You have just received your flu vaccine in your deltoid muscle. explain all of the layers and structures the needle passed through, from impact to muscle.

The deltoid muscle is very common in the world of healthcare and is most generally used for vaccination of influenza flu. The deltoid muscle is a triangular and rounded muscle situated on the top of the shoulder and the uppermost part of the arm.  

The site for intramuscular injections in the deltoid muscle is the thickest and the central part of the muscle, that is, above the level of the armpit and about two to three fingerprints underneath the acromion process.  

one would be wind another would be water density

the answer is:

carbon dioxide