So we're entering the loop of Henle, and it dips down, and then comes back up. And so most of the length of the nephron is the loop of Henle. And if I go back to this diagram right here, if I'm talking about the loop of Henle, I'm talking about this whole thing right there. And you can see something interesting here. It crosses the border between the cortex, this light brown part, and the renal medulla, this kind of reddish or orange part right there, and it does that for a very good reason.
I'm going to draw it here. So let's say this is the dividing line right here. This right here was the cortex. This right here is the medulla. So the whole point-- well, there's two points of the loop of Henle. One point is to make the renal medulla salty, and it does this by actively pumping out salts. So it actively pumps out salts, and it does that in the ascending part of the loop of Henle. So it actively pumps out salts: sodium, potassium, chloride, or chlorine, I should say.
Chlorine ions. It actively pumps out these salts right here to make the entire medulla salty, or if we think about it in terms of kind of osmosis, make it hypertonic. You have more solute out here than you have in the filtrate that's going through the tubules. And it uses ATP to do this. All of this stuff requires ATP to actively pump against a concentration gradient.
So this is salty and it's salty for a reason. It's not just to take back these salts from the filtrate, although that's part of the reason, but by making this salty, the ascending part is only permeable to these salts and these ions.
It's not permeable to water. The descending part of the loop of Henle is only permeable to water. So what's going to happen? If this is all salty because the ascending part is actively pumping out salt, what's going to happen to water as it goes down the descending loop?
Well, it's hypertonic out here. Water will naturally want to go and kind of try to make the concentrations balance out. I've done a whole video on that. It doesn't happen by magic. And so the water will-- because this is hypertonic, it's more salty, and this is only permeable to water, the water will leave the membrane on the descending part of the loop of Henle right now.
And this is a major part of water reabsorption. I've thought a lot about why don't we use ATP somehow to actively pump water? And the answer there is, there's no easy way to do that. Biological systems are good at using ATP to pump out ions, but it can't actively pump out water. Water's kind of a hard thing for proteins to operate on. So the solution is to make it salty out here by pumping out ions and then water, if you make this porous only to water, water will naturally flow out.
So this is a major mechanism of gaining back a lot of the water that gets filtered out up here. And the reason why this is so long is to give time for this water to secrete out, and that's why it dips nice and pretty far down into this salty portion. So then we'll leave the loop of Henle and then we're almost done with the nephron.
Then we're in another convoluted tubule, and you might even guess the name of this convoluted tubule. If this was the proximal one, this is the distal one. And actually, just to make my drawing correct, it actually passes very close to the Bowman's capsule, so let me do it in a different color. The distal convoluted tubule actually goes pretty close to the Bowman's capsule. And once again, I've made it all convoluted in two dimensions, but it's actually convoluted in three.
And it's not that long, but I just had to get over here and I wanted to get over that point right there. It's called distal. Distal is further away.
It's convoluted and it's a tubule. So this right here is the distal convoluted tubule, and here we have more reabsorption: calcium, more sodium reabsorption. We're just reabsorbing more things that we didn't want to lose in the first place. There's a lot of things we could talk about what get reabsorbed, but this is just the overview.
And we're also reabsorbing a little bit of more water. The medulla is full of renal tubules , but does not contain renal corpuscles.
All the renal corpuscles are found in the cortex. Urinary: Nephron The nephron consists of the renal corpuscle and the renal tubule. The end product is urine. The third part of the renal tubule is called the distal convoluted tubule DCT ; this part is also restricted to the renal cortex. This last part of the nephron connects with and empties its filtrate into collecting ducts that line the medullary pyramids.
The collecting ducts amass contents from multiple nephrons, fusing together as they enter the papillae of the renal medulla. Urine leaves the medullary collecting ducts through the renal papillae, emptying into the renal calyces, the renal pelvis, and finally into the bladder via the ureter. The convoluted portion of the tubule leads into a straight segment that descends into the medulla within a medullary ray and becomes the loop of Henle. The loop of Henle forms a hair-pin structure that dips down into the medulla.
It contains four segments: the pars recta the straight descending limb of proximal tubule , the thin descending limb, the thin ascending limb, and the thick ascending limb.
The turn of the loop of Henle usually occurs in the thin segment within the medulla, and the tubule then ascends toward the cortex parallel to the descending limb. The end of the loop of Henle becomes the distal convoluted tubule near its original glomerulus.
The loops of Henle run in parallel to capillary loops known as the vasa recta. Recall from Physiology that the loop of Henle serves to create high osmotic pressure in the renal medulla via the counter-current multiplier system. Such high osmotic pressure is important for the reabsorption of water in the later segments of the renal tubule.
The distal convoluted tubule is shorter and less convoluted than the proximal convoluted tubule. Further reabsorption and secretion of ions occur in this segment. The initial segment of the distal convoluted tubule lies right next to the glomerulus and forms the juxtaglomerular apparatus. The juxtaglomerular apparatus is a specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole.
It is located near the vascular pole of the glomerulus. The main function of the apparatus is the secretion of renin, which regulates systemic blood pressure via the renin-angiotensin-alodosterone system. The juxtaglomerular apparatus is composed of:. The terminal portion of the distal tubule empties through collecting tubules into a straight collecting duct in the medullary ray. The collecting duct system is under the control of antidiuretic hormone ADH.
When ADH is present, the collecting duct becomes permeable to water. Numerous collecting ducts merge into the renal pelvis, which then becomes the ureter. The ureter is a muscular tube, composed of an inner longitudinal layer and an outer circular layer. The lumen of the ureter is covered by transitional epithelium also called urothelium. Recall from the Laboratory on Epithelia that the transitional epithelium is unique to the conducting passages of the urinary system.
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