Which heart sound is caused by the closure of the semilunar valves?

A. Arteries have thicker walls than veins: Arteries have thick, muscular, and elastic walls to withstand high pressure from the heart’s pumping action. This structural feature allows them to maintain blood flow and pressure throughout the circulatory system.

B. Arteries have valves inside them: Arteries generally do not have valves because blood flows under high pressure from the heart. Valves are primarily found in veins to prevent backflow.

C. Veins have thicker walls than arteries: Veins have thinner walls than arteries since they carry blood under lower pressure. Their thinner walls allow veins to expand and accommodate larger volumes of blood.

D. Veins have more elastic tissue than arteries: Arteries contain more elastic tissue to handle pulsatile blood flow. Veins have less elastic tissue and rely on valves and surrounding muscles to help return blood to the heart.

A. Connective tissue: Connective tissue provides structural support and connects other tissues, but it does not contract to produce movement. It cannot generate the force needed for breathing.

B. Skeletal muscle: The diaphragm is a skeletal muscle that contracts voluntarily and involuntarily to facilitate breathing. Its rhythmic contraction increases thoracic volume, allowing air to enter the lungs.

C. Smooth muscle: Smooth muscle is found in the walls of internal organs and blood vessels, controlling involuntary movements such as peristalsis, but it is not involved in respiratory mechanics.

D. Cardiac muscle: Cardiac muscle is found only in the heart and is responsible for pumping blood. It does not participate in ventilation or diaphragm movement.