Why do veins pop out when exercising?

Contrary to expectations, perhaps, bulging veins during exercise have nothing to do with an increase in either blood volume or pressure in these vessels. In fact, both are known to decrease during stepped-up activity, including exercise. When exercise begins, the heart’s rate and strength of contraction increases and blood is quickly pumped into the arteries. As this is occurring, systolic blood pressure increases linearly with exercise intensity, rising to nearly 200 mmHg during high intensity aerobic exercise (and to more than 400 mmHg during weight lifting). Diastolic pressure, on the other hand, changes very little with aerobic exercise (although it rises during weight lifting). Simultaneously, the internal diameters of veins and venules narrow in a process called venoconstriction, forcing the flow of blood forward to the heart and enhancing their ability to receive blood coming from the capillaries. Overall, this process helps decrease the pressure in the venules and veins to at most about five mmHg. Venous volume and pressure thereby decrease and are thus not the basis for the bulging. Instead, the process occurring in the capillaries as a result of the rise in arterial blood pressure during exercise causes plasma fluid otherwise resting in these tiny tributaries to be forced out through the thin vessel walls and into compartments surrounding the muscles. This process, known as filtration, causes a swelling and hardening of the muscle that is noticed during exercise. As a result of this swelling, cutaneous veins are pushed toward the skin surface, flatten to some extent, and appear to bulge. Such veins are more visible in persons with less subcutaneous fat. This bulging is neither good nor bad but simply a result of normal physiological mechanisms that result from the rise in arterial blood pressure during exertion.