What substance is most important for maintaining a stable thermal environment within the human body?

Water.

Explanation:

The human body uses water in the cells, tissues and organs to regulate its thermal environment and maintain other bodily functions. This is because water has the capacity to regulate the internal temperature of the body in response to the external temperature. This can be done through sweat, since when the external temperature is high, sweat prevents overheating of the body. So at a high temperature, the evaporation of sweat brings a loss of heat and reduces the internal temperature.

Because the body loses water through sweating, digestion and urination, especially during exercise and high temperatures, it's important to rehydrate by drinking fluids.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.