Syndrome of inappropriate antidiuretic hormone secretion (SIADH)

How does your body know when to retain fluids and when to get rid of them?

It’s not like you just think to yourself “I’ve had too much water, better get rid of some.” (If you do, and it works, call us).

Your body uses chemicals called hormones to send widespread messages, kind of like how the P.A. system at school tells everyone to ignore the smoke billowing out of the science wing.

The antidiuretic hormone, abbreviated as ADH, is the hormone that controls water retention in the body.

It also constricts blood vessels, and incidentally the vasoconstrictor drug called vasopressin is just ADH. Cool! But that’s not what we’re talking about right now.

Anyways, the more ADH floating around in your blood, the more fluid you retain.

The less ADH in your blood, the more fluid you excrete.

The nephrons in the kidneys are the structures that physically control how much water is excreted from your body.

Nephrons are mostly a series of tubes attached end-to-end that type fluids and wastes towards the bladder.

These tubes though also allow fluids and electrolytes to move through the tube walls and back into the blood if needed.

ADH affects the last two-thirds of these tubes, called the distal convoluted tubule and the collecting ducts.

These tubes focus almost exclusively on reabsorbing water back into the blood.

The wall of these tubes are unsurprisingly made up of cells, a common trait of living things, but these cells have proteins called aquaporins.

Aquaporins allow water to move quickly in and out of the cells.

The more ADH floating around in the blood, the more aquaporins are available to... ahem...facilitate water movement through the cell (yo, wata, come over here for a sec).

So when ADH is low, most of the water flows through the distal convoluted tubule and the collecting duct, giving us diluted urine.

When ADH is high, aquaporins grab much of the water passing through the these tubes and throws them back into the blood.

When I drink a glass of water and that water is absorbed into my blood, my plasma osmolality drops, which means I’m diluting my blood with the water.