This image shows the left side of a preserved sheep heart that has been sectioned to reveal its internal chambers.  Note the thick, muscular  walls of the left ventricle, reflecting its role in pumping oxygenated blood to all parts of the body through the systemic circulatory loop.  Oxygenated blood returning from the lungs via the pulmonary veins enters the left atrium and then the left ventricle through a one-way bicuspid (mitral) valve.