Overview
The lecture explains the journey of an oxygen molecule from inhalation to its absorption into red blood cells, focusing on the structural barriers in the lung and equations governing oxygen diffusion.
Oxygen Pathway from Air to Blood
- Oxygen enters through the mouth or nose, travels down the trachea, and enters the left or right lung.
- Each lung contains millions of alveoli, where gas exchange occurs.
- Oxygen passes through several layers: alveolar fluid, epithelial cells, basement membrane, connective tissue, more basement membrane, endothelial cells, plasma, and finally enters the red blood cell.
Alveolar and Capillary Interface
- Gas exchange occurs between the alveolus (air sac) and the capillary (blood vessel).
- Alveoli are lined with a thin fluid layer, followed by epithelial cells and multiple structural membranes.
- Oxygen must pass through both cellular and extracellular (liquid) barriers to reach the blood.
Oxygen Binding and Transport
- Oxygen enters red blood cells and binds to hemoglobin, which can carry four oxygen molecules.
- Hemoglobin-bound oxygen is then transported throughout the body.
Gas Laws and Diffusion Equations
- The alveolar gas equation calculates the partial pressure of oxygen (PO2) in the alveolus.
- Fick's law describes how oxygen diffuses across the alveolar-capillary barrier, depending on gradient, area, diffusion coefficient, and thickness.
Clinical Considerations for Oxygen Diffusion
- Factors affecting oxygen diffusion: FiO2 (inspired oxygen concentration), altitude, alveolar surface area, and barrier thickness.
- Reduced working alveoli decrease surface area for gas exchange.
- Increased fluid in the lung layers increases thickness, reducing diffusion efficiency.
- Diffusion coefficient and returning blood PO2 are typically stable and not major sources of variation.
Key Terms & Definitions
- Alveolus — tiny air sac in the lung where gas exchange occurs.
- Hemoglobin — red blood cell protein that binds and carries oxygen.
- Partial Pressure (PO2) — pressure exerted by oxygen in a mixture of gases or liquid.
- Fick's Law — formula relating diffusion rate to surface area, concentration gradient, and membrane thickness.
- FiO2 — fraction of inspired oxygen, i.e., the percentage of oxygen in inhaled air.
Action Items / Next Steps
- Review the alveolar gas equation and Fick's law formulas.
- Understand factors that can alter oxygen diffusion and how to use equations to identify underlying causes.