Gas Exchange and Transport of Oxygen and Carbon Dioxide

Key Concepts of Gas Exchange in the Human Body

Overview of Gas Exchange

• Gas Exchange Types:

  • Pulmonary Gas Exchange: This involves the exchange of gases between the blood in the pulmonary capillaries and the air in the alveoli. This process predominantly involves oxygen (O2) loading into the blood and carbon dioxide (CO2) unloading from the blood.

  • Tissue Gas Exchange: This refers to the exchange of gases between the blood in systemic capillaries and the systemic cells, where oxygen is released into tissues, and carbon dioxide is absorbed from tissues into the blood.

• Key Influences:

  • Physical properties of gases affect their movement and exchange.

  • Composition of alveolar gas impacts the efficiency of gas exchange.

Dalton’s Law of Partial Pressures

• Definition: This law states that the total pressure in a mixture of gases equals the sum of the individual partial pressures exerted by each gas.

• Movement of Gases: Gases shift down their partial pressure gradients to balance concentrations, facilitating efficient exchange.

Partial Pressure Gradients

• Systemic vs. Pulmonary Circulation: Importance of understanding partial pressure of O2 (PO2) and carbon dioxide (PCO2) levels across different compartments for efficient gas exchange:

  • Typically, higher PO2 in alveoli leads to oxygen diffusion into blood, while higher PCO2 promotes carbon dioxide escape from blood into alveoli.

Henry’s Law

• Application: Describes the solubility of gas in the liquid phase.

• Key Points:

  • Gas solubility in liquid is directly proportional to its partial pressure and the solubility coefficient of the gas.

  • CO2 is markedly more soluble than O2, being roughly 24 times more soluble, highlighting the need for larger pressure gradients for gases with lower solubility.

Pulmonary Gas Exchange

• Oxygen Uptake:

  • The steep gradient between alveolar oxygen and blood facilitates the rapid diffusion of O2 from alveoli to blood, reaching equilibrium within approx. 0.25 seconds across the respiratory membrane.

• Carbon Dioxide Release:

  • The gradient favoring CO2 diffusion from blood to alveoli is less steep than for O2, leading to the accumulation of CO2 in the blood before it diffuses out.

Other Influencing Factors

• Alveolar Gas Exchange: Influences include the thickness and surface area of the respiratory membrane.

• Ventilation-Perfusion Coupling: Balance between ventilation (air reaching alveoli) and perfusion (blood reaching alveoli) is crucial for optimal gas exchange.

Tissue Gas Exchange

• Oxygen Movement: O2 diffuses from blood (high PO2) into tissues (low PO2) based on concentration gradients.

• Carbon Dioxide Movement: CO2 produced in tissues diffuses into blood due to the higher concentration of CO2 in the tissues compared to the blood.

Oxygen Transport in Blood

• Transport Mechanisms:

  • Over 98% of O2 is bound to hemoglobin in red blood cells (Hb) where it is stored for transportation.

  • Less than 2% of O2 is carried dissolved in plasma.

Carbon Dioxide Transport in Blood

• Forms:

  • 7% dissolved in plasma.

  • 23% bound to globin component of hemoglobin (forming carbaminohemoglobin).

  • 70% transported as bicarbonate ions (HCO3–) within plasma.

CO2 Transport Mechanism in Capillaries

1. In Systemic Capillaries:

   • CO2 diffuses into erythrocytes and forms carbonic acid (H2CO3) via the enzyme carbonic anhydrase.

   • H2CO3 dissociates into bicarbonate (HCO3–) and hydrogen ions (H+); the latter being buffered by hemoglobin and the bicarbonate exiting the RBC while Cl– enters (Chloride shift).

2. In Pulmonary Capillaries:

   • HCO3– re-enters RBC, combines with H+ to reform H2CO3, which dissociates to release CO2 and water (H2O).

   • CO2 then diffuses into alveoli for exhalation.

Oxygen-Hemoglobin Saturation

• Curve Influences: The shape of the curve illustrates how PO2 levels influence the loading and unloading of O2 by hemoglobin.

• Factors that promote O2 release include increased temperature, acidosis (increased H+), and the presence of 2,3-BPG.

Factors Influencing Hemoglobin Saturation

• Right Shift (decrease in oxygen affinity): Triggered by increased temperature, increased CO2 concentrations, and decreased pH (acidosis).

• Left Shift (increase in oxygen affinity): Caused by decreased temperature, decreased CO2 concentrations, and increased pH (alkalosis).

Homeostatic Imbalances

• Hyperventilation:

  • Characterized by rapid breathing that lowers CO2 levels (hypocapnia), leading to potential respiratory alkalosis.

  • Symptoms can include dizziness, tingling sensations, and in severe cases, loss of consciousness.

• Hypoventilation:

  • Inadequate airflow results in increased CO2 levels (hypercapnia), which may lead to respiratory acidosis.

  • Symptoms may involve lethargy, cyanosis (bluish skin), and headaches.

• Hyperpnea:

  • This refers to an increase in the depth of breathing while maintaining a normal breathing rate, commonly occurring during physical exercise.

• Apnea:

  • The cessation of breathing, which may occur when CO2 levels drop excessively.

  • Apnea Vera: A condition characterized by longer periods of apnea, typically observed in individuals with sleep disorders or respiratory issues.