One surprising thing about the structure and organization of the heart is the way in which it is divided into four separate chambers. The heart has two upper chambers called the atria, and two lower chambers called the ventricles. The atria are responsible for receiving blood from the body and the ventricles are responsible for pumping blood out to the body. The heart is also divided into right and left halves, with the right half responsible for pumping blood to the lungs to be oxygenated, and the left half responsible for pumping oxygenated blood out to the rest of the body. This unique four-chamber structure allows the heart to efficiently pump blood and oxygen throughout the body.