Renal clearance

In order for the body to function properly, it needs a way to get rid of toxins and other waste materials. That’s where the kidneys come in. Their main function is to filter the blood and remove any unwanted substances from the body. Now, the first step in blood filtration happens at the glomerulus - a tiny bed of capillaries surrounded by the Bowman’s capsule. The glomerular filtration barrier is made up of three layers and together they separate the blood inside the glomerular capillaries from the fluid inside Bowman’s capsule. They work like a sieve, allowing water and some solutes in the plasma like sodium, to pass into Bowman’s space, while keeping negatively charged particles like proteins, or large particles like red blood cells in the blood. The filtered fluid, now called pre-urine, leaves the Bowman’s space and travels through the nephron.

The nephron is the basic unit of the kidney, and is essentially one long tube bent into a “U” shape. Different sections of this tube either reabsorb substances back into systemic circulation or actively secrete them into the nephron to be excreted in urine.

Renal clearance of a substance refers to how quickly a particular substance is removed from the plasma by the kidney and excreted in urine. So something with a high renal clearance means that it will be quickly removed from the blood, and vice versa. There’s a formula to calculate renal clearance for some substance X.

In this formula, C stands for the renal clearance which is the volume of blood plasma that’s cleared of that substance over time in minutes. C equals the concentration of the substance in urine [U]x multiplied by the urine flow rate (V) which is the amount of urine excreted over time in minutes. All of that’s divided by the plasma concentration of the substance [P]x. So, if the urine concentration is high but the plasma concentration is low, then that must mean that a lot of the substance was removed from the blood, leading to a high renal clearance. As a general rule, small, uncharged substances like inulin, which is a small inert polysaccharide molecule, have a relatively easy time passing through the glomerulus.