Blood Flow

Blood flow is driven by the heart which must be strong enough to push blood throughout the body into all of the major arteries, arterials and smallest capillary beds.

When blood leaves through the aorta and enters smaller arterials and capillaries, its high velocity and high pressure decrease, as a result of the increased combined diameters of the many blood vessels in comparison to the diameter of the aorta alone.

Such slow rate of movement provides adequate time for gas and nutrient exchange through the walls of the small blood vessels.

In contrast, blood travels faster through the venules, veins and back to the vena cava with the aid of smooth muscle in the vessel walls and compression from neighboring skeletal muscles preventing blood from pooling. Plus one way valves in veins prevent backflow despite the pull of gravity.

Ultimately, the process of blood flow is controlled by the body's needs and can be regulated by neurological signaling and hormones. For example, while exercising, blood is targeted to muscles due to vasodilation and away from the digestive system via vasoconstriction, directing blood where it is needed most.

Thus not all of the capillary beds have blood flowing through at all times.