Breathing and Exchange of Gases

2.1 Core concepts

• Breathing (commonly called respiration) is the exchange of O2 from the atmosphere with CO2 produced by cells; O2 is needed to break down glucose, amino acids and fatty acids, while CO2 released is harmful and must be expelled (NCERT §14 intro, p. 183).
• Respiratory strategies vary by habitat and organisation: sponges, coelenterates and flatworms exchange gases by simple diffusion over the entire body surface; earthworms use moist cuticle; insects use tracheal tubes; aquatic arthropods and molluscs use gills (branchial respiration); terrestrial forms use lungs (pulmonary respiration). Fishes use gills; amphibians, reptiles, birds and mammals respire through lungs; frogs additionally use cutaneous respiration through moist skin (NCERT §14.1, p. 183).
• Human respiratory passage: external nostrils → nasal chamber → pharynx (common food–air passage) → larynx (sound box; glottis covered by epiglottis during swallowing) → trachea → primary bronchi (trachea divides at the 5th thoracic vertebra) → secondary and tertiary bronchi → bronchioles → terminal bronchioles → alveoli. Trachea, primary, secondary and tertiary bronchi and initial bronchioles are supported by incomplete cartilaginous rings (NCERT §14.1.1, p. 184).
• Lungs are paired and enclosed in a double-layered pleura with pleural fluid between the layers to reduce friction; the outer pleural membrane contacts the thoracic lining and the inner contacts the lung surface (NCERT §14.1.1, p. 184).
• The conducting part (external nostrils → terminal bronchioles) transports, cleans, humidifies and warms air; the respiratory/exchange part (alveoli + ducts) is the site of actual O2/CO2 diffusion (NCERT §14.1.1, p. 185).
• Thoracic chamber boundaries: dorsally the vertebral column, ventrally the sternum, laterally the ribs, and below the dome-shaped diaphragm; the chamber is air-tight so changes in its volume are mirrored in pulmonary volume (NCERT §14.1.1, p. 185).
• Respiration involves five steps: (i) pulmonary ventilation, (ii) diffusion across alveolar membrane, (iii) transport of gases by blood, (iv) diffusion between blood and tissues, (v) cellular utilisation of O2 (NCERT §14.1.1, p. 185).
• Inspiration: diaphragm contracts → thoracic volume increases on antero-posterior axis; external intercostals contract → ribs and sternum lift up → thoracic volume increases on dorso-ventral axis; pulmonary volume rises, intra-pulmonary pressure falls below atmospheric, air rushes in. Expiration: diaphragm and intercostals relax → thoracic and pulmonary volumes decrease → intra-pulmonary pressure rises slightly above atmospheric → air is expelled. Healthy human breathes 12–16 times/minute; a spirometer measures volumes for clinical assessment (NCERT §14.2, pp. 185–186).
• Respiratory volumes: Tidal Volume (TV) ≈ 500 mL (a healthy man inspires/expires ~6000–8000 mL per minute); Inspiratory Reserve Volume (IRV) 2500–3000 mL; Expiratory Reserve Volume (ERV) 1000–1100 mL; Residual Volume (RV) 1100–1200 mL (NCERT §14.2.1, pp. 186–187).
• Respiratory capacities: Inspiratory Capacity (IC) = TV + IRV; Expiratory Capacity (EC) = TV + ERV; Functional Residual Capacity (FRC) = ERV + RV; Vital Capacity (VC) = ERV + TV + IRV; Total Lung Capacity (TLC) = RV + ERV + TV + IRV = VC + RV (NCERT §14.2.1, p. 187).
• Gas exchange at alveoli (and at tissues) is by simple diffusion driven by pressure/concentration gradients; solubility of gases and thickness of the diffusion membrane also affect the rate. Partial pressures (mm Hg) — O2: atmospheric 159, alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40; CO2: atmospheric 0.3, alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45. CO2 solubility is 20–25 times higher than O2, so CO2 diffuses more per unit pressure difference (NCERT §14.3, Table 14.1, pp. 187–188).
• Diffusion membrane has three layers: thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and the basement substance between them; total thickness less than a millimetre (NCERT §14.3, p. 188).
• Transport of gases: ~97% of O2 carried by RBCs (as oxyhaemoglobin), ~3% dissolved in plasma; ~20–25% of CO2 carried by RBCs as carbamino-haemoglobin, ~70% as bicarbonate, ~7% dissolved in plasma (NCERT §14.4, p. 189).
• Each haemoglobin molecule binds a maximum of four O2 molecules; binding is mainly governed by pO2 and modulated by pCO2, H+ concentration and temperature. The plot of % saturation versus pO2 is the sigmoid oxygen dissociation curve; alveolar conditions (high pO2, low pCO2, low H+, low temperature) favour oxyhaemoglobin formation, while tissue conditions (low pO2, high pCO2, high H+, high temperature) favour dissociation. Every 100 mL of oxygenated blood delivers ~5 mL O2 to tissues (NCERT §14.4.1, p. 189; Figure 14.5).
• CO2 transport by bicarbonate route uses the enzyme carbonic anhydrase (concentrated in RBCs, traces in plasma) catalysing CO2 + H2O ⇌ H2CO3 ⇌ HCO3⁻ + H+; reaction proceeds forward at tissues (high pCO2) and reverses at alveoli (low pCO2). Every 100 mL of deoxygenated blood delivers ~4 mL CO2 to the alveoli (NCERT §14.4.2, p. 190).
• Regulation: respiratory rhythm centre in the medulla oblongata maintains rhythm; pneumotaxic centre in the pons can reduce the duration of inspiration and thus alter respiratory rate; a chemosensitive area adjacent to the rhythm centre senses CO2 and H+ (not O2 primarily); receptors in the aortic arch and carotid artery also sense CO2 and H+ and signal the rhythm centre. Role of O2 in respiratory regulation is insignificant (NCERT §14.5, p. 190).
• Disorders: Asthma — difficulty in breathing with wheezing due to inflammation of bronchi and bronchioles; Emphysema — chronic damage to alveolar walls reducing respiratory surface, major cause cigarette smoking; Occupational respiratory disorders — long exposure to dust in industries (grinding, stone-breaking) causes inflammation, fibrosis and serious lung damage; protective masks advised (NCERT §14.6, pp. 190–191).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 14.1 (p. 184) — Human respiratory system with sectional view of the left lung, showing epiglottis, larynx, trachea, bronchus, bronchiole, alveoli, pleural membranes, pleural fluid and diaphragm.
• Figure 14.2 (p. 186) — (a) Inspiration: diaphragm contracted, ribs and sternum raised, thoracic volume increased; (b) Expiration: diaphragm relaxed and arched upward, ribs and sternum returned to original position, thoracic volume decreased.
• Figure 14.3 (p. 188) — Exchange of gases at alveolus and at body tissues, showing pO2 and pCO2 values in pulmonary artery (deoxygenated, 40/45) and pulmonary vein/systemic arteries (oxygenated, 95/40).
• Figure 14.4 (p. 188) — Section of an alveolus with pulmonary capillary; three-layered diffusion membrane: squamous epithelium of alveolar wall, basement substance, endothelium of blood capillary.
• Figure 14.5 (p. 189) — Sigmoid oxygen dissociation curve (% saturation of Hb with O2 vs pO2 in mm Hg).
• Table 14.1 (p. 187) — Partial pressures (mm Hg) of O2 and CO2 in atmospheric air, alveoli, deoxygenated blood, oxygenated blood and tissues.

2.4 Common confusions / NTA trap points

• Trachea divides at the level of the 5th thoracic vertebra — students often misremember this as the 4th or 6th.
• Volumes vs capacities: a capacity is always a sum of two or more volumes. VC = IRV + TV + ERV (does NOT include RV); TLC = VC + RV; FRC = ERV + RV (does NOT include TV).
• O2 transport split is 97% bound + 3% dissolved, whereas CO2 transport split is 70% bicarbonate + 20–25% carbamino-Hb + 7% dissolved — distractors swap these numbers.
• The chemosensitive area and aortic/carotid receptors respond to CO2 and H+, not to O2; the role of O2 in regulation is described as insignificant.
• Inflammation of bronchi and bronchioles is asthma; damage to alveolar walls is emphysema — examiners often swap the targets.
• pCO2 in atmospheric air is 0.3 mm Hg (very low), but in alveoli it is 40 — atmospheric pCO2 is not zero.
• Solubility of CO2 is 20–25 times higher than O2 (not the diffusion coefficient or the partial pressure).
• Hering-Breuer reflex is mediated by stretch receptors in alveoli, not by chemoreceptors (NCERT §14.5, p. 271).
• Hb is a tetramer of 4 polypeptide chains; each iron-containing haem binds one O2 molecule, so one Hb binds 4 O2 (p. 269).
• Diffusion membrane thickness is less than a millimetre — examiners ask for "thin" not exact figure (p. 268).
• Inspiration vs expiration drivers — Inspiration is active (diaphragm contracts and flattens, external intercostals lift ribs and sternum, intra-pulmonary pressure falls below atmospheric); normal expiration is passive (elastic recoil), but forced expiration uses internal intercostals and abdominal muscles (NCERT §14.3, p. 267).
• Smoking and emphysema — Long-term smoking damages alveolar walls, reducing the gas-exchange surface; pulmonary fibrosis (from silica/asbestos) leads to a stiff, non-compliant lung (p. 271).

2.5 Quick comparison table — respiratory system at a glance