Breathing and gas exchange
Breathing is pressure-driven: air flows from high to low pressure. Gas exchange then depends on diffusion down partial pressure gradients.
Pressure and volume
Boyle's law says that at constant temperature, gas pressure and volume are inversely related. When the chest cavity gets bigger, the pressure inside the lungs drops below atmospheric pressure and air flows in. When it gets smaller, pressure rises and air flows out.
Inspiration and expiration
During quiet inspiration, the diaphragm contracts and flattens and the external intercostal muscles lift the ribs, enlarging the chest. Quiet expiration is mostly passive: muscles relax and the elastic lungs recoil. Forced breathing, during exercise, adds neck muscles for inspiration and abdominal and internal intercostal muscles for expiration.
Lung volumes
Tidal volume is the air moved in a normal breath, about 500 mL. Vital capacity is the maximum you can exhale after a maximum inhalation, about 4.5-5 litres in a young adult. Residual volume, about 1.2 litres, always stays in the lungs. Spirometry measures these volumes to diagnose lung diseases.
Partial pressures
Each gas in a mixture exerts its own partial pressure. Oxygen is about 21% of air, so at sea level its partial pressure is about 160 mmHg; in alveoli it is about 100 mmHg, and in returning venous blood about 40 mmHg. Gases diffuse from high to low partial pressure, so oxygen moves from alveoli into blood and carbon dioxide moves out.
Oxygen transport
About 98.5% of oxygen in blood is carried bound to haemoglobin; each haemoglobin can carry four oxygen molecules. Haemoglobin releases oxygen more readily where carbon dioxide is high, pH is low, and temperature is higher, exactly the conditions in active muscle. Carbon monoxide binds haemoglobin over 200 times more strongly than oxygen, which makes it deadly.
Carbon dioxide transport
Carbon dioxide is carried in three ways: about 70% as bicarbonate ions, formed when carbon dioxide reacts with water in red cells with the enzyme carbonic anhydrase; about 20% bound to haemoglobin; and about 7-10% dissolved in plasma. In the lungs, the reactions reverse and carbon dioxide is exhaled.
Control of breathing
Breathing rhythm is set by centres in the medulla and pons. Chemoreceptors mainly respond to carbon dioxide, which lowers pH when it rises. Higher carbon dioxide triggers faster, deeper breathing. Low oxygen becomes a major stimulus only at high altitude or in some lung diseases.
Respiratory disorders
Asthma narrows airways through inflammation and smooth muscle spasm, treated with inhaled bronchodilators and steroids. Chronic obstructive pulmonary disease (COPD), mostly from smoking, includes emphysema, which destroys alveolar walls and reduces surface area. Pneumonia fills alveoli with fluid. Lung cancer is the leading cause of cancer death worldwide.
Key takeaways
Breathing follows Boyle's law: a larger chest lowers pressure and draws air in. The diaphragm drives quiet breathing; expiration is mostly passive recoil. Tidal volume is about 500 mL. Gases diffuse down partial pressure gradients. Oxygen rides on haemoglobin; carbon dioxide travels mostly as bicarbonate. Carbon dioxide levels drive breathing. Asthma, COPD, and pneumonia impair it.
Flashcard
Pressure and volume inversely related
Check your understanding
Why does haemoglobin release more oxygen in exercising muscle?