Heart

The heart is the organ that pumps blood to all parts of the body, and it is the center of the circulatory system. It is made of muscle tissue (technically making it both an organ and a muscle) and is mostly hollow, and it is located in your chest slightly left of the breast bone. Its job is to take oxygen-depleted blood that go to it through the blood vessels called veins, replace it with oxygenated blood, and pump that to the rest of the body through blood vessels called veins. Oxygen is required for cellular respiration, so the heart provides all of your body with resources to function, so arguably it is the most important organ in the body. The human heart has four chambers, and each one has its own unique function. The chambers are called the left atrium, right atrium, left ventricle, and right ventricle. The atria are the top part of your heart, and the ventricles are the bottom part of your heart. In the process of a heartbeat, the right atrium sucks in oxygen-poor blood sent by two giant veins called the superior and inferior vena cava (see circulatory system) and passes into the right ventricle. The right ventricle takes this oxygen-poor blood and sends it through pulmonary arteries to the lungs which oxygenate the blood. Now, this oxygenated blood travel through pulmonary veins to the left atrium, which holds this blood. Finally, the left ventricle takes this blood and pumps it to all the organs that need it. Now for one second to rest before doing this all again. This is how the heat works.

Source: Cleveland clinic my.clevelandclinic.org/health/body/23074-heart-chambers