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Mechanism Of Respiration

Mechanism of external respiration :

Pulmonary ventilation : 

This is basically, intake of oxygen rich air and giving out carbon dioxide concentrated air by our natural ventilator ; lungs.

External respiration / breathing involves inspiration; inhaling of air and expiration ; exhaling of air. This is all aided by muscles  associated with the respiratory system.

  Muscles involved are named as follows : 

  1. Intercostal muscles 

  2. Diaphragm/phrenic  (also the chief respiratory muscle )

  3. Abdominal muscles 

  4.  Sternocleidomastoid

  5. Scalene 

 Inspiration : 

 Here ,we  discuss of normal quiet inspiration 

  1. First , external intercostal muscles contract; causing ribs to move forward then, upward 

  2. Then, phrenic muscles’ radial muscle  contract ; causing dome shaped phrenic muscle to fall.

  3. Abdominal muscles relax ; pushing abdominal organs downward

 Overall these aims ,  reduction in intra intrapulmonary pressure  so, air  can be inspired.

The  normal quiet inspiration takes about 2 seconds .

 For forced inspiration : scalene, sternocleidomastoid , external intercostals and diaphragm are involved. 

Quiet and forced both inspirations  are  active processes.  

 Expiration : 

 Quiet expiration : 

  1. External  intercostal muscles relax.

  2. Phrenic muscles relax .

  3. Abdominal muscles contract letting organs push back.

 Overall this aims to increase intra pulmonary pressure  to let go air inside lungs.

 Quiet expiration is  passive while forced one is active.

 For, normal healthy adult breathing rate is   12- 16 per minute.

   Pulmonary volumes and capacities : 

This can be measured using a device called a spirometer and its recording can be called a spirogram .

Some of them are : 

  1. Tidal  volume : normal volume of air passing in and out the lungs in quiet breathing.  It’s value is 500 ml.

 Out of this there are regions where air reaches but  can’t enter the capillaries and gets exhaled without any exchange  called dead spaces .

Dead space are of 2 types : 

a.Anatomical : here, no exchange of gases due to anatomy not favouring it. Like thick bronchi.

b.Alveolar :  alveoli without blood capillaries spaces

  Dead space volume : 150 ml

 Both types of spaces make up physiological dead space.

  1. Inspiratory reserve volume/IRV :  volume  of air that can be forcefully  inspired after a quiet inspiration. It’s  value is 3100 ml.

  2. Expiratory reserve volume /ERV: volume of air that can be forcefully expired after  quiet expiration. It’s value is 1200 ml .

  3. Residual volume /RV: volume of air  that remains in the lung even after forceful expiration  and cannot be expelled out. It’s volume is 1200 ml .

  4. Vital capacity /VC:  volume  of  air that can be expired  forcefully after forceful  inspiration.

Mathematically  it is sum of  ERV , IRV and  Tidal volume 

 It’s  value  is: 1200 + 3100+ 500 = 4800 ml

  1. Total lung capacity : maximum amount of air that lungs can hold.

 Mathematically it is the sum of  IRV , ERV , RV  and tidal volume or VC AND RV.

It’s value is : 4800 + 1200 ml = 6000 ml 

  1. Inspiratory capacity :  IRV + tidal volume  

value  is 3100 + 500 = 3600ml

  1.  Functional residual capacity/FRC : ERV + RV

 Value is 1200 + 1200 = 2400  ml

 Inspired air is what is in the atmosphere so the composition of it is the same as of atmosphere.

But expired air has  16% oxygen , 4 % carbon dioxide  and 78 % nitrogen .

 Note : spirometer fails to  find TLC,RV and FRC

Exchange of gases : 

This occurs in the alveolar region where gases pass cells of alveoli , fused basement membrane and endothelial cells of blood capillaries .

 This way of exchange occurs due to differences in partial pressure of gases.

 In alveoli , Partial pressure of oxygen  in alveoli  and carbon dioxide in  capillaries   is 100  and 40 mm of Hg respectively . so due to this  pressure deficit oxygen diffuses in and carbon dioxide out .

 This above type of diffusing capacity is 21 ml per minute per mm of Hg for oxygen.

While in tissue cells   partial pressure of  carbon dioxide and oxygen in capillaries are 40 and 44 mm of Hg respectively.

Haldane’s effect: this effect happens to Co2 carrying haemoglobin in alveoli ; causing it’s dissociation under   high Ph and oxygen presence.

Bohr’s effect : this effect happens to oxyhaemoglobin around  cells causing it’s dissociation under low Ph and carbon dioxide presence.

 in fig :  left one is Haldane's  and right one is Bohr's

 Oxygen transport : 

Oxygen once diffused in can be transported to various   parts for  internal respiration as follows : 

  1. In dissolved form :  about 1 -3 % of total oxygen is transported dissolving  ;by plasma‘s water mainly. As oxygen is slightly soluble in it.

  2. As oxyhaemoglobin : about 97%-99% of  4 molecules of oxygen getting in contact with a molecule of haemoglobin.

 Carbon dioxide transport :

  1. .As carbonic acid  : about 7 % of total released is transported in this form . Carbon dioxide reacts with water in blood to get into this form.

  2.  As bicarbonates :  about 70% of the total are transported in this form. 

 The reaction can be as shown in below diagram: 


In the above diagram , chloride enters to maintain neutrality  . This is called the hamburger's effect.

And percentage there it's just tentative.

  1. As carbaminohemoglobin :  about 23 % of it is transported  when the amino group and hemoglobin bind with carbon dioxide.


 You can even go through these videos in : 




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