b. left ventricle
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The left ventricle would have the thickest wall of all the chambers of the human heart. Atrial walls are rather thin because there is no need for much heart muscle in these two upper chambers. The atria merely pump blood into the ventricles; this does not require much force. The right ventricle pumps blood to the lungs, which is close to the heart and, thus, does not take much force. The left ventricle, however, must pump blood to the rest of the body; therefore, much more heart muscle is needed to provide enough force to be able to force blood to travel to all parts of the body from the head to the toes.