One of the big risk factors for erectile dysfunction, stroke and other nasty conditions is high blood pressure, or hypertension. High blood pressure is very hard on the arteries in particular and the added pressure actually increases the inflammation and damage to the lining of the arteries. This can be a vicious circle, because as plaque builds up in the arteries and limits the flow space, blood pressure is increased.
Some guys think that Viagra and Cialis may help with high blood pressure. After all, don't these two drugs relax the arteries and allow more blood flow? Well, the answer is not as easy as it might seem.
NOTE: If you need to actually clear out your arteries, consider a Low Fat Diet, which actually reduces arteriosclerosis. Also,
Just how much can Viagra and Cialis lower high blood pressure and help with pulmonary hypertension? It all started with a 1999 study that documented an nice average reduction of 10 and 7 for systolic and diastolic presure, respectively.  This was verified by a follow up study in 2001. 
CAUTIONS: These PDE5 Inhibitors have many side effects and issues associated with them, which I cover in my link on Viagra and Cialis.
Of course, the problem with Viagra is that few guys can take it daily, due to expense, side effects and other issues becuase its effect only last for a few hours. Cialis is much more practical in the regards and its effects are generally considered to last about 36 hours. In fact, there is a new daily version out now. 
These facts led to several follow-up studies using Cialis. Unfortunately, the results from Cialis were not nearly as dramatic.  A typical Cialis dose is 20 mg, although some guys might split the pill in half, and 20 mg was found to have to no significant effect on high blood pressure. A follow up study with 40 mg found some benefit, an average drop of 4 mm, but this is a rather high dosage level.  
Therefore, there is really no practical way for most guys to use PDE5 inhibitors to control blood pressure. Viagra cannot be taken daily and Cialis is not just effective at any reasonable dosage. If you have chronic hypertension, you best bet is to read my link on How to Naturally Lower Blood Pressure and then work with your doctor to come up with a plan.
1) Am J Cardiol, 1999 Mar 4, 83(5A):35C-44C, "Overall cardiovascular profile of sildenafil citrate"
2) J Hum Hypertens, 2001 Oct, 15(10):707-13, "Effect of sildenafil on blood pressure and arterial wave reflection in treated hypertensive men"
4) Eur Heart J Suppl, 2002, 4(suppl H):H32-H47, "Overview of the cardiovascular effects of tadalafil"
5) Circulation, 2009, 119:2894-2903, "Tadalafil Therapy for Pulmonary Arterial Hypertension"