If you're pumping iron at Venice Beach -- or just ogling those pumping iron at Venice Beach -- you may notice the Fabio lookalike who just finished working out has veins bulging among his rippling deltoids.

Once you get over your awe (or envy) of his sculpted physique, you may stop to wonder: What makes a muscleman (or musclewoman's) veins stand out?

Though a body builder's veins might appear to be bulging out of his skin, it's not because they're more swollen with blood, according to Mark A.W. Andrews, a professor of physiology at the Lake Erie College of Osteopathic Medicine.

Andrews told Scientific American in 2006 that blood pressure inside veins, which carry oxygen-spent blood back to the heart, isn't the cause of bulging. It's actually primarily due to a rise in blood pressure in the arteries, which carry oxygen-rich blood away from the heart to the muscles. This rise in arterial blood pressure forces plasma fluid out of the walls of thin blood vessels known as capillaries.

When the plasma is forced out, the surrounding muscle becomes swollen and harder and pushes subcutaneous veins -- located closer to the skin -- up to the surface.

This effect is more noticeable if a person has a lower body-fat percentage -- less fat right underneath the skin means it won't mask the bulging veins, according to Stephen Ball, an associate professor of nutrition and exercise physiology at the University of Missouri.

In general, the leaner the person is the more the veins you will see, Ball said in an e-mail.