Wednesday, August 5, 2020

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


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.
Part of respiratory tract (from nostrils to terminal bronchi) not involved in gaseous exchange is called dead space.

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.
👇 Select Your Topic Here 👇


  1. really helpful. thank you so much.
    but would be nice if we could select and copy the text

  2. Really it's very helpful.....thank u