The Respiratory System
The respiratory system domain tests ventilation, pressure changes, gas exchange, oxygen and carbon dioxide transport, and neural control of breathing. Questions often ask you to predict how changing volume, resistance or altitude affects alveolar ventilation and blood gases.
Core rules and concepts
- Inspiration is active at rest: the diaphragm contracts and flattens, external intercostals lift the ribs, thoracic volume increases, and intrapulmonary pressure falls below atmospheric pressure so air enters.
- Expiration is usually passive at rest as elastic recoil increases intrathoracic pressure. Forced expiration uses abdominal and internal intercostal muscles.
- Boyle’s law explains ventilation: at constant temperature, pressure is inversely related to volume. Air flows down a pressure gradient.
- Surfactant reduces alveolar surface tension and prevents collapse. Without surfactant, alveoli are hard to inflate and tend to collapse.
- Oxygen is mostly carried bound to hemoglobin. Carbon dioxide is transported as bicarbonate, dissolved in plasma, and bound to hemoglobin. Carbonic anhydrase speeds bicarbonate formation.
- The medulla controls resting breathing via central chemoreceptors sensitive to CO2 and pH; peripheral chemoreceptors respond strongly to low oxygen.
Worked example
A person has a tidal volume of 500 mL and a respiratory rate of 12 breaths per minute. Minute ventilation is 500 × 12 = 6,000 mL/min. If 150 mL of each breath fills dead space, alveolar ventilation is (500 − 150) × 12 = 4,200 mL/min. A rapid, shallow pattern can raise minute ventilation while lowering alveolar ventilation, which is why rate alone does not guarantee effective gas exchange.
Traps that cost marks
Do not say oxygen is mostly dissolved in plasma; it is mostly bound to hemoglobin. Do not call expiration always active. Do not confuse surfactant with mucus. Remember that low oxygen stimulates peripheral chemoreceptors, but carbon dioxide and pH are the main drivers of normal breathing. A rightward shift in the oxygen-hemoglobin curve means hemoglobin releases oxygen more readily.
Worked example
At high altitude, alveolar oxygen pressure falls, hemoglobin saturation drops, and peripheral chemoreceptors increase ventilation.