The heart feeds itself first
Here's a surprise that trips up many beginners: the blood roaring through the heart's chambers does not feed the heart muscle itself. The walls are too thick for oxygen to soak inward. Instead the myocardium has its own dedicated supply, the coronary arteries, which branch off the aorta just above the aortic valve and wrap over the heart's surface like a crown — which is exactly what “coronary” means.
There are two main coronary arteries, left and right, and the left one quickly splits. Its most famous branch is the left anterior descending artery, which runs down the front of the heart and feeds a large part of the muscular left ventricle. Because it supplies so much vital muscle, a blockage there is serious enough to have earned a grim nickname among clinicians.
Draining the muscle
Once coronary blood has given its oxygen to the muscle, it has to get out. Small cardiac veins run alongside the arteries, gather into a large vein on the back of the heart called the coronary sinus, and empty into the right atrium. So the heart's own used blood rejoins the main stream at exactly the point where the body's returning blood arrives — neat and economical.
You don't need to memorise every vein. The point to carry forward is that the coronary supply is a complete little circulation of its own — arteries in, capillaries through the muscle, veins out — running in parallel with everything else the heart is doing for the body.
The great vessels: the heart's gateways
Finally, how does blood get in and out of the whole organ? Through a handful of great vessels at the top of the heart. Two great veins, the upper and lower vena cava, bring oxygen-poor blood from the body into the right atrium. The pulmonary artery carries blood from the right ventricle to the lungs, and the pulmonary veins bring it back to the left atrium. And the grandest of all, the aorta, leaves the left ventricle to carry oxygen-rich blood to the entire body.
Tracing one full circuit through the great vessels
Body (low O2)
-> Vena cava (superior + inferior)
-> RIGHT ATRIUM
-> [tricuspid valve]
-> RIGHT VENTRICLE
-> [pulmonary valve]
-> Pulmonary ARTERY -> LUNGS (pick up O2)
-> Pulmonary VEINS
-> LEFT ATRIUM
-> [mitral valve]
-> LEFT VENTRICLE
-> [aortic valve]
-> AORTA -> Body (high O2)
Note the one tricky naming point:
* the pulmonary ARTERY carries oxygen-POOR blood
* the pulmonary VEINS carry oxygen-RICH blood
Artery/vein names describe DIRECTION (away from / toward the heart),
NOT oxygen content. The lung circuit is the exception that proves it.