Question: what are the causes of pulmonary hypertension

Reference:
Pulmonary hypertension (Causes): Your heart has two upper chambers (atria) and two lower chambers (ventricles). Each time blood passes through your heart, the lower right chamber (right ventricle) pumps blood to your lungs through a large blood vessel (pulmonary artery). In your lungs, the blood releases carbon dioxide and picks up oxygen. The oxygen-rich blood then flows through blood vessels in your lungs (pulmonary arteries, capillaries and veins) to the left side of your heart. Ordinarily, the blood flows easily through the vessels in your lungs, so blood pressure is usually much lower in your lungs. With pulmonary hypertension, the rise in blood pressure is caused by changes in the cells that line your pulmonary arteries. These changes can cause the walls of the arteries to become stiff and thick, and extra tissue may form. The blood vessels may also become inflamed and tight. These changes in the pulmonary arteries can reduce or block blood flow through the blood vessels. This makes it harder for blood to flow, raising the blood pressure in the pulmonary arteries. Pulmonary hypertension is classified into five groups, depending on the cause. Group 1: Pulmonary arterial hypertension - Cause unknown, known as idiopathic pulmonary arterial hypertension   - A specific gene mutation that can cause pulmonary hypertension to develop in families, also called heritable pulmonary arterial hypertension   - Certain drugs - such as certain prescription diet drugs or illegal drugs such as methamphetamines - or certain toxins   - Heart abnormalities present at birth (congenital heart disease)   - Other conditions, such as connective tissue disorders (scleroderma, lupus, others), HIV infection or chronic liver disease (cirrhosis) Group 2: Pulmonary hypertension caused by left-sided heart disease - Left-sided valvular heart disease, such as mitral valve or aortic valve disease   - Failure of the lower left heart chamber (left ventricle) Group 3: Pulmonary hypertension caused by lung disease - Chronic obstructive pulmonary disease, such as emphysema   - Lung disease such as pulmonary fibrosis, a condition that causes scarring in the tissue between the lungs' air sacs (interstitium)   - Sleep apnea and other sleep disorders   - Long-term exposure to high altitudes in people who may be at higher risk of pulmonary hypertension Group 4: Pulmonary hypertension caused by chronic blood clots - Chronic blood clots in the lungs (pulmonary emboli) Group 5: Pulmonary hypertension associated with other conditions that have unclear reasons why the pulmonary hypertension occurs - Blood disorders   - Disorders that affect several organs in the body, such as sarcoidosis   - Metabolic disorders, such as glycogen storage disease   - Tumors pressing against pulmonary arteries Eisenmenger syndrome and pulmonary hypertension Eisenmenger syndrome, a type of congenital heart disease, causes pulmonary hypertension. It's most commonly caused by a large hole in your heart between the two lower heart chambers (ventricles), called a ventricular septal defect. This hole in your heart causes blood to circulate abnormally in your heart. Oxygen-carrying blood (red blood) mixes with oxygen-poor blood (blue blood). The blood then returns to your lungs instead of going to the rest of your body, increasing the pressure in the pulmonary arteries and causing pulmonary hypertension.

Answer: With Pulmonary hypertension, blood pressure in the pulmonary arteries is  increased because the walls of the pulmonary arteries stiffen and thicken. These changes make it harder for blood to flow, which raises the blood pressure in the pulmonary arteries.