**Core Concept**
The concept being tested is the relationship between the number of perfused capillaries and the exchange of substances between blood and tissue, a fundamental principle in tissue perfusion and exchange.

**Why the Correct Answer is Right**
At a constant blood flow, increasing the number of perfused capillaries enhances the exchange between blood and tissue due to the increased surface area available for diffusion. This is described by the Fick's law of diffusion, which states that the rate of diffusion is directly proportional to the surface area and the concentration gradient. With more capillaries perfused, there is a greater surface area for oxygen, nutrients, and waste products to diffuse across, thereby improving tissue exchange.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because increasing the number of perfused capillaries at a constant blood flow does not affect the blood flow velocity or the transit time of blood through the capillaries.

**Option B:** This option is incorrect because increasing the number of perfused capillaries does not directly affect the blood flow resistance or the pressure gradient driving blood flow.

**Option C:** This option is incorrect because increasing the number of perfused capillaries does not directly affect the oxygen or nutrient concentration in the blood.

**Clinical Pearl / High-Yield Fact**
A key principle to remember is that increasing the surface area for exchange, such as with more perfused capillaries, can compensate for a decrease in blood flow velocity or concentration gradient, thereby maintaining tissue exchange.

**Correct Answer:** B.