the

How does Antidiuretic Hormone (ADH) work?

so the main factor that controls how

much water the kidney excretes is ADH

and now let's talk about how it works so

the part of the kidney that ADH acts on

is here it's the collecting duct so

let's zoom in on a little piece of the

collecting duct so here's a zoomed in a

version of the collecting duct and what

ADH does is it comes over and we'll draw

it as these little circles and it binds

to receptors on the principle cells in

the collecting duct

so not the intercalated cells but the

principle cells and what happens when it

binds to these receptors is that it

causes the cell to put aquaporins into

its apical membrane so these are

aquaporins and if you recall aquaporins

are channels that allow water to pass

freely and so these aquaporins

allow water from this fluid which is

basically urine at this point they allow

water to pass through and therefore get

reabsorbed so water molecules pass that

way and now hopefully this leaves you

with a lot of questions if I stop the

video now you should be really

unsatisfied and there are a couple of

reasons why one is okay so you put

aquaporins on the apical membrane but

how is water getting through the

basolateral membrane well it turns out

that the basolateral membrane actually

always has little aquaporins in it so

that it's really easy for water to pass

through that side so that's one

complaint you might have the next

complaint you might have is that when we

talked about fluid shifts we said that

water shifts in and out of cells through

aquaporins so that made it seem like

cells always have aquaporins and water

can always get into them and that's true

to an extent but the number of

aquaporins is limited so that that can't

actually happen so fast so when we add

more aquaporins into the membrane it

allows this allows a lot more water to

get through and faster so that you can

reabsorb more so that might be complaint

to number two and now complaint number

three you should have probably the

biggest complaint you should have is you

know we spent so much time talking about

in the proximal convoluted tubules and

the loop of Henle and so on we spend so

much time talking about what

Rives the reabsorption of sodium and

water and so on we talked about all

those osmotic forces that caused that to

happen so what is the force driving

water to cross here

the aquaporins allow water to pass

through but what causes water to go this

way and the answer to that is that

actually out here we have an extremely

high concentration of solutes so what is

out here well out here is interstitial

right we're not inside cells and we're

not inside the blood so it's

interstitial and we're actually in the

part of the kidney that we call the

medulla and so I'm drawing this dotted

line to show the separation between the

cortex and the medulla and so the high

concentration is here in the medulla so

it's in the medullary meaning in the

medulla the medullary interstitial so

that's where we have our high

concentration and because we have a very

high concentration out here the driving

force for water to be absorbed is our

usual driving force which is osmotic now

this just brings up more questions like

how do we have a high concentration of

solutes here in the medulla and that

will be the subject of upcoming videos

but for now just understand that the

driving force for water reabsorption is

this high concentration and the high

concentration is always here but it's

only when you have ADH which comes along

and which puts aquaporins here in the

apical membrane it's only then that

you're actually using this high

concentration to drive reabsorption of

water if you don't have a th you don't

have the aquaporins and therefore you

can't make use of that osmotic

difference