Study Notes on Breathing and Exchange of Gases

Study Notes on Breathing and Exchange of Gases


  • The process of breathing is very among different kinds of animals depending majorly on their habitats and level of organisation.
  • example like lower vertebrates which is sponges, coelenterates etc. They exchange O2 with CO2 by the process of simple diffusion over their entire body surface.
  • special vascularized structure called gills which are used for the most aquatic arthropods.
  • Molluscs in which vascularised bags called lungs. It is used for exchange of gases.
  • amphibians light frogs can respire through their moist skin.


  • we all have a pair of external nostrils opening out above the upper lips which is leads to nasal chamber through the nasal passage.
  • then the national chamber open into the pharynx, a portion which is very common for food and air.
  • The pharynx open through a larynx region called trachea. It’s straight you extending up to the mid thoracic cavity which divides at the level of 5th thoracic vertebra into the right and left primary bronchi.
  • Larynx is a cartilaginous flap which helps in sound production known as sound box.
  • each bronchi undergoes repeated division to form the secondary and tertiary bronchi and bronchioles and of in very thin terminal bronchioles.
  • each terminal bronchioles give rise to a number of very thin irregular walled vascularised bag-like structure which is known as alveoli.
  • we have two lungs which are covered by double layered pleura by which fluid between them it reduced friction on the lungs surface and the outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface.
  • the parts starting with the external nostrils up to the terminal bronchioles institute the conducting part whereas the alveoli and their ducts from the respiratory or exchange part of the respiratory system.
  • The exchange part is the site of the actual diffusion of O2 and CO2 between blood and atmospheric air.
  • Lungs are situated in the thoracic chamber which is anatomically an air tight chamber.
  • respiratory involves the following steps:
  • breathing or pulmonary ventilation by which atmospheric air is drawn in and to reach alveolar air is released out.
  • diffusion of gases which is O2 and CO2 across the alveolar membrane.
  • diffusion of O2 and CO2 between blood and tissues.
  • transport of gases by the blood.

Study Notes on Breathing and Exchange of Gases

    • The breathing process is consists of two stages one is inspiration during which atmospheric air is drawn in and expiration by which the alveolar air is released out.
    • Inspiration can occur if the pressure within the lungs is less than the atmospheric pressure for example there is a negative pressure in the lungs with respect to atmospheric pressure.
    • In the expression, process takes place when the intrapulmonary pressure is higher than the atmospheric pressure.
    • Basically, inspiration is initiated by the contraction of the diagram which increases the volume of the thoracic chamber in the anterior-posterior axis.
    • The overall increase in thoracic volume cause by a similar increase in pulmonary volume.
    • An increase in pulmonary volume decreases the intrapulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs example inspiration.
    • Relaxation of the diagram the intercostal muscles returns the diagram and sternum to their normal position and reduce the thoracic volume and thereby the pulmonary volume they lead to an increase in intrapulmonary pressure to slightly above the atmospheric pressure causing the explosion of air from the lungs is example expiration.



The volume of air inspired are expired during normal respiration. It is approx 500 ml.


The additional volume of air a person can inspire by a forcible inspiration. This average 2500 ml to 3000 ml.


The additional volume of air a person can expire by a forcible expiration. This average is 1000ml to 1100ml.


The volume of air remaining in the lungs even after a forcible expiration. The average is 1100 ml to 1200 ml.


The total volume of air a person can inspire after a normal expiration . This also include tidal volume and inspiratory reserve volume.


The total volume of air a person can expire after a normal inspiration which is includes tidal volume and expiratory reserve volume.


The volume of air that will remain in the lungs after a normal expiration.


The maximum volume of air a person can breathe in after a forced expiration which is include IRV, ERV and TV we are the maximum volume of air a person can breathe out after a forced inspiration.


Total volume of air accommodated in the lungs at the end of a forced inspiration which is include RV, ERV, TV and IRV.

Study Notes on Breathing and Exchange of Gases


  • Basically alveoli are the primary sites of exchange of gases in which exchange of gases also occur between blood and tissues.
  • O2 and CO2 are exchange in this sides by simple diffusion mainly based on pressure or concentration gradient.
  • Pressure contributed by an individual gas in the mixture of gases is called partial pressure.
  • The concentration gradient for oxygen from alveoli to blood and blood to tissues similarly a gradient is represent for CO2 in the opposite direction.
  • The solubility of O2 the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher as compared to that of O2.
  • The diffusion membrane is made of three major layer which is the thin squamous epithelium of alveoli , the endothelium of alveoli capillaries and the basement substance.
  • Just because of the thickness all the factors in our body are favourable for diffusion of O2 from alveolar to tissues and that of CO2 from tissues to alveoli.

Study Notes on Breathing and Exchange of Gases

  • Blood is the medium of transport for O2 and CO2 in 97% of O2 is transported by RBC in the blood and the remaining 3% of auto is carried in a dissolved state through the plasma.
  • Around 7% of CO2is carried in a dissolved state through plasma.


  • Haemoglobin is a red coloured iron containing pigment present in the RBC which O2 can bind with haemoglobin in a reversible manner to form oxyhaemoglobin.
  • Hemoglobin molecule contain 4 molecule of O2.
  • Partial pressure of CO2 hydrogen ion concentration and temperature are the other factor which can interfere with this binding.
  • A curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the PO2. This curve is known as oxygen dissociation curve.
  • In this really there is high po2 and low low pco2,lesser h + concentration and lower temperature the factors are all favourable for the formation of oxyhaemoglobin square in the tissue Low PO2,high PO2,hign h + concentration and high temperature exist the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin.
  • This clearly indicate that to gets bound to hemoglobin in the long surface and gets dissociated at the tissues.

Study Notes on Breathing and Exchange of Gases


  • CO2 is carried by hemoglobin as carbaminohemoglobin.
  • When PCO2 is high and PO2 is low as in tissues more binding of carbon dioxide occur where the pco2 is low and PO2 is high as in the alveoli.
  • Dissociation of CO2 from carbamino haemoglobin takes place.
  • CO2 which is bound to hemoglobin from the tissues is deliberate at the alveoli and RBC contain a very high concentration of the enzyme in which carbonate anhydrous and minute qualities of the same is present in the plasma too.


  • Human being have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of body tissues which is done by the neural system.
  • A centre present in the medula reason of the brain which is known as respiratory rhythm centre is primarily responsible for the regulation.
  • Another centre present in the pond region of the brain called pneumotaxic centre can moderate the function of respiratory rhythm centre.
  • A chemo sensitive area is situated adjacent to the rhythm centre which is highly sensitive to CO2 and hydrogen ions.
  • Increase these substances can active this centre which in turn can the signal the rhythm centre to make necessary adjustment in the respiratory process by which the substance can be eliminated.


  • Asthma is a difficulty in breathing causing due to inflammation of bronchi and bronchioles.
  • Emphysema is a chronic disease in which alveolar wall are get damage due to which respiratory surface is decreased in which one of the major cause is cigarette smoking.


  • In specific industries specially those involving grinding or stone breaking in which so much dust is produced that the defence mechanism of the body cannot fully cope with the situation so that it can give rise to inflammation leading to fibrosis and cause serious lung damage.