Mechanism of Breathing

Human Physiology

Lesson 34 : Mechanism of Breathing & Respiratory Volumes

Mechanism of Breathing

A normal breathing process involves two processes:

  1. Inspiration (inhalation) is act of taking the atmosphere air into lungs.
  2. Expiration (exhalation) is act of bringing out alveolar air into atmosphere. Inspiration and expiration both are involuntary process and take place without any conscious (willful) effort on the part of individual. However, both processes can be modified voluntarily (with conscious effort) in man.

Inspiration begins with contraction of diaphragm under nerve stimulation from brain. Mechanical contraction of diaphragm causes movement of diaphragm towards abdominal cavity, thus reducing pressure (intra-thoracic pressure) in the thorax. This results in flow of atmospheric air into respiratory tract along the pressure gradient.

Expiration is brought about passively i.e. without involving nerve activation. After inspiration, the nerve stimulation of diaphragm is stopped, it undergo relaxation and pushed back towards thorax. Elastic recoil of lung tissue compresses the air out of respiratory system. This function is comparable to inflation & deflation of rubber balloon. To inflate a balloon, the air is required to be pushed into by force, using some energy. When mouth of inflated balloon is released, the air moves out automatically due to return of rubber balloon to its original shape (elastic recoil). The above mechanism holds true for normal and effortless breathing process.

For forceful and extra breathing additional voluntary efforts are made through contraction and relaxation of muscles of abdominal wall. By forceful efforts one can inspire more air, expire additional volume and hold breath. Act of inspiration and expiration can be demonstrated through simple bell jar experiment as shown below.

Intrapulmonic pressure (i.e. pressure inside the alveoli) is maintained sub-atmospheric around -5cm of water which is essential for maintaining alveoli in partially inflated condition even after the expiration. A surfactant (polysaccharide) chemical is also secreted in alveoli which prevents the alveoli from collapsing. In the absence of sub atmospheric pressure and surfactant, the alveoli may be collapsed and render them unfit for respiration.

Last modified: Tuesday, 10 April 2012, 12:40 PM