BMS1032: Respiratory Physiology Notes lecture bite 3

Alveolar Gas Exchange Influences:

• O₂ reaching the alveoli: The availability of oxygen to the alveoli is critical, as it directly influences the saturation of hemoglobin in the blood. Factors such as altitude, air quality, and ventilation rates can affect this.

• Gas diffusion between alveoli and blood: Efficient diffusion is facilitated by a greater concentration gradient, where O₂ moves from high concentration in the alveoli to low concentration in the blood. This process depends on the partial pressures of gases.

• Composition of inspired air: The proportions of oxygen, carbon dioxide, and other gases in the inhaled air, along with air pollutants, influence gas exchange efficiency.

• Adequate perfusion of alveoli: Sufficient blood flow to the alveoli is essential for optimal gas exchange. Conditions such as pulmonary embolism can impair perfusion, reducing oxygen uptake.

• Rate and depth of breathing: Hyperventilation increases gas exchange, while shallow breathing can compromise it. Effective ventilation maintains optimal alveolar gas levels.

• Lung resistance and compliance: Resistance refers to the effort required to expand the lungs, while compliance indicates how easily the lungs can stretch. High resistance or low compliance impedes efficient gas exchange.

• Airway conditions: Conditions like inflammation, bronchoconstriction, or foreign body obstruction can significantly alter airflow and gas diffusion.

• Thickness of the barrier: The alveolar-capillary membrane must be sufficiently thin to allow for rapid gas exchanges. Any thickening due to pathological conditions can slow diffusion rates.

• Amount of fluid present: The presence of fluid can hinder gas exchange by increasing the distance that gases must diffuse. Excessive fluid may occur in pulmonary edema or other conditions affecting the lungs.

Principles of Gas Movement

• Gas movement occurs from high pressure to low pressure areas: This fundamental principle governs the direction of gas diffusion, heavily reliant on the differences in partial pressures of gases involved.

• Fick's Law of Diffusion states:
  V_{gas} = A \times D \times \frac{(P1 - P2)}{T}
  Where:

• V_{gas} = volume of gas diffusing through the membrane (mL/min)

• A = area available for diffusion; larger areas enhance gas exchange

• D = diffusion coefficient of the gas; this value varies based on gas properties

• P1 and P2 are the partial pressures of the gas on both sides of the membrane, driving diffusion

• T = thickness of the membrane; thinner membranes permit faster diffusion

Key Points of Gas Diffusion:

• Greater pressure difference increases flow rate: A larger gradient between P1 and P2 enhances the rate at which gases diffuse.

• Larger surface area permits more gas exchange: Conditions that improve lung surface areas, such as increased functional lung units, optimize gas exchange.

• Thinner membranes enhance diffusion rates: Physiological conditions that aid in reducing membrane thickness (like healthy lung function) are beneficial for gas exchange.

• Higher gas solubility in the membrane raises diffusion rates: Gases that are more soluble in blood will diffuse more readily across the alveolar-capillary membrane.

Disease Impacting Alveolar Diffusion

• Emphysema:
  Destruction of alveoli reduces the surface area for gas exchange significantly, leading to decreased oxygenation of blood and less efficient removal of carbon dioxide.

• Fibrotic Lung Disease:
  Thickened alveolar membranes delay gas exchange and reduce lung compliance, leading to symptoms such as shortness of breath and reduced exercise tolerance.

• Pulmonary Edema:
  Fluid accumulation in the alveolar spaces increases diffusion distance, potentially leading to impaired gas exchange, while arterial P_{CO₂} levels may remain normal due to CO₂’s high solubility in water.

• Asthma:
  Increased airway resistance diminishes alveolar ventilation, which limits airflow and can cause symptoms like wheezing and breathlessness, particularly during an asthma exacerbation.

Gas Movement Flow Summary
Gases move according to their partial pressures:

• Oxygen (O₂) moves from areas of higher partial pressure (P_{O2}) to lower, facilitating oxygen uptake by the blood.

• Carbon Dioxide (CO₂) moves in the opposite direction, from areas of high P{CO₂} (in tissues) to areas of low P{CO₂ (in alveoli), allowing for effective carbon dioxide removal.

Key Partial Pressures:

• Atmospheric air: P{O2} = 159 ext{ mmHg}, P{CO₂} = 0.3 ext{ mmHg}

• Alveoli: P{O2} = 105 ext{ mmHg}, P{CO₂} = 40 ext{ mmHg}

• Deoxygenated blood: P{O2} = 40 ext{ mmHg}, P{CO₂} = 45 ext{ mmHg}

• Oxygenated blood: P{O2} = 100 ext{ mmHg}, P{CO₂} = 40 ext{ mmHg}

Gas exchange relation in pulmonary and systemic tissues follows:

• External respiration occurs in the lungs where oxygen is taken in and carbon dioxide is expelled.

• Internal respiration occurs within tissues, where oxygen is released to the cells, and carbon dioxide is picked up to be transported back to the lungs.

As blood circulates:

• P{O2} decreases and P{CO₂} increases as it moves towards the heart and lungs due to metabolic activity, reflecting the consumption of oxygen for cellular processes and production of carbon dioxide as a waste product.