Sweat all afternoon and your urine turns dark and scant; drink a litre of water and it runs pale and plentiful. The difference is set by a single hormone, ADH, which decides how many water channels sit in the walls of the collecting ducts.
Osmoregulation is the control of the water potential of the blood. Osmoreceptors in the hypothalamus detect it. When the water potential falls (after sweating, or eating salty food), they lose water by osmosis and shrink, stimulating neurosecretory cells; these made ADH (antidiuretic hormone) in the hypothalamus and it is released from their endings in the posterior pituitary gland into the blood. ADH binds to receptors on the cells of the collecting ducts, triggering vesicles containing aquaporins (water channel proteins) to fuse with the membrane facing the lumen. The duct wall becomes more permeable, more water moves by osmosis into the concentrated tissue fluid of the medulla and then the blood, and a small volume of concentrated urine is made. When the water potential rises, less ADH is released, aquaporins are taken back into vesicles and a large volume of dilute urine results. This is negative feedback.
Fig. 1: negative feedback control of the water potential of the blood by ADH
Section 01
Sensing water potential, releasing ADH.
Choose dehydrated, normal and drunk a lot of water in turn. Watch the ADH and URINE readings and count the blue aquaporins in the collecting duct wall: many when ADH is high, almost none when it is low.
Receptors: osmoreceptors in the hypothalamus. When blood water potential falls, water leaves them by osmosis and they shrink; this stimulates neurosecretory cells.
Coordinator: the hypothalamus. ADH is made in the cell bodies of the neurosecretory cells, travels down their axons and is stored in their endings in the posterior pituitary gland. Impulses cause its release into the blood.
Effector: the cells of the collecting duct (and the distal convoluted tubule). ADH binds to receptors on the membrane facing the blood; through a second messenger (cAMP) this causes phosphorylation of aquaporin proteins and moves vesicles of aquaporins to the membrane facing the lumen.
Response: vesicles fuse with the luminal membrane, so it contains many more aquaporins. Water moves out of the duct by osmosis, down the water potential gradient into the medulla, where the loop of Henle has made the tissue fluid very concentrated, and is taken into the blood.
Switching off: as blood water potential rises, the osmoreceptors are stimulated less, less ADH is released, ADH already in the blood is broken down, and aquaporins are removed from the membrane by endocytosis.
Examiner trap
ADH does not pump water. It only changes the permeability of the collecting duct to water; the water moves passively by osmosis into the concentrated medulla.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the simulator you choose dehydrated. What do the ADH and URINE readings show?
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What do the blue channels in the collecting duct wall represent?
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In the simulator, where is ADH released into the blood?
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You switch from dehydrated to drunk a lot of water. What happens to the aquaporins?
Section 02
What the urine tells you.
Compare the three states shown in the simulator with what happens to urine.
State
Blood water potential
ADH released
Collecting duct
Urine
dehydrated (sweating, salty meal)
low (more negative)
high
many aquaporins, very permeable
small volume, concentrated
normal
normal
moderate
some aquaporins
normal
after drinking a lot of water
high (less negative)
low
few aquaporins, much less permeable
large volume, dilute
Antidiuretic means against diuresis (the production of a large volume of dilute urine), which describes exactly what ADH does.
Endocrine control: ADH is a hormone, carried in the blood to target cells with specific receptors, so its effect takes minutes to develop and to wear off.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
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Which sequence describes the response to a fall in the water potential of the blood?
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Why does water leave the collecting duct when aquaporins are present?
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A person has a condition in which the collecting duct cells have no functional ADH receptors. Which symptom is expected?
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Which statement about ADH is correct?
Skill unlocked
Osmoregulation and ADH, mastered.
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