Left heart is a term used to refer collectively to the left atrium and left ventricle of the heart; occasionally, this term is intended to reference the left atrium, left ventricle, and the aorta collectively.
The left atrium receives oxygenated pulmonic blood from the pulmonary veins. The blood is then pumped through the mitral valve into the left ventricle, which in turn pumps the blood through the aortic valve into the aorta.
The left side of the heart is thicker than the right because of the requirement to pump blood from the left throughout the body, as opposed to the right side pumping only through the lungs.
See also: Right heart