Osmoregulation and the nephron - TCE Biology (Tasmania)
“Analyse osmoregulation in humans, including stimulus-response models, feedback loops and the associated hormone (antidiuretic hormone) and organ structures, including kidney nephron function”
Osmoregulation keeps the water and solute concentration of the blood stable. The kidneys filter the blood in nephrons (about a million in each kidney): water, glucose, salts and urea are filtered out of the glomerulus into Bowman's capsule, then useful substances and most of the water are reabsorbed as the filtrate passes along the tubule. When the blood is too concentrated, osmoreceptors in the hypothalamus detect it, the posterior pituitary releases more antidiuretic hormone (ADH), and ADH makes the collecting ducts more permeable to water, so more water is reabsorbed and a small volume of concentrated urine is produced. When there is too much water, less ADH is released and more dilute urine is produced.
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What this dot point is asking
Osmoregulation in animals is part of criterion 6 (Section C of the TASC exam). The external assessment specifications include the role of the hypothalamus, a basic understanding of kidney function for osmoregulation, the mechanism of ADH on the collecting tubule, kidney structure and function including filtration and reabsorption, and the components of urine.
The nephron
| Part | Structure | Function |
|---|---|---|
| Glomerulus | A knot of capillaries | Blood is under high pressure, which forces small molecules out (filtration) |
| Bowman's capsule | Cup surrounding the glomerulus | Collects the filtrate |
| Proximal convoluted tubule | Coiled tube after the capsule | Reabsorbs all glucose and amino acids, and most water and salts |
| Loop of Henle | Hairpin loop dipping into the medulla | Reabsorbs water and salts; creates a high salt concentration in the medulla that helps later water reabsorption |
| Distal convoluted tubule | Second coiled section | Further reabsorption of salts and water |
| Collecting duct | Tube collecting filtrate from many nephrons | Water reabsorption controlled by ADH; carries urine to the renal pelvis |
Filtration and reabsorption
- Filtration: blood pressure forces water, glucose, amino acids, salts and urea out of the glomerulus into Bowman's capsule. Blood cells and large proteins are too large and stay in the blood.
- Reabsorption: as the filtrate moves along the tubule, useful substances are taken back into the surrounding capillaries: all glucose and amino acids, most salts and most water.
- Urine: what remains is water, urea, excess salts and other wastes. Urine does not normally contain glucose, protein or blood cells.
ADH and the feedback loop
Blood too concentrated (dehydration, sweating, salty food)
- Stimulus: blood solute concentration rises.
- Receptor and control centre: osmoreceptors in the hypothalamus detect the change.
- Message: the posterior pituitary releases more ADH into the blood (ADH is made in the hypothalamus).
- Effector: the collecting ducts of the nephrons.
- Response: ADH makes the collecting duct walls more permeable to water, so more water moves by osmosis into the blood. Urine is lower in volume and more concentrated.
- Result: blood concentration falls to normal; ADH release decreases.
Blood too dilute (drinking a lot of water)
Less ADH is released, the collecting ducts become less permeable to water, less water is reabsorbed, and a large volume of dilute urine is produced.
ADH does not make the kidneys "produce more water". It increases the permeability of the collecting ducts to water so that more water is reabsorbed into the blood. (Some students describe water channels called aquaporins; TASC's 2025 assessment report notes these are not in the course document, so they are optional detail.)
Two days in summer
On Day 1 a student drinks 3 L of water and produces a large volume of pale urine. On Day 2 they play sport in the heat, drink very little, and produce a small volume of dark urine.
- Day 1: blood becomes dilute, the hypothalamus detects this, less ADH is released, the collecting ducts reabsorb less water, so urine is plentiful and dilute.
- Day 2: water is lost in sweat, blood becomes concentrated, more ADH is released, more water is reabsorbed in the collecting ducts, so urine is scant and concentrated.
Marker's note: include the receptor, the hormone, the target, the change in permeability and the effect on urine.
- Confusing the kidney with the liver
- Filtration and water reabsorption happen in the kidney's nephrons.
- Saying ADH is released when there is too much water
- More ADH means more water is kept; it is released when the blood is too concentrated.
- Leaving glucose in the urine of a healthy person
- All glucose is reabsorbed in the proximal convoluted tubule unless blood glucose is abnormally high.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksAfter a long hike without drinking, a student's blood becomes more concentrated. Explain how negative feedback restores water balance.Show worked answer →
One mark for each step.
- Stimulus: the solute concentration of the blood rises (water deficit).
- Receptor and control centre: osmoreceptors in the hypothalamus detect the rise.
- Message: the hypothalamus causes the posterior pituitary to release more ADH into the blood.
- Effector: ADH acts on the collecting ducts of the nephrons in the kidneys, making their walls more permeable to water.
- Response: more water is reabsorbed into the blood by osmosis, producing a smaller volume of more concentrated urine.
- Result: blood concentration returns towards normal, so the stimulus is reduced and ADH release decreases.
Original4 marksExplain why glucose is present in the filtrate in Bowman's capsule but not normally in urine, while urea is present in both.Show worked answer →
- Filtration (1 mark)
- Glucose and urea are small molecules, so both are forced out of the glomerulus into Bowman's capsule under pressure.
- Glucose (2 marks)
- All of the glucose is reabsorbed back into the blood from the proximal convoluted tubule, so none normally remains in urine.
- Urea (1 mark)
- Urea is a waste product and is not reabsorbed to the same extent, so it stays in the filtrate and is excreted in urine.