How are cancer and lymph nodes related to your respiratory system?

Metastasis: Many cancers spread to lymph nodes that are in the chest or directly to the lungs themselves. The enlarging tumors compress vital structures including the lungs themselves, arterial supply or venous return to or from the lungs which may fill with fluid. The common denominator is that gas exchange becomes impaired so the blood oxygen level falls.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.