What is hematocrit, and how does it relate to oxygen transport?

• A. Proportion of blood volume occupied by plasma
• B. Proportion of blood volume occupied by white blood cells
• C. Proportion of blood volume occupied by red blood cells
• D. Ratio of platelets to plasma

Answer: (C)

Hematocrit is the proportion of blood volume that is made up of red blood cells. Plasma is the liquid part of blood, while white blood cells and platelets are part of the remaining formed elements; hematocrit specifically measures how much of the blood’s volume comes from red cells. Those red blood cells house hemoglobin, the protein that binds oxygen in the lungs and releases it to tissues as blood circulates.

So, when hematocrit is higher, there are more red cells and more hemoglobin available to carry oxygen, increasing the blood’s oxygen-carrying capacity (up to practical limits, since very high hematocrit raises blood viscosity and can hinder flow). If hematocrit is low, there are fewer red cells to carry oxygen, which can reduce oxygen delivery to tissues and is a hallmark of anemia or blood loss. Remember, hematocrit indicates red cell mass, not the speed of delivery or how well tissues extract oxygen; other factors like cardiac output and microvascular perfusion also play roles.