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Microcirculation Factor: Hematocrit
The principal factor affecting the viscosity of blood is hematocrit.
Bifurcations
Shown above is the distribution of red blood cells at three successive bifurcations in the cremaster muscle of the mouse. The fraction of red cells present (hematocrit) and plasma vary due to the skimming of plasma into the left branch of the arteriole. The hematocrit in the capillary on the right is greatly reduced because the red cells speed up relative to the plasma as they squeeze through the capillary. Since they must travel faster than the plasma, there must be fewer of them present to maintain the same proportions of cells and plasma as blood exits the capillary. This is the so-called Fahraeus Effect.
Volume Fraction
Shown here is the distribution of red blood cells at three successive bifurcations in the cremaster muscle of the mouse. The fraction of red cells present (hematocrit) and plasma vary due to the skimming of plasma into the left branch of the arteriole. The hematocrit in the capillary on the right is greatly reduced because the red cells speed up relative to the plasma as they squeeze through the capillary. Since they must travel faster than the plasma, there must be fewer of them present to maintain the same proportions of cells and plasma as blood exits the capillary. This is the so-called Fahraeus Effect.