Final answer:
The pulmonary veins carry oxygenated blood to the left atrium from the lungs, playing a vital role in connecting the pulmonary and systemic circuits of the body.
Step-by-step explanation:
Pulmonary veins are responsible for bringing oxygenated blood to the left atrium from the lungs. After the exchange of gases in the pulmonary capillaries, oxygen-rich blood travels through these vessels to enter the left atrium. The left atrium, which does not contain pectinate muscles, has an auricle that includes pectinate ridges. As the primary receiving chamber for oxygenated blood, the left atrium allows blood to flow nearly continuously from the pulmonary veins back into it and then through an opening into the left ventricle.
Most of the ventricular filling occurs due to passive blood flow when both the atria and ventricles are relaxed. However, around 20 percent of the filling occurs due to the contraction of the left atrium. The mitral valve guards the opening between the left atrium and ventricle, ensuring unidirectional flow and preventing backflow into the atrium.
In summary, the pulmonary veins perform the critical role of connecting the lungs' pulmonary circuit with the heart's systemic circuit by delivering oxygen-enriched blood back to the left atrium, which then pumps it into the left ventricle, culminating in the distribution of this vital blood to the rest of the body via the aorta.