Why do veins pop out
Anyone who has been to a public gym has seen that some people have veins popping out from their bodies, especially arms and deltoids. Although there may be other people in the gym who exercise following the same routine as those people but they do not have such vascularity. This may be surprising to a newcomer and may seem too common to a regular guy, but the reasons behind it may not be known properly by either. Your vascularity depends on the amount of body fat and the volume of lean muscle that you have built on your body. You may have veins visibly popping out of your arms and shoulders if you have a lean structure but the veins may not be visible if the percentage of fat in your body is significant enough.
Make no mistake though, all bodies that work out, go through a great deal of increase in their systolic blood pressure. Systolic blood pressure increases with the intensity of the exercise as blood gets pumped by the cardiac muscle very quickly into the arteries, meaning that the heart has to contract with a lot of pressure as the exercise session commences. During exercise, the systolic blood pressure can reach anywhere from in between 200 mmHg to 400 mmHg, depending on the manner of the work out. Diastolic pressure also rises during exercise, especially while lifting weights. As the pressure in and from the arteries increase, the plasma fluid is pushed directly out from the thin walls of the blood capillaries and onto the muscle compartments. This process is named filtration and is responsible for the temporary bulge and stiffness that we see and feel in our muscles during a heavy work out session. Now, the cutaneous veins around the muscles obviously press onto the skin due to the pressure exerted by the bulging muscles on to these veins and appear to be bulging out themselves, and thus we see the phenomenon of popping veins.
I have mentioned why we see popping veins during work outs especially, but whether these cutaneous veins would be visible or not, is largely dependent on the quantity of fat that the person has right below his skin, or in other words, the vascualarity depends on the amount of subcutaneous fat levels of the person.