A hollow organ, the heart s walls are made up by three distinct layers. The endocardium tissue is also the type of tissue that makes up the valves of the heart. The main portion of the heart is split into two different sides with an actual partition called the septum. Each side of the heart works as a separate pump and have two chambers apiece or as a whole, the heart has four distinct chambers. Right atrium is the thin-walled area that receives the venous or used blood returning to the body by the veins.
Right ventricle is the pump area of the heart s right side.
The atrium dumps the blood into the ventricle where it is then pumped out the pulmonary arteries and to the lungs. Left atrium receives the oxygenated blood returning from the lungs. Left ventricle has the thickest walls of all. It is from this chamber the blood is pumped out of the heart, into the aorta and out to the rest of the body. Since blood flow needs to be a one-way affair, there are valves at the entrance and exit of each ventricle.