Exchange of Gases

Exchange of gases means O₂ and CO₂ move across a thin membrane by simple diffusion. In humans it happens at two places:

  1. Alveoli ↔ Blood (Pulmonary exchange)
  2. Blood ↔ Tissues (Tissue exchange)

Gases always diffuse from higher partial pressure (p) to lower partial pressure.

Why “Partial Pressure” is used?

Air and blood contain mixtures of gases. Each gas exerts its own pressure.

  • pO₂ = partial pressure of oxygen
  • pCO₂ = partial pressure of carbon dioxide

Partial pressure is the pressure contributed by an individual gas in a mixture.

Sites of Exchange

1. Alveoli (Main site)

  • Alveoli have huge surface area, thin wall, and rich capillary network.
  • So diffusion is fast and efficient.

2. Tissues

  • Cells use O₂ for respiration and continuously produce CO₂.
  • Therefore tissues have low pO₂ and high pCO₂, driving diffusion.

Partial Pressure

Partial pressure is the pressure contributed by an individual gas in a mixture of gases. It is represented as pO2pO_2 for oxygen and pCO2pCO_2 for carbon dioxide.

Partial Pressure Values (in mm Hg):

Respiratory Gas Atmospheric Air Alveoli Deoxygenated Blood Oxygenated Blood Tissues
O₂ (pO₂) 159 104 40 95 40
CO₂ (pCO₂) 0.3 40 45 40 45

Direction of diffusion

Oxygen

  • Alveoli (104) → Blood (40) because 104 > 40
  • After loading, blood becomes oxygenated (~95) and goes to tissues.
  • Blood (95) → Tissues (40) because 95 > 40

Key point: O₂ moves Alveoli → Blood → Tissues.

Carbon dioxide

  • Tissues (45) → Blood (40–45) because tissues produce CO₂
  • Deoxygenated blood (45) → Alveoli (40) because 45 > 40

Key point: CO₂ moves Tissues → Blood → Alveoli.

Factors Affecting Diffusion

Diffusion becomes faster when:

1. Partial pressure gradient (Main driver)

  • Bigger difference in partial pressure → faster diffusion.

2. Solubility of gases (CO₂ wins here)

  • CO₂ is 20–25 times more soluble than O₂.
  • So even with a smaller gradient, CO₂ diffuses quickly.

CO₂ diffuses 20–25 times faster than O₂ because it is more soluble.

3. Thickness of diffusion membrane

  • Thinner membrane → faster diffusion.
  • In normal lungs, the diffusion path is extremely thin (very small micrometre range), so diffusion is rapid.
  • If membrane thickens (e.g., disease), diffusion slows.

4. Surface area

  • Larger surface area → greater diffusion.
  • Alveoli provide a huge surface area.

Structure of the Diffusion (Respiratory) Membrane

The diffusion membrane has three layers:

  1. Thin squamous epithelium of alveoli
  2. Basement membrane / basement substance
  3. Endothelium of alveolar capillaries

Key point: Even though there are 3 layers, total thickness is extremely small → diffusion is efficient.

Quick Points

  • Gas exchange occurs by simple diffusion.
  • O₂ gradient: Alveoli → blood → tissues.
  • CO₂ gradient: Tissues → blood → alveoli.
  • CO₂ diffuses faster due to higher solubility.
  • Thin membrane + large area = efficient exchange.

Exchange of Gases

Rule: Gas diffuses High p → Low p

O₂ path: Alveoli (104) → Blood (40) → Tissues (40) CO₂ path: Tissues (45) → Blood (45) → Alveoli (40)

3 factors to shout in exam:

  1. Partial pressure gradient (main)
  2. CO₂ is 20–25× more soluble → diffuses faster
  3. Membrane is extremely thin + large surface area

3 layers of diffusion membrane: Alveolar epithelium + Basement membrane + Capillary endothelium

💡 Questions and Answers

Q1. What is the pO₂ in alveoli and deoxygenated blood? Why is it important?

A1: In alveoli, pO₂ is about 104 mm Hg and in deoxygenated blood it is about 40 mm Hg. Because alveolar pO₂ is higher, oxygen naturally diffuses from alveoli into blood.

Key points:

  • Alveoli 104
  • Deoxygenated blood 40
  • O₂ diffuses high → low

Q2. Why does CO₂ diffuse faster than O₂ even if its pressure gradient is smaller?

A2: CO₂ diffuses faster because it is much more soluble in blood and tissues—about 20–25 times more than O₂. So it crosses the membrane easily.

Key points:

  • CO₂ solubility >> O₂
  • 20–25× faster diffusion

Q3. Name the three layers of the diffusion membrane.**

A3: The diffusion membrane includes:

  1. Alveolar squamous epithelium
  2. Basement membrane
  3. Capillary endothelium

Key points:

  • Epithelium + Basement membrane + Endothelium

Q4: What is the main mechanism of gas exchange in humans?

A4: Gas exchange happens mainly by simple diffusion, because gases move from higher partial pressure to lower partial pressure across a thin membrane.

Key points:

  • Simple diffusion
  • Driven by partial pressure

Q5. Compare pCO₂ in tissues and oxygenated blood. What does it cause?

A5: Tissues have pCO₂ ~45 mm Hg because cells produce CO₂. Oxygenated blood has pCO₂ ~40 mm Hg. So CO₂ diffuses from tissues into blood.

Key points:

  • Tissues 45
  • Oxygenated blood 40
  • CO₂ moves tissues → blood