Breathing

Breathing, or pulmonary ventilation, is the movement of air into and out of the lungs. It enables gas exchange that supplies oxygen for cellular respiration and eliminates carbon dioxide, a waste product of metabolism. The respiratory system includes the upper airways (nose, pharynx, larynx) and the lower airways (trachea, bronchi, bronchioles) plus the lungs, where alveoli provide the site of gas exchange with the blood. Oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveolar air to be exhaled.

Breathing is driven by the musculoskeletal system. Inhalation is typically an active process in which the diaphragm contracts and flattens, the chest expands, and air is drawn into the lungs. Exhalation is usually passive as these muscles relax, allowing elastic recoil to push air out; forced exhalation uses abdominal and intercostal muscles. The amount of air moved per breath and the rate of breathing determine minute ventilation, which increases during exercise or stress.

Neural control of breathing resides mainly in the brainstem, with the medulla oblongata and pons generating a rhythmic pattern. Chemoreceptors monitor carbon dioxide, oxygen, and pH levels, adjusting rate and depth. Voluntary control is possible through the cerebral cortex, though automatic control predominates during routine breathing. Gas transport involves oxygen binding to hemoglobin in red blood cells and carbon dioxide being carried as bicarbonate in plasma; at the lungs, these processes reverse to reoxygenate blood and remove CO2.

Breathing patterns vary: eupnea (normal), tachypnea (rapid), bradypnea (slow), apnea (cessation), and hyperventilation. Health issues such as asthma, chronic obstructive pulmonary disease, infections, or exposure to pollutants can affect ventilation and gas exchange.