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Exchange of Gases


Introduction:

  • The primary sites of the exchange of gases in the lungs are alveoli.
  • The exchange of gases not only occurs at the level of alveoli but also between blood and tissues.
  • O2 and CO2 are exchanged in these sites by the means of simple diffusion.
  • The exchange of gases is based on some important factors that can affect the rate of diffusion.

Topics covered:

  • Factors Affecting Exchange of Gases
  • Respiratory Membrane
  • Gaseous Exchange and Partial Pressure


Factors Affecting Exchange of Gases


Introduction:

Factors affecting the exchange of gases are:
1. Partial pressure/ concentration gradient of gases - A gas always diffuse from an area of its high partial pressure to an area of its low partial pressure
2. Solubility of the gases - A gas having high solubility, diffuses at a faster rate than the gas having low solubility.
3. Thickness of the respiratory membrane/ diffusion membrane - Thinner membrane supports a faster rate of diffusion.

 

Respiratory Membrane (Alveolar-capillary membrane)

  • Diffusion takes place efficiently if the respiratory or diffusion membrane is thin.
  • Diffusion membrane is made up of three major layers:
    - The thin squamous epithelium of alveoli
    - The endothelium of the alveolar capillaries
    - Basement membrane
  • Basement membrane is present between the squamous epithelium of alveoli and endothelium of alveolar capillaries
  • Total thickness of all three layers of diffusion membrane together is 0.2 mm
     

alveolar-cavity


epithelial-cell


Gaseous Exchange and Partial Pressure


Definition:

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

Detailed Explanation:

  • Gases diffused from their higher partial pressure to their lower partial pressure
  • Partial pressure for oxygen and carbon dioxide are represented as pO₂ and pCO₂ respectively.
  • Partial pressure (in mmHg) of oxygen and carbon dioxide at different parts involved in diffusion in comparison to those in the atmosphere is given below in the table.
     

respiratory-gas


Exchange of gases between alveoli and blood (Pulmonary gas exchange):

  • The pO₂ in the atmospheric air is higher than that in the alveoli and pO₂ in the alveoli is higher than that in the deoxygenated blood in the capillaries of the pulmonary arteries.
  • O₂ moves from atmospheric air to alveoli and then to blood.
     



 

  • The pCO₂ is higher in deoxygenated blood than that in alveioli and pCO₂ is higher in alveoli than that in atmospheric air.
  • CO₂ moves from deoxygenated blood to alveoli and finally to the atmosphere.



 

Exchange of gases between blood and tissue (Systemic gas exchange):

  • O2 and CO2 gases are exchanged between blood capillaries and body cells and vice versa.
  • The pO₂ is higher in systemic arteries carrying oxygenated blood than in tissues or body cells.
  • So, O2 moves from systemic arteries to body cells
     



 

  • In body cells, O2 is utilised for catabolic reaction to produce CO2, H2O and energy (ATP).
  • As CO2 is produced in body cells, so pCO₂ is higher in body cells than that in blood capillaries.
  • So, CO₂ moves from body cells to blood capillaries through tissue fluid.
  • Presence of CO₂ makes the blood deoxygenated, which is carried to the heart and then to lungs via pulmonary artery.
     

body-tissues


iii. Solubility of the gases:

  • A gas having high solubility, diffuses at a faster rate than that having low solubility
  • Solubility of CO2 is 20-25 times higher than that of O2
  • Therefore, the amount of CO2 diffuses through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O2.

Frequently Asked Questions - FAQs

Q1. Which epithelium lines the alveoli?
Ans. - Thin squamous epithelium lines the alveoli

Q2. Which is more soluble O2 or CO2?
Ans. - Solubility of CO2 is 20-25 times higher than that of O2.

Q3. Why is CO2 gas transferred from alveoli to atmospheric air?
Ans. - Gases diffuse from higher partial pressure to lower partial pressure
         - pCO2 in alveoli (40 mm Hg) is more than that in atmospheric air (0.3 mm Hg)
         - So, CO2 gas moves from alveoli to atmospheric air

Q4. Why does O2 gas move from atmospheric air to alveoli?
Ans. - Gases diffuse from higher partial pressure to lower partial pressure
         - pO2 in atmospheric air (159 mm Hg) is more than that in alveoli (104 mm Hg)
         - So, O2 gas moves from atmospheric air to alveoli

Q5. How partial pressure is related to the rate of diffusion?
Ans. - Partial pressure is one of the major factors that affect the rate of exchange of gases
         - A gas always diffuse from an area of its high partial pressure to an area of its low partial pressure

Q6. How is the thickness of the membrane related to the rate of diffusion?
Ans. - Thickness of diffusing membrane is one of the factors that affect the rate of exchange of gases
         - Thinner membrane supports a faster rate of diffusion.

Q7. What is the difference in partial pressure of oxygen between arterial and venous blood?
Ans.- Partial pressure of O2 in arterial blood (oxygenated blood) is more than that in venous blood (deoxygenated blood)
        - pO2 in arterial blood - 95 mm Hg
        - pO2 in venous blood - 40 mm Hg

Q8. What are the three layers that form the diffusion membrane or alveolar-capillary membrane?
Ans. The three layers forming diffusion membrane are:
         i. Squamous epithelium of alveoli that lines in
        ii. Endothelial lining of alveolar capillaries
        iii. Basement membrane

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