Breathing
Right now, without a single conscious thought, your diaphragm is contracting and pulling air into your lungs. Breathing, also called respiration or ventilation, is the rhythmic movement of air into and out of the lungs. Inhalation draws air in. Exhalation pushes it back out. The whole purpose is gas exchange: taking in oxygen and removing carbon dioxide. Every aerobic organism needs oxygen for cellular respiration, the process that extracts energy from food and leaves carbon dioxide as waste. Yet the lungs cannot inflate themselves. They expand only when something else makes room for them. The air you breathe out is not the air you breathed in, and the difference can reveal disease. How does a process that runs on autopilot also bend to your will when you sing or hold your breath underwater? Why does the body guard carbon dioxide so carefully, and what happens to that careful guarding on the summit of Mount Everest? And why does the word for spirit trace back, in Latin, to the simple act of taking a breath?
In mammals, the diaphragm does most of the work, with the intercostal muscles lifting the rib cage to a lesser degree. The thoracic cavity grows in volume, and only then do the lungs follow. During forceful inhalation, accessory muscles join in, augmenting the pump-handle and bucket-handle movements of the ribs to widen the chest further.
Exhalation at rest takes almost no effort. The inhalatory muscles relax, and the elastic recoil of the lungs and chest wall returns the chest to its resting position. At that resting point the lungs are not empty. They hold the functional residual capacity, roughly 2.5 to 3.0 liters in an adult human.
Heavy breathing changes the picture. During hyperpnea, such as exercise, the abdominal muscles actively contract, pushing the diaphragm upward and reducing the lung volume left at the end of exhalation. Even at maximum exhalation, a normal mammal keeps residual air inside. When breathing becomes labored, the strain shows. Diaphragmatic or abdominal breathing produces visible movement of the belly, while clavicular elevation and accessory muscles appear in severe asthma or chronic obstructive pulmonary disease exacerbations.
Air is ideally inhaled and exhaled through the nose, not the mouth. The nasal cavities, divided by the nasal septum and lined with convoluted conchae, expose incoming air to a wide mucosal surface. There the air is warmed and humidified, and particulate matter is trapped in mucus before it travels deeper. During exhalation, some heat and moisture are recovered as air passes back over the cooler, partly dried mucus.
Below the nose, the system is often described as a respiratory or tracheobronchial tree. Larger conducting airways branch again and again into smaller bronchi and bronchioles, averaging about 23 branching generations in humans. The trachea and major bronchi begin outside the lungs, and most of the branching happens within them, all the way to the blind-ended alveoli.
Not all of that air ever reaches the exchange surface. The conducting airways form an anatomical dead space, about 150 milliliters in an adult, that takes no part in gas exchange. The terminal divisions, the respiratory bronchioles, alveolar ducts and alveoli, are the parts specialized for moving gases across the membrane.
Diffusion does the real exchange, across a structure thinner than it sounds. The respiratory membrane is built from alveolar epithelium, capillary endothelium and the basement membrane, fused into a blood-gas barrier. Across that barrier, oxygen and carbon dioxide pass between alveolar air and the blood in the pulmonary capillaries.
Each breath changes less than you might expect. After exhalation the lungs still hold the functional residual capacity, so on a typical inhalation only a small volume of fresh atmospheric air mixes with what is already there. Alveolar gas composition stays fairly constant from one breath to the next. Capillary blood therefore equilibrates with a relatively steady mixture.
Because the inner sky shifts so slowly, the sensors respond to drift rather than to sudden swings. Peripheral and central chemoreceptors detect gradual changes in dissolved gases. Homeostatic control of breathing thus centers on the arterial partial pressures of carbon dioxide and oxygen, and on holding blood pH in place.
Respiratory centers in the brainstem set both the rate and depth of breathing, drawing on input from two kinds of chemoreceptors. Central chemoreceptors in the medulla react keenly to pH and carbon dioxide in the blood and cerebrospinal fluid. Peripheral chemoreceptors in the aortic and carotid bodies watch mainly the arterial oxygen.
The pons and medulla integrate these signals and adjust ventilation to restore blood gas tensions, for instance pulling arterial carbon dioxide back toward normal during exercise. Motor nerves carry these orders outward, among them the phrenic nerves running to the diaphragm.
Though breathing is mostly automatic, it yields to conscious command. People modify it deliberately for speaking, singing, swimming, or breath-holding training, and conscious breathing techniques may promote relaxation. Reflexes can override it too. The diving reflex alters breathing and circulation during submersion to conserve oxygen.
Inhaled air is, by volume, 78% nitrogen and 20.95% oxygen, plus small amounts of argon, carbon dioxide, neon, helium and hydrogen. What comes back out tells a different story.
Exhaled air carries 4% to 5% carbon dioxide by volume, about a hundredfold increase over what went in. The oxygen drops by roughly a quarter. A typical exhaled mixture runs about 5.0 to 6.3% water vapor, 79% nitrogen, 13.6 to 16.0% oxygen, 4.0 to 5.3% carbon dioxide and 1% argon. Within that breath sit fainter traces. Hydrogen appears in parts per million from microorganisms in the large intestine, carbon monoxide from the breakdown of heme proteins, about 4.5 ppm of methanol and 1 ppm of ammonia.
Hundreds of volatile organic compounds also ride along, especially isoprene and acetone, and the presence of certain organic compounds signals disease. Not all breathing draws on ordinary air. Technical divers may breathe oxygen-rich, oxygen-depleted or helium-rich mixtures, patients receive oxygen and analgesic gases, and space suits hold pure oxygen kept at around 20% of Earth's atmospheric pressure to control the rate of inspiration.
At the summit of Mount Everest, 8848 meters up, the air is still 21% oxygen, yet a climber gasps. Atmospheric pressure falls exponentially with altitude, roughly halving every 5500 meters, while the composition of the air stays nearly constant below 80 kilometers. At sea level the ambient pressure is about 100 kPa and oxygen's partial pressure is 21 kPa. On Everest, where total pressure is 33.7 kPa, that partial pressure falls to 7.1 kPa, so a greater volume of air must be drawn in to capture the same oxygen.
Water vapor complicates the high country. Saturated vapor pressure depends only on temperature, holding at 6.3 kPa at a body core of 37 degrees Celsius regardless of altitude. In sea level tracheal air the oxygen pressure is 19.7 kPa, but at Everest's summit it drops to 5.8 kPa, below what the lower atmospheric pressure alone would explain.
The body answers low pressure by raising the respiratory minute volume automatically. At sea level the system guards arterial carbon dioxide near 5.3 kPa and lets oxygen vary widely. Below 75% of sea level pressure, at about 2500 meters, oxygen homeostasis takes priority instead. An abrupt switch can trigger hyperventilation, raising arterial pH into respiratory alkalosis, one contributor to high altitude sickness.
Descend below the waterline and the trend reverses. Pressure rises about one atmosphere, slightly more than 100 kPa, for every 10 meters. Divers breathe air at the ambient pressure of the surrounding water, which carries risks like pulmonary barotrauma, decompression sickness, nitrogen narcosis and oxygen toxicity. A diving regulator reduces the high pressure in a cylinder to ambient pressure, and a good one uses the Venturi effect to make each draw of air nearly effortless.
The word spirit comes from the Latin spiritus, meaning breath, and that link runs deep through human belief. The Hebrew Bible describes God breathing the breath of life into clay to make Adam a living soul, the nephesh, and the breath returning to God when a mortal dies. The same thread ties together spirit, prana, the Polynesian mana, the Hebrew ruach and the psyche of psychology.
Disciplines built whole practices around the inhale and exhale. Tai chi pairs aerobic movement with breathing exercises to strengthen the diaphragm and improve posture. A Buddhist meditation called anapanasati, meaning mindfulness of breath, was first introduced by the Buddha, and breathing methods run through pranayama yoga and the Buteyko method used to treat asthma. Wind instrument players use circular breathing, and singers depend on breath control.
Breathing and mood move together, which is why deeper diaphragmatic and abdominal breathing is so often recommended to encourage relaxation. The same pattern serves the body under strain. During exercise a deeper breathing pattern lowers the diaphragm, generating intra-abdominal pressure that strengthens the lumbar spine, which is why people are told to take a deep breath when lifting heavy weights.
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Common questions
What is breathing and what is its main purpose?
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.
How do the lungs expand during breathing?
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.
What is the composition of exhaled air compared to inhaled air?
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.
How is breathing controlled in the body?
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.
Why is breathing harder at high altitude like Mount Everest?
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.
Why does the word spirit relate to breathing?
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.