The blood circulation has a systemic circuit and a pulmonary circuit. The latter operates at considerably lower pressure to accommodate oxygenation of blood in the pulmonary capillaries. The microscopic anatomy of both systems is similar, although systemic vessels tend to have thicker walls to accommodate higher pressures.
The blood circulation is composed arteries, arterioles, microcirculation (capillaries and sinusoids), venules, and veins. Arteries serve to conduct high-pressure blood to an organ. These transition into arterioles and metaarterioles that distribute and regulate pressure to a vast network of delicate capillaries. Following exchange at the level of the microvasculature, blood collected by venules is returned to the heart by veins. Generally speaking, blood vessels closer to the heart have wider diameter and thicker walls; arteries have thicker walls and smaller lumens than veins.