Why does the left ventricle have the thickest walls?

The left ventricle has the thickest walls primarily because it is responsible for pumping blood into the aorta and systemic circulation. This chamber must generate a significant amount of pressure to effectively circulate oxygenated blood throughout the entire body, reaching all the tissues and organs. The thick muscular walls of the left ventricle allow it to produce the high contractile force needed to overcome the resistance of the systemic circulation, which is much greater than that of the pulmonary circulation.

The other choices highlight functions of the heart that do not necessitate the same level of muscular strength. For example, the right ventricle, not the left, pumps blood to the lungs, which require less pressure than the full-body circulation. Though the left ventricle does use oxygen to function, it does so in a way that supports its role in systemic circulation, hence it does not use less oxygen than other chambers. Additionally, while the left ventricle does receive blood, it is not directly from systemic circulation; rather, it receives oxygenated blood from the lungs via the left atrium. Thus, the unique pressure requirements of systemic circulation justify the thickness of the left ventricle's walls.