Why is the heart muscle thicker in the ventricles than the Atria?

The heart muscle is thicker in the ventricles than in the atria because the ventricles are responsible for pumping blood throughout the body and need to generate higher pressure to do so. The left ventricle, in particular, has the thickest walls as it must overcome the systemic circulation's resistance. In contrast, the atria only need to push blood into the ventricles, requiring less muscular strength and pressure. This structural difference supports the heart's function in efficiently circulating blood.