Respiratory muscles
Gas exchange
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
- Bronchi: all
- Bronchioles
- No cartilage
- Pseudostratified epithelium → Simple epithelium
- – Globlet cells and basal cells
- + Club cells
- Terminal bronchioles
- Columnar ciliated cells → Cuboidal ciliated cells
- Respiratory bronchioles
- + Squamous cells
- 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.