Renal function: filtration and clearance

Published on February 29, 2024   16 min
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Hello, and welcome back to The Fundamentals of Human Physiology. My name is Jessica Briffa, and today we'll be going through the second lecture in the 'Kidney Function' series, which is on filtration and clearance.
The first thing that comes to mind on this lecture topic is what exactly is renal clearance? Renal clearance is defined as the rate of urinary excretion of a substance per unit time, relative to its plasma concentration or in other words, how quickly a particular substance is removed from the plasma by the kidneys and excreted in urine. This concept will be described in more detail in subsequent slides with examples of the clearance of different substances.
Before we discuss renal clearance more, let's consider the following question to start to get an idea about renal clearance. Why do you think inulin, which is a type of dietary fiber, is the ideal substance for determining glomerular filtration rate (GFR)? For any compound to be considered an ideal marker for determining glomerular filtration rate, we must first think back to what the glomerular filtration rate is. Glomerular Filtration Rate is defined as the volume of plasma filtered by the glomerulus over a specified period. This in combination with the fact that inulin clearance is measured in the urine, it means the following must happen for it to be considered an ideal substance for glomerular filtration rate. It must freely be filtered by the glomerulus. It must not be secreted or reabsorbed in the nephron, which will change its concentration in the urine compared to the glomerular filtrate. It must not be synthesized or metabolized by the kidney, which will again change its concentration in the urine compared to the filtrate. Lastly, it must be exogenous such that normal biological body processes must not normally produce a substance. As you can see, this is a bit of a trick question as all of the options are correct except for option 3.