In basic tissue distribution models, which assumption is commonly made about the rate of blood flow through arteries and veins?

• A. The rate is different and must be modeled separately.
• B. The rate is zero.
• C. The rate is the same.
• D. The rate only matters for lipophilic drugs.

Answer: (C)

In basic tissue distribution models, a key simplification is that the blood flow rate to a tissue is described by a single perfusion rate, and the rate of blood entering the tissue through arteries equals the rate leaving through veins in steady state. This mass-balance idea means you can use the same flow value to characterize both inflow and outflow, rather than modeling them separately. It’s not about the flow being zero or about lipophilicity; it’s about using a single, common flow rate to simplify the model while conserving mass. In more detailed models, you could distinguish arterial and venous flows, but the common approach uses the same rate for both.