Pulmonary respiration is the process of exchanging oxygen and carbon dioxide between the air in your lungs and your bloodstream. It happens across millions of tiny air sacs called alveoli, where oxygen passes into the blood and carbon dioxide passes out to be exhaled. This is distinct from cellular respiration, which is the separate process of cells using that oxygen to produce energy. When doctors or textbooks say “pulmonary respiration,” they mean the gas exchange event in the lungs specifically.
How Air Reaches the Lungs
Air enters through the nose or mouth, passes through the pharynx (the back of the throat), and moves past the larynx, where the epiglottis acts as a trapdoor to keep food and liquid out of the airway. From there it travels down the trachea, a rigid tube reinforced with cartilage rings that sits in the center of the chest.
At a junction called the carina, the trachea splits into the right and left main bronchi, one for each lung. These bronchi branch repeatedly into smaller and smaller airways, often called the bronchial tree because casts of the airways look like a bare deciduous tree in winter. The larger branches are conducting airways: their job is simply to move air deeper into the lung. As the branches get smaller, their walls begin to sprout alveoli, the thin-walled sacs where gas exchange actually happens. By the deepest level, the airways are entirely made up of alveoli clustered together in alveolar sacs.
How Breathing Creates Airflow
Your lungs don’t have muscles of their own. Instead, they rely on pressure changes in the chest cavity. At rest, the pressure inside the space surrounding the lungs (the intrapleural space) sits at about negative 5 centimeters of water pressure relative to the atmosphere. This slight vacuum keeps the lungs gently expanded.
When you inhale, the diaphragm contracts and flattens downward while the external intercostal muscles between your ribs pull the rib cage outward. This increases the volume of the chest cavity. Because volume and pressure are inversely related (a principle known as Boyle’s law), the expansion drops the pressure inside the alveoli to slightly below atmospheric pressure. That tiny pressure difference is enough to draw air in through the nose or mouth and all the way down to the alveoli.
Exhaling at rest is mostly passive. The inspiratory muscles relax, the chest cavity shrinks back to its resting size, pressure inside the alveoli rises above atmospheric pressure, and air flows out. During exercise or forceful breathing, abdominal muscles and internal intercostal muscles actively compress the chest to push air out faster.
Gas Exchange Across the Alveolar Membrane
The actual exchange of gases, the core event in pulmonary respiration, happens by simple diffusion. Oxygen moves from where its concentration is high (the air inside the alveoli) to where it is low (the blood in the surrounding capillaries), and carbon dioxide moves the opposite direction. No energy is required for this; the gases flow down their concentration gradients automatically.
To cross from air to blood, oxygen passes through four layers: a thin coating of surfactant fluid on the alveolar surface, the alveolar wall itself (a single cell thick), a shared basement membrane, and the wall of the capillary. The entire barrier is remarkably thin, which speeds diffusion. Several factors make gas exchange more or less efficient: a larger surface area, a bigger pressure difference between the alveolar air and the blood, and a thinner membrane all increase the rate of diffusion.
The numbers tell a clear story. Deoxygenated blood arriving at the lungs through the pulmonary arteries carries oxygen at a partial pressure of about 40 mmHg. The air inside the alveoli holds oxygen at roughly 100 mmHg. That 60 mmHg difference drives oxygen rapidly into the blood. Diffusion reaches equilibrium only about one-third of the way along the capillary, meaning there is a large built-in safety margin. Carbon dioxide moves in the other direction, dropping from about 46 mmHg in the incoming blood to 40 mmHg by the time blood leaves the alveolar capillaries. By the time blood exits the lungs, arterial oxygen sits near 100 mmHg and carbon dioxide near 40 mmHg.
What Surfactant Does
The alveoli are lined with a thin film of water, and water creates surface tension that would naturally cause these tiny sacs to collapse, especially at the end of an exhale when they are at their smallest. Pulmonary surfactant, a mixture of fats and proteins produced by specialized cells in the alveolar wall, dramatically lowers that surface tension to less than 1 millinewton per meter during compression. This keeps alveoli open between breaths and reduces the muscular effort needed to inflate them again on the next inhale. Without surfactant, breathing would require far more energy, and large portions of the lung would collapse (a condition called atelectasis). Surfactant also plays a role in immune defense, helping to trap and kill pathogens that reach the deepest parts of the lung.
How Your Brain Controls Breathing Rate
You rarely think about breathing because your brainstem manages it automatically. A cluster of neurons in an area called the retrotrapezoid nucleus (RTN) acts as the brain’s primary carbon dioxide sensor. These neurons respond vigorously when CO2 levels in the blood rise. When activated, they signal the respiratory pattern generator to increase both the rate and depth of breathing. Selective stimulation of the RTN produces large increases in breathing even in conscious animals, and inhibiting it consistently weakens the body’s response to rising CO2.
Serotonin-producing neurons in another brainstem region also boost breathing frequency and amplitude. And a group of neurons that produce orexin, a chemical better known for its role in wakefulness, independently stimulates both breathing and cardiovascular output. This layered system ensures breathing adjusts smoothly to changing metabolic demands, whether you’re asleep, sitting at a desk, or sprinting.
Peripheral sensors also contribute. Chemoreceptors in the carotid bodies, located near the carotid arteries in the neck, detect drops in blood oxygen and relay that information to the brainstem. Under normal conditions, however, CO2 level is the dominant driver of breathing rate. A healthy resting adult breathes 12 to 20 times per minute.
Pulmonary vs. Internal Respiration
Pulmonary respiration and internal respiration are two halves of the same cycle. Pulmonary respiration loads oxygen into the blood and offloads carbon dioxide in the lungs. Internal respiration does the reverse at the tissue level: oxygen leaves the capillaries and enters cells, while carbon dioxide produced by cellular metabolism diffuses back into the blood. By the time blood returns to the lungs, its oxygen content has dropped from about 100 mmHg to 40 mmHg, and its carbon dioxide has risen from 40 mmHg to 46 mmHg, resetting the gradients that drive the next round of gas exchange in the alveoli.
When Gas Exchange Goes Wrong
Efficient pulmonary respiration depends on a good match between ventilation (airflow to the alveoli) and perfusion (blood flow through the capillary network). The ideal ratio of ventilation to perfusion is about 0.8. Even in healthy lungs, this ratio varies by region: blood flow is higher at the base of the lung due to gravity, while the apex receives relatively less blood flow. Ventilation also increases toward the base, but not as steeply, so the ratio is naturally lower at the base and higher at the apex.
When disease disrupts this balance, oxygen levels in the blood fall. Ventilation-perfusion mismatch is the most common cause of low blood oxygen (hypoxemia). In conditions like asthma, COPD, and cystic fibrosis, some alveoli receive too little air relative to their blood supply, so blood passes through without picking up enough oxygen. In pulmonary embolism, a clot blocks blood flow to part of the lung, creating areas that are ventilated but not perfused, wasting that ventilation. Interstitial lung diseases thicken the alveolar membrane, slowing diffusion. In liver disease, increased blood flow through the lungs with normal ventilation also creates a mismatch. The end result in all these cases is the same: less oxygen reaches the bloodstream than the body needs.