pulmonary circulation
Pulmonary circulation is the short trip the blood takes to the lungs to ‘recharge’ with oxygen. Unlike the long journey of the systemic loop, this is a brief in-and-out: blood goes from the right heart to the lungs, picks up oxygen and dumps carbon dioxide, then comes straight back to the left heart, ready to be sent out to the body again.
Anatomically, the right ventricle pumps deoxygenated blood into the pulmonary artery, which branches to both lungs. In the dense capillary network surrounding the air sacs (alveoli), carbon dioxide diffuses out and oxygen diffuses in. The now-oxygenated blood returns through the pulmonary veins to the left atrium. Note the naming quirk: pulmonary arteries here carry deoxygenated blood and pulmonary veins carry oxygenated blood, the reverse of the usual convention.
The pulmonary circuit is a low-pressure, low-resistance system: normal pulmonary artery pressures are roughly a quarter of systemic arterial pressures. This matters clinically, because the thin-walled right ventricle is built for low pressure—when lung disease or other conditions push pulmonary pressures up (pulmonary hypertension), the right heart can struggle and eventually fail.
In severe pulmonary hypertension, the right ventricle must generate much higher pressures to push blood through stiffened lung vessels, and over months it thickens and may fail.
The low-pressure design of the pulmonary circuit explains why the right ventricle is thin-walled.
The pulmonary circuit handles the same cardiac output as the systemic circuit but at far lower pressure, because pulmonary vascular resistance is low.