Breathing and Exchange of Gases - Notes | Class 11 | Part 2: Mechanism of Breathing

Mechanism of Breathing (Inspiration & Expiration)

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.

• Normal respiratory (breathing) rate: 12-16 times/min.
• Spirometer: An instrument used to estimate the volume of air involved in breathing movements. It helps in clinical assessment of pulmonary functions.

Select a Topic 👇

1. Human Respiratory System 2. Mechanism of Respiration 3. Gas Exchange 4. Gas Transport 5. Regulation of Respiration, Respiratory Disorders

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