The blood in the pulmonary artery is under less pressure since it is only being pumped a short distance to the lungs and so does not need a thick wall. The aorta (which transports the blood over greater distances) receives blood under greater pressure from the powerful left ventricle and so needs a thicker wall. The pulmonary valve (pulmonic valve) is the semilunar valve of the heart that lies between the right ventricle and the pulmonary artery. Valves are used to prevent backflow of blood. 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.

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