How blood reaches the breast

The breast does not hang there in isolation: it is plugged into the circulation from several sides at once, and one of them dominates. At least 60% of its blood comes from the internal mammary artery, which is the one that runs right up against the sternum, the flat bone in the center of the chest. That is where most of the plumbing comes in.

Another portion comes in from behind. The intercostal arteries, which are the ones that run between one rib and the next, send branches that cross the chest muscles and reach the deep parenchyma, which is the glandular tissue at the back of the breast.

The skin on top has its own network, the subdermal plexus, which is a mesh of tiny vessels just under the skin; that mesh connects with deeper arterioles, so the skin and the gland are not disconnected from each other.

On the way back, blood leaves through the internal thoracic vein toward the center, through the lateral thoracic veins toward the side, and through superficial veins toward the neck. And those veins have a quirk: they almost never contain valves, and they communicate with one another inside the breast itself.