Which device is commonly used to demonstrate capillary action in hematology tests?

Which device is commonly used to demonstrate capillary action in hematology tests?

• A. Petri dish
• B. Hematocrit tube
• C. Microscope slide
• D. Test tube

Answer: (B)

Capillary action is the tendency of a liquid to move into narrow spaces or tubes because adhesive forces between the liquid and the surface pull it in, aided by cohesive forces within the liquid. In hematology, this mechanism lets a tiny amount of blood be drawn into a capillary tube without a syringe. The microhematocrit tube is designed for this purpose: its narrow bore creates the capillary space, and after filling, the tube is sealed so the sample can be centrifuged to measure packed cell volume. Because it directly demonstrates and utilizes capillary action to obtain a precise blood volume, it is the best device for this purpose. The other items—Petri dish, microscope slide, and plain test tube—do not rely on capillary action to collect small volumes of blood and are used for different steps in lab work.

Capillary action is the tendency of a liquid to move into narrow spaces or tubes because adhesive forces between the liquid and the surface pull it in, aided by cohesive forces within the liquid. In hematology, this mechanism lets a tiny amount of blood be drawn into a capillary tube without a syringe. The microhematocrit tube is designed for this purpose: its narrow bore creates the capillary space, and after filling, the tube is sealed so the sample can be centrifuged to measure packed cell volume. Because it directly demonstrates and utilizes capillary action to obtain a precise blood volume, it is the best device for this purpose. The other items—Petri dish, microscope slide, and plain test tube—do not rely on capillary action to collect small volumes of blood and are used for different steps in lab work.