M.D. Although ALS does not affect the lungs themselves, the indirect effects on the lungs are profound. The lung is a passive organ in which air is pumped in and out. Blood flowing through the lung picks up oxygen from the fresh air inhaled into it.
Carbon dioxide leaves that same blood when air is exhaled from the lung out of the body.
The diaphragm and other muscles which pump the air in and out are skeletal muscles. When their strength decreases to a critical level, the act of breathing itself will cause the respiratory muscles to fatigue , causing further weakness. Eventually, the respiratory muscles become so weak that they cannot sustain an adequate level of breathing and respiratory failure ensues. Deterioration of the respiratory muscles also impairs the ability to cough and clear secretions. If lung secretions from a minor respiratory infection are not cleared, breathing becomes more difficult, oxygen levels fall, and pneumonia may develop.