Blood is composed of various components, but which of the following is not a component of blood?

HCT, or hematocrit, is not a component of blood itself but rather a measurement or a value that indicates the percentage of red blood cells in the blood volume. It is used in clinical settings to assess conditions such as anemia or polycythemia but does not constitute a physical part of blood like plasma, red blood cells, or white blood cells do.

In contrast, plasma is the liquid component that carries cells and various substances; red blood cells are responsible for oxygen transport, and white blood cells are a crucial part of the immune system. Each of those components plays a direct role in the functions of blood, whereas HCT serves as a diagnostic tool rather than a structural component. This distinction is important in understanding blood composition versus the metrics clinicians use to assess it.