Naga Vayu is one of the vital airs of the body. It is one of the five auxiliary airs of the body, the other being kurma, krkara, devadatta and dhananjaya. Naga Vayu brings about activities like spitting, belching, vomiting etc, as mentioned in Darshana Upanishad (IV.33) and Trisikhi Brahmana Upanishad (86).
Kurma (tortoise) controls blinking of eyes; krkara (patridge) controls sneezing, hunger and thirst; devadatta (god-given) controls yawning and dozing and dhananjaya (treasure winning) controls hiccupping and stays in the body even after death.
Any activity or movement in the body involves expenditure of energy. That energy can be derived from one single source, namely, the vital air called prana, which is present everywhere. The individual living beings are supposed to receive this universal source of energy into their bodies through the process of respiration. So long as the process of respiration goes on in the body, the supply of prana (the principal energy) can be maintained without a break. Any discontinuity in the supply of prana means death. Life goes on only as long as respiration continues unhindered. When it stops, life comes to an end.
Perhaps this fact has caused the concept of vayu or simply air from the atmosphere, to be associated with life and to be taken as a fundamental requirement of life. Since there are many different life activities going on at different places in the human body, the vayus came to be grouped into ten different varieties. Five of them were later recognized as principal vayus (prana, apana, vyana, udana and samana) and five as subordinate vayus. All these are said to be different names of prana alone (Yoga Vasistha V: 78:11)