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 :
Diaphragm/phrenic (also the chief respiratory muscle )
Here ,we discuss of normal quiet inspiration
First , external intercostal muscles contract; causing ribs to move forward then, upward
Then, phrenic muscles’ radial muscle contract ; causing dome shaped phrenic muscle to fall.
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.
Quiet expiration :
External intercostal muscles relax.
Phrenic muscles relax .
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 :
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.
Inspiratory reserve volume/IRV : volume of air that can be forcefully inspired after a quiet inspiration. It’s value is 3100 ml.
Expiratory reserve volume /ERV: volume of air that can be forcefully expired after quiet expiration. It’s value is 1200 ml .
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 .
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
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
Inspiratory capacity : IRV + tidal volume
value is 3100 + 500 = 3600ml
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 :
In dissolved form : about 1 -3 % of total oxygen is transported dissolving ;by plasma‘s water mainly. As oxygen is slightly soluble in it.
As oxyhaemoglobin : about 97%-99% of 4 molecules of oxygen getting in contact with a molecule of haemoglobin.
Carbon dioxide transport :
.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.
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.
As carbaminohemoglobin : about 23 % of it is transported when the amino group and hemoglobin bind with carbon dioxide.
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