NHA Phlebotomy Certification Practice Exam

What fluid comprises approximately 55% of the total blood volume?

1. Hematocrit

2. Plasma

3. Tissue fluid

4. Serum

The correct answer is: Plasma

Plasma is the correct answer as it constitutes about 55% of the total blood volume. Plasma is the liquid component of blood that acts as a medium for transporting cells, nutrients, hormones, and waste products throughout the body. It is primarily composed of water (about 90-92%), but it also contains proteins, electrolytes, and other substances. Understanding the role of plasma is essential for phlebotomy because it reflects the body's biochemical status and is vital for functions such as maintaining blood pressure and volume, facilitating the transport of cells and nutrients, and playing a role in immune responses. The composition of plasma can also provide important information during medical evaluations, including various tests that assess liver function, kidney function, and overall metabolic health. Hematocrit refers specifically to the percentage of blood volume that is made up of red blood cells, not the total fluid component. Tissue fluid is the fluid found in the spaces between cells, while serum is the liquid left after blood has clotted and does not contain clotting factors. These definitions clarify the unique role of plasma in the circulatory system.