The ability to achieve and sustain erections require: In the past, erection problems were thought to be "all in the man's mind." Men often were given unhelpful advice such as "don't worry" or "just relax and it will take care of itself." Today, doctors believe that when the problem is not temporary or does not go away on its own, physical factors are often the cause. where to buy cialis cialis online sale usa “We’re talking about blood flow in the male penile shaft,” explains urologist Ajay Nehra, MD, professor of urology at the Mayo Clinic in Rochester, Minn. Arteries in both the heart and the penis are lined with endothelium, says Dr. Nehra. When the endothelium in these blood vessels doesn’t respond well to the release of nitric oxide (a key part of the chain reaction that allows blood to flow as needed), then blood doesn’t flow into organs, including the penis.