1. Respiratory Organs Across Animals
| Animal group / example | Respiratory structure | Type of exchange |
|---|---|---|
| Sponges, coelenterates and flatworms | Entire body surface | Simple diffusion |
| Earthworms | Moist cuticle | Cutaneous exchange |
| Insects | Tracheal tubes | Atmospheric air reaches body tissues through a tube network |
| Aquatic arthropods, molluscs and fishes | Vascularised gills | Branchial respiration |
| Amphibians, reptiles, birds and mammals | Vascularised lungs | Pulmonary respiration |
| Frogs | Lungs and moist skin | Pulmonary plus cutaneous respiration |
2. Conducting Part vs Respiratory Part
| Feature | Conducting part | Respiratory / exchange part |
|---|---|---|
| Extent | External nostrils up to terminal bronchioles | Alveoli and alveolar ducts |
| Primary function | Transports atmospheric air to alveoli | Actual diffusion of O2 and CO2 between blood and air |
| Additional role | Removes foreign particles, humidifies air and brings it to body temperature | Provides a large, thin and vascular surface for exchange |
| Key structures | Nostrils, nasal passage, pharynx, larynx, trachea, bronchi and bronchioles | Alveoli surrounded by pulmonary capillaries |
Pathway Nostrils -> nasal chamber -> pharynx -> larynx -> trachea -> bronchi -> bronchioles -> terminal bronchioles -> alveoli.
3. Inspiration vs Expiration
| Feature | Inspiration | Expiration |
|---|---|---|
| Diaphragm | Contracts, increasing thoracic volume in antero-posterior axis | Relaxes and regains normal position |
| External intercostal muscles | Contract; ribs and sternum lift up | 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 |
| Direction of air | Atmosphere to lungs | Lungs to atmosphere |
4. Respiratory Volumes
| Volume | Definition | Approximate value |
|---|---|---|
| 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 after a forcible expiration | 1100-1200 mL |
| Normal breathing rate | Breaths per minute in healthy human | 12-16 per minute |
5. Pulmonary Capacities
| Capacity | Definition | Formula |
|---|---|---|
| Inspiratory capacity (IC) | Total air that can be inspired after normal expiration | TV + IRV |
| Expiratory capacity (EC) | Total air that can be expired after normal inspiration | TV + ERV |
| Functional residual capacity (FRC) | Air remaining in lungs after normal expiration | ERV + RV |
| Vital capacity (VC) | Maximum inspired after forced expiration, or maximum expired after forced inspiration | ERV + TV + IRV |
| Total lung capacity (TLC) | Air accommodated in lungs after forced inspiration | RV + ERV + TV + IRV = VC + RV |
6. Gas Exchange: Alveoli vs Tissues
| Feature | At alveoli | At tissues |
|---|---|---|
| O2 gradient | pO2 is 104 mm Hg in alveoli versus 40 mm Hg in deoxygenated blood; O2 enters blood. | pO2 is about 95 mm Hg in oxygenated blood versus 40 mm Hg in tissues; O2 enters tissues. |
| CO2 gradient | Blood pCO2 is 45 mm Hg versus 40 mm Hg in alveoli; CO2 enters alveoli. | Tissue pCO2 is 45 mm Hg versus 40 mm Hg in oxygenated blood; CO2 enters blood. |
| Direction summary | O2: alveoli -> blood; CO2: blood -> alveoli | O2: blood -> tissues; CO2: tissues -> blood |
| Diffusion factors | Partial-pressure gradient, gas solubility and thickness of diffusion membrane. CO2 is 20-25 times more soluble than O2. | |
7. Transport of Oxygen vs Carbon Dioxide
| Feature | Oxygen | Carbon dioxide |
|---|---|---|
| Main transport form | About 97% as oxyhaemoglobin in RBCs | About 70% as bicarbonate (HCO3-) |
| Other forms | About 3% dissolved in plasma | 20-25% as carbamino-haemoglobin; about 7% dissolved in plasma |
| Key protein / enzyme | Haemoglobin; one molecule binds a maximum of four O2 molecules reversibly | Haemoglobin and carbonic anhydrase, abundant in RBCs |
| Binding favoured at | Alveoli: high pO2, low pCO2, low H+ concentration and lower temperature | Tissues: high pCO2 and low pO2 |
| Release favoured at | Tissues: low pO2, high pCO2, high H+ concentration and higher temperature | Alveoli: low pCO2 and high pO2 |
8. Respiratory Regulation and Disorders
| Topic | Key NCERT point |
|---|---|
| Respiratory rhythm centre | Located in medulla; primarily regulates respiratory rhythm. |
| Pneumotaxic centre | Located in pons; can reduce inspiration duration and alter respiratory rate. |
| Chemosensitive area | Near rhythm centre; highly sensitive to CO2 and H+. Their rise prompts adjustments that eliminate them. |
| Aortic and carotid receptors | Recognise changes in CO2 and H+ and signal the rhythm centre. Oxygen has insignificant role in rhythm regulation. |
| Asthma | Breathing difficulty and wheezing due to inflammation of bronchi and bronchioles. |
| Emphysema | Chronic damage to alveolar walls reduces respiratory surface; cigarette smoking is a major cause. |
| Occupational respiratory disorders | Long-term industrial dust exposure may cause inflammation and fibrosis; protective masks are necessary. |
Quick recall An oxygen dissociation curve plots percentage saturation of haemoglobin with O2 against pO2; it is sigmoid in shape.