Final answer:
Hemoconcentration in a blood sample commonly results from a reduction in plasma volume due to dehydration or hypovolemia, leading to a relatively higher concentration of blood cells.
Step-by-step explanation:
Hemoconcentration of a venous blood sample is most commonly caused by a situation where there is a reduction in plasma volume, resulting in a higher concentration of red blood cells (RBCs) and other cellular elements in the blood. This can be due to dehydration, where water intake is inadequate or water losses are excessive, leading to a fall in plasma volume and a consequent rise in hematocrit levels.
Another cause could be hypovolemia, which is a condition of abnormally low levels of fluid and blood within the body due to factors such as bleeding, severe burns, or certain medications for hypertension. These conditions lead to a relative increase in the concentration of blood components without an actual increase in cellular elements, which is evident in cases like polycythemia where there is an actual increase in RBC production.