How do intrinsic clearance (CLint) and hepatic blood flow interact to determine hepatic clearance?

• A. ClH is determined by the balance of Q and fu × CLint; high CLint or high fu increases ClH, but the max is limited by Q.
• B. ClH equals fu × CLint only.
• C. ClH equals Q.
• D. ClH increases indefinitely with CLint.

Answer: (B)

Hepatic clearance is controlled by the interplay between how much blood reaches the liver (Q) and how efficiently the liver can clear the drug at the cellular level (intrinsic clearance times the unbound fraction, fu × CLint). The well-stirred model puts this together as CL_H = (Q × fu × CLint) / (Q + fu × CLint). This shows that clearance grows as fu × CLint increases, but it cannot exceed the amount of drug that can be delivered by blood flow.

If fu × CLint is much smaller than Q, the equation simplifies to CL_H ≈ fu × CLint, so clearance tracks the intrinsic capability. If fu × CLint is much larger than Q, CL_H ≈ Q, meaning clearance is limited by how fast blood can bring drug to the liver. So the concept is a balance: clearance rises with intrinsic ability but is capped by hepatic blood flow. The idea that clearance simply equals fu × CLint or increases without bound is not accurate.