Gas Exchange and Fick's Law of Diffusion

• Gas exchange between lungs and tissues happens through simple diffusion.
• The partial pressure of oxygen (PO2) decreases gradually from 159 mm Hg when inspired to 40 mm Hg or less in the capillaries.
• Intracellular PO2 (around 5 mm Hg) drives O2 diffusion into the cell.
• Normal values for PaCO2 are 35 to 45 mm Hg while healthy is 40 mm Hg.
• If PaCO2 increases then Va/Ve decreases.

Fick's Law of Diffusion

• Diffusion increases with greater surface area, diffusion constant, and pressure gradient.

Alveolar-Capillary (A/C) Membrane

• The A/C membrane has three barriers:
  • Alveolar epithelium.
  • Interstitial space and structures.
  • Capillary endothelium.

Mechanism of Diffusion

• Gas molecules move from high to low partial pressure areas.
• O2 and CO2 must pass through several barriers to diffuse into and out of the lungs and tissues.

Barriers to Gaseous Diffusion

• The alveolar-capillary membrane is the major barrier.
• CO2 and O2 must pass through these three barriers:
  • Alveolar epithelium.
  • Interstitial space.
  • Capillary endothelium.
• Gases must also pass through the red blood cell membrane.

Time Limits to Diffusion

• Blood must stay in contact long enough with the alveolus for proper oxygenation.
• Normal red blood cell transit time in the pulmonary capillary is about 0.75 seconds.
• Blood PO2 reaches equilibrium with alveolar PO2 before the capillary end.
• During exercise transit time is .25 seconds.

Anatomic Shunts

• A shunt returns cardiac output to the left heart without oxygenation from ventilated alveoli.
• Two types of right-to-left anatomic shunts:
  • Bronchial venous drainage.
  • Thebesian venous drainage.
• Normal A-a gradient is 5-10 mmHg, where A = alveolar and a = arterial.

V/Q Imbalance

• V/Q imbalance is the most common cause of hypoxemia in respiratory diseases
• Shunting is also a common issue, especially in critically ill patients
• If FiO2 is greater than 50% and PaO2 is less than 50 mm Hg, then significant shunting is present.
• Otherwise, the hypoxemia is mainly due to a V/Q imbalance.

Ventilation/Perfusion Ratio Alterations

• In healthy lungs, regional ventilation differences match perfusion differences, maintaining a V/Q ratio close to 1.
• Patients with ARDS have gas exchange abnormalities from fluid in the alveoli.
  • Requires ventilation with high FiO2 and PEEP.

Oxygen Transport

• Blood carries O2 in two forms:
  • A small amount dissolves in plasma and erythrocyte fluid.
  • Most O2 binds to hemoglobin (Hb) inside red blood cells.
• 1 g of normal Hb can carry about 1.34 mL of O2.
• The average blood Hb content of 15 g/dL determines the blood's O2-carrying capacity.
• The oxyhemoglobin dissociation curve indicates the likely hood for oxygen to offload at the lungs/tissues.

Hemoglobin

• Binds with iron.
• When 02 binds with Hb it changes its shape so it will reflect or absorb light differently which means:
  • Bright red is arterial.
• Spectrophotometry (oximetry) measures the amount of Hb saturated.
• If 10 Hb molecules all carry four O2 molecules, the saturation is 100%.
• Normal Hb value is 12-16.

Hemoglobin Saturation

• Saturation measures the proportion of available Hb carrying oxygen.
• SaO2 = (HbO2/total Hb) x 100.

CaO2 Calculation

• CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003).
• Normal CaO2 value is 17-20%.

Explore gas exchange mechanisms, Fick's Law, and the alveolar-capillary membrane. Learn how oxygen and carbon dioxide diffuse between the lungs, tissues, and cells. Understand the impact of partial pressure and membrane barriers on diffusion efficiency.