The purpose of the lungs is to allow oxygen to be delivered to the blood, and to remove carbon dioxide , or waste gas, from the blood.
Blood travels to the right side of the heart through veins, from all over the body.
After blood has travelled through the lungs, picking up oxygen, it travels through the pulmonary veins to the left side of the heart. The chambers that make up the left side of the heart then pump the blood through arteries, to all parts of the body. Unfortunately, if all parts of the lung don't contain enough oxygen, this normal process becomes harmful, because it results in every pulmonary artery constricting. This is caused Pulmonary Hypertension . When Pulmonary Hypertension happens, the right side of the heart is forced to try to pump blood through very-narrowed arteries, which is much harder work.