What causes pulmonary artery dilation?

Each artery in the human body has a baro-receptor. These baroreceptors, when innervated, cause dilation and constriction of the artery. For instance, if someone is losing a vast amount of blood, the baro-receptor notices a decrease in volume and sends an impulse to the brain which changes the amount of hormone released from the pituitary gland. The hormones are known as prostaglandins and have been pharmacologically refined to assist with the dilation in the pulmonary vessels. You may know this drug to be Viagra.