Exchange of Gases

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.

Inspired air in the alveoli remains in close contact with blood.

The partition between the two include.

1. Alveolar epithelium

2. Alveolar epithelial basement membrane

3. Thin interstitial space

4. Capillary basement membrane

5. Capillary endothelial membrane

This whole part is called respiratory membrane and cumulatively form a membrane of 0.5 mm thickness.

The limit of exchange between alveoli and pulmonary blood is diffusing capacity.

It is defined as volume of gas that diffuses through respiratory membrane in one minute for a
particle pressure difference of 1 mm Hg.

Solubility of the gases in lipids also affects diffusing capacity.

Diffusing capacity of CO₂ is 20 times more than that of oxygen and the diffusing capacity of O₂ is about double than that of nitrogen.

Surfactant:

A surface active agent -lecithin secreted by type II alveolar epithelial cells, reduces surface tension between the alveolar fluid and air.

It prevents collapsing of lung alveoli.

The exchange of gases between the alveoli and blood in the lungs, and the blood and the tissues is the result of difference in partial pressure of the respiratory gases, that is, oxygen and carbon dioxide.

External Respiration or Exchange at Alveoli Level

Partial pressure of O₂ in the atmospheric air is 159 mm of Hg and that of CO₂ is 0.3 mm of Hg.

Partial pressure of oxygen pO₂ in alveolar air is 104 mm Hg and it is only 40 mm Hg in the arterial capillaries of the lungs.

Therefore, oxygen from the alveolar air rapidly diffuses into the blood capillaries due to its higher pO­₂.

In the expired air, it changes to 116 mm of Hg.

Similarly the pCO₂ in the blood reaching the alveolar capillaries is 45 mm Hg whereas pCO₂ in alveolar air is 40 mm Hg.

Therefore, CO₂ rapidly leaves the blood capillaries and reaches the alveoli.

The pCO₂ level in expired air is 32 mm Hg.

Internal Respiration or Exchange at Tissues Level

The gaseous exchange between the blood and body tissues is also due to the differential partial pressures. The pO₂ and pCO₂ of the arterial blood reaching the tissues is 95 mm Hg and 40 mm Hg respectively. The pO₂ and pCO₂ of tissues is 20 mm Hg and 52 mm Hg respectively. Therefore, oxygen quickly leaves the blood and enters the cells whereas CO₂ produced in the tissues leaves the cells and enters the blood.

FACTORS THAT DECREASE OXYGENATION

1. Low blood volume

2. Anaemia

3. Low Hb

4. Poor blood flow

5. Pulmonary diseases