Pneumonia and Pulmonary Edema

Normal Physiology of Breathing and the Cough Reflex

Jennifer Kong

Learning Objectives

By the end of this section, you will be able to:

  • define coughing and explain how it is a protective mechanism for the lungs
  • identify the components needed for effective coughing
  • briefly explain the mechanism behind involuntary coughing


For a review of the physiology of breathing, please refer to the  The Process of Breathing in the Emphysema chapter.


Cough is a physiological response for both defense or aid ventilation.  Coughing can be both involuntary (ie. reflex) or voluntary (ie. conscious control).  The involuntary cough reflex that acts both as a defensive physiological mechanism against the inhalation of harmful substances  and for the clearing excessive bronchial secretions (mucous).   This act of closing the glottis while initiating exhalation will also physically prop open airways with the building positive pressure.

The mechanism of coughing – whether voluntary or involuntary – involves a quick inhalation to max inhalation volume followed by a closing of the glottis and then – upon opening of the glottis –  a sudden, explosive exhalation.    In order to produce an effective cough, three components must be in place:   lung volume should be close to normal, respiratory and glottis muscles are functional, and neurological sensation and control of the prementioned muscles are in tact.  A deficiency in any of these components will result in both ineffective coughing (resulting in possibly inhalation of harmful substances and pathogens) and insufficient clearing of mucous from the airways.

In the situation of involuntary cough reflex, stimulation of receptors around the bronchial tree begin the cough reflex.  There are also receptors in areas adjacent to the respiratory tract (albeit, at a lesser extent): esophagus, ear, sinuses, pleura, diaphragm, and pericardium. Thus, irritation/stimulation of any of these receptors will begin the cough reflex.  For example, excessive mucous secretions built up in the bronchioles will irritate the cough receptors initiating the cough reflex.  The cough centre in the brain stem will interpret this message and then send motor messages to both the glottis and muscles responsible for exhalation: causing an explosive exhalation which will (hopefully) clear the secretions which will no longer irritate the cough receptors.  And if a harmful pathogen happens to be trapped in this secretion, then the cough reflex is successful in expelling it from the body.

Section Summary

Coughing is a protective mechanism intended to prevent inhalation of irritants and noxious substances into the lower respiratory tract.  Coughing can be both voluntary (e.g. to make a sound to alert someone of your presence) or involuntary (e.g. food accidentally going into airways).  Involuntary coughing is initiated by stimulation or physical irritation of receptors in the airway and surrounding structures. This sensation is sent to the cough centres in the brain which, in turn, initiate 3 motor messages:  1) a quick inhalation; 2) closing of the glottis muscle); 3) exhalation through a closed glottis.  A cough results as the pressure from the closed glottis explodes with enough force to expel the irritant that stimulated the receptors.

Review questions


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Pathology Copyright © 2022 by Jennifer Kong is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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