PDE5’s main function is to break down cGMP, a molecule that helps relax smooth muscle tissue and dilates blood vessels. By breaking down cGMP, PDE5 limits the relaxation of blood vessels and reduces blood flow. PDE5 is an enzyme that regulates blood flow by breaking down cGMP, a molecule that helps relax smooth muscle tissue. Inhibition of PDE5 is important in treating conditions like erectile dysfunction (ED) and pulmonary arterial hypertension (PAH), where promoting the relaxation of blood vessels and increasing blood flow is beneficial. PDE-5 inhibitors, by preventing the breakdown of cGMP, help to dilate blood vessels, improving circulation and reducing blood pressure in certain clinical situations.