Mark A. W. Andrews, professor of physiology at the Lake Erie College of Osteopathic Medicine, offers this answer.
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. To explain the prominence of veins during exercise, it helps to understand the vascular system and its components. Blood that circulates throughout the body is pumped from the left ventricle of the heart. It first enters into the high pressure arteries, where systolic blood pressure, the highest pressure exerted there, is recorded around 120 mmHg (millimeters of mercury), and diastolic pressure, the minimal pressure exerted in these vessels, is recorded at around 80 mmHg. (Thus, normal blood pressure is typically around 120/80 mmHg.) The blood flows into smaller and smaller branches of arteries called arterioles. As it continues along, its pressure decreases due to the resistance of the walls of the arterioles themselves. The blood then enters the capillaries--the smallest blood vessels--which provide nourishment to, and remove waste material from, active cells. There are more than one billion of these in the human body and they are extremely small and thin. The pressure exerted by the blood as it enters the capillaries is approximately 30 mmHg.
This pressure decreases even further as the blood completes its nourishment functions and then leaves the capillaries to flow back toward the heart via the smallest veins--the venules. The venules combine into larger and larger veins until they feed into the right atrium of the heart as the vena cava. By the time blood enters the largest veins, pressure exerted by the blood stream is only a few mmHg and its return to the heart is moved along more by muscle activity and breathing than its own inherent force.
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.