Blood has a liquid component (plasma) and a particulate component (blood cells). The denser blood cells (most of which are red blood cells) will settle in a tube, particularly if the blood is spun in a device called a centrifuge. The fraction of the resultant column composed of red blood cells relative to the entire column is the hematocrit, which normally is in the range of 40 percent. The upper portion of the separated blood is yellowish plasma. Red blood cells carry oxygen, and blood with a low hematocrit (e.g., 20%) leaves the tissues relatively oxygen-starved and weak. A high hematocrit (e.g., 70%) produces problems as well, but is uncommon.
KENNETH R. BRIDGES
(SEE ALSO: Hemoglobin)
Corash, L. (1995). "Laboratory Hematology: Methods for the Analysis of Blood." In Blood: Principles and Practice of Hematology, eds. R. I. Handin, S. E. Lux, and T. P. Stossel. Philadelphia, PA: J. B. Lippincott Company.