left ventricle
The left ventricle is the powerhouse of the heart — the muscular pump that drives blood out to the entire body. If the right ventricle is a short-haul delivery van, the left ventricle is the long-haul engine that pushes blood from your scalp to your toes.
Anatomically, it is the thick-walled lower left chamber. It fills with oxygen-rich blood through the mitral valve from the left atrium, then contracts forcefully to eject that blood through the aortic valve into the aorta, the body's largest artery.
Its wall is roughly three times thicker than the right ventricle's because it must generate high pressure to overcome the resistance of the whole systemic circulation. The fraction of blood it ejects with each beat, the ejection fraction, is one of the most important numbers in cardiology, and a weak left ventricle is the central problem in many forms of heart failure.
The visible thump on the left chest, the apex beat, is produced largely by the left ventricle; its position helps doctors judge whether the chamber is enlarged.