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Inspiration and Expiration

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For the mechanism of Breathing the human should have the lungs and in that lungs, the inhalation and exhalation process mechanism is under the different Respiratory organs, etc the instrument used for measuring the volume of air involved in breathing is the spirometer 

Respiratory Organs

Mechanisms of breathing differ for different groups of animals depending on their living and levels of the organization. Lower invertebrates like sponges, and coelenterates, exchange O with CO, by diffusion process by their entire body surface. Earthworms use their cocktails and insects have a tracheal tube to transport atmospheric air within the body, Special structures called gills branchial respiration are used by most aquatic arthropods and Mollusca whereas vascularized bags called lungs are used by the terrestrial forms for the exchange of gases. Among vertebrates, fishes use gills whereas amphibians and reptiles. Birds and mammals respire through the lungs. Amphibians like frogs can respire through their moist skin. 

Human Respiratory System

The human Respiratory system is based on the exchange of gases that are oxygen and carbon dioxide through the organs that organs are the lungs, which internally have the alveoli and different nerves and tissue that trachea divides the two lungs, and the trachea is connected to the pair of the external nose opening out above the upper lips. It leads to a nasal chamber through the nasal passage. The nasal chamber opens into the pharynx, which is common for the passage of food and air. The pharynx opens through the larynx region into the trachea. The larynx helps in sound production and is hence called the sound box. During swallowing, the glottis can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx. The trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into the right and left primary bronchi. Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings. Each bronchiole gives rise to a number of very thin, irregular-walled, and vascularized bag-like structures called alveoli. The branching network of bronchi, bronchioles, and alveoli comprise the lungs. We have two lungs that are covered by a double-layered pleura, and between the two layers, the pleura fluid is present. It reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic cavity.

The lungs are situated in the thoracic chamber, which is anatomically an airtight chamber. The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and on the lower side by the inverted U-shaped diaphragm. The anatomical setup of lungs in the thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung cavity. Such arrangement is essential for breathing so that we cannot directly alter the pulmonary volume. 

Respiration involves the following steps: 

  • Diffusion of gases (O2, and CO2) across the alveolar membrane.
  • Transport of gases by the blood.
  • Diffusion of O2, and CO2, between blood and tissues.
  • Utilization of O2, by the cells for catabolic reactions and resultant release of CO2 

Mechanism of Breathing

The mechanism of Breathing involves two stages: inspiration during which the air is going in and expiration by which the alveolar air goes out to the lungs. The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere. 

Breathing Process


These mechanisms of breathing have two ways of breathing in the lungs. They are: 

  • Inspiration 
  • Expiration


  • Inspiration is involved in the mechanism of breathing.
  • This is the process of inhalation is moving the air from the environment to the lungs of the human body. 
  • The air first passes from the nasal chamber nose to the pharynx and then to the air pipe bronchitis. Then the air reaches the small air sacs called the Alveoli. 
  • The alveoli help the air to move to the heart to oxygenate blood 
  • Inspiration can occur if the pressure within the lungs is less than the atmospheric pressure.
  • In this inspiration time, the diaphragm gets contracted in the body and comes here to the lungs. 
  • Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the anteroposterior axis.
  • The contraction of external muscles lifts the ribs and the sternum, causing an increase in the volume of the thoracic chamber in the dorsoventral. 
  • The overall increase in the thoracic volume causes a similar increase in pulmonary volume. 
  • An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure, which forces air from outside to move into the lungs is the inspiration. 


  • Expiration is involved in the movement of air from the lungs to the outside. Expiration is the process of moving the air from the lungs to the outside of the environment.
  • The total reverse process of the inspiration from start to end, like lungs to the nose. 
  • Expiration is the process of moving carbon dioxide from the alveoli of the lungs to the environment through the parts of the alveoli. 
  • Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure. 
  • The diaphragm and a specialized set of muscles-external and Internal intercostal between the ribs, help in the generation of such gradients. 
  • Relaxation of the diaphragm and the muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume.
  • This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure, causing the expulsion of air from the lungs.
  • We have the ability to increase the strength of Inspiration and expiration with the help of additional muscles in the abdomen.
  • A healthy human breathes 12-16times per minute. In breathing, movements can be estimated by using a spirometer, which helps in the physical assessment of pulmonary function. 

FAQs on Breathing Mechanism

Question 1: Where did the exchange of gases take place in the human body? And what are that gases?


The exchange of gases takes place in the human body in the lungs by the inhalation and exhalation of air .and the gases are oxygen and carbon dioxide 

Question 2: What are the different types of respiratory Volumes and Capacities?


  • Tidal volume (TV)
  • Residual volume(RV)
  • Inspiratory reserve volume (IRV)
  • Expiratory reserve volume(ERV)
  • Inspiratory capacity (IC)
  • Expiratory capacity(EC)
  • Vital capacity(VC)
  • Total lung capacity (TLC) 

Question 3: Explain the Tidal volume


The amount of air inhaled or exhaled in normal respiration is 500 ml and a person can respire nearly 5000 to 6000 ml per minute is called Tidal volume (TV) 

Question 4: Which measuring instrument is used for measuring the volume of air involved in breathing?


A spirometer is used to measure breathing volume.

Question 5: At which inhalation or exhalation time does the diaphragm relaxation take place? Explain.


During the exhalation time, diaphragm relaxation takes place. Because the exhalation is a passive process and that external intercostal  muscle is present 

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Last Updated : 11 Oct, 2022
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