Topic Overview

Ventilation and Perfusion in Lungs: Diffusion

Introduction

• Gas exchange in lungs occurs by diffusion across the respiratory membrane
• Oxygen diffuses from alveoli into pulmonary capillary blood
• Carbon dioxide diffuses from blood into alveoli
• Efficient gas exchange requires proper matching of ventilation and perfusion
• Important for maintaining normal oxygenation of blood

---

Diffusion Across the Respiratory Membrane

 

 

 

• Process of diffusion in lungs
• Diffusion refers to the movement of gases from an area of higher partial pressure to lower partial pressure.

• Oxygen diffusion
• Oxygen moves from alveolar air (higher PO₂) → pulmonary capillary blood (lower PO₂).

• Carbon dioxide diffusion
• Carbon dioxide moves from blood (higher PCO₂) → alveoli (lower PCO₂).

• Respiratory membrane
• Diffusion occurs across a very thin membrane consisting of:
• Alveolar epithelium
• Interstitial space
• Capillary endothelium

• Importance of ventilation–perfusion matching
• Effective gas exchange requires adequate air reaching alveoli and adequate blood flow in pulmonary capillaries.

The respiratory membrane is astonishingly thin—about 0.5 micrometers. Nature essentially stretched a delicate film between air and blood to make diffusion extremely efficient.

---

Factors Controlling Diffusion

• Several physical and physiological factors determine the rate of gas diffusion in lungs

---

Factors Affecting Diffusion of Gases

 

 

 

4

• Partial pressure gradient
• Diffusion rate depends on the difference in partial pressure of gases between alveoli and blood.

• Surface area of respiratory membrane
• Larger surface area increases diffusion.
• Human lungs provide about 70–100 square meters of exchange surface.

• Thickness of respiratory membrane
• Diffusion decreases if membrane thickness increases.
• Seen in diseases like pulmonary fibrosis and pulmonary edema.

• Solubility of gases
• Carbon dioxide diffuses about 20 times faster than oxygen because of greater solubility.

• Molecular weight of gases
• Diffusion rate is inversely proportional to the square root of molecular weight.

This relationship is summarized by Fick’s law of diffusion, which governs how gases move across membranes.

---

Pulmonary Diffusing Capacity for O₂

• Measure of efficiency of lungs in transferring oxygen from alveoli to blood
• Reflects functional status of respiratory membrane

---

Pulmonary Diffusing Capacity

 

 

 

4

• Definition
• Pulmonary diffusing capacity refers to the volume of oxygen that diffuses through the respiratory membrane per minute for each mmHg difference in pressure.

• Normal diffusing capacity
• Diffusing capacity of oxygen at rest is approximately 20–25 mL/min/mmHg.

• Factors influencing diffusing capacity
• Surface area of alveoli
• Thickness of respiratory membrane
• Pulmonary blood flow
• Hemoglobin concentration

• Clinical significance
• Diffusing capacity decreases in:
• Pulmonary fibrosis
• Emphysema
• Pulmonary edema

 

Diffusion of CO₂

• Movement of carbon dioxide from pulmonary capillary blood to alveolar air
• Occurs across the respiratory membrane by diffusion
• Driven by partial pressure gradient between blood and alveoli
• Important for elimination of metabolic carbon dioxide

---

Diffusion of Carbon Dioxide in Lungs

 

 

 

4

• Direction of diffusion
• Carbon dioxide diffuses from pulmonary capillary blood (higher PCO₂) to alveolar air (lower PCO₂).

• High solubility of CO₂
• Carbon dioxide is about 20 times more soluble than oxygen, so it diffuses very rapidly across the respiratory membrane.

• Small pressure gradient required
• Only a small pressure difference is needed:
• Pulmonary capillary PCO₂ ≈ 45 mmHg
• Alveolar PCO₂ ≈ 40 mmHg

• Efficiency of carbon dioxide removal
• Because of its high diffusibility, CO₂ elimination remains effective even if the respiratory membrane becomes slightly thickened.

This is a clever piece of evolutionary engineering. Carbon dioxide leaves the blood easily, even when the lungs are not working perfectly.

---

Perfusion

• Refers to blood flow through pulmonary capillaries surrounding alveoli
• Essential for effective gas exchange
• Must be properly matched with ventilation

---

Pulmonary Perfusion

 

 

 

4

• Definition of perfusion
• Perfusion refers to the flow of blood through pulmonary capillaries available for gas exchange.

• Importance in gas exchange
• Oxygen uptake and carbon dioxide removal require adequate blood flow around ventilated alveoli.

• Relationship with ventilation
• Efficient respiration depends on proper matching between ventilation (airflow) and perfusion (blood flow).

• Ventilation–perfusion ratio (V/Q ratio)
• The ratio between alveolar ventilation and pulmonary blood flow.
• Normal average V/Q ratio ≈ 0.8.

• Clinical importance
• Mismatch between ventilation and perfusion can lead to:
• Hypoxemia
• Pulmonary diseases such as embolism, COPD, and pneumonia