How does blood flow through the heart?

The heart is the muscle responsible for pumping blood around the body. It has four chambers: an atrium and ventricle on the left, and an atrium and ventricle on the right. Atrioventricular and semilunar valves are found between the atria and ventricles, and ventricles and outflow vessels respectively. These valves prevent the backflow of blood. 
 There are also four important blood vessels to consider: the vena cava (which carries blood to the right side); the pulmonary artery (which carries blood away from the heart to the lungs); the pulmonary vein (which carries blood to the left side of the heart); and, the aorta (which carries blood away from the heart and around the body).
 Remember that when we talk about the heart, we label ‘left’ and ‘right’ based on the patient’s left and right and not ours! Therefore on a diagram of a heart, the right atrium and ventricle can be found on the left of the diagram and the left atrium and ventricle is on the right! 
 On the right side: 
 Deoxygenated blood from the body enters the right atrium via the vena cava. It then flows into the right ventricle through the right atrioventricular valve (or tricuspid valve). When the ventricles contract blood then exits the right side of the heart through the pulmonary valve and the pulmonary artery. This artery takes the blood to the lungs. In the lungs, the deoxygenated blood can pick up oxygen which is transported inside red blood cells. 
 On the left side:
 Oxygenated blood returns to the left side of the heart via the pulmonary vein. The first chamber on the left that blood enters is the left atrium. It then passes into the left ventricle through the left atrioventricular valve (also known as the bicuspid, or mitral valve). After the left ventricle contracts blood passes through the aortic valve into the aorta and is then transported to respiring tissues in the body. The muscular wall of the left ventricle has to be thicker than the wall of the right ventricle. This is because a lot of pressure is required to pump blood around the entire body.