Breathing and Exchange of Gases
[PREMIUM NCERT NOTES • HUMAN PHYSIOLOGY SERIES]
Mechanism of breathing varies among animal groups depending on their habitats and level of organisation.
- Lower Invertebrates: Simple diffusion (Sponges, Coelenterates).
- Earthworms: Moist cuticle (Cutaneous).
- Insects: Network of tubes (Tracheal).
- Aquatic Arthropods/Molluscs/Fishes: Gills (Branchial).
- Terrestrial Forms: Lungs (Pulmonary).
Consists of a conducting part and an exchange part.
- Conducting Part: External nostrils to terminal bronchioles. Clears air, humidifies, and brings air to body temp.
- Exchange Part: Alveoli and their ducts. Actual site of diffusion of O2 and CO2 between blood and atmospheric air.
Breathing involves two stages: Inspiration (active) and Expiration (passive).
- Inspiration: Diaphragm and External Intercostal muscles contract. Thoracic volume increases, pressure decreases.
- Expiration: Muscles relax. Thoracic volume decreases, pressure increases, air is forced out.
--> Tidal Volume (TV): ~500 mL (normal breath).
--> Vital Capacity (VC): ERV + TV + IRV.
--> Residual Volume (RV): Air remaining after forceful expiration (cannot be measured by spirometer).
Primary site is Alveoli. Exchange occurs by Simple Diffusion based on pressure gradient.
| Gas | Alveoli | Blood (De-oxy) | Blood (Oxy) | Tissues |
|---|---|---|---|---|
| pO2 | 104 | 40 | 95 | 40 |
| pCO2 | 40 | 45 | 40 | 45 |
Oxygen: 97% by Hemoglobin (Oxyhemoglobin), 3% dissolved in plasma. Every 100ml blood delivers 5ml O2 to tissues.
Carbon Dioxide: 70% as Bicarbonate, 20-25% as Carbamino-hemoglobin, 7% dissolved in plasma.
--> Left Shift: High pO2, Low pCO2, Low H+, Low Temp (occurs in Alveoli).
--> Right Shift: Low pO2, High pCO2, High H+, High Temp (occurs in Tissues).
- Asthma: Inflammation of bronchi/bronchioles; wheezing.
- Emphysema: Alveolar walls damage; caused by smoking.
- Occupational: Silicosis, Asbestosis; leads to fibrosis (proliferation of fibrous tissues).
Breathing & Gas Exchange: HOTS
[ ANALYTICAL & NEET TARGETED ]
At high altitudes, the partial pressure of oxygen (pO2) in the atmosphere is low. To compensate for the reduced oxygen availability, the body produces more RBCs and hemoglobin (Polycythemia) to increase the oxygen-carrying capacity of the blood, ensuring tissues receive sufficient O2.
The sigmoid shape indicates the cooperative binding of oxygen to hemoglobin. As one O2 molecule binds, it changes the Hb structure to make subsequent O2 binding easier. It also ensures that Hb can bind O2 efficiently in lungs (high pO2) and release it rapidly in tissues (low pO2).
The Bohr effect is the decrease in hemoglobin's affinity for oxygen in the presence of high CO2 or low pH (high H+). In active tissues, pCO2 is high, causing Hb to release O2 more readily, thus matching the increased metabolic demand.
Diffusion depends on both the pressure gradient and solubility. Since CO2 is 20-25 times more soluble than O2, it diffuses much faster across the respiratory membrane even with a smaller partial pressure difference (5 mmHg for CO2 vs 64 mmHg for O2).
The rising levels of pCO2 and H+ ions (acidity). Human respiratory centers (medulla) are highly sensitive to CO2/H+ changes and relatively insensitive to oxygen levels. The buildup of CO2 stimulates the chemoreceptors to signal the brain to resume breathing.
TLC includes Residual Volume (RV), VC does not.
Maintains ionic balance as HCO3- moves out of RBC.
Fastest enzyme; facilitates CO2 + H2O <-> H2CO3.
Destroys walls (Emphysema), reducing Surface Area.
Due to elastic recoil of lungs and relaxation of diaphragm.
Pons; acts as "switch-off" point for inspiration.
104 mmHg vs 40 mmHg.
Approx 5 ml.
As Bicarbonate (70%).
ERV + RV.
Prevents collapse of trachea while allowing oesophagus to expand.
Less than 1 mm (approx 0.5mm).
Haldane is about CO2 loading/unloading based on O2 levels.
Low intrapulmonary pressure created by diaphragm contraction.
Chemosensitive area adjacent to Medullary rhythm center.
Breathing: 50 Mastery Facts
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