The respiratory system is three different jobs stacked on top of each other, and students lose marks because the textbook treats them as one topic. Ventilation is the mechanical movement of air in and out of the lungs. Gas exchange is the diffusion of oxygen and carbon dioxide across the alveolar membrane. Gas transport is how the blood carries those gases between the lungs and the tissues. Each step has its own driving force, and once you separate them, the whole respiratory system stops blurring together.
Ventilation: How Air Moves In and Out
Breathing in (inspiration) is not a vacuum-cleaner pull. It is governed by one rule — Boyle's Law: at a fixed temperature, pressure and volume are inversely related. Expand a closed space and pressure inside drops; air rushes in from outside to equalize.
The diaphragm, a dome of muscle below the lungs, contracts and flattens. The external intercostal muscles lift the rib cage outward and upward. Together they enlarge the thoracic cavity. With more volume, intra-alveolar pressure drops about 1 mmHg below atmospheric — and air flows down that gradient into the lungs. Roughly 500 mL enters in a quiet breath; this is the tidal volume.
Expiration at rest is the reverse and almost entirely passive. The diaphragm relaxes, the rib cage drops, the thorax shrinks, intra-alveolar pressure now exceeds atmospheric by about 1 mmHg, and air flows out. The lungs are also elastic — they recoil like a stretched balloon released. Only during forced expiration (coughing, exercise) do the internal intercostals and abdominal muscles actively squeeze air out.
Gas Exchange: What Happens at the Alveoli
Once air reaches the lungs, the real business happens in the alveoli — tiny grape-like sacs at the end of the airway tree. Each is wrapped in a dense net of pulmonary capillaries. The combined surface area is enormous: roughly 70 square meters in an adult, about the size of a tennis court, packed inside the chest.
Two features make diffusion fast. The wall is paper-thin — the respiratory membrane is about 0.5 μm across, just an alveolar epithelial cell plus a capillary endothelial cell. And the surface area is huge. Both terms appear in Fick's Law of Diffusion: rate is proportional to surface area and inversely proportional to thickness. Lose either — to emphysema (less surface area) or pulmonary edema (greater thickness) — and gas exchange drops.
What drives diffusion is the partial pressure gradient of each gas. In alveolar air, oxygen sits at about 104 mmHg; in venous blood arriving from the body, it is only 40 mmHg. Oxygen moves from the alveolus into the blood down that 64-mmHg drop. Carbon dioxide moves the other way: it is at 45 mmHg in the arriving blood and 40 mmHg in the alveolus, so it diffuses out. Both gases move passively; the only "decision" is which way the gradient points.
Gas Transport: How Blood Carries the Gases
Once oxygen enters the blood, almost none of it dissolves freely — water is a poor solvent for O₂. Instead, about 98.5% of oxygen is bound to hemoglobin inside red blood cells, and only 1.5% rides in plasma. Each hemoglobin molecule has four iron-containing heme groups and can carry four oxygen molecules. Fully loaded, it is oxyhemoglobin.
Hemoglobin loads and unloads cooperatively, which gives the oxygen-hemoglobin dissociation curve its sigmoid shape. In the lungs, where partial pressure of O₂ is high, hemoglobin saturates almost completely. In active tissue, where partial pressure of O₂ is low and the environment is warmer, more acidic, and richer in CO₂, hemoglobin releases its oxygen more readily — the curve shifts right, which is called the Bohr effect. This is exactly the behavior you want: hemoglobin holds tight where oxygen is plentiful and lets go where it is needed.
Carbon dioxide travels three ways. About 7% dissolves in plasma. About 23% binds directly to hemoglobin (on amino-acid groups, not the iron) as carbaminohemoglobin. The largest share, about 70%, is carried as bicarbonate ion (HCO₃⁻), after CO₂ reacts inside red blood cells with water — the enzyme carbonic anhydrase speeds it up — to form carbonic acid that immediately dissociates to bicarbonate and H⁺. The bicarbonate then leaves the red blood cell in exchange for a chloride ion (the "chloride shift"). At the lungs, every step reverses: bicarbonate re-enters the red blood cell, recombines with H⁺, becomes CO₂, and diffuses out into the alveolus.
Putting the Three Steps Together
Trace one oxygen molecule and you cover the whole respiratory system. You inhale: the diaphragm flattens, lung volume grows, alveolar pressure drops, and air flows in (ventilation). The O₂ reaches an alveolus and diffuses across the 0.5-μm respiratory membrane into a capillary because its partial pressure is higher in the alveolus than in venous blood (gas exchange). It binds hemoglobin, rides the systemic arteries to a working muscle, and unloads because tissue conditions push the dissociation curve to the right (gas transport). The CO₂ produced by that muscle catches a ride back, mostly as bicarbonate, and is breathed out as the diaphragm relaxes — closing the loop.
Getting Help
The respiratory and circulatory systems are tightly coupled — once the blood is loaded with oxygen, the cardiac cycle pumps it out to the body. For more physiology walkthroughs, see the full set of Anatomy & Physiology study guides.
Conclusion
The respiratory system explained as one process is overwhelming; explained as three is straightforward. Ventilation moves air by pressure differences set up by the diaphragm. Gas exchange moves O₂ and CO₂ across the alveolar membrane down partial-pressure gradients. Gas transport carries those gases in blood — oxygen on hemoglobin, carbon dioxide mostly as bicarbonate. Name the step you are looking at and the driving force becomes obvious.
