According to the University of Cincinnati's Clermont College, human beings have double circulation, which means that there are two separate loops through which blood travels. One loop, called the systemic loop, takes oxygenated blood to the body, while the other loop, called the pulmonary loop, carries blood to and from the lungs so that blood can absorb oxygen.
Blood from the body, or systemic loop, enters the heart via the right atrium. The oxygen-poor blood then travels from the right atrium through the atrioventricular valve and into the right ventricle. The right ventricle pumps the blood under low pressure into the lungs. Once the blood absorbs oxygen from the lungs, it returns to the heart via the left atrium. The blood then passes through the left atrioventricular valve and into the left ventricle. The left ventricle then pumps the blood out of the heart and into the rest of the body under high pressure, notes UC Clermont.
The mammalian heart is divided into four different chambers by structures called septums. These septums help to keep the oxygen-rich blood separate from the oxygen-poor blood. This makes the mammalian four-chambered heart much more efficient than the two- and three-chambered hearts of reptiles, amphibians and fish, according to PetEducation.com.