74. Renal blood flow. Clearance of PAH. Extraction ratio. Filtration fraction
Renal blood and plasma flow
The kidneys receive large amounts of blood flow every minute. This allows the kidneys to filter large amounts of blood every minute. The renal blood flow (RBF) is normally 1200 mL/min, a whopping 20% of the cardiac output.
The kidneys receive so much blood flow because they should filter as much blood as possible, not because the kidneys have a large oxygen requirement. In cases where blood flow is needed elsewhere, like during exercise, the RBF will decrease significantly. The kidney will still work fine, but it won’t filter as much as long as the RBF is decreased.
The renal plasma flow (RPF) is the amount of plasma that enters the kidneys every minute. It can be calculated if we know the RBF and the haematocrit, like this:
Renal plasma flow = RBF x (1 – haematocrit)
The RPF is normally 600 – 800 mL/min.
Clearance of PAH
See previous topic.
Filtration fraction
The filtration fraction is the fraction of the renal blood flow which is filtered across the glomerular capillaries. It can be calculated like this:
Filtration fraction = GFR / RPF
It is normally around 0,20, meaning that around 20% of the renal plasma flow is filtered. The remaining 80% of the plasma leaves the glomerulus through the efferent arteriole.
Extraction ratio
The extraction ratio shows how much of a substance is cleared from the plasma after the plasma has passed through the kidneys. More specifically it is the percentage of the substance entering the kidney that was excreted into the urine.
For a given substance the extraction ratio can be calculated like this:
Pa is the concentration of the substance in the renal artery.
Pv is the concentration of the substance in the renal vein.
The extraction ratio of a substance depends on how much the substance is filtered, reabsorbed and secreted by the nephron. Here is a table of some substances and their extraction ratios:
| Substance | Extraction ratio |
| Glucose | 0 |
| Amino acids | 0 |
| Inulin | 0,2 |
| Creatinine | 0,2 |
| PAH | 0,9 |
It is because the extraction ratio of PAH is close to 1 that we can use it to estimate the renal plasma flow.