Wednesday, August 5, 2020

Breathing and Exchange of Gases - Notes | Class 11 | Part 3: Gas Exchange

17. BREATHING AND EXCHANGE OF GASES

GAS EXCHANGE
Gas exchange occurs between
  1. Alveoli and blood 
  2. Blood and tissues 
Alveoli are the primary sites of gas exchange.

O2 & CO­2 are exchanged by simple diffusion. It depends upon the following factors:
  • Pressure/ concentration gradient: The Partial pressures (individual pressure of a gas in a gas mixture) of O2 and CO2 (pO2 and pCO2) are given below. 

Respiratory gas

pO2 (in mm Hg)

pCO2 (in mm Hg)

Atmospheric air

159

0.3

Alveoli

104

40

Deoxygenated blood

40

45

Oxygenated blood

95

40

Tissues

40

45


pO2 in alveoli is more (104 mm Hg) than that in blood capillaries (40 mm Hg). So O2 diffuses into capillary blood. pCO2 in deoxygenated blood is more (45 mm Hg) than that in alveoli (40 mm Hg). So, CO2 diffuses to alveoli.
  • Solubility of gases: Solubility of CO2 is 20-25 times higher than that of O2. So, the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is higher than that of O2.
  • Thickness of membranes: The diffusion membrane is made up of 3 layers:
a) Squamous epithelium of alveoli.
b) Endothelium of alveolar capillaries.
c) Basement substance between them.
Its total thickness is only 0.5 mm. It enables easy gas exchange.
  • Surface area: Presence of alveoli increases the surface area of lungs. It increases the gas exchange.

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