Solution:

• The aorta receives blood under greater pressure from the powerful left ventricle and so needs a thicker wall.
• The pulmonary vein carries oxygenated blood from the lungs to the left side of the heart, from the pulmonary circulation.
• The heart contraction is enough to keep the blood flowing as the distance between the lungs and back to the heart is minimal compared to the systemic circulation.
• So, the pressure of the heart exerting is enough to keep the blood in one direction.
• Semilunar valves are present in opening of Aorta and Pulmonary artery.
• Unlike arteries, veins contain valves that ensure blood flows in only one direction.
• But, Arteries don’t require valves because pressure from the heart is so strong that blood is only able to flow in one direction.
• Valves also help blood travel back to the heart against the force of gravity.