Not gas dispersion, but it's crazy how fast some compounds can disperse in the human circulatory system when introduced by IV. If you've ever had IV saline flush you may know that metallic taste that seems to show up in your mouth almost instantly.

Similarly, there is a sensation from Adenosine for chemical cardioversion that creates a hot flushing feeling inside your body as it spreads, and it's quite the sensation to feel it going from your chest down to your extremities in a few seconds.