The respiratory system is fundamental to human performance, responsible for the intake of oxygen and the removal of carbon dioxide — processes essential for energy production and acid-base balance. According to the new IB SEHS specification, students are required to study the structure and function of the respiratory system, the mechanics of breathing, pulmonary ventilation, gas exchange at the lungs and muscles, and the regulation of breathing during exercise. This knowledge provides the foundation for understanding how the body supports aerobic energy production, maintains pH levels, and responds to increasing physical demand through both acute and long-term adaptations.

The system itself includes the lungs, airways (nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles), and the muscles of ventilation (primarily the diaphragm and intercostals). Its central role is to enable pulmonary ventilation, external respiration (gas exchange at the alveoli), and internal respiration (gas exchange at the tissues).

Pulmonary ventilation is the process of moving air into and out of the lungs, achieved through the coordinated action of respiratory muscles. During inspiration, the diaphragm contracts and flattens, and the external intercostals lift the rib cage, increasing thoracic volume and drawing air in. Expiration is usually passive at rest but becomes active during exercise, assisted by the internal intercostals and abdominal muscles.

Two key variables determine ventilation:

Tidal volume (TV): the volume of air moved in and out per breath

Respiratory rate (RR): the number of breaths per minute

Minute ventilation (VE): the total volume of air ventilated per minute (VE = TV × RR)

During exercise, both tidal volume and breathing rate increase significantly, raising minute ventilation to meet the heightened demand for oxygen and the need to expel accumulating carbon dioxide.

Gas exchange relies on diffusion gradients and occurs across the alveolar-capillary membrane in the lungs and the capillary-muscle interface in tissues. Oxygen diffuses from areas of high partial pressure (alveoli) to low (capillaries), while carbon dioxide diffuses in the opposite direction. At the muscle level, oxygen is delivered for aerobic metabolism, and carbon dioxide produced by cellular respiration is carried away.

Control of breathing is managed by respiratory centres in the medulla oblongata, which receive input from chemoreceptors detecting CO₂, pH, and O₂ levels in the blood. During exercise, rising carbon dioxide levels and falling pH stimulate an increase in respiratory rate and depth. Additionally, neural inputs from proprioceptors and motor cortex activity further stimulate ventilation even before significant chemical changes occur.

With long-term aerobic training, the respiratory system becomes more efficient. Improvements include increased tidal volume, stronger respiratory muscles, improved alveolar ventilation, and enhanced diffusion capacity. Although the lungs themselves do not grow significantly in healthy individuals, the overall efficiency of the system improves, reducing the oxygen cost of breathing and supporting higher aerobic workloads.