Breathing and Exchange of Gases

Mechanism of Breathing

Do any of us have to think too much or make a lot of effort while breathing? No, right? It comes naturally to us the moment we are born. But, did you know that while breathing, an act that seems simple to us, our respiratory system performs a lot of work! Our respiratory system is made of organs and muscles that help make this process smooth and easy. Let’s learn how this system works.

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Mechanism Of Breathing

The respiratory system goes through two stages – Inspiration and Expiration. Let’s understand what happens during these two stages. During inspiration, atmospheric air is drawn in while during expiration, air is expelled out. This movement of air in and out of the lungs is due to a pressure gradient between the atmosphere and the lungs.

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If the pressure in the lungs is lower than the atmospheric pressure, then inspiration can take place. On the other hand, if the pressure in the lungs is higher than that of the atmosphere, then expiration takes place. This pressure gradient is due to the diaphragm and a special set of muscles in the respiratory system – the internal and external intercostals that are situated between the ribs.


What causes inspiration? The contraction of the diaphragm increases the volume of the thoracic chamber in the front and back. This volume is further increased by the contraction of external intercostal muscles which lifts up the ribs and sternum. Increase in the thoracic volume increases the pulmonary volume such that the intrapulmonary pressure is less than the atmospheric pressure. This forces the air to move from outside into the lungs i.e. inspiration.

Respiratory system

Process of inspiration (Source: Wikimedia Commons)


How does expiration happen? When the diaphragm and the intercostal muscles relax, the ribs and sternum return to their normal positions. This reduces the thoracic volume and consequently, the pulmonary volume. This makes the intra-pulmonary pressure higher than the atmospheric pressure causing the release of air from the lungs i.e. expiration.

Respiratory system

Process of expiration (Source: Wikimedia Commons)

Additional muscles in the abdomen help to increase the strength of inspiration and expiration. The average rate of respiration of a healthy human is 12-16 breaths/minute. We can estimate the volume of air during breathing movements using a Spirometer. It helps to clinically assess pulmonary function.

Respiratory Volumes And Capacities

During clinical assessments, the following terms are used to describe respiratory volumes and capacities.

  • Tidal Volume (TV): It is the volume of air inspired or expired during normal breathing. A healthy human can inspire or expire approximately 6000 to 8000 ml of air/minute. Therefore, the tidal volume is approximately 500 ml per inspiration.
  • Inspiratory Reserve Volume (IRV): This refers to the additional volume of air a person can forcefully inspire. It is approximately 2500-3000 ml.
  • Expiratory Reserve Volume (ERV): This refers to the additional volume of air a person can forcefully expire. It is approximately 1000-1100 ml.
  • Residual Volume (RV): This is the volume of air that remains in the lungs even after a forcible expiration. This is approximately 1100-1200 ml.

The pulmonary capacities used in clinical assessment are derived by adding a few respiratory volumes described above.

  • Inspiratory Capacity (IC): This is the volume of air a person can inspire after a normal expiration. It is the sum of tidal volume and inspiratory reserve volume (TV+IRV).
  • Expiratory Capacity (EC): This refers to the volume of air a person can expire after a normal inspiration. It is the sum of tidal volume and expiratory reserve volume (TV+ERV).
  • Functional Residual Capacity (FRC): This is the volume of air that remains in the lungs following a normal expiration. It is the sum of expiratory reserve volume and residual volume (ERV+RV).
  • Vital Capacity (VC): It is the maximum volume of air a person can breathe out, following a forced inspiration. It is the sum of inspiratory and expiratory reserve volume and tidal volume (IRV+ERV+TV).
  • Total Lung Capacity: It refers to the total volume of air the lungs can accommodate after a maximal inspiration. It is the sum of all the lung volumes (TV+RV+IRV+ERV) or vital capacity + residual volume (VC+RV).

A Solved Example For You

Q: If the tidal volume is 500 ml and the expiratory reserve volume is 1100 ml, what is the expiratory capacity?

  1. 1100 ml
  2. 500 ml
  3. 1600 ml
  4. 1500 ml

Solution: The answer is ‘c’. The expiratory capacity is tidal volume + expiratory reserve volume. Therefore, it is 500+1100 = 1600 ml.

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