Which of the following is crucial for blood flow when performing a capillary blood collection?

Orienting the puncture site vertically downward is essential during capillary blood collection as it helps facilitate gravity-driven blood flow. By positioning the puncture site this way, the blood naturally flows away from the puncture point and into the collection device, reducing the risk of clots and ensuring a steady sample collection. This orientation also helps minimize the risk of excessive tissue fluid contamination, which can occur if the puncture is not correctly aligned.

In contrast, horizontal orientation may not effectively direct the blood flow, possibly leading to inadequate sample volume. Using plastic microhematocrit tubes and applying a tourniquet tightly around the site are not practices associated specifically with improving blood flow in capillary collections and can introduce complications, such as hemolysis or clotting, respectively.