Regional hematocrit changes related to blood flow conditions in the arterial bed

Present study was aimed at specifying both the anatomical and physiological factors determining the regional hematocrit alterations. These latter were investigated in blood samples from specific vascular branches of rabbits, as well as from the skin and the jugular veins (reflecting the cerebral microcirculation) of humans. Principal findings were as follows: (1) Blood in the heart left ventricle has a higher hematocrit than blood flowing in the hind legs. (2) Blood flowing towards the cerebral vessels has a higher hematocrit than blood flowing to the hind legs. (3) Hematocrit in blood from the right brachial artery is higher than from the left one. (4) Analogous is the red cell/plasma relationship in the microcirculation from the humans' right and left hands' fingers. (5) Local hematocrit in the individual arterial branches of the rabbits' aortic arch is dependent on the blood flow velocity in them: the faster flow provides higher hematocrit in their lumen. (6) Microvascular hematocrit in rabbits was found significantly lowered during ischemia in the respective vascular beds. (7) Microvascular hematocrit in the human skin decreases during lowering of blood inflow, and vice versa, thus evidencing that the capillary hematocrit depends on the functional state of the feeding arterioles. We concluded, therefore, that the two basic factors determining the local hematocrit in arterial branches and in the respective microvascular networks are the anatomical outlines and the comparative velocities of blood flow in their lumina. However, in patients with strokes the hematocrit in blood flowing in the jugular veins was found lower than in blood from the carotid arteries. This might be related to the enhanced RBC aggregation and retention in the cerebral microvessels producing multiple blood stases in their lumina.