3.1 Diffusion of Gases

Respiratory System Overview

• The respiratory system combines three main processes to provide oxygen to cells: respiration, circulation, and metabolism.

Gas Movement

• Gas movement occurs through two mechanisms: bulk flow and diffusion.
• Bulk flow:
  • Gas movement through the conducting airways results from differences in total pressure.
  • External convection (mass movement or convection) and internal convection also occur.
• Diffusion:
  • Gas movement in the respiratory airways occurs according to their own partial pressures.
  • Oxygen must dissolve in and diffuse through the alveolar-capillary barrier.

Henry's Law

• States that at a constant temperature, the amount of gas that dissolves in a solution is directly proportional to the partial pressure of that gas over the solution.
• Factors affecting solubility: temperature (lower temperature = increased solubility, higher temperature = decreased solubility) and solubility coefficient (O2 = 0.003 mL/dL/mmHg, CO2 = 0.06 mL/dL/mmHg).

Dissolved Oxygen

• Only the part of the gas that is dissolved contributes to the partial pressure of oxygen in blood.
• Gases diffuse according to their individual partial pressure gradients.

Fick's Law of Diffusion

• Describes the rate of diffusion of a gas through a tissue sheet.
• The rate of diffusion of a gas is directly proportional to:
  • The surface area available for diffusion (increased surface area increases the rate of diffusion)
  • The diffusion coefficient (solubility and molecular weight of the gas)
  • The partial pressure difference driving diffusion (higher gradient = faster rate of diffusion)
  • Inversely proportional to the thickness of the membrane (increased thickness decreases the rate of diffusion)

Limitations of Gas Transfer

• Important partial pressures of a gas when determining the rate of diffusion include: alveolar, mixed venous blood, and pulmonary capillary blood.
• Perfusion-limited transfer equilibrates, while diffusion-limited transfer does not.

Oxygen and CO2 Transfer

• Oxygen is normally a perfusion-limited gas, while CO2 is more soluble and does not have difficulty with diffusion.
• Factors that limit O2 diffusion: loss of surface area, reduced alveolar partial pressure, decreased time available for exchange, and increased thickness of the alveolar-capillary membrane.

Diffusing Capacity or Transfer Factor

• Measures the lungs' ability to exchange gas.
• The diffusing capacity of the lung is determined by both the diffusing capacity of the membrane and reaction with hemoglobin.

Oxygen Consumption

• At rest, humans consume about 250 mL O2/minute.
• Oxygen consumption is limited by maximal cardiac output, mitochondrial density, and respiratory limitations.

Altered Rates of Ventilation

• Hyperventilation: composition of alveolar air approaches that of tracheal or ambient air, resulting in increased PO2 and decreased PCO2 (hypocapnia).
• Hypoventilation: composition of arterial blood partial pressures change, resulting in decreased PO2 and increased PCO2 (hypercapnia).### Respiratory System Overview
• The respiratory system combines three main processes to provide oxygen to cells: respiration, circulation, and metabolism
• Respiration involves gas movement through bulk flow and diffusion
• Circulation involves the transport of oxygen and carbon dioxide through the bloodstream
• Metabolism involves the diffusion of oxygen from peripheral capillaries to mitochondria

Gas Movement

• Gas movement occurs through bulk flow and diffusion
• Bulk flow occurs due to differences in total pressure, resulting in the movement of gases together along the total pressure gradient
• Diffusion occurs due to differences in partial pressures of individual gases, with each gas moving according to its own partial pressure gradient

Henry's Law

• States that the amount of gas that dissolves in a solution is directly proportional to the partial pressure of that gas over the solution
• The higher the partial pressure of the gas, the more it will dissolve in the liquid
• Temperature affects solubility, with lower temperatures increasing solubility and higher temperatures decreasing solubility
• Each gas has its own solubility coefficient, with oxygen being less soluble in plasma than carbon dioxide

Oxygen Dissolution and Transport

• Only dissolved oxygen contributes to the partial pressure of oxygen in blood
• Oxygen diffuses from the alveoli into the bloodstream, where it binds to hemoglobin
• Oxygen is transported from the lungs to the tissues through the bloodstream
• At the tissues, oxygen diffuses from the capillaries into the mitochondria

Fick's Law of Diffusion

• Describes the rate of diffusion of a gas through a tissue sheet
• The rate of diffusion is directly proportional to the partial pressure gradient of the gas
• The rate of diffusion is also affected by the surface area and thickness of the tissue sheet

Limitations of Gas Transfer

• The rate of gas transfer across the alveolar-capillary unit is limited by the rate of pulmonary blood flow
• The amount of gas transferred across the alveolar-capillary unit is also limited by the diffusing capacity of the lung
• The diffusing capacity of the lung is determined by the ability of the gas to diffuse through the respiratory membrane and react with hemoglobin

Diffusing Capacity or Transfer Factor

• Measures the lung's ability to exchange gas
• Is determined by the diffusing capacity of the membrane and the reaction with hemoglobin
• Is affected by factors such as edema, pneumonia, and exercise

Oxygen Consumption

• At rest, humans consume about 250 ml O2/minute
• Oxygen consumption can increase 10-20 times during exercise
• Cardiac output and ventilation increase during exercise to meet the increased oxygen demand
• Oxygen consumption is limited by the maximal cardiac output, mitochondrial density, and respiratory limitations

Altered Rates of Ventilation

• Hyperventilation occurs when the rate of ventilation is increased, resulting in a decrease in PCO2 and an increase in PO2
• Hypoventilation occurs when the rate of ventilation is decreased, resulting in an increase in PCO2 and a decrease in PO2
• Both hyperventilation and hypoventilation can have significant effects on the body's pH and electrolyte balance

Pulmonary Function Tests

• Measure static lung volumes, dynamic lung volumes, and diffusing capacity
• Used to diagnose and evaluate the severity of pulmonary disorders
• Can be used to predict postoperative pulmonary complications
• Interpretation involves comparing recorded values to predicted values, with a value of +20% of predicted considered normal

Interpreting Pulmonary Function Tests

• Evaluate lung volumes and spirometry to determine whether the disorder is obstructive or restrictive
• Review lung volumes, such as FRC, TLC, and RV, to determine if hyperinflation or restriction is present
• Look at FEV1/FVC ratio and FEF25-75% to determine if obstruction is present
• Use the results to diagnose and evaluate the severity of pulmonary disorders