How long poison stays in the bite and why there are seven stages

The text says poison stays at the bite for at most 450 matra units of time, then, carried by vata, passes from one dhatu to the next, each passage making one stage, so there are exactly seven stages. Snake poison is said to kill like a weapon or lightning unless removed between the stages with medicines or mantras, and the author announces the antidote section (agada tantra).