A famous saying in Chinese Medicine goes: "Phlegm has its source in the Kidneys, is then moved by the Spleen, and resides in the Lungs."

This couldn't be more obvious in Cold-Phlegm patterns. The "Cold" aspect of it is that it stems from a Fire Deficiency of the Gate of Life (Ming Men), which is the source of warmth for all physiological processes. This Deficiency inhibits the Spleen's ability to transform and transport the Body Fluids, which accumulate and become Phlegm and thin mucus.

The thin mucus is then sent to the Lungs, the natural next step for Body Fluids after they're filtered in the Spleen. Hence there are the symptoms of white and watery sputum as well as chest oppression in this pattern.