In health, the Yin also represents the inner body or subconscious functions (respiration, circulations, digestion, etc) and the Yang is for the outer conscious functions (muscle control, waking and sleeping, eating, etc.).
And you are right one can not be without the other, the out body supports the inner and vice versa. As example the emotions can affect the skin and the skin can affect the emotions.
Nice topic @sirwinchester.