17. BREATHING AND EXCHANGE OF GASES
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.
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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