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We've talked a lot about the kidney,
but I want to point out

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that the kidney is not the only organ 
involved in controlling the blood pressure.

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In fact the liver plays a really really important role in helping you create the right enzymes and proteins to control blood pressure.

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So I'm gonna sketch out real quick some liver cells here.

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These liver cells are busy making a lot of different proteins.

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One of them is called angiotensinogen.

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Angiotensinogen gets made by the liver cells 
and they dump it out into the blood vessel.

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If we were to draw angiotensinogen
it's actually a pretty large molecule,

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it has lots and lots of amino acids.

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Amino acids are these tiny little circles that I'm drawing.

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I'm connecting them with little bonds.

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You can imagine amino acids like pearls on a necklace.

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In total angiotensinogen has about 450+ amino acids.

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So it's a pretty long chain.
542 amino acids actually.

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It's a pretty long chain, I'm not gonna draw all of it, 
but you get the idea that it's some crazy long chain like that.

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It's little amino acids all the way through.

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So this huge protein gets put into the blood vessels
by the liver cells and starts floating around.

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If you would zoom in on the little protein,
if you could give it a little face,

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it would maybe look something like this, because 
even though it's floating through the body,

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it's basically asleep, it's not really active.

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I just want you to keep that in the back of your mind.

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Even though it's there, it's not truly active.

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It's not really doing a lot.

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At the same time you've got the kidney.

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The kidney is not idle, 
the kidney is busy making a hormone of its own.

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You know there's the afferent arteriole.

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The afferent arteriole is the blood vessel 
that's headed into the glomerulus.

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It's headed into the part of the kidney 
where all of the urine is initially made.

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In the afferent arteriole blood is going that way.

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Lining this afferent arteriole, there are these little cells 
all the way through like this.

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These are called the juxtaglomerular cells.

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I'm doing a simplified version of it,
because this is the part I really want to focus on.

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These juxtaglomerular cells have in them little granules, 
they're sometimes even called granular cells.

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These granular cells sense,either themselves 
or one of the neighbors helped them sense

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when the blood pressure is low.

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Then these granules get dumped into the blood stream.

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If you look at the microscopic level,
you can see these little proteins called renin.

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These are proteins that act at a distant site, 
we call those hormones.

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These peptide or protein hormones are basically 
gonna work on cells far away.

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If the renin is floating along in the blood and the angiotensinogen is floating in the blood, they might meet up.

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In a blood vessels in your arm, 
or maybe in a blood vessel in your leg.