USMLE Step 1 Renal clearance

Tyler York

This video explains the concept of renal clearance for the USMLE exam. Renal clearance is mainly affected by glomerular filtration, tubular reabsorption and tubular secretion. It is plotted on a graph using the ratio of tubular fluid to plasma concentration of a substance and the percent proximal tubule length. Inulin, creatinine and PAH clearance is important to understand renal clearance.

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Full USMLE Step 1 Renal clearance video transcript:

Hi, this is Sujatha. In this video lets discuss renal clearance. Renal clearance is the volume of plasma that is completely cleared of a substance in a given period of time. It is given by the formula C equals to Ux divided by PX multiplied by V where Ux is the urine concentration of substance X PX.
Is the plasma concentration of substance X&V is the urine flow rate. Renal clearance is mainly affected by three factors, glomerular filtration, tubular reabsorption, and tubular secretion. Any factor that changes either of these will have an effect on the renal clearance. This slide gives the many factors that have an effect on the renal clearance.
Important among them are size, state of ionization, protein binding and presence of renal disease. Let us look at the renal clearance of three important products that are tested often in the usmly these are creatinine, inulin and pH or paraminohipuric acid, which is an organic acid. Creatinine is not only freely filtered at the glomerulus, but it is slightly excreted at the tubules as well.
So the creatinine clearance is always going to be slightly higher than the GFR. On the other hand, inulin clearance is equal to the actual GFR. Because inulin is only filtered at the glomerulus, it is neither reabsorbed nor actively secreted by the tubules. pH and other organic acids have the highest renal clearances.
Because they are not only filtered at the glomerulus, but they are highly secreted at the tubules as well. This renal clearance graph is plotted by the tubular fluid concentration to the plasma concentration of a substance on the Y axis and the percent proximal tubular length on the X axis.
The ratio of tubular fluid to plasma concentration of a substance will be more than one in those substances which have a net excretion, and it will be less than one. In those substances that have a net reabsorption, creatinine and urea are excreted, while electrolytes, glucose and amino acids are reabsorbed. Sodium stays close to 1, mirroring the urine osmolarity.
Let's look at a few scenarios which are likely to be tested on renal clearance in the USMLE. So on the graph, which plot will most likely represent pH clearance? You can take a few seconds to pause the video and then come with an answer. The answer is line D because it has a very high tubular concentration relative to the plasma concentration.
Choose the lines on the graph for glucose and inulin respectively. The answer is lines A and D respectively, because glucose is one of the most highly reabsorbed substances, while inulin is not reabsorbed, it is only filtered at the glomerulus. What will be the tubular fluid to plasma concentration ratio for albumin in a healthy individual? The answer is 0.
Just because albumin is not filtered by a healthy Glomodalus, nor is it secreted at the tubules. Hope this video clears some of your concepts on renal clearance. Thank you for watching.
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