Answer: Erythrocytes and plasma proteins
Viscosity is a measure of resistance to flow of fluid due to internal friction of the particles of the fluid. Honey flows slowly or has high viscosity ( resistance to flow,) because of the greater internal friction of its molecules, However water has low viscosity , low resistance to flow because, the internal frictional forces of its molecules is very low.
Therefore an increase in erythrocytes, proteins, levels contributes to blood high viscosity, and the relatively slow flow (high resistance of the blood), but a reduction in the blood concentration of these materials reduces the internal frictions, and therefore relatively fast movements of the molecules(low viscosity).
Basically surrounding tissues and cells receive blood supply containing oxygen,metabolites,from the blood capillaries while the cells and tissues returns wastes of metabolites and carbondioxide. The total molarity of dissolved particles (protein, blood cells) which can not pass through capillary fenestration but retained in the blood is called Osmolarity. The higher the molarity of these materials(protein, blood cells) the higher the blood Osmolarity and viscosity.
If the blood osmolarity increases, due to high RBC and plasma proteins, the solute potential, and blood viscosity also increases compare to that of the surrounding tissues and cells. Therefore water is retained in the cells and tissues resulting in edema. To solve this, water flows into the blood capillaries by osmosis,this rises the blood volume, thus reducing the viscosity, and osmolarity allowing free flow of blood
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