Each breath refreshes only a small share of the air in your lungs
A resting breath brings in roughly 350 millilitres of fresh air, but it lands in about 2.5 litres of gas that never leaves the lungs between breaths. That reservoir, the functional residual capacity, has its own composition, and keeping it steady is the real job of respiration.
In physiology, respiration means carrying oxygen from the surroundings to the body's tissues and removing carbon dioxide, using a respiratory system. It is not the same as cellular respiration, the chemistry inside each cell that oxidises nutrients to release energy as ATP, producing carbon dioxide and water along the way. Physiological respiration keeps the cells supplied, but it concerns the movement of gases between organism and environment rather than what happens within cells.
Gas exchange in the lungs depends on two things: ventilation, the flow of air in and out, and perfusion, the circulation of blood through the tiny pulmonary capillaries. Breathing in is normally active. The diaphragm contracts, creating a pressure change that balances the elastic, resistive and inertial forces of the respiratory system, and air reaches the alveoli, where it trades gases with the blood. Breathing out is usually passive, with many exceptions: speaking, singing, laughing, coughing and sneezing, exhaling underwater, hard exertion such as running or throwing, and some breath-control meditation. Humans' sustained breath control for speech and song is something many mammals cannot manage.
Because inhaled air mixes into the larger resident volume, respiration involves mechanisms that keep the composition of that reservoir constant and in balance with gases dissolved in the blood, and so throughout the body. Strictly, then, breathing and ventilation are narrower terms falling under respiration, not synonyms for it. Even most health professionals ignore that distinction, since respiratory rate is a firmly established clinical term that would otherwise have to become ventilation rate.
Intensive care and emergency medicine add their own measures, such as diffusing capacity, which gauges how readily gas passes from the lungs into red blood cells, and mechanical ventilation, which assists or replaces a patient's own breathing.
Source: Respiration (physiology)
