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The Renal System [Lesson + Study Guide]


Next now we're gonna change gears a little bit, move onto Kidney Anatomy. So what we have in the kidney is an afferent arteriole coming in, afferent arteriole in. We have this convolution of blood vessels here that's called the glomerulus. And than exiting there, we have the efferent arteriole, okay? And the same equation that we talked about that governs net filtration pressure also governs a kidney anatomy.

Except for here it's the hydrostatic pressure in the glomerulus. Plus the oncotic pressure in what's on the other side here, that's called the Bowman's capsule. So oncotic pressure in the Bowman's capsule minus hydrostatic pressure in the Bowman's capsule plus oncotic pressure in the glomerulus. That's gonna determine what filters out.

And this value is so high here, the hydrostatic pressure in the glomerulus, that we're always filtering stuff out of the glomerulus into the Bowman's capsule. And this initial stuff that goes from the glomerulus into the Bowman's capsule, is called our initial filtrate. And that's the process of filtration, so you need to know that term. That's going from the glomerulus into the Bowen's Capsule.

Another thing I want you to know is the setup of afferent and efferent arterioles that will maximize filtration. So how do you think we could maximize filtration? Specifically, how could we maximize hydrostatic pressure in the glomerulus? By either vasoconstricting or vasodilating are afferent or efferent arterioles? This is the kinda critical reasoning.

This may not be something that you encounter in you're reading. And sometimes it's not presented in your standard MCAT course, but this is the kinda critical reasoning you need to be able to do. So what's gonna maximize glomerular pressure? Well, a maximally dilated afferent arteriole and a maximally constricted efferent arteriole is going to maximize the net filtration.

Because you'll have a huge pressure there in the glomulerus, okay. So filtration glomerulus to Bowman's capsule. Now we have some stuff in our Bowman's capsule. It's the beginning of what will ultimately be urine. We have proximal convoluted tubule, this is simple rendition of a kidney. Wee have a descending loop of Henle and we have an ascending loop of Henle that gradually gets thicker.

And again this is not 100% detailed. And then, as we come up here, we come right there next to our arterioles. We have a distal convoluted tubule that terminates in a collecting duct here. Then we'll have other nephrons terminating in the same collecting duct. And they have their distal convoluted tubule is out here, okay? And this collecting duct flows down and eventually the contents of the kidney go into the ureter.

From the ureter they go into the bladder. And from the bladder they go into the urethra. And from the urethra they go into our toilet or if we've chosen another location, some other location to urinate. Okay now, you should also know this efferent arteriole continues on and remains in close proximity to the rest of the nephron.

And this is where we get what's called reabsorption or filtration. All right, so reabsorption is movement of things from the kidney tubule back into the blood plasma. And this is often refered to as the vasa recta, it's blood vessels that are in close proximity there to the nephron that exchange things back and forth between the kidney.

Filtration is from the plasma back to the kidney tubule. So reabsorption is from the kidney tubule to the plasma. And filtration, I don't know why I put the filtration there again, apologize. This is called secretion. Secretion is from the plasma to the kidney tubule again. And we're gonna talk about all these things in a little bit more detail.

Before we do that though, we have a short time here together. And I wanna give you the highest yield kidney topics that I can give you in the shortest amount of time. So the main things that seem to be most popular in terms of testing are what happens to sodium. And what happens to water in this kidney tubule or the nephron.

So we're gonna start out here with sodium. And sodium is prominently reabsorbed in the proximal convoluted tubule. Remember reabsorption. It's going back into the blood plasma. So prominently reabsorbed in the proximal convoluted tubule and also in the ascending loop of Henle.

Significant reabsorption there. Water, it seems to be the other hot topic. And it is also prominently reabsorbed in the proximal convoluted tubule. And it's also probably reabsorbed in the descending loop of Henle. It is not reabsorbed in the ascending loop of Henle. And likewise sodium, which is reabsorbed in the proximal convoluted tubule and ascending loop of Henle, is not reabsorbed in the descending loop of Henle.

Another big MCAT point is, our filtration is actually huge. The amount of stuff that we filtrate out from the glomerulus into the Bowman's capsule is huge. It's almost 180 liters a day. And we pee, I don't know how much everybody pees. It depends, obviously, as you all know, on what you are drinking.

But we could pee two or three liters, maybe more a day. So, we are reabsorbing a ton of stuff after it's filtered out. So, a 180 liters a day filtered from glomerulus into Bowman's capsule. And we actually only excrete, pee out, a few liters a day. So we are doing a lot of reabsorption throughout that kidney tubule. Now, let's revisit these concepts.

Filtration, again, from gromerulus to Bowman's capsule. Secretion from after filtration, it's anything that goes from blood plasma back into the kidney tubule. Reabsorption is from kidney tubule back into blood plasma. And then excretion is the end product of everything, it's what we actually, leaves the collecting duct.

And eventually exits the urethra onto our location of choice. So excretion equals, and this should make sense to you, filtration plus secretion, that's everything that goes into the kidney tubulal. Minus reabsorption, which is everything that goes out of it. You should be able to use that equation, it should be fairly intuitive to you I hope.

All right, we're gonna talk about a few hormones that prominently affect the kidney. And what you need to know about them for the MCAT. Everything we talk about today, the topic that we are going to talk about that is the most heavily tested. And shows up year after year on just about everybody's test, are hormones, okay?

And so we'll start here with ADH. ADH, antidiuretic hormone, is the same thing as vasopressin. It is a hormone of the posterior pituitary, and we'll get to talk about that again in just a minute. And what it does is it increases water reabsorption In the distal convoluted tubal and collecting duct.

And it does that by opening aquaporin channels. So it opens aquaporin channels and we retain water. And I'm gonna move down to this last point here, and you should know the two signals that cause for ADH secretion. One is low blood volume. And that makes sense, right?

If the blood volume is low the body is gonna want to secrete something that's gonna retain more water and increase blood volume. And if we have increased blood osmolarity. So if the blood is too concentrated, we'll re-absorb water and that's also a good thing. So ADH, its basic action is to increase blood pressure.

It does that by increasing water reabsorption and it does that in response to these two signals. Next, aldosterone, so if this is the kidney tubule, what aldosterone does is cause sodium to be reabsorbed in exchange for potassium that gets secreted and eventually excreted. And the thing you should know about aldosterone and the kidney in general is that when sodium gets reabsorbed, water often follows sodium.

Okay, so water, will follow that sodium out. And the net result then is obviously increased blood pressure. So aldosterone is something also that's also secreted in response to increased blood pressure. And we're gonna talk a little bit more about aldosterone later in this lesson. Addison's Disease now is something.

It's a condition where there's insufficient aldosterone production. And less important than knowing Addison's Disease by name is being able to think, when I have low aldosterone, what would I expect my blood levels of sodium and potassium to be? So again, this is insufficient aldosterone. So we would expect low sodium in our blood and high potassium in our blood compared with the opposite in our urine.

So if we lack aldosterone, we're gonna have a lot of sodium in our urine and not very much potassium in our urine. And then renin we're gonna talk about. It's a hormone of the kidney so it's put here, but we're going to discuss in more detail at the very end of this lesson

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