Breathing and Exchange of Gases
How the human respiratory system draws in oxygen and expels carbon dioxide, and how these gases diffuse, get transported in blood, and have their breathing rhythm regulated.
Basic human physiology is a staple of Prelims general-science MCQs — expect direct factual questions on respiratory volumes/capacities, gas-transport percentages, partial-pressure gradients and the regulatory centres. For Mains GS-III it underpins air-pollution health impacts (PM2.5/PM10 driving asthma and COPD) and the science behind medical-oxygen logistics and public-health interventions. The oxygen dissociation curve and bicarbonate transport are recurring conceptual favourites.
Understand the chapter
What Breathing Is and the Diversity of Respiratory Organs
Breathing (commonly called respiration) is the exchange of atmospheric O2 with the CO2 produced by cells during catabolism. O2 is needed to break down glucose, amino acids and fatty acids for energy, while the CO2 released is harmful and must be expelled continuously. The mechanism of breathing differs across animal groups depending on habitat and level of organisation.
- Simple diffusion over the whole body surface: sponges, coelenterates, flatworms
- Moist cuticle (earthworm); tracheal tubes (insects)
- Gills/branchial respiration: aquatic arthropods, molluscs, fishes
- Lungs/pulmonary respiration: amphibians, reptiles, birds, mammals; frogs also use moist skin (cutaneous)
The Human Respiratory Pathway: Conducting vs Exchange Parts
Air travels: external nostrils to nasal chamber to pharynx (the common food-air passage) to larynx to trachea to primary bronchi, then to secondary/tertiary bronchi, bronchioles, terminal bronchioles and finally alveoli. The larynx is the cartilaginous sound box; the epiglottis seals the glottis during swallowing to keep food out. The trachea divides at the level of the 5th thoracic vertebra. The system splits functionally into a conducting part and a respiratory (exchange) part.
- Conducting part (nostrils to terminal bronchioles): transports, filters, humidifies and warms air to body temperature
- Exchange part (alveoli and ducts): actual site of O2/CO2 diffusion with blood
- Lungs covered by a double-layered pleura with pleural fluid that reduces friction
- Thorax: dorsally the vertebral column, ventrally the sternum, laterally the ribs, floor the dome-shaped diaphragm; an air-tight chamber
Mechanism of Breathing: Pressure Gradients in Action
Because we cannot alter lung volume directly, breathing works by changing thoracic volume to create pressure gradients. Inspiration occurs when intra-pulmonary pressure falls below atmospheric (a negative pressure); expiration occurs when it rises above atmospheric. The diaphragm plus external and internal intercostal muscles generate these gradients.
- Inspiration: diaphragm contracts (raises antero-posterior volume); external intercostals lift ribs and sternum (raises dorso-ventral volume), lowering intra-pulmonary pressure so air rushes in
- Expiration: diaphragm and intercostals relax, thoracic and pulmonary volume fall, pressure rises and air is expelled
- Normal rate: 12-16 breaths per minute
- Spirometer measures respiratory volumes for clinical assessment of pulmonary function
Respiratory Volumes and Capacities
Lung function is quantified by four primary volumes and five derived capacities. Capacities are simply additive sums of two or more volumes and are used in clinical diagnosis. Memorising the values and the additive formulae is the single most testable part of this chapter.
- Volumes: TV about 500 mL; IRV 2500-3000 mL; ERV 1000-1100 mL; RV 1100-1200 mL
- IC = TV + IRV; EC = TV + ERV; FRC = ERV + RV
- VC = ERV + TV + IRV (max air expelled after a forced inspiration)
- TLC = VC + RV = RV + ERV + TV + IRV
Exchange of Gases: Partial Pressures and the Diffusion Membrane
Alveoli are the primary sites of gas exchange; gases move purely by simple diffusion along partial-pressure (concentration) gradients. O2 flows alveoli to blood to tissues, while CO2 flows tissues to blood to alveoli. Diffusion rate also depends on gas solubility and membrane thickness; CO2 is 20-25 times more soluble than O2, so far more of it diffuses per unit pressure difference.
- pO2 (mm Hg): atmosphere 159, alveoli 104, oxygenated blood 95, tissues 40
- pCO2 (mm Hg): tissues 45, deoxygenated blood 45, alveoli 40, atmosphere 0.3
- Diffusion membrane has 3 layers: alveolar squamous epithelium, capillary endothelium, and basement substance between them
- Total membrane thickness is less than a millimetre, favouring rapid diffusion
Transport of O2 and CO2 in Blood
Blood is the transport medium, with RBCs doing most of the carrying. About 97% of O2 binds haemoglobin (each molecule carrying up to 4 O2) as oxyhaemoglobin, leaving 3% dissolved in plasma. CO2 is mostly carried as bicarbonate (70%), aided by the RBC enzyme carbonic anhydrase, with 20-25% as carbamino-haemoglobin and about 7% dissolved in plasma. The sigmoid oxygen dissociation curve shows how loading and unloading shift with conditions.
- Alveoli (high pO2, low pCO2, low H+, low temperature): O2 loads onto haemoglobin
- Tissues (low pO2, high pCO2, high H+, high temperature): O2 unloads to tissues
- CO2 + H2O reversibly forms H2CO3, then HCO3- + H+, catalysed both ways by carbonic anhydrase
- 100 mL oxygenated blood delivers about 5 mL O2; 100 mL deoxygenated blood delivers about 4 mL CO2
Regulation of Respiration and Disorders
Breathing rhythm is controlled neurally, not by oxygen levels. The respiratory rhythm centre in the medulla sets the basic rhythm; the pneumotaxic centre in the pons moderates it by shortening the duration of inspiration. A chemosensitive area near the rhythm centre, plus receptors on the aortic arch and carotid artery, respond chiefly to rising CO2 and H+; oxygen's regulatory role is insignificant.
- Respiratory rhythm centre: medulla oblongata (primary control)
- Pneumotaxic centre: pons; reduces inspiration duration and alters rate
- CO2 and H+ are the main chemical drivers; O2 role insignificant
- Disorders: Asthma (inflammation of bronchi/bronchioles, wheezing), Emphysema (alveolar wall damage, mainly from cigarette smoking), Occupational respiratory disorders (dust/silica exposure causing fibrosis)
Key terms
- Breathing
- Exchange of atmospheric O2 with CO2 produced by cells; also called pulmonary ventilation.
- Alveoli
- Thin-walled, vascularised sac-like lung structures that are the primary sites of gas exchange.
- Pleura
- Double-layered membrane covering the lungs, with pleural fluid that reduces friction during breathing.
- Tidal Volume (TV)
- Volume of air inspired or expired in a normal breath, about 500 mL.
- Vital Capacity (VC)
- Maximum air expelled after a forced inspiration; equals ERV + TV + IRV (excludes residual volume).
- Partial pressure
- Pressure contributed by an individual gas in a mixture; pO2 and pCO2 drive diffusion.
- Oxyhaemoglobin
- Reversible compound of O2 with haemoglobin; each Hb molecule carries up to 4 O2 molecules.
- Carbonic anhydrase
- RBC enzyme catalysing CO2 + H2O to HCO3- + H+ in both directions, enabling bicarbonate transport.
- Oxygen dissociation curve
- Sigmoid plot of haemoglobin percentage saturation against pO2, showing O2 loading and unloading.
- Pneumotaxic centre
- Pons centre that moderates the medullary rhythm centre by shortening inspiration.
Must-know facts exam-ready
- Trachea divides at the level of the 5th thoracic vertebra into right and left primary bronchi.
- Normal breathing rate is 12-16 times per minute; a healthy person moves 6000-8000 mL air per minute.
- Tidal Volume is about 500 mL; Residual Volume 1100-1200 mL; IRV 2500-3000 mL; ERV 1000-1100 mL.
- O2 transport: 97% by RBCs (haemoglobin), 3% dissolved in plasma.
- CO2 transport: 70% as bicarbonate, 20-25% as carbamino-haemoglobin, 7% dissolved in plasma.
- Each haemoglobin molecule carries a maximum of 4 O2 molecules.
- CO2 is 20-25 times more soluble than O2 across the diffusion membrane.
- pO2 (mm Hg): atmosphere 159, alveoli 104, oxygenated blood 95, tissues 40.
- pCO2 (mm Hg): tissues 45, alveoli 40, atmosphere 0.3.
- Respiratory rhythm centre is in the medulla; pneumotaxic centre is in the pons.
- 100 mL oxygenated blood delivers about 5 mL O2; 100 mL deoxygenated blood delivers about 4 mL CO2.
- Diffusion membrane has 3 layers, total thickness under 1 mm; oxygen's role in regulating respiration is insignificant.
Memory tricks remember it for good
Traps to avoid
- Vital Capacity excludes Residual Volume (VC = ERV + TV + IRV); only Total Lung Capacity includes RV.
- Residual Volume (1100-1200 mL) stays after a forced expiration; Expiratory Reserve Volume (1000-1100 mL) is the extra you can forcibly breathe out, do not equate them.
- Respiration is regulated chiefly by CO2 and H+, NOT by O2; oxygen's regulatory role is insignificant.
- Rhythm centre is in the medulla; the pneumotaxic centre (which only moderates it) is in the pons, do not swap them.
- CO2 is carried mostly as bicarbonate (70%), not mainly by haemoglobin; it is O2 that is about 97% haemoglobin-bound.
- Carbamino-haemoglobin is CO2 bound to Hb, while oxyhaemoglobin is O2 bound to Hb, do not confuse the two.
Exam focus
🧠 Prelims angles
- Respiratory volumes and capacities and their additive formulae (VC, TLC, FRC, IC, EC).
- Percentage split of O2 and CO2 transport in blood.
- Partial-pressure gradient values (pO2/pCO2 across atmosphere, alveoli, blood, tissues).
- Location and function of the respiratory rhythm centre (medulla) versus pneumotaxic centre (pons).
- Match-the-following on respiratory organs across animal groups (gills, tracheal tubes, cutaneous, pulmonary).
- Role of carbonic anhydrase and the bicarbonate (CO2 + H2O) reaction.
✍️ Mains angles GS-III
- Air pollution and the respiratory system: how particulate matter (PM2.5/PM10) drives asthma and COPD in Indian cities.Link alveolar gas-exchange physiology to pollution sources and the National Clean Air Programme, and cite health-burden data.
- Tobacco smoking, emphysema and India's public-health response.Connect alveolar-wall damage to COTPA regulation and a preventive, primary-healthcare framing.
- Medical-oxygen infrastructure: why O2 supply chains matter in health emergencies.Use O2 transport and exchange physiology to justify PSA oxygen plants and logistics preparedness.
Last-minute revision tick as you recall
- Breathing = O2 in, CO2 out; conducting part warms/filters, alveoli exchange.
- Inspiration = diaphragm contracts, negative intra-pulmonary pressure pulls air in.
- TV 500, IRV ~3000, ERV ~1100, RV ~1200; VC = ERV+TV+IRV; TLC = VC+RV.
- O2: 97% haemoglobin + 3% plasma; CO2: 70% bicarbonate + ~23% carbamino + 7% plasma.
- Each Hb carries 4 O2; CO2 is 20-25 times more soluble than O2.
- pO2 alveoli 104 vs tissues 40; pCO2 tissues 45 vs alveoli 40.
- Rhythm centre = medulla; pneumotaxic = pons; CO2 and H+ regulate, not O2.
- Trachea splits at the 5th thoracic vertebra; normal rate 12-16 per minute.
- Carbonic anhydrase in RBCs runs CO2 + H2O to HCO3- + H+ in both directions.
Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.