Gas Exchange and Diffusion in Lungs

Gas Exchange in the Lungs

• Applies the physical characteristics of diffusion of gases and gas pressures to the exchange of gases in the lungs.
• Explains the diffusion of gases through the respiratory membrane, analyzing influencing factors.
• Argues the importance of ventilation/perfusion ratio, evaluating the effects of different conditions.

Molecular Basis of Gas Diffusion

• Net gas diffusion occurs from high to low concentration areas.
• Gas pressure is directly proportional to gas molecule concentration.
• The most important gases in respiration are O2, N2, and CO2.
• The diffusion rate of each gas is directly proportional to its partial pressure.
• Air contains 79% N2 and 21% O2, with a total sea level pressure of 760 mm Hg.
• Nitrogen's partial pressure is 600 mm Hg, while oxygen's is 160 mm Hg.
• Symbolically represented as PO2, PCO2, and PN2.

Partial Pressure

• Represent the pressure exerted by a specific gas alone.
• It is determined by dissolved gas concentration and solubility coefficient (Henry's law).
• Formula: Partial pressure of a gas = concentration of released gas/solubility coefficient
• CO2 solubility is 20x more than O2, PCO2 is less than 1/20 of PO2.
• The solubility coefficient of CO2 is 0.57, and O2 is 0.024.
• Partial pressure of each gas in the alveolar respiratory gas mixture forces gas molecules into alveolar capillaries.

Partial Pressures of Respiratory Gases (mm Hg)

• Partial pressures of respiratory gases entering and leaving the lungs at sea level are as follows:

Vapor Pressure of Water

• Water evaporates when non-humidified air is inhaled into the respiratory tract.
• PH2O is the partial pressure exerted by water molecules escaping the water's surface.
• At a normal body temperature, PH2O = 47 mm Hg.
• PH20 depends only on water temperature.
• Higher temperatures increase the kinetic activity of molecules, increasing PH2O.

Factors Influencing Gas Diffusion in Liquid

• Formula: D ∝ (ΔP × A × S) / (d × √MW)
  • D: Diffusion rate
  • P: Pressure
  • A: Cross-section of liquid area
  • S: Gas solubility
  • d: Distance over which gas diffuses
  • M: Molecular mass of the gas
  • W: Liquid temperature

Composition of Air

• Alveolar and atmospheric air compositions differ.
• Alveolar air is only partially displaced by atmospheric air with each breath.
• Atmospheric air is humidified before reaching the alveoli.
• O2 diffuses to the pulmonary blood and CO2 diffuses to the alveoli.

Humidified Air

• Atmospheric air enters the respiratory tract, exposed to fluids covering respiratory surfaces.
• Alveolar air is slowly replaced, preventing sudden changes in blood gas concentrations.
• It stabilizes the respiratory control mechanism.
• It prevents excessive increases/decreases in tissue oxygenation, CO2 concentration, and pH during short respiration periods.

Effect of Alveolar Ventilation and O2 Uptake on Alveolar PO2

• O2 is continuously absorbed into the blood, and fresh O2 is released from the atmosphere upon inspiration.
• Alveolar O2 concentration and PO2 is controlled by:
  • The rate of O2 absorption into the blood.
  • The rate at which fresh O2 enters the lungs.
• Normal operation occurs at 4.2 L/min ventilation and 250 ml/min O2 consumption.
• At a normal alveolar ventilation rate of 4.2 L/min and O2 consumption of 250 ml / min, is the normal operating point at point A.
• Increasing alveolar ventilation 4x to normal only maintains alveolar PO2 at 104 mm Hg when 1000 mL of O2 is absorbed per minute
• Increased alveolar ventilation cannot raise alveolar PO2 above 149 mmHg with normal atmospheric air at sea level (maximum PO2 in humidified air)

Effect of Alveolar Ventilation and CO2 Excretion on Alveolar PCO2

• CO2 is continuously produced and transported by the blood to the alveoli and is removed via ventilation.

Composition of Exhaled Air

• Depends on dead space air and alveolar air amounts.
• Dead space air from the respiratory tract is humidified.
• Alveolar air mixes with dead space air until only alveolar air is exhaled.

Structure of the Respiratory Unit & Membrane

• Respiratory unit:
  • Includes the respiratory bronchiole, alveolar ducts, atria, and alveoli.
  • Lungs contain ~300 million alveoli, each 0.2 mm in diameter.
  • There is an almost solid capillary network between the thin alveolar walls
  • The gas finds exchange between alveolar air and pulmonary blood through the membranes of all terminal parts of the lungs, not just in the alveoli.
  • All membranes are known as the respiratory or pulmonary membranes.
• Respiratory membrane:
  • It has several layers
    • Liquid layer with surfactant to align alveoli and reduce surface tension.
    • Alveolar epithelium of thin epithelial cells.
    • Epithelial basement membrane.
    • Thin interstitial space between alveolar epithelium and capillary membrane.
    • Capillary basal membrane uniting with alveolar epithelial basement membrane in many places.
    • Capillary endothelial membrane.
  • About 0.2 µm thick in certain areas.
  • About 0.6 µm where nuclei are present.

Factors Affecting Diffusion Rate through the Respiratory Membrane

• Thickness.
• Surface area.
• Diffusion coefficient of the gas.
• Partial pressure difference.

Diffusion Capacity

• The volume of gas diffusing through the respiratory membrane per minute for each 1 mm Hg partial pressure difference.
• O2 diffusion capacity is 21 ml/min/mm Hg at rest; it increases to 65 ml/min/mm Hg (in men) during exercise.
• Blood oxygenation is not limited to exercise performed by elevated alveolar ventilation increases, but facilitated by increased respiratory membrane diffusion capacity.
• CO2 diffusion capacity isn't measured because CO2 diffuses so quickly through the membrane, but PCO2 in pulmonary blood isnt differnt from the PCO2 in the alveoli at 400-450 ml / min / mm Hg at rest and 1200-1300 ml / min / mm Hg during exercise

Ventilation-Perfusion Ratio (VA/Q)

• Represents the relationship between alveolar ventilation and perfusion of alveolar capillaries.
• VA/Q → 0:
  • No alveolar ventilation but normal blood flow.
  • Partial gas pressures in alveoli equal those in the blood.
  • PO2 = 40mmHg and PCO2 = 45mmHg.
  • Physiological "shunt" (blood diversion).
• VA/Q → ∞:
  • No blood flow, but ventilation is normal (no gas exchange).
  • Gas pressures in alveoli equal those in humidified air.
  • PO2 = 149mmHg and PCO2 = 0 mmHg.
  • Physiological dead space.
  • Formation of fibrous alveoli.
  • Extreme lung tissue destruction

Shunted Blood

• Venous blood passing pulmonary capillaries without oxygenation.

Physiological Diversion of Blood (Physiological Shunt)

• Defined as the total quantitative amount of "shunted" blood per minute.

Abnormalities of the Ventilation-Perfusion Ratio

• At the apex (top) of the lungs, the pulmonary capillaries increase blood flow, the VA/Q is 2.5 times greater than the normal value, thus creating physiological dead space.
• During exercise, increased blood flow to the apex reduces physiological dead space.
• At the base (lower end) of the lungs, alveolar ventilation decreases, with the VA/Q 6 times smaller than normal, causing physiological blood diversion.
• In chronic obstructive pulmonary disease:
  • Smoking causes bronchial obstruction (emphysema) = 'N VA/Q = 0
  • In damaged alveolar wall areas, a VA/Q =-

Effect of Atelectasis

• There is reduced blood flow through the atelactic lung.
• 5/6 of the blood is transported by the lung containing air and 1/6 through the ate lactic lung.
• The VA/Q is only moderately affected, because through the aortic blood only moderate, the loss of ventilation in one lung.

Effect of Various Conditions on Alveolar PO2 and PCO2

1. Height above sea level: lowered alveolar PO2 from decreased PO2 in inhaled air
2. Hyperventilation: alveolar PCO2 decreases via alveolar ventilation exceeding CO2 levels
3. low alveolar ventilation:
   • Alveolar PO2 decreases because less fresh air enters per unit time.
   • Alveolar PCO2 increases due to reduced CO2 exhalation.

Key questions answered

1. What is the basis for the diffusion of gases through the respiratory membrane?

• The diffusion of a gas occurs when the gas from an area with a high concentration moves to an area with a low concentration
• Gas pressure is directly proportional to the concentration of gas molecules
• The important gases in respiration are O2, N2, and CO2, and the diffusion of each gas is proportional to the pressure(partial)
• Air consists of 79% N2 and 21% O2 (total pressure at sea level = 760 mm Hg).
• The partial pressure of N2 is 600 mm Hg and partial pressure of O2 is 160 mm Hg, represented as: PO2, PCO2 and PN2

2. Why are the atmospheric air and alveolar air consistency different? |Gas|Atmospheric air|Alveolar pressure| |---|---|---| |N₂|597 (78.62%)|579 (74.9%)| |O₂|159 (20.84%)|104 (13.6%)| |CO₂|0.3 (0.04%)|40 (5.3%)| |H2O|3.7 (0.50%)|47 (6.2%)| |Total|760 (100%)|760 (100%)|

• O2 in the atmospheres needs to travel down the concentration so the body utilize O2 in the respiratory system
• N₂ gas has no physiological role other than to keep the alveoli open not fully collapse and is not utilized or produced by the cells.
• CO2 needs to diffuse out of the alveoli from the cells, gradient
• Alveoli is much higher in H2O

3. Is alveolar air replaced quickly or gradually?

• Alveolar air is replaced gradually not quickly to avoid sudden changes in gas concentrations in blood
• Prevent excessive decrease in carbon dioxide concentrations and increase in oxygen and changes in the pH

4. What effect do alveolar ventilation and the absorption of oxygen have?

• Controlled by rate of absorption
• controlled by Rate at which O2 moves into the lungs
• Proportional to thus, alveolar pressure increased, so does ventilation
• A large increase in alveolar ventilation can never the PO2 above 149