That pump is tangible, real-time biofeedback to let you know that blood is flowing to your muscle cells, beginning a chain of events that stimulates protein synthesis. Maybe that's why it's easy to overlook how important good nutrition is in the mass-building equation. When you choose to eat, say, chicken instead of ice cream, there's no immediate muscle gratification -- no pump to keep you motivated.
Remember my special answer: here it is… MOUTHTAPERS exist. People out there will tape their mouth closed during sleep so that they can breathe their nose. Even during the day sometimes. Why? Because the nose is directly connected to the diaphragm while the mouth is connected to the chest. Sure, increase chest mobility and your lungs may be able to expand more which will allow more air to be held, BUT HOW CAN YOU HOLD THAT MORE AIR IF YOUR DIAPHRAGM is weak? You won’t. So everything I just talked about must be done through the nose. Notice yourself breathing with your mouth and SWITCH right away. It takes work and energy. How you breathe during the day rolls over to how you breathe during the night.