The heart is generally thought of as having a right and left side. In reality, the heart is one organ and not divided into two separate organs. The heart is made up of four chambers, two on the right and two on the left. The chambers are known as atria and ventricles. Each side of the heart is composed of one atrium and one ventricle. The atria are the receiving chambers of the heart, receiving blood flowing back to the heart. The ventricles are the chambers of the heart that pump the blood out of the heart.