7. Diffusion (video)

Diffusion and Its Role in the Respiratory System

• Focus of the video is on diffusion as a key component of the respiratory system.

• Discussion revolves around the differences between venous and alveolar oxygen and carbon dioxide levels, indicated as follows:

  • V = Venous

  • A = Alveolar (with a sometimes indicating Arterial)

Oxygen and Carbon Dioxide Gradients

• Blood enters pulmonary capillaries from the pulmonary artery, considered venous blood due to low oxygen content.

• Partial pressures (p) of gases:

  • Oxygen (O2):

  • Enters alveoli at a partial pressure of approximately 40 mmHg.

  • Leaves alveoli at a higher partial pressure of about 104 mmHg.

  • Carbon Dioxide (CO2):

  • In venous blood at about 45 mmHg.

  • In alveoli at around 40 mmHg.

• Movement of gases is determined by pressure gradients, leading to:

  • Oxygen diffusing from the alveoli into the bloodstream (due to the higher pressure in alveoli).

  • Carbon dioxide diffusing from the bloodstream into the alveoli to be exhaled (higher in venous blood).

Mechanism of Gas Exchange

• Diffusion occurs via passive mechanisms, meaning it doesn’t require energy, unlike active transport which is energy-dependent.

Factors Affecting the Rate of Diffusion

Fick’s Law of Diffusion

• Describes factors influencing diffusion rates and is typically not used for explicit calculations, rather for conceptual understanding. The formula indicates:

  • Pressure gradient P = P (alveolar) - P (capillary)

  • Rate of Diffusion ∝ (Surface Area × Diffusion Constant × Pressure Gradient) / Thickness

  • Surface Area (A) is how many “locations” can exchange occur. 

  • Thickness (T)  is how far does the gas have to travel.

  • Diffusion Constant (D) is specific to each gas; carbon dioxide has a higher constant than oxygen. 

  • Variables:

  • Numerator (increased leads to higher diffusion):

    • Surface Area

    • Diffusion Constant (specific for each gas)

    • Pressure Gradient

  • Denominator (increased leads to lower diffusion):

    • Thickness (distance of diffusion)

  • Example Application for Oxygen:

  • (Surface Area x Diffusion constant of oxygen x Pressure gradient of oxygen) / Thickness (T)

  • Notably, carbon dioxide has a higher constant than oxygen, suggesting that it diffuses more rapidly than oxygen under similar conditions due to its affinity for water.

Factors Determining Alveolar Oxygen and Carbon Dioxide Levels

1. Partial Pressure in Inspired Air

   • Higher oxygen or carbon dioxide content in inhaled air increases respective concentrations in alveoli.

2. Rate of Alveolar Ventilation

   • Faster ventilation rates increase oxygen entry while enhancing carbon dioxide removal.

3. Cellular Oxygen Use and CO2 Production

   • Increased oxygen consumption lowers alveolar oxygen; increased CO2 production raises alveolar CO2 levels.

Impact of Altitude on Gases

• When ascending to high altitudes:

  • Inspired Air:

  • Oxygen levels are lower at high altitudes, impacting alveolar oxygen levels but not significantly altering alveolar carbon dioxide because it approximates zero in the inspired air.

  • Alveolar Ventilation:

  • Increased breathing rate attempts to compensate for lower oxygen; this raises alveolar oxygen but may also decrease carbon dioxide.

  • Oxygen Consumption and CO2 Production:

  • Generally stable and unaffected by altitude changes while at rest, leading to constant alveolar levels.

Cellular Respiration

• Oxygen is critical in cellular respiration which produces ATP, resulting in low oxygen and high CO2 conditions within cells:

  • Enhanced gradients facilitate oxygen entry into cells and efflux of CO2 back to the blood.

  • Blood: Low oxygen and high CO2 in venous circulation return to lungs.

Physiological Responses to Changes in O2 and CO2 Levels

• Changes in Systemic Arterioles and Bronchioles:

  • Low O2 / High CO2 enhances blood flow through vasodilation in systemic arterioles (to supply oxygen) and Bronchodilation in bronchioles (to increase airflow).

• Pulmonary Arterioles respond oppositely:

  • Low O2 causes vasoconstriction to divert blood away from poorly ventilated areas, aiming to improve overall gas exchange efficiency.

Summary of Responses by Vascular Structures:

Key Points to Remember

• Assess and list the partial pressures of O2 and CO2 in veins and alveoli.

• Identify 4 factors affecting diffusion and their implications.

• Explain the 3 determinants for alveolar oxygen and CO2.

• Discuss the physiological responses (arterioles and bronchioles) to variations in oxygen and carbon dioxide levels during gas exchange processes.