Heart anatomy. Historical artwork of a heart that has been vertically sliced to reveal the anatomy of the four internal chambers. The two at top are the atria. The two at bottom are the ventricles. This view is from the front. Blood arrives in the right atrium (upper left), is passed to the right ventricle (lower left) and is pumped to the lungs. After oxygenation in the lungs, the blood arrives back in the left atrium (upper right) and passes to the left ventricle (lower right), which pumps it round the body. The ventricles have thicker walls than the atria because they pump the blood under higher pressure. The heart beats around 70 times a minute, billions of times over a lifetime.