Osmoregulation and the Action of ADH
Osmoregulation keeps blood water potential near its usual range. ADH changes water permeability in collecting duct cells, adjusting water reabsorption and the volume and concentration of urine.
What a learner can do afterwards
- Traces the loop from a fall in blood water potential through to a change in urine volume.
- Explains how ADH increases collecting duct water permeability and water reabsorption.
- Predicts what happens to urine concentration after a large drink of water.
1 · Read
Osmoregulation keeps blood water potential near its usual range. Water loss in sweat, or salt gain without enough water, can lower it. Osmoreceptors in the hypothalamus detect the fall. ADH means antidiuretic hormone: anti means against, and a diuretic increases urine volume. ADH reduces water loss in urine. It is made in the hypothalamus and released into the blood from the posterior pituitary.
ADH travels in the blood to collecting duct cells. It increases aquaporin water channels in the membranes facing the fluid inside the duct. This makes the duct wall more permeable to water. In the kidney medulla, water can leave the duct by osmosis into tissue fluid with a lower water potential, then return to blood. This increases reabsorption of filtered water; it is different from forming filtrate by pressure at the glomerulus.
With healthy kidneys, more ADH increases water reabsorption and leaves a smaller volume of more concentrated urine. Returning water raises blood water potential towards its usual range. As the fall is corrected, the stimulus for ADH release decreases. This is negative feedback: the response opposes the change that started it. Urine colour can change for other reasons, so colour alone does not tell you the ADH level.
After a large drink has been absorbed, blood water potential may rise. Less ADH is then released, and fewer aquaporin channels remain in the tubule-facing membranes. The collecting duct becomes less permeable to water. Less water is reabsorbed, so a larger volume of dilute urine forms. This prediction assumes healthy kidneys. Dehydration means too little body water; diuresis means increased urine production.
ADH increases collecting duct water permeability. More water returns to blood by osmosis, leaving a smaller volume of more concentrated urine.
Original lesson, questions and flashcards written for this dataset. Anatomy and Physiology and Khan Academy renal physiology were consulted for facts and teaching structure.
Three original readings written for this dataset. Anatomy and Physiology and Khan Academy renal physiology were consulted for facts and teaching structure.
More to read
Two directions through the water-balance loop
Start with the quantity being controlled: blood water potential. A water loss that makes blood more concentrated lowers its water potential. Absorbing enough drinking water can produce the opposite change.
When water potential falls, osmoreceptors in the hypothalamus detect it. More ADH is released from the posterior pituitary. Collecting duct cells respond by increasing water channels in their tubule-facing membranes. More water is reabsorbed, and less is lost in urine.
Now start with a rise in blood water potential after drinking. ADH release decreases. With fewer water channels in those membranes, the collecting duct is less permeable. Less water is reabsorbed, so a larger volume of dilute urine leaves the body.
Both responses oppose their starting changes. Retaining water helps correct a fall in blood water potential. Losing more water helps correct a rise. As water potential approaches its usual range, the stimulus driving the response decreases. This is negative feedback, not a command to keep ADH permanently high or permanently low.
A hormone signal, a channel and a gradient
ADH is also called vasopressin. It is made in the hypothalamus, then stored and released from the posterior pituitary. Making a hormone and releasing it into blood are different jobs.
In the kidney's water-balance response, ADH acts on collecting duct cells. Aquaporins are channels through which water can cross cell membranes. ADH increases the number of these channels in the membranes facing the fluid inside the duct.
Adding a channel makes water movement easier, but it does not pump water. In the medulla, water can move by osmosis from the duct into surrounding tissue fluid with a lower water potential. Water then returns to the circulation.
The name antidiuretic describes reduced water loss in urine. A diuretic increases urine production. Diuresis means increased urine production, while dehydration means that the body has too little water. These terms do not identify a single cause or a particular ADH level. Urine colour alone does not measure ADH either.
Filtered water is not all lost in urine
Compare what enters a nephron with what finally leaves it. Pressure at the glomerulus moves water and small dissolved substances from blood into the capsule. The filtration barrier normally retains blood cells and most plasma proteins. The fluid entering the tubule is filtrate, not finished urine.
As filtrate travels along the tubule, water and useful dissolved substances are reabsorbed. In the previous topic, you followed glucose returning to blood through transport proteins. Water can be reabsorbed by osmosis, so different substances need different explanations.
Imagine two periods in which the same volume of filtrate is formed. During one period, more of its water returns to blood. During the other, less returns. The amount filtered is the same, but the amount left to become urine differs.
ADH helps make this later adjustment by changing collecting duct water permeability. A smaller urine volume can therefore result from increased reabsorption, without requiring a decrease in filtration. Keep those two processes separate when explaining an ADH response.
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