ADH and water balance sim
Give a teaching torso a glass of water, a salty snack or an hour of hard exercise in the heat, wait 15 minutes at a time, and watch the hypothalamus, ADH, the collecting duct and the urine bring the plasma back towards normal, with your own readings plotted on a graph.
Homeostasis: osmoregulation by ADH and negative feedback · covers the O-Level, IGCSE, GCSE topic
What the sim shows
ADH and water balance: a model. A teaching torso stands for a person of about 15 with about 29 L of body water. Drag the glass to the mouth to drink 500 cm³ of water, drag the snack to eat salt, or turn the dial for exercise and heat (sweating). Wait 15 minutes or an hour at a time. Osmoreceptors in the hypothalamus detect the concentration of the blood plasma. The pituitary gland releases more ADH into the blood when the plasma is too concentrated, and less when it is too dilute. ADH makes the walls of the collecting ducts in the kidneys more permeable to water, so more water leaves the duct by osmosis into the concentrated tissue of the medulla and goes back into the blood. With lots of ADH the kidneys make a small volume of concentrated urine; with little ADH, a large volume of dilute urine. The response reverses the change, so the plasma moves back towards normal: negative feedback. In feedback words, the change in plasma concentration is the stimulus, the osmoreceptors in the hypothalamus are the receptors, and the kidneys are the effector (some books call the pituitary gland, which releases the hormone, the effector). The kidneys can only save water, not replace it: after sweating, the plasma comes back slowly, from the water in food and normal drinking (a background gain of 60 cm³ an hour in this model), and quickly when the person drinks. Water potential is the other way round: more concentrated plasma has a lower water potential. Simplified on purpose: the numbers are illustrative (≈); ADH is in arbitrary units (a.u.); the plasma concentration is in mOsm per kg (how much is dissolved in each kg of water) and changes by 1 mOsm per kg for each 100 cm³ of water, counted in 10 cm³ lots; the urine volume and concentration for each ADH level are a table; one kidney's duct stands for all the collecting ducts in both kidneys; the brain and pituitary gland are shown schematically, and ADH is made in the hypothalamus and released from the pituitary gland; ADH also acts on the distal convoluted tubule, which is not shown; the time course is simplified (water and salt are absorbed in 30 minutes; ADH moves at least half-way to its new level in each 15 minutes); the extra salt from a snack is not removed by the kidneys in this model (it really takes several hours); thirst is not modelled; the torso, glass, snack and measuring cylinder are about a third of life size, and the duct close-up is not to scale. The blue arrows in the close-up are a key for water, which is colourless, and the purple dots are a key for ADH; urine runs from pale straw to dark amber as it gets more concentrated.
- Lit ring: drop the glass or snack on the mouth
- Close-up: the control loop and a collecting duct (schematic, not to scale)
- Graph: dashed lines, the normal band of the plasma
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