One factor affecting blood flow is peripheral resistance. This is the resistance to blood flow in the peripheral vessels . . . the arterioles.
What kinds of things can cause blood to "resist" flowing as freely as possible? Well, one factor is the viscosity of blood. One way to think of viscosity is "thickness" of a fluid. What makes blood thick? One factor is how concentrated the red blood cells (RBCs) are in each drop of blood. Is is often expressed as the PCV (packed cell volume) or hematocrit of blood. The more blood cells there are in a drop of blood, the thicker--or more viscous--the blood is. And the more it resists flow.
An analogy we can use to visualize this rather abstract concept is any of the classic TV commercials once used by Heinz to promote their ketchup. Heinz claimed that their ketchup is better than other brands because it has more tomatoes per bottle of ketchup. And that, presumably, can be demonstrated by the comparative thickness of their ketchup. In other words, Heinz ketchup is more viscous than some other brands of ketchup.
As the commmercial below demonstrates, the extra thickness or viscosity of Heinz ketchup means that it will flow at an incredibly slow rate.
So to summarize this analogy:
- More tomatoes per bottle of ketchup make the ketchup thicker, or more viscous. The higher the viscosity of ketchup, the more it resists flow and thus the slower it will flow out the narrow neck of a tipped bottle.
- Likewise, more RBCs per drop of blood make the blood thicker, or more viscous. The higher the viscosity of blood, the more it resists flow. This resistance can be significant where the blood vessels narrow at the peripheral vessels called arterioles.