Respiratory muscles

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Gas exchange

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Types of gas exchange

• Perfusion-limited gas exchange: Gas exchange is limited by the rate of blood flow through the pulmonary capillaries.
  • Gases (e.g., O2, CO2, N2O) can diffuse freely across the blood-air barrier.
  • The concentration of gases in the plasma will become equal to the concentration in the alveoli before the blood reaches the end of the capillary.
  • An increase in blood flow causes an increase in gas exchange .
  • Occurrence: under normal conditions (i.e., at rest)
• Diffusion-limited gas exchange: Gas exchange is limited by the diffusion rate of the gas (e.g., O2, CO) across the blood-air barrier.
  • The gas concentration in the plasma will not be in equilibrium at the end of the capillary.
  • Occurrence
    • Strenuous exercise
    • In certain pathological conditions that affect the blood-air barrier (e.g., emphysema, lung fibrosis)

Tip

Under normal physiological conditions, O2 transport is generally perfusion-limited, while it can become diffusion-limited in pathological states or during intense exercise.

Anatomy

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1. Bronchi: all
2. Bronchioles
   • No cartilage
   • Pseudostratified epithelium → Simple epithelium
     • – Globlet cells and basal cells
     • + Club cells
3. Terminal bronchioles
   • Columnar ciliated cells → Cuboidal ciliated cells
4. Respiratory bronchioles
   • + Squamous cells
5. Alveolar sacs
   • All gone

Pneumocytes

• Type I pneumocytes: thin squamous cells that line the alveoli that allow for transcellular gas exchange
  • Comprise 95% of the total alveolar area
  • Connected to each other by tight junctions
  • Form the blood-air barrier, together with the endothelial cells of the capillaries and the basement membrane between the two cells.
• Type II pneumocytes: cuboidal alveolar cells
  • Comprise 5% of the total alveolar area, but 60% of total number of cells
  • Surfactant formation: Type II pneumocytes contain lamellar bodies, which secrete surfactant (surface-activating lipoprotein complex).
    • Surfactant is mainly composed of the phospholipids dipalmitoylphosphatidylcholine (DPPC or lecithin) and phosphatidylglycerol.
    • It reduces alveolar surface tension and thereby prevents the alveoli from collapsing.
  • Lung regeneration and repair: Type II pneumocytes can proliferate to replace type I or type II pneumocytes following lung damage.