What I’ve gathered is that in Traditional Chinese Medicine, organs can be grouped into complimentary pairs that work together and compliment each other, like the concept of yin yang. In the body, yin is about accepting and storing, in this case, the lungs. You breathe in, holding the air, your body directing it throughout you wherever it needs to go, before breathing back out and doing it again. Yin allows our bodies to take in what we need. To compliment yin, yang is about transport, in this case our large intestine. This organ transports and expels from our bodies what does not serve us. It is a cycle of in and out that the lungs and large intestine work together in.