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A Handbook of Biology

1. 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 mmHg)

pCO2 (in mmHg)

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.

2. SØLÜBÏLÏTÝ ØF GÅSËS: 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.

3. THÏÇKÑËSS ØF MËMBRÅÑËS

The diffusion membrane is made up of 3 layers:

a) Squamous epithelium of alveoli.

b) Endothelium of alveolar capillaries.

c) Basement substance between them.(AIPMT 2011)

Expired air

Inspired air

Alveolus

Alveolar air

pO2 = 104 mm Hg

pCO2 = 40 mm Hg

Pulmonary

vein

Pulmonary

artery

Body tissues

Systemic arteries

(carrying oxygenated blood)

pO2 = 95 mm Hg

pCO2 = 40 mm Hg

Systemic veins

(carrying deoxygenated blood)

pO2 = 40 mm Hg

pCO2 = 45 mm Hg

CO2

CO2

CO2

CO2

O2

O2

O2

O2

Its total thickness is only 0.5 mm. It enables easy gas exchange.