BREATHING OR RESPIRATION PART-2:MECHANISM OF EXCHANGE OF GASES AND RESPIRATORY DISORDERS

      In this post we are going to discuss about the mechanism of exchange of gases and Respiratory disorders in brief. For the breathing and human respiratory system post, click here

        Transverse Section of an     

                           Alviolus  

Respiratory System and Capacities:

•Tidal Volume (TV): Volume of air inspired or expired during a normal respiration. It is approx. 500 mL., i.e., a healthy man can

inspire or expire approximately 6000 to 8000 mL of air per minute.

•Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration. This averages 2500

mL to 3000 mL.

•Expiratory Reserve Volume (ERV): Additional volume of air, a person can expire by a forcible expiration. This averages 1000

mL to 1100 mL. 

•Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL. By adding up a few respiratory volumes described above, one can derive various pulmonary capacities, which can be used in clinical diagnosis.

•Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration. This includes tidal volume and inspiratory

reserve volume ( TV+IRV).

•Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve

volume (TV+ERV).

•Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration. This includes ERV+RV.

•Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration. This includes ERV, TV and IRV or the maximum volume of air a person can breathe out after a forced inspiration.

•Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration. This includes RV, ERV, TV and

IRV or vital capacity + residual volume.

  Process of Gaseous Exchange

Exchange of Gases:

•Alveoli are the primary sites of exchange of gases. Exchange of gases also occur between blood and tissues. 

•O2 and CO2 are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient. 

•Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion.

•Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO2

 for oxygen and pCO2 for

carbon dioxide. 

•The data given in the table clearly indicates a concentration gradient for oxygen from alveoli to blood and blood to tissues. 

•Similarly, a gradient is present for CO2 in the opposite direction, i.e., from tissues to

blood and blood to alveoli. 

•As the solubility of CO2 is 20-25 times higher than that of O2, the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O2. 

•The diffusion membrane is made up of three major layers

 namely, the thin squamous

epithelium of alveoli, the endothelium of alveolar capillaries and the basement

substance (composed of a thin basement membrane supporting the squamous

epithelium and the basement membrane surrounding the single layer endothelial

cells of capillaries) in between them.

•However, its total thickness is much less than a millimetre. Therefore, all the factors

in our body are favourable for diffusion of O2 from alveoli to tissues and that of CO2

from tissues to alveoli.

Table of pCo2 and pO2 in various locations of Respiratory System

Transport of Gases:

•Blood is the medium of transport for O2 and CO2.

•About 97 per cent of O2 is

transported by RBCs in the blood.

•The remaining 3 per cent of O2 is carried in a dissolved state through the plasma.

•Nearly 20-25 per cent of CO2 is transported by RBCs whereas 70 per cent of it is carried as bicarbonate.

•About 7 per cent of CO2 is carried in a dissolved state through plasma.

Transport of Oxygen:

     Oxygen Dissociation Curve

•Haemoglobin is a red coloured iron containing pigment present in the RBCs. 

•O2 can bind with haemoglobin in a reversible manner to form

oxyhaemoglobin. 

•Each haemoglobin molecule can carry a maximum of four molecules of O2. 

•Binding of oxygen with haemoglobin is primarily related to partial pressure of O2. 

•Partial pressure of CO2, hydrogen ion concentration and temperature are the other factors which can interfere

with this binding. 

•A sigmoid curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the pO2. 

•This curve is called the Oxygen

dissociation curve and is highly useful in studying the effect of factors like pCO2, H+ concentration, etc., on binding of O2 with haemoglobin. 

•In the alveoli, where there is high pO2, low pCO2, lesser H+

 concentration and lower temperature, the factors are

all favourable for the formation of oxyhaemoglobin, whereas in the tissues, where low pO2, high pCO2, high H+ concentration and higher temperature exist,

the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin. 

•This clearly indicates that O2

 gets bound to haemoglobin in the lung surface and gets

dissociated at the tissues. 

•Every 100 ml of oxygenated blood can deliver around 5 ml of

O2 to the tissues under normal physiological conditions.

Transport of Carbon-Di-Oxide:

•CO2 is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent). 

•This binding is related to the partial pressure of CO2.

•pO2 is a major factor which could affect this binding. 

•When pCO2 is high and pO2

 is low as in the tissues, more binding of carbon dioxide occurs

whereas, when the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from carbamino-haemoglobin takes place, i.e., CO2 which is bound

to haemoglobin from the tissues is delivered at the alveoli. 

•RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute

quantities of the same is present in the plasma too. 

•This enzyme facilitates the following reaction in both directions.

•At the tissue site where partial pressure of CO2 is high due to

catabolism, CO2 diffuses into blood (RBCs and plasma) and forms HCO3– and H+. 

•At the alveolar site where pCO2

 is low, the reaction proceeds in

the opposite direction leading to the formation of CO2 and H2O.

•Thus, CO2 trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2. 

•Every 100 ml of deoxygenated

blood delivers approximately 4 ml of CO2 to the alveoli.

Regulation of Respiration:

•Human beings have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues. 

•This is done by the neural system. 

•A specialised centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for this regulation. 

•Another centre present in the pons region of the brain called pneumotaxic centre can moderate the functions of the respiratory rhythm centre. 

•Neural signal from this centre can reduce the duration of

inspiration and thereby alter the respiratory rate. 

•A chemosensitive area is situated adjacent to the rhythm centre which is highly sensitive to CO2 and hydrogen ions.

•Increase in these substances can activate this centre,

which in turn can signal the rhythm centre to make necessary adjustments

in the respiratory process by which these substances can be eliminated.

•Receptors associated with aortic arch and carotid artery also can recognise changes in CO2 and H+ concentration and send necessary signals to the

rhythm centre for remedial actions. 

•The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

Disorders of Respiratory System:

Asthma: It is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.

Emphysema: It is a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. One of the major causes of this is cigarette smoking.

Occupational Respiratory Disorders: In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage.

Workers in such industries should wear protective masks.