The Lab / Unit 1: The heart, the kidney and blood pressure / Lesson 22
The body controls its water apart from its salt. Watch the brain sense how concentrated the blood is and send out ADH, a hormone that tells the kidney to keep water, then see the urine change when that loop breaks in four different ways.
Build the loop
Blood is mostly water carrying dissolved particles, called solutes. Osmolality is how concentrated the blood is: how many solute particles there are for its water. Lose water and it rises; drink water and it falls. A healthy body keeps it in a narrow range.
Build the loop
The hypothalamus is a small region at the base of the brain. In it sit osmoreceptors: sensor cells that check how concentrated the blood is, especially its sodium, the main part of table salt. When the blood grows too concentrated, they fire.
Build the loop
Firing osmoreceptors make the posterior pituitary, the back of a small gland under the brain, release a hormone: a chemical messenger carried in the blood. It is ADH, antidiuretic hormone, also called vasopressin. The hypothalamus makes it; the pituitary stores and releases it.
Build the loop
In the kidney, urine flows through tiny tubes. The last stretch is the collecting duct, lined by cells with urine on one side and blood on the other. ADH fits a receptor on those cells: a docking site shaped for one messenger. Lesson 2 follows the tubes.
Build the loop
Docked ADH makes the cell move its aquaporins, water channels kept in little bubbles inside it, into the membrane that faces the urine. Each channel is a tiny pore that water can pass through. More ADH, more channels in the membrane.
Build the loop
With channels in place, water moves out of the urine, through the cell and back into the blood. Taking something back from the urine like this is called reabsorption. The water kept dilutes the blood.
Build the loop
Water kept is water not lost. More ADH: less urine, more concentrated. Less ADH: more urine, more dilute. The flask marks a typical day’s output and the least the body needs to clear its wastes; the Clinical values tab gives their numbers.
Build the loop
As water returns, the blood becomes less concentrated. The osmoreceptors calm, less ADH is released, and the channels are pulled back out of the membrane. This is negative feedback: a result that turns down its own cause, holding the blood in its narrow range.
Build the loop
Sodium is the main solute outside cells, so extra water dilutes it and lost water concentrates it. Too little is hyponatremia; too much is hypernatremia: the two shaded ends of the sodium gauge.
Build the loop
Too concentrated: osmoreceptors fire, ADH rises, water channels move in, and less, concentrated urine is made until the blood is back in range. Too dilute: ADH falls, channels are removed, more dilute urine is made. Watch three measures: urine volume, urine concentration and blood sodium.
What changes it
Dehydration is a net loss of water, as after a long, hot run without drinking. The blood grows more concentrated, the osmoreceptors fire and ADH rises. More water channels open, so the kidney keeps water: urine becomes less, and more concentrated.
What changes it
Drink a large amount of water and the blood becomes more dilute. The osmoreceptors calm, ADH falls, and water channels are pulled out of the membrane. The kidney lets the extra water go as more, dilute urine: diuresis, making more urine than normal.
What changes it
Diabetes insipidus means large volumes of dilute urine. In AVP deficiency, or central diabetes insipidus, too little ADH (AVP, arginine vasopressin) is made and released. Few channels open, so water is not kept: polyuria, passing far more urine than normal, and dilute.
What changes it
In AVP resistance, or nephrogenic diabetes insipidus, ADH is made and reaches the kidney, but the collecting duct does not respond to it. Channels stay inside the cells, so the result looks like AVP deficiency: polyuria with dilute urine, though ADH is in the blood.
What changes it
In SIADH, the syndrome of inappropriate antidiuretic hormone, ADH keeps being released when it should be turned down, and water excretion is impaired: the water held back dilutes the blood’s sodium into hyponatremia. The source gives no direction for the water channels or the urine, so those gauges read “not stated”.
What changes it
Primary polydipsia is drinking far more fluid than the body needs. It leads to polyuria with diluted urine, as in diabetes insipidus, but caused by the drinking. Ultimately, sodium is diluted into hyponatremia. The source gives no direction for ADH, so its gauge reads “not stated”.
Start with Next, or use the arrow keys; Space plays or pauses a moving picture. Under it: pause, step forward, drag to any moment, slow it down or replay. Each change asks for your prediction first, every time it plays. Underlined terms light up their part of the picture. The pictures show directions only, never measurements.
Educational, not for clinical use
Loading reference values…
Each bar has its own unit, printed exactly as its source writes it; bars in different units are never compared. A shaded band is a reference range; a red line with a shaded side is a threshold, and the shaded side is the condition the source names; a black line is a single typical value or a minimum.
The dashed line is the baseline and the blue line is the state you chose. The sources give directions, not amounts, so the blue line only moves a fixed step from baseline, and crosses a threshold only where a source names that condition for the state. “Not stated in sources” means no cited excerpt gives a direction, so none is drawn. Choosing a state asks for a prediction first.
The whole loop in one picture. Choose a state; first say what you expect urine volume to do compared with the baseline, then watch the picture change from the state you were on, over dashed outlines of the baseline. Every gauge reads from a table of quoted directions; where the sources say nothing, the gauge stays at normal and says “not stated”.
Baseline: everything in balance.