Final answer:
A tourniquet is used in venipuncture procedures to temporarily restrict blood flow in a vein, making the vein more prominent and easier to puncture. It should be released as soon as blood flow has started.
Step-by-step explanation:
A tourniquet is a device used in venipuncture procedures to temporarily restrict blood flow in a vein. It is typically placed around the upper arm and tightened to compress the vein and make it easier to locate and access for blood collection. The purpose of using a tourniquet is to engorge the veins, making them more prominent and easier to puncture, improving the chances of successful blood collection.
However, it's important to note that the tourniquet should not be left on for an extended period of time, as it can lead to complications such as tissue damage or hemoconcentration. Therefore, it is essential to release the tourniquet as soon as the vein is successfully accessed and blood starts to flow.