Final answer:
During venipuncture, a "pop" is often felt as the needle penetrates the tunica externa and enters the lumen of a vein, with slight resistance followed by a giving way of the vessel's wall.
Step-by-step explanation:
A "pop" felt during venipuncture typically corresponds to the needle entering the lumen of a vein. To elaborate, the venule is a small vein that enables blood to return from the capillary beds to larger blood vessels. The puncture occurs when a needle passes through the outer tissue and reaches the thin-walled vessel; this may sometimes be sensed as a 'pop' due to the transition through the tunica externa and entering the much softer lumen of the venule. The sensation can also be attributed to the slight resistance followed by a giving way as the needle penetrates the endothelium layer and enters the venous system during blood sampling or the insertion of an intravenous line.