Short answers, pulled from the story.
Breathing is the rhythmic process of moving air into and out of the lungs to drive cellular respiration in every aerobic organism on Earth. This continuous exchange of oxygen and carbon dioxide provides the energy required to power the heart, brain, and muscles. The process begins with inhalation and concludes with exhalation, repeating roughly twelve to twenty times per minute in a resting adult.
The lungs are passive organs that rely on the expansion of the thoracic cavity to draw air into the body. In mammals, this expansion is primarily generated by the contraction of the diaphragm, a dome-shaped muscle that separates the chest from the abdomen. During normal rest, exhalation is a largely passive event where the inhalatory muscles relax and the elastic recoil of the lungs and chest wall returns the chest to its resting position.
Inhaled air is approximately 78 percent nitrogen and 20.95 percent oxygen, while exhaled air contains 4 to 5 percent carbon dioxide and 5.0 to 6.3 percent water vapor. The volume of oxygen is reduced by about a quarter during exhalation, and the typical composition of exhaled air includes 79 percent nitrogen and 13.6 to 16.0 percent oxygen. Trace amounts of other substances such as hydrogen, carbon monoxide, methanol, and ammonia are also present in the exhaled air.
Atmospheric pressure decreases with height above sea level, and the pressure in the lungs decreases at the same rate with altitude. At the summit of Mount Everest, the total atmospheric pressure is 33.7 kilopascals, meaning a greater volume of air must be inhaled to breathe in the same amount of oxygen as at sea level. When the atmospheric pressure falls to below 75 percent of its value at sea level, oxygen homeostasis is given priority over carbon dioxide homeostasis at an elevation of about 15,000 feet.
Breathing compressed gases underwater may lead to several diving disorders which include pulmonary barotrauma, decompression sickness, nitrogen narcosis, and oxygen toxicity. Air is provided by a diving regulator which reduces the high pressure in a diving cylinder to the ambient pressure of the surrounding water. Underwater divers practicing technical diving may breathe oxygen-rich, oxygen-depleted, or helium-rich breathing gas mixtures to manage these risks.
The word spirit comes from the Latin spiritus meaning breath, and historically breath has often been considered in terms of the concept of life force. Different forms of meditation and yoga advocate various breathing methods, and a form of Buddhist meditation called anapanasati meaning mindfulness of breath was first introduced by Buddha. Breathing disciplines are incorporated into meditation, certain forms of yoga such as pranayama, and the Buteyko method as a treatment for asthma and other conditions.