Short answers, pulled from the story.
Breathing, also called respiration or ventilation, is the rhythmic process of moving air into and out of the lungs. Its primary purpose is gas exchange, taking in oxygen and removing carbon dioxide so aerobic organisms can perform cellular respiration.
The lungs cannot inflate themselves and expand only when the thoracic cavity grows in volume. In mammals this is produced mainly by contraction of the diaphragm, with the intercostal muscles lifting the rib cage to a lesser degree. Exhalation at rest is largely passive through elastic recoil.
Inhaled air is by volume 78% nitrogen and 20.95% oxygen. Exhaled air contains 4% to 5% carbon dioxide, about a hundredfold increase, with oxygen reduced by roughly a quarter, alongside water vapor, argon and trace compounds such as isoprene and acetone.
Breathing rate and depth are regulated by respiratory centers in the brainstem that receive input from central and peripheral chemoreceptors. Central chemoreceptors in the medulla sense pH and carbon dioxide, while peripheral chemoreceptors in the aortic and carotid bodies sense arterial oxygen, with the phrenic nerves carrying signals to the diaphragm.
Atmospheric pressure falls exponentially with altitude, roughly halving every 5500 meters, so although air stays 21% oxygen, oxygen's partial pressure drops. At the summit of Mount Everest, 8848 meters, oxygen's partial pressure is only 7.1 kPa compared to 21 kPa at sea level, requiring a greater volume of air to be inhaled.
The word spirit comes from the Latin spiritus, meaning breath, reflecting a long history of treating breath as a life force. The Hebrew Bible describes God breathing the breath of life into clay to make Adam a living soul, and terms like prana, mana, ruach and psyche are all related to the concept of breath.