Simplify the following text for a Grade 10 native English-speaking person.
Original: In medicine , pulmonary hypertension ( PH ) is an increase in blood pressure in the pulmonary artery , pulmonary vein , or pulmonary capillaries , together known as the lung vasculature , leading to shortness of breath , dizziness , fainting , and other symptoms , all of which are exacerbated by exertion . Pulmonary hypertension can be a severe disease with a markedly decreased exercise tolerance and heart failure .
Simplified: The most common cause of pulmonary hypertension is left heart failure .