Breathing and Exchange of Gases - Notes | Class 11 | Part 2: Mechanism of Breathing
17. BREATHING AND EXCHANGE OF GASES
a. Inspiration
Active intake of air from atmosphere into lungs.
During this, the diaphragm contracts (flattens) causing an increase in vertical thoracic volume (antero-posterior axis).
Contraction of external intercostal muscles (muscles found between ribs) lifts up the ribs and sternum causing an increase in thoracic volume in the dorso-ventral axis.
Increase in thoracic volume reduces thoracic pressure. So, lungs expand. Thus, pulmonary volume increases resulting in decrease of intra-pulmonary pressure to less than the atmospheric pressure. So, air moves into lungs.
b. Expiration
Passive expelling of air from the lungs.
During this, intercostal muscles & diaphragm relax causing a decrease in thoracic volume and thereby pulmonary volume. So, air moves out.
During forceful expiration, abdominal muscles and internal inter-costal muscles contract.
Respiratory volumes and capacities
- Tidal volume (TV): Volume of air inspired or expired during a normal respiration. It is about 500 ml. i.e., 6000-8000 ml per minute.
- Inspiratory reserve volume (IRV) or complemental air: Additional volume of air that can inspire by forceful inspiration. It is 2500-3000 ml.
- Expiratory reserve volume (ERV) or supplemental air: Additional volume of air that can expire by a forceful expiration. It is 1000-1100 ml.
- Residual volume (RV): Volume of air remaining in lungs after a forcible expiration. It is 1100-1200 ml.
- Inspiratory capacity (IC): Total volume of air inspired after a normal expiration (TV + IRV). It is 3000-3500 ml.
- Expiratory capacity (EC): Total volume of air expired after a normal inspiration (TV + ERV). It is 1500-1600 ml.
- Functional residual capacity (FRC): Volume of air remaining in the lungs after a normal expiration (ERV + RV). It is 2100-2300 ml.
- Vital capacity (VC): Volume of air that can breathe in after a forced expiration or Volume of air that can breathe out after a forced inspiration (ERV + TV + IRV). It is 3500-4500 ml.
- Total lung capacity (TLC): Total volume of air in the lungs after a maximum inspiration. (RV + ERV + TV + IRV or VC + RV). It is 5000-6000 ml.
Dead air volume is about 150 ml.
- Respiratory cycle= an inspiration + an expiration
- Normal respiratory (breathing) rate: 12-16 times/min.
- Spirometer (respirometer): To measure respiratory rate.
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