Chapter 14: Breathing and Exchange of Gases | Summary

Breathing involves two stages: inspiration (drawing atmospheric air in) and expiration (releasing alveolar air out). Air movement is driven by pressure gradients between the lungs and atmosphere. Inspiration occurs when intra-pulmonary pressure falls below atmospheric pressure; expiration occurs when it rises above. The diaphragm and external and internal intercostal muscles generate these gradients.

During inspiration, the diaphragm contracts (flattens) and external intercostal muscles lift the ribs and sternum, increasing thoracic volume in both antero-posterior and dorso-ventral axes. This increases pulmonary volume, reduces intra-pulmonary pressure, and air flows in. During expiration, relaxation of these muscles reduces thoracic and pulmonary volume, raising intra-pulmonary pressure and expelling air. A healthy adult breathes 12–16 times per minute. A spirometer is used to estimate air volumes clinically.

Key respiratory volumes include: Tidal Volume (TV) — approximately 500 mL inspired or expired in normal breathing; Inspiratory Reserve Volume (IRV) — 2500–3000 mL inspired additionally by forcible effort; Expiratory Reserve Volume (ERV) — 1000–1100 mL expired additionally by force; and Residual Volume (RV) — 1100–1200 mL remaining in lungs even after forcible expiration.

Pulmonary capacities derived from these volumes are important for clinical diagnosis: Inspiratory Capacity (IC) = TV + IRV; Expiratory Capacity (EC) = TV + ERV; Functional Residual Capacity (FRC) = ERV + RV; Vital Capacity (VC) = ERV + TV + IRV (the maximum volume breathable after forced expiration); Total Lung Capacity (TLC) = RV + ERV + TV + IRV, or VC + RV.

Alveoli are the primary sites of gas exchange. Exchange also occurs between blood and tissues. O2 and CO2 diffuse across membranes based on their partial pressure gradients, and the rate of diffusion is additionally affected by the solubility of gases and thickness of the membrane. The partial pressure of O2 in atmospheric air is 159 mm Hg, in alveoli 104 mm Hg, in deoxygenated blood 40 mm Hg, in oxygenated blood 95 mm Hg, and in tissues 40 mm Hg. For CO2, the gradient runs in the opposite direction — from tissues (45 mm Hg) to blood to alveoli (40 mm Hg).

The diffusion membrane across which exchange occurs consists of three layers: the thin squamous epithelium of the alveoli, the basement substance, and the endothelium of alveolar capillaries. Its total thickness is much less than a millimetre. CO2 has a solubility 20–25 times higher than O2, so it diffuses more rapidly per unit pressure difference. All physiological conditions in the body are arranged to favour O2 diffusion from alveoli to tissues and CO2 diffusion in the reverse direction.

Blood transports both O2 and CO2. About 97% of O2 is carried by RBCs bound to haemoglobin as oxyhaemoglobin, while 3% is dissolved in plasma. Each haemoglobin molecule can carry a maximum of four O2 molecules. O2 binding to haemoglobin is primarily influenced by pO2; it is also affected by pCO2, H+ concentration, and temperature. The oxygen dissociation curve is sigmoid in shape. In alveoli, high pO2, low pCO2, low H+ and lower temperature favour oxyhaemoglobin formation; in tissues, low pO2, high pCO2, high H+ and higher temperature favour dissociation. Every 100 mL of oxygenated blood delivers about 5 mL of O2 to tissues.

CO2 is transported in three ways: about 70% as bicarbonate ions (HCO3–) in plasma and RBCs with the help of the enzyme carbonic anhydrase; 20–25% as carbamino-haemoglobin bound to haemoglobin; and about 7% dissolved in plasma. At tissue sites, high pCO2 drives CO2 into blood where it reacts with water to form carbonic acid, then HCO3– and H+. At the alveoli, low pCO2 reverses the reaction, releasing CO2 to be exhaled. Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO2 to the alveoli.

Respiratory rhythm is regulated by the neural system. The respiratory rhythm centre in the medulla oblongata is primarily responsible for maintaining the breathing rhythm. The pneumotaxic centre in the pons region can moderate the rhythm centre by reducing the duration of inspiration, thereby altering the respiratory rate.

A chemosensitive area adjacent to the rhythm centre in the medulla is highly sensitive to CO2 and H+ ions. Elevated levels of these substances activate this area, which signals the rhythm centre to increase breathing rate to eliminate them. Receptors in the aortic arch and carotid artery also detect changes in CO2 and H+ and relay signals to the rhythm centre. The role of O2 in regulating respiratory rhythm is described as quite insignificant.

Asthma is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles. Emphysema is a chronic disorder in which alveolar walls are damaged, thereby reducing the respiratory surface; cigarette smoking is a major cause. Occupational respiratory disorders arise in industries involving grinding or stone-breaking where excessive dust exposure overwhelms the body's defence mechanisms, leading to inflammation, fibrosis (proliferation of fibrous tissue), and serious lung damage; workers in such settings should wear protective masks.

These disorders highlight the vulnerability of the respiratory system to environmental, lifestyle, and occupational hazards. Understanding their causes and mechanisms is crucial both for clinical management and for prevention through appropriate safety measures.

Chapter 14 provides a comprehensive account of how the human body takes in oxygen and eliminates carbon dioxide through an intricately coordinated system. From the structural organisation of the respiratory tract — nostrils to alveoli — to the mechanics of breathing driven by pressure gradients, the precise quantification of lung volumes and capacities, the diffusion of gases across ultra-thin membranes, and the dual transport mechanisms of haemoglobin and bicarbonate, every aspect is precisely tuned to meet cellular metabolic demands. Neural regulation by the medulla and pons ensures continuous adjustment of respiratory rate, while chemosensitive receptors respond to rising CO2 and H+ levels.

For board examinations, this chapter is among the most numerically and conceptually rich in Unit 5. Students are frequently tested on respiratory volumes and their formulas, the partial pressure table and direction of gas diffusion, the oxygen dissociation curve and factors affecting it, the three modes of CO2 transport with percentages, the role of carbonic anhydrase, and the neural centres regulating respiration. Mastery of these topics, along with the ability to apply them to clinical and real-world scenarios, is essential for scoring well in both short-answer and long-answer questions.