Final answer:
Plasma constitutes the majority of whole blood by volume at about 55%, followed by erythrocytes at approximately 45%, and leukocytes and platelets combined make up less than 1%.
Step-by-step explanation:
Among the constituents of whole blood by volume, plasma represents the majority. Plasma makes up about 55% of the total blood volume. The other elements are erythrocytes (red blood cells), which comprise about 45%, and leukocytes (white blood cells), along with platelets, make up less than 1%. Erythrocytes are the most abundant of the formed cellular elements, essential for carrying oxygen due to the hemoglobin they contain, while leukocytes play a crucial role in the body's immune response.
A hematocrit test, which measures the percentage of RBCs in blood, utilizes centrifugation to separate these cellular components based on density, emphasizing the distinction in their respective volumes within the blood.