Mechanism of Breathing



1. Mechanism of breathing occurs in two Stages:


  • Inspiration: Drawing atmospheric air into the lungs. 
  • Expiration: Releasing alveolar air out of the lungs.


2. Pressure Gradient:


  • Air movement between lungs and atmosphere occurs due to pressure differences. 
  • Inspiration happens when intra-pulmonary pressure is lower than atmospheric pressure, creating a negative pressure in the lungs. 
  • Expiration occurs when intra-pulmonary pressure exceeds atmospheric pressure, leading to air expulsion.


3. Muscular Involvement:


  • Diaphragm: Primary muscle involved in breathing. Contracts during inspiration and relaxes during expiration. 
  • Intercostal Muscles: External and internal muscles between the ribs. Contraction lifts ribs and sternum during inspiration.







4. Inspiration:


  • Diaphragm contracts, increasing thoracic chamber volume in antero-posterior axis. 
  • External intercostal muscles lift ribs and sternum, increasing thoracic chamber volume in dorso-ventral axis. 
  • Thoracic volume increase leads to pulmonary volume increase, reducing intra-pulmonary pressure below atmospheric pressure. 
  • Atmospheric air rushes into the lungs due to pressure gradient, causing inspiration.


5. Expiration:


  • Diaphragm and intercostal muscles relax, returning to their resting positions. 
  • Thoracic volume decreases, leading to pulmonary volume reduction. 
  • Intra-pulmonary pressure rises slightly above atmospheric pressure. 
  • Air is expelled from the lungs due to pressure gradient, causing expiration.


6. Additional Muscles and Rate:


  • Additional abdominal muscles can aid in increasing the strength of inspiration and expiration. 
  • On average, a healthy human breathes 12-16 times per minute. 
  • Clinical assessment of pulmonary functions can be done using a spirometer to estimate the volume of air involved in breathing movements.


Lungs Volumes and Capacities


The human respiratory system performs the vital function of exchanging gases, primarily oxygen and carbon dioxide, between the body and the environment. To understand respiratory function comprehensively, various volumes and capacities of the lungs are measured. These parameters provide valuable insights into lung health, efficiency, and capacity.


1. Tidal Volume (TV):


  • Tidal volume refers to the volume of air inspired or expired during a normal breath. 
  • In healthy individuals, tidal volume averages around 500 mL. 
  • It represents the amount of air that moves in and out of the lungs with each breath, contributing to a total of approximately 6000 to 8000 mL of air exchanged per minute.


2. Inspiratory Reserve Volume (IRV):


  • Inspiratory reserve volume is the additional volume of air a person can forcefully inspire beyond the tidal volume. 
  • It ranges between 2500 mL to 3000 mL on average. 
  • IRV allows for deep, vigorous inhalation, such as during exercise or when extra oxygen is needed.


3. Expiratory Reserve Volume (ERV):


  • Expiratory reserve volume refers to the additional volume of air a person can forcefully expire beyond the tidal volume. 
  • It averages between 1000 mL to 1100 mL. 
  • ERV enables forceful exhalation, such as during physical exertion or when expelling excess carbon dioxide.


4. Residual Volume (RV):


  • Residual volume is the volume of air remaining in the lungs after maximal expiration. 
  • It ranges from 1100 mL to 1200 mL on average. 
  • RV ensures that the lungs remain partially inflated, preventing lung collapse and facilitating gas exchange even after forceful exhalation.


Pulmonary Capacities:- Pulmonary capacities involve inspiration capacity, expiratory capacity, functional residual capacity, vital capacity and total lung capacity.


5. Inspiratory Capacity (IC):


  • Inspiratory capacity represents the total volume of air a person can inspire after a normal expiration. 
  • IC includes tidal volume and inspiratory reserve volume (TV + IRV). 
  • It reflects the maximum amount of air that can be inhaled after a relaxed exhalation.


6. Expiratory Capacity (EC):


  • Expiratory capacity refers to the total volume of air a person can expire after a normal inspiration. 
  • EC includes tidal volume and expiratory reserve volume (TV + ERV). 
  • It represents the maximum amount of air that can be exhaled after a relaxed inhalation.


7. Functional Residual Capacity (FRC):


  • Functional residual capacity is the volume of air remaining in the lungs after a normal expiration. 
  • FRC includes expiratory reserve volume and residual volume (ERV + RV). 
  • It represents the equilibrium point where the forces of lung recoil and chest wall expansion are balanced, maintaining lung integrity.


8. Vital Capacity (VC):


  • Vital capacity is the maximum volume of air a person can exhale after a maximal inhalation or inhale after a maximal exhalation. 
  • VC includes expiratory reserve volume, tidal volume, and inspiratory reserve volume (ERV + TV + IRV). 
  • It reflects the overall lung function and respiratory muscle strength.


9. Total Lung Capacity (TLC):


  • Total lung capacity represents the maximum volume of air the lungs can accommodate at the end of a maximal inspiration. 
  • TLC includes residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume (RV + ERV + TV + IRV). 
  • It signifies the total amount of air contained within the lungs, including the amount that cannot be exhaled forcefully.


Exchange of Gases 






  • The exchange of gases is a fundamental process essential for sustaining life in organisms. 
  • In the human respiratory system, this exchange primarily involves oxygen (O2) and carbon dioxide (CO2) and occurs at specialized sites such as the alveoli in the lungs and between the bloodstream and body tissues. This process is driven by simple diffusion, where gases move across membranes from areas of higher concentration to areas of lower concentration. 
  • Factors such as partial pressure, gas solubility, and membrane thickness influence the rate and efficiency of gas exchange.  
  • Understanding the mechanisms of gas exchange is crucial for comprehending how the respiratory system ensures the delivery of oxygen to tissues for cellular respiration and the removal of carbon dioxide waste from the body, maintaining overall physiological balance and function.


1. Primary Sites:


  • Alveoli: These tiny air sacs in the lungs are the primary sites where gases are exchanged between inhaled air and the bloodstream. Oxygen enters the blood, while carbon dioxide exits into the air in the alveoli.
  • Blood and Tissues: Once oxygen is absorbed into the blood in the lungs, it's transported throughout the body via blood vessels. In tissues, oxygen is released from the blood and used for cellular functions, while carbon dioxide produced by cells is picked up by the blood to be carried back to the lungs for exhalation.


2. Mechanism:


  • Gases are exchanged through simple diffusion, driven by pressure/concentration gradients. 
  • Simple Diffusion: The movement of gases occurs naturally from areas of higher concentration to areas of lower concentration. This process, known as simple diffusion, is the driving force behind gas exchange in the respiratory system. 
  • Factors affecting diffusion rate include gas solubility and membrane thickness.


3. Partial Pressure:


  • Partial pressure refers to the pressure exerted by each gas in a mixture. 
  • Oxygen partial pressure is denoted as pO2, while carbon dioxide partial pressure is represented as pCO2.


4. Concentration Gradient:


  • There's a gradient for oxygen concentration, meaning oxygen moves from areas of higher partial pressure (such as in the alveoli) to areas of lower partial pressure (like in the bloodstream and tissues). 
  • Oxygen concentration gradient exists from alveoli to blood and blood to tissues. 
  • Conversely, carbon dioxide moves in the opposite direction, from areas of higher partial pressure (in tissues) to areas of lower partial pressure (in the bloodstream and alveoli).


5. Solubility Factor:


  • CO2 is 20-25 times more soluble than O2, facilitating higher diffusion rates for CO2. 
  • Carbon dioxide is much more soluble in blood than oxygen, which means it can dissolve and diffuse more readily across membranes. 
  • This higher solubility of carbon dioxide allows for faster diffusion rates compared to oxygen, despite oxygen being less soluble.


6. Diffusion Membrane:


  • The thin layers involved in gas exchange include the squamous epithelium of the alveoli, the endothelium of alveolar capillaries, and the basement substance between them.





  • Despite being composed of multiple layers, the total thickness of the diffusion membrane is very thin, ensuring efficient gas exchange.