Hi Aspirant,

There are two types of blood that pours in and out of human heart.Like one is oxygen rich and other is oxygen poor.inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium. The pulmonary vein empties oxygen-rich blood, from the lungs into the left atrium.This is the functioning of the blood entering in to the heart and leaving it.

Hope it helps!!