scienceClose article
Why this matters
NIDDK’s overview says kidneys filter wastes, including urea, and return substances the body needs. Use that sentence as a check that your longer answer has not put urea production in the kidney or left glucose in the urine as if that were normal. The page is not the countercurrent account. 7.17–7.22: hormones, kidney, ADH and thermoregulation. 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.
- 7.18 Gross and microscopic kidney structure.
Use the science
Which specification detail is still missing if an answer only says “the kidney filters urea and keeps useful nutrients”?
Explore a possible response
Any of: ultrafiltration under pressure at the glomerulus, selective reabsorption in the proximal tubule, the loop as a countercurrent multiplier, or ADH control of the collecting duct. The public sentence is true and incomplete.
Explore the source
- NIDDK: your kidneys and how they workUS 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 7.18Read & 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.
- 7.18 Gross and microscopic kidney structure.
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