Why is the part labelled 5 thicker than 6?

• A.5 is the left ventricle and 6 the right ventricle
• B.6 is the left ventricle and 5 the right ventricle
• C.5 receives blood from the left auricle and 6 from the right
• D.5 pumps blood round the body and 6 pumps blood into the lungs
• E.5 deals with oxygenated blood and 6 deals with deoxygenated blood