Chapter overview: breathing and gas exchange
Cells require a continuous supply of oxygen for catabolic reactions and release carbon dioxide. Breathing or pulmonary ventilation draws atmospheric air in and releases CO2-rich alveolar air; respiration also involves gas exchange, transport by blood and cellular use of oxygen.
- Pulmonary ventilation
- Diffusion across the alveolar membrane
- Transport of gases by blood
- Diffusion between blood and tissues
- Cellular utilisation of O2 and release of CO2
14.1 Respiratory organs in animals
| Animal group / example | Respiratory surface or organ |
|---|---|
| Sponges, coelenterates and flatworms | Simple diffusion across the whole body surface |
| Earthworm | Moist cuticle |
| Insects | Tracheal tubes carrying atmospheric air throughout the body |
| Aquatic arthropods and molluscs; fishes | Vascularised gills - branchial respiration |
| Terrestrial vertebrates | Vascularised lungs - pulmonary respiration |
| Frogs | Lungs plus moist skin for cutaneous respiration |
Human respiratory system
Air passes through external nostrils -> nasal passage -> nasal chamber -> pharynx -> larynx -> trachea -> primary bronchi -> secondary and tertiary bronchi -> bronchioles -> terminal bronchioles -> alveoli.
The larynx is a cartilaginous sound box. During swallowing, the elastic cartilaginous epiglottis covers the glottis and prevents food entering the larynx. The trachea and bronchi are supported by incomplete cartilaginous rings.
| Part | Extent and function |
|---|---|
| Conducting part | External nostrils to terminal bronchioles. Transports air, clears foreign particles, humidifies air and brings it to body temperature. |
| Respiratory / exchange part | Alveoli and alveolar ducts. Site of actual O2 and CO2 diffusion between air and blood. |
| Pleura | Double-layered covering of lungs; pleural fluid between layers reduces friction. Outer layer contacts thoracic lining and inner layer contacts lung surface. |
The airtight thoracic chamber is bounded dorsally by vertebral column, ventrally by sternum, laterally by ribs and below by dome-shaped diaphragm. A change in thoracic volume changes pulmonary volume.
14.2 Mechanism of breathing: inspiration vs expiration
Air moves due to pressure gradients between lungs and atmosphere. Inspiration occurs when intra-pulmonary pressure is lower than atmospheric pressure; expiration occurs when it is higher.
| Feature | Inspiration | Expiration |
|---|---|---|
| Diaphragm | Contracts; increases thoracic volume in the antero-posterior axis | Relaxes and returns to normal position |
| Intercostal muscles / ribs | External intercostals contract, lifting ribs and sternum | Intercostals relax; ribs and sternum return to normal position |
| Thoracic and pulmonary volume | Increase | Decrease |
| Intra-pulmonary pressure | Falls below atmospheric pressure | Rises slightly above atmospheric pressure |
| Air movement | Atmospheric air enters lungs | Alveolar air is expelled |
Respiratory volumes and capacities
| Term | Definition / formula | Approximate value where given |
|---|---|---|
| Tidal volume (TV) | Air inspired or expired during a normal respiration | 500 mL |
| Inspiratory reserve volume (IRV) | Additional air that can be inspired by forcible inspiration | 2500-3000 mL |
| Expiratory reserve volume (ERV) | Additional air that can be expired by forcible expiration | 1000-1100 mL |
| Residual volume (RV) | Air remaining in lungs even after forcible expiration | 1100-1200 mL |
| Inspiratory capacity (IC) | Maximum inspired after a normal expiration = TV + IRV | - |
| Expiratory capacity (EC) | Maximum expired after a normal inspiration = TV + ERV | - |
| Functional residual capacity (FRC) | Air remaining after normal expiration = ERV + RV | - |
| Vital capacity (VC) | Maximum air inspired after forced expiration, or expired after forced inspiration = ERV + TV + IRV | - |
| Total lung capacity (TLC) | Air accommodated after forced inspiration = RV + ERV + TV + IRV = VC + RV | - |
At normal TV of 500 mL and 12-16 breaths per minute, a healthy person moves approximately 6000-8000 mL air per minute.
14.3 Exchange of gases
Alveoli are the primary site of exchange, and gas exchange also occurs between blood and tissues. O2 and CO2 diffuse down their partial-pressure gradients. Diffusion rate also depends on gas solubility and membrane thickness.
| Site | pO2 (mm Hg) | pCO2 (mm Hg) |
|---|---|---|
| Atmospheric air | 159 | 0.3 |
| Alveoli | 104 | 40 |
| Deoxygenated blood | 40 | 45 |
| Oxygenated blood | 95 | 40 |
| Tissues | 40 | 45 |
O2 moves from alveoli to blood and from blood to tissues. CO2 moves in the opposite direction: tissues to blood and blood to alveoli. CO2 is 20-25 times more soluble than O2, so it diffuses much more readily for a given pressure difference.
14.4 Transport of oxygen
Blood transports gases. About 97% of O2 is carried by RBCs and about 3% is dissolved in plasma. Haemoglobin, the iron-containing red pigment of RBCs, reversibly binds oxygen to form oxyhaemoglobin; one haemoglobin molecule carries a maximum of four O2 molecules.
| Condition | Alveoli | Tissues |
|---|---|---|
| pO2 | High | Low |
| pCO2, H+ concentration and temperature | Low | High |
| Effect on haemoglobin | Favour oxyhaemoglobin formation | Favour dissociation of O2 from oxyhaemoglobin |
A graph of haemoglobin percentage saturation against pO2 is the sigmoidal oxygen dissociation curve. Under normal conditions, every 100 mL of oxygenated blood delivers around 5 mL O2 to tissues.
Transport of carbon dioxide
| Mode | Proportion | Key detail |
|---|---|---|
| As bicarbonate (HCO3-) | About 70% | Major mode; carbonic anhydrase in RBCs catalyses CO2 + H2O reversible H2CO3 reversible H+ + HCO3-. |
| As carbamino-haemoglobin | About 20-25% | Binding is favoured at high pCO2 and low pO2 in tissues; dissociation occurs at low pCO2 and high pO2 in alveoli. |
| Dissolved in plasma | About 7% | Carried directly in dissolved state. |
At tissues, high pCO2 from catabolism drives CO2 into RBCs and plasma and favours bicarbonate formation. At alveoli, low pCO2 reverses the reaction so CO2 is released. Every 100 mL of deoxygenated blood delivers approximately 4 mL CO2 to alveoli.
14.5 Regulation of respiration
Respiratory rhythm is regulated primarily by a specialised respiratory rhythm centre in the medulla. The pneumotaxic centre in the pons moderates it; signals from this centre can reduce inspiration duration and thereby alter respiratory rate.
A chemosensitive area adjacent to the rhythm centre is highly sensitive to CO2 and H+. Their increase activates it, leading to adjustments that eliminate them. Receptors in the aortic arch and carotid artery also detect changes in CO2 and H+ and signal the rhythm centre. Oxygen has an insignificant role in regulating respiratory rhythm.
14.6 Disorders of the respiratory system
| Disorder | NCERT description |
|---|---|
| Asthma | Difficulty in breathing with wheezing due to inflammation of bronchi and bronchioles. |
| Emphysema | Chronic disorder in which alveolar walls are damaged, decreasing respiratory surface; cigarette smoking is a major cause. |
| Occupational respiratory disorders | Long exposure to industrial dust, especially in grinding or stone-breaking work, can cause inflammation and fibrosis, resulting in serious lung damage. Protective masks are essential. |
NCERT revision prompts
- Trace the path of air from nostrils to alveoli and distinguish conducting and exchange parts.
- Explain inspiration using thoracic volume and pressure changes.
- Differentiate TV, IRV, ERV and RV; derive the pulmonary capacities.
- State the pO2 and pCO2 gradients at alveoli and tissues.
- Why does CO2 diffuse more readily than O2?
- Explain the oxygen dissociation curve and factors that favour oxyhaemoglobin formation.
- State the three modes of CO2 transport.
- How do medulla, pons and chemoreceptors regulate breathing?
- Differentiate asthma, emphysema and occupational respiratory disorders.