Urea and creatinine concentration, the urea:creatinine ratio
Clinicians have long been aware of a bleeding tendency in uremic patients. Recently there have been several reports that the platelets of uremic patients have. Creatininemia Versus Uremia: The Relative Significance of Blood Urea Relationship between urine volume and the rate of urea excretion by normal adults. Richards1 has proved by examinations of the glomerular urine of the frog that urea is filtered through the glomerular zolyblog.info reabsorption of urea in the tubu.
This is a Bowman's capsule right here. It looks something like that, and the whole nephron is going to be convoluted like this. It's going to dip down into the medulla, and then come back, and then it's going to eventually dump into a collecting duct, and I'll talk more about that.
So what I've drawn just here, this is a zoomed-in version of that part right there. Now what I want to do is zoom out a little bit because I'm going to run out of space.
So let me zoom out. So we had our arteriole go in. It gets all bunched in the glomerulus, and then most of the blood leaves, but one-fifth of it gets essentially filtered in to the Bowman's capsule. That's the Bowman's capsule right there. I've just zoomed out a little bit. So we have our filtrate here.
Maybe I'll make it a little bit yellow. The filtrate that just comes out at this point, sometimes it's called the glomerular filtrate because it's been filtered by the glomerulus, but it's also been filtered by those podocyte cells on the inside of the Bowman's capsule.
But now it's ready to go to the proximal tubule. Let me draw something like this. And obviously, this is not exactly what it looks like, but it gives you the sense.
This right here, this is the proximal tubule. And it sounds like a very fancy word, but proximal just means near and tubule, you can imagine, is just a small tube. So it's a small tube that's near the beginning.
That's why it's called a proximal tubule.
And it has two parts. The whole thing is often called a proximal convoluted tubule. That's because it's all convoluted. The way I've drawn it is all curvy. And I just drew it curvy in two dimensions. It's actually curvy in three dimensions.
- The kidney and nephron
- Urea and creatinine concentration, the urea:creatinine ratio
- Effects of Urea and Creatinine on Human Platelet Aggregation.
But the reality is there's a curvy part and then there's a straight part near the end of the proximal tubule. So we'll call this whole thing the proximal tubule. This is the convoluted part. That's the straight part, but we don't have to get too picky. But the whole point of this part of the nephron-- and just to remember where we are, we're now at this point of the nephron right there-- the whole point is to start reabsorbing some of the stuff that is in the filtrate that we don't want to lose.
We don't want to lose glucose. That's hard-earned stuff that we ate that was good for energy. We don't want to lose necessarily as much sodium. We've seen in multiple videos that that's a useful ion to have around.
We don't want to lose amino acids. Those are useful for building up proteins and other things. So these are things we don't want to lose so we start absorbing them back.
Urea – Creatinine Ratio
I'll do a whole video on exactly how that happens, but it's done actively. Since we're using ATP, and just as a bit of a summary, you're using ATP to actually pump out the sodium and then that actually helps bring in the other things. That's just kind of a tidbit on what's happening. So we're reabsorbing, so just imagine what's happening. You have cells lining the proximal tubule right now. And actually, they have little things that jut out. I'll do a whole video on that because it's actually interesting.
So you have cells out here. On the other side of the cells, you have an arterial system, or a capillary system, I should say, actually. So let's say you have a capillary system here that is very close to the cells lining the proximal tubule, and so this stuff actually gets actively pumped, especially the sodium, but all of it, using energy, gets pumped back into the blood selectively, and maybe a little bit of our water.
So we're pumping back some sodium, some glucose, and we'll start pumping a little bit of the water back in because we don't want to lose all of that water. If all of the water that was originally in the filtrate we were just leaving in our urine, we'd be excreting gallons and gallons of water every day, which we do not want to do. So that's the whole point.
We're starting the absorption process. And then we'll enter the loop of Henle, and actually, this is, in my mind, the most interesting part of the nephron. 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.
The kidney and nephron (video) | Khan Academy
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: 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. Srygley FD et tal. Does this patient have a severe upper gastrointestinal bleed?
JAMA ; Pumphrey CW et tal. Raised blood urea concentration indicates considerable blood loss in acute upper gastrointestinal haemorrhage.
UREA REABSORPTION AND RELATION BETWEEN CREATININE AND UREA CLEARANCE IN RENAL DISEASE
Br Med J ; Blatchford O et tal. A risk score to predict need for treatment for upper-gastrointestinal haemorrhage. Lancet ; Stevenson J et tal. Validating the Glasgow-Blatchford upper GI bleeding scoring system. Gut ; 62, 2: Cheng DW et tal.
A modified Glasgow Blatchford Score improves risk stratification in upper gastrointestinal bleed: Aliment Pharmacol Ther ; 36, 8: Rahman M et tal.
Am Fam Physician ; 86, 7: Pathophysiology of pre-renal azotemia. Kidney Int ; 53, 2: Agrawal M et tal. Am Fam Physician ; 61, 7: Uchino S et tal. Clinical Kidney Journal ; 5, 2: Rachoin J et tal.
The fallacy of the BUN: Nephrol Dial Transplant ; 27, 6: Beier K et tal. Crit Care Med ; 39, 2: Damman K et tal. The kidney in heart failure: Eur Heart J ; 36, Gotsman I et tal. The significance of serum urea and renal function in patients with heart failure. Medicine Baltimore ; 89, 4: Sood MM et tal.
The urea-to-creatinine ratio is predictive of worsening kidney function in ambulatory heart failure patients. J Card Fail ; 21, 5: Takaya Y et tal. Circ J ; 79, 7: Aronson D et tal.
Elevated blood urea nitrogen level as a predictor of mortality in patients admitted for decompensated heart failure. Am J Med ;7: Ranson JH et tal. Objective early identification of severe acute pancreatitis. Am J Gastroenterol ; 61, 6: Corfield AP et tal. Prediction of severity in acute pancreatitis: Lancet ; 2, Wu BU et tal. Blood urea nitrogen in the early assessment of acute pancreatitis: Arch Intern Med ;7: Haemodialysis — clinical standards and targets Chapter 3 In: Treatment of Adults and Children with Renal Failure: Standards and Audit Measures: Royal College of Physicians of London;