就是水。
水 Water, the entity which flows between blood vessel walls.
《黃帝内經》- 中醫的源頭：
地氣升為云
上焦如霧
中焦如漚 （水汽混雜的狀態）
下焦如凟
Besides water, now it is discovered in the West, something called interstitial fluid 組織間液, which also flows between blood vessel walls. 《黃帝内經》：
營氣走在血管内；衛氣走在血管外
as taught in《傷寒論》
So, our Qi/Chi/氣, rises from feet to head as it heats up and dropped back down as it cooled, such is the circulation of Qi. If this is halted/slowed down (i.e. Air Conditioning), then pain begins. So the whole process of recovery is a matter of changes in pressure 壓力 and temperature 溫度.