Capillaries

A microscopic view of the network of blood capillaries around a small bunch of muscle fibres. The capillaries have been injected with a red dye.

Blood flows from the heart through arteries, which branch and narrow into arterioles, and then branch further into capillaries where nutrients and wastes are exchanged. The capillaries then join and widen to become veinules, which in turn widen and converge to become veins, which then return blood back to the heart through the great veins. 

Is it not amazing that we can construct such complex things for ourselves without any conscious involvement whatsoever?