A 28-year-old woman presents to your pulmonary clinic with worsening shortness of breath. She has been taking sildenafil, iloprost, and diuretics, but they did not improve her condition. She would like to try a new medication that acts by blocking endothelin receptors in the pulmonary bed. After going through her options, she expresses interest in trying Bosentan. What is the mechanism of action of this medication?