8.1.3—Main blood vessels

The named vessels connect the two linked circuits of a closed double circulation. Capillary beds are the exchange networks between the large vessels and the lungs or body tissues; arteries carry blood away from the heart and veins carry it towards the heart.

1. Pulmonary outflow: right side of heart → pulmonary artery → lung capillary beds. Deoxygenated blood reaches the lungs for gas exchange.
2. Pulmonary return: lung capillary beds → pulmonary vein → left side of heart. Blood returns oxygenated before entering the systemic circuit.
3. Systemic outflow: left side of heart → aorta → systemic arteries and arterioles → body-tissue capillary beds. Oxygenated blood is delivered for exchange with tissues.
4. Systemic return: body-tissue capillary beds → systemic venules and veins → vena cava → right side of heart. Deoxygenated blood returns, completing the route back to the pulmonary circuit.

Whole route cue: heart right side → pulmonary artery → lungs → pulmonary vein → heart left side → aorta → body capillaries → vena cava → heart right side.

Do not define a vessel by oxygen content alone: the pulmonary artery carries deoxygenated blood away from the heart, while the pulmonary vein carries oxygenated blood towards it. The vessel names above describe direction and route; capillaries are the exchange beds, not an extra third circulation.