Answer:
Pulmonary semilunar
Explanation
Generally heart chambers are separated from each other by valves.Likewise blood vessels of the heart. When the heart contracts(systole) to ejects blood,theses valves close up,to prevent back flow and the mixture of oxygenated with deoxygenated blood. When these occurs the forces of closure leads to certain distinctive sounds in the heart, detected by the stethoscope.Theses are called heart sounds,.
The pulmonary arteries take deoxygenated blood away from the heart to the pulmonary circulation in the lungs,while the aorta takes oxygenated blood away from the heart into the systemic circulation,The back flow of the blood into the pulmonary artery is prevented by the pulmonary valves,while the aortic semi lunar valves guide against this in the aorta,The sounds produced by these closure is called the dupp sound.,
However, because the the left sternal border of the second intercostal space is the location of the Pulmonary semiminular valves.Therefore the closure is the actual sound of dupp.
