These veins are so prominent because of extremely low body fat when compared to a regular person. They are also more flexible and can expand and contract easier. This is also due to a fact that he leads a healthy life style, and nobody else are watch it after their diet as these guys do. Sometimes of course due to extreme physical loads they have some issues with heart muscle, but i never heard them complaining about the blood pressure :) And of course if they use steroids extensively that is already a totally different story :)