Figure 8.13: Schematic diagram of a vein.Figure 8.14: Diagram representing the branching of an artery into arterioles. These subsequently
form the capillary bed which empties into several venules, leading to the vein.
Valves
Once the blood has passed through the capillaries very little blood pressure remains to
return blood to the heart.Instead of pressure from the heart veins use a series of valves to
force blood to return to the heart. Contraction of the muscles squeezes the veins, pushing
the blood through them. The valves cause the blood to flow in only one direction, back to
the heart.
Chapter 8. Transport systems in animals 237