Final answer:
Making a fist contracts the muscles around the veins in your forearm and hand, compressing them and causing blood to be pushed towards the heart, temporarily making the veins more visible. This is due to the skeletal muscle pump mechanism, which assists in venous blood return.
Step-by-step explanation:
When you make a fist a few times, the veins in your hand and forearm may become more visible. This happens because as you clench your fist, the muscles in your arm and hand contract. The contraction of skeletal muscles surrounding a vein compresses the blood inside and increases the venous pressure in that area. As a result, blood is pushed through the veins toward the heart where the pressure is lower. This process is part of the skeletal muscle pump mechanism that helps move blood back to the heart, especially from the lower parts of the body.
The digital veins in the fingers drain into the palmar venous arches which feed into the radial and ulnar veins followed by the brachial vein and eventually the axillary vein. When you repeatedly make a fist, you're essentially aiding the muscle pump in propelling blood, momentarily causing the veins to bulge somewhat as they accommodate the increased flow and pressure.
In addition to the muscular action, the lack of subcutaneous fat in certain individuals, such as bodybuilders, can make the veins appear more prominent, as the surface veins are pushed closer to the skin surface. Meanwhile, factors such as aging or dysfunction of the one-way venous valves can lead to conditions like varicose veins, where veins become enlarged and blood flows backwards due to the valves not closing properly.