systemic vs pulmonary circulation

Blood doesn't take one big lap around the body — it runs two connected loops, and telling them apart is one of the most useful ideas in cardiology. The big loop, systemic circulation, supplies the whole body; the small loop, pulmonary circulation, runs only through the lungs.

In the systemic loop the left side of the heart pumps oxygen-rich blood out through the aorta to every organ and limb, where the tissues take the oxygen, and the now oxygen-poor blood returns to the right side of the heart. In the pulmonary loop the right side pumps that oxygen-poor blood to the lungs, where it releases carbon dioxide and reloads with oxygen, then returns to the left side. The two loops are arranged in series, so all the blood passes through both.

A key practical difference is pressure: the systemic circuit runs at high pressure to reach distant tissues, while the pulmonary circuit is a low-pressure system, because the delicate lungs sit right next door and high pressure would flood them. Diseases that raise pulmonary pressure or weaken one side of the heart disturb this carefully balanced arrangement.