by the liver and kidneys. Therefore, total clearance is the sum of the clearance caused by the liver (hepatic clearance, CLH) and the kidneys (renal clearance, CLR).
Clearance at an individual organ is a function of two factors, blood flow (Q) and extraction ratio (E). Blood flow to an organ is simply the volume of blood that passes through an organ per unit time. Extraction ratio is a bit more complicated. If an organ clears a drug, the plasma concentration of a drug that enters the organ (Cpin) is higher than the plasma concentration of the drug that leaves the organ (Cpout). The difference between these two concentrations divided by Cpin is the extraction ratio. Extraction ratio is a dimensionless number that falls within the range of 0 to 1.
With these ideas in mind, one can re‐express CLT as a combination of the blood flow and extraction ratio of both the liver and kidneys.
Values for Q are easy to handle. For a 70‐kg human, blood flow to the liver is approximately 1,500 mL/min (QH = 1,500 mL/min). Blood flow to the kidneys is around 1,100 mL/min, but the kidneys are only able to filter approximately 220 mL/min (QR = 220 mL/min). The extraction ratios for the kidneys and liver are more challenging. Assuming we can determine CLT (to be covered in the next chapter section), we can estimate EH and ER. To approach these two variables, we need to put a drug into one of three categories: cleared by kidneys only, cleared by liver only, or cleared by both the liver and kidneys.