Exchange of Gases

Hey guys ! Today I’m going to talk about the Exchange of Gases. The major sites of gaseous exchange are alveoli of lungs.The exchange of gases also occurs between the tissues and the blood. The oxygen and carbon dioxide is exchanged at these sites by simple diffusion method mainly based on pressure gradient.

1.Gaseous exchange at alveoli level :-

The diffusing respiratory membrane in lungs consists of alveolar surfactant, alveolar epithelium, epithelial basement membrane, a thin interstitial space, capillary basement membrane and capillary endothelial membrane as shown here

The limit of exchange of gases between alveoli and pulmonary blood is called diffusing capacity. This diffusing capacity is 20 times more for CO2 than O2.
The PO2 in alveolar air is about 104 mm Hg and that in arterial capillaries of alveoli is 40mm Hg. Therefore, oxygen from the alveolar air rapidly diffuses into the blood capillaries. As the result, the PO2 becomes 95 mm of Hg in the blood leaving lungs.
Similarly, pCO2 in alveolar air is 40 mm of Hg and that in arterial capillaries of alveoli is 45 mm of Hg. Therefore, CO2 rapidly leaves blood capillaries and reaches alveoli. As the result PCO2 becomes 40 mm of Hg in the blood leaving lungs.Gaseous exchange at

2.Tissue level :-

The gaseous exchange between blood and body tissue fluid is aIso due to differential partial pressures. pO2 in a respiring cell tissue fluid is 40 mm Hg, while it is 95 mm Hg in capillary blood. Therefore, O2 diffuses from blood into tissue uid and from there into cells. As the result, the pO2 becomes 40 mm of Hg in the blood leaving tissue fluid.

On the other hand, pCO2 of blood capillaries is 40 mm Hg and that of cells is 50 mm Hg. Hence, CO2 diffuses out of the cells into tissue fluid and from tissue fluid into blood. This results to increase in pCO2 of blood to 45 mm of Hg.

Table showing partial pressure (in mm Hg) of oxygen and carbon dioxide at different parts involved in diffusion in comparison to those in atmosphere.

Respiratory GasO2CO2
Atmospheric air1590.3
Alveoli10440
Bood ( Deoxygenated )40 45
Blood ( Oxyganated )9540
Tissue4045
Inside cells2050

TRANSPORT OF GASES :-

The transport of respiratory (O2 and CO2) gases takes place in the following events: –

1.External respiration,
2.Internal respiration,
3.Cellular respiration
.

1.External respiration :-
It includes the respiratory processes which take place in the lungs. Oxygen from the lungs diffuses in the lung capillaries and similarly CO2 from the lung capillaries diffuses into lungs depending on partial pressure of each gas. (Partial pressure is the pressure contributed by the individual gas in a mixture of gases.)

External respiration includes three events :

i. Exchange of gases :
Concentration of oxygen is higher in the inspired air (PO2 = 104mmHg) than in the alveolar blood (PO2= 40mmHg) and the concentration of carbon-dioxide is higher in the alveolar blood (PCO2 = 45mmHg) than in the inspired air (PCO2 = 40mmHg). This results in the exchange of oxygen from the air into the blood and carbon-dioxide from blood into the air which is exhaled out.

ii. Formation of oxy-haemoglobin :
The absorbed oxygen combines with the haemoglobin of RBCs.Haemoglobin is a respiratory protein pigment. It forms the unstable oxy-haemoglobin
Haemoglobin + Oxygen—-> Oxyhaemoglobin
Hb + 402——> Hb (402)

iii.Release of carbon-di-oxide :
Carbon-dioxide from the blood is released in the air. CO2 is brought by the blood from the tissue cells in the form of sodium and potassium bicarbonates in the blood plasma. Some amount of CO2 is also brought by haemoglobin in the from of carbamino-haemoglobin. CO2 brought in all these forms is released.
The bicarbonates break down to liberate carbonic acid.

H+
NaHCO2———–> Na+ H2CO3 Sodium Sodium carbonic


Bicarbonate Release of carbon-di-oxide: Carbon-dioxide from the blood is released in the air. CO2 IS brought by the blood from the tissue cells in the form of sodium and potassium bicarbonates in the blood plasma. Some amount of CO, is also brought by haemoglobin in the from of carbamino-haemoglobin. CO, brought in all these forms is released.

2.Internal respiration :-
It includes the respiratory processes which take place in the tissue cells. Oxygen brought by the blood (PO2 = 95mmHg) and carbon-dioxide from the tissues (PCO2 = 40mmHg), is passed into the blood. When the blood reaches the tissue cells, the unstable oxy- haemoglobin breaks down to form haemoglobin and oxygen.

i. Carbon-dioxide dissolves in the cellular fluid and passes into the plasma. CO2 dissolves in the water to form carbonic acid

CO2 + H20 —————> H2CO3

Carbonic acid reacts with sodium and potassium to convert it into sodium and potassium bicarbonates respectively.
About 80% to 85% carbon-dioxide is carried by the blood in the form of bicarbonates.

ii. A small amount of carbon-dioxide combines with haemoglobin to form carbamino- haemoglobin.

iii. Some carbon-dioxide dissolves in the plasma which is carried to the lungs

3.Cellular respiration:-
The ultimate purpose of respiration is to release energy. This is carried out in the cells by oxidation of food. It results in the formation of ATP molecules. Energy is stored in this form. This energy is used to carry out vital life processes. So ATP is called energy currency of cell.
The and Cellular respiration takes place mainly in the mitochondria of cells. ATP is formed as the main product while byproducts are CO2 and water vapour which are transported by the blood to the lungs. Energy released as heat to certain extent is used to maintain the body temperature.

Regulation of Respiration :-

Neural control:-

  • Respiratory rhythm centre is located in medulla.
  • Pneumotaxic centre is located in pons of brain.
  • Increase in these substances activates the chemosensitive area.

Chemical control:-

  • Chemosensitive area :- Adjacent to rhythm centre, highly sensitive to CO2 and H+ ion concentration in blood.
  • Increase in these substances activates the chemosensitive area.
  • Chemoreceptors in carotid artery and aortic arch can recognise changes in CO2 and H+ concentration.
  •  Necessary neural signals are sent to rhythm centre when CO2 and H+ concentration changes.

Thank you for visit !

Be the first to comment

Leave a Reply

Your email address will not be published.


*