The Respiratory System and Gas Exchange
Breathing and Gas Exchange
Every sporting action requires energy, and most energy for sustained activity comes from aerobic respiration, which demands a continuous supply of oxygen and an efficient means of removing carbon dioxide. The respiratory system provides both. It moves air in and out of the body, extracts oxygen from inhaled air and expels the CO2 produced by working muscles. Understanding the pathway of air, the mechanics of breathing and the process of gas exchange at the alveoli underpins everything else in applied anatomy and physiology.
The Pathway of Air
Air enters through the nose or mouth and travels down the trachea (windpipe) into the chest. The trachea divides into two bronchi (singular: bronchus), one entering each lung. Inside the lungs, each bronchus branches repeatedly into smaller and smaller airways called bronchioles. At the end of each bronchiole sits a cluster of tiny air sacs called alveoli (singular: alveolus). It is here, at the alveoli, that oxygen passes into the bloodstream and carbon dioxide passes out.
The diaphragm is the primary muscle of breathing. It sits below the lungs as a dome-shaped sheet of muscle. The external intercostal muscles run between the ribs and assist breathing by raising the ribcage. Together, these structures create the pressure changes that move air in and out.
Breathing Mechanics
During inhalation, the diaphragm contracts and flattens downward, while the external intercostal muscles contract and lift the ribcage upward and outward. This increases the volume of the thoracic (chest) cavity, reducing the air pressure inside. Air rushes in down the pressure gradient from the higher-pressure atmosphere into the lower-pressure lungs.
During exhalation at rest, the diaphragm and external intercostals relax. The elastic recoil of the lung tissue and the weight of the ribcage cause the thoracic volume to decrease, pressure rises above atmospheric, and air is pushed out. During exercise, when faster and deeper breathing is needed, the internal intercostal muscles and abdominal muscles actively assist exhalation by compressing the thoracic cavity further.
Tidal Volume and Vital Capacity
Tidal volume is the amount of air moved per breath. At rest, a typical adult’s tidal volume is about 0.5 litres. During maximal exercise, tidal volume can rise to around 3 litres as the body demands more oxygen. This increase is due to the deeper contractions of the diaphragm and the active engagement of the intercostal and abdominal muscles.
Vital capacity is the maximum amount of air that can be exhaled after a maximal inhalation. It is used as a measure of lung capacity and typically ranges from 3 to 6 litres depending on body size, sex and training status. Endurance athletes tend to have higher vital capacities than untrained individuals, partly due to strengthening of the respiratory muscles and increased chest wall flexibility. Regular aerobic training can modestly increase vital capacity over time.
Air pathway: nose/mouth → trachea → bronchi (one per lung) → bronchioles → alveoli. At the alveoli: O2 diffuses into blood; CO2 diffuses out. Tidal volume rises from ~0.5 L at rest to ~3 L during maximal exercise. Vital capacity is the maximum air exhaled after a full breath in.
Gas Exchange at the Alveoli
Gas exchange works by diffusion — gases move from an area of high concentration to an area of low concentration across a thin membrane. The alveoli are designed to maximise the rate of diffusion. Each alveolus is surrounded by a dense network of pulmonary capillaries. The combined wall of the alveolus and the capillary is only two cells thick, providing the shortest possible diffusion distance.
Inhaled air is richer in oxygen (about 21%) than the deoxygenated blood arriving from the working muscles (which has used most of its oxygen). Oxygen therefore diffuses from the alveolar air into the blood. Carbon dioxide concentration is higher in the blood (produced by aerobic respiration in the muscles) than in the alveolar air, so CO2 diffuses from the blood into the alveolus to be exhaled. This simultaneous exchange happens continuously — as long as you are breathing and your heart is beating.
Inhaled air contains approximately 21% oxygen and 0.04% CO2. Exhaled air contains approximately 16% oxygen and 4% CO2, reflecting the oxygen taken up and the CO2 expelled. The nitrogen content (~78%) is virtually unchanged because the body neither uses nor produces it.
Inhaled vs exhaled air: O2 falls from ~21% to ~16%; CO2 rises from ~0.04% to ~4%. This difference reflects the oxygen absorbed and CO2 produced by working tissues. At the alveoli, diffusion is maximised by: large surface area, thin membrane, rich blood supply and steep concentration gradients maintained by breathing and circulation.
The respiratory and cardiovascular systems must operate in synchrony. Oxygen absorbed at the alveoli immediately enters the pulmonary capillaries and is carried by haemoglobin in red blood cells via the pulmonary vein to the left side of the heart. From there, the left ventricle pumps it out through the aorta to the working muscles. At the muscle capillaries, oxygen diffuses from the blood into the muscle fibres while CO2 diffuses from the fibres into the blood. The blood, now deoxygenated and CO2-rich, returns via the vena cava to the right side of the heart, is pumped to the lungs, and the cycle repeats. During intense exercise, both systems up-regulate together: breathing rate and depth increase to bring more O2 to the alveoli; heart rate and stroke volume increase to transport it faster. If either system is limited (e.g. by asthma reducing airflow, or anaemia reducing haemoglobin), exercise capacity is compromised.
During maximal, short-duration anaerobic exercise (e.g. a 100 m sprint), the demand for oxygen exceeds what the cardiovascular and respiratory systems can deliver. Muscles shift to anaerobic respiration and accumulate lactic acid. The lungs are still exchanging gases as fast as possible, but the limiting factor is the delivery rate, not the exchange mechanism itself. After the sprint, breathing rate remains elevated to repay the oxygen debt (excess post-exercise oxygen consumption, EPOC) — the extra oxygen needed to remove lactic acid and restore the body’s pre-exercise state.