Lecture 16 Flashcards
During quiet breathing, why does pleural pressure become more negative during inspiration?
The chest expands, pulling the lungs outward. Since the lungs are adhered to the chest wall via pleural fluid, this creates a greater vacuum, decreasing pleural pressure.
What is transmural pressure (TPP), and why is it highest at the end of inspiration?
TPP = Alveolar pressure (PA) – Intrapleural pressure (Ppl). At end-inspiration, alveoli are fully expanded, and Ppl is most negative, making TPP highest.
How does increasing tidal volume (TV) improve alveolar ventilation more effectively than increasing respiratory rate (RR)?
Increasing TV is more efficient because it reduces the proportion of air lost in dead space, ensuring more fresh air reaches the alveoli per breath.
According to Fick’s Law, what factors influence the diffusion of gases across the alveolar membrane?
Gas diffusion depends on:
Surface area (A) (↑ A = ↑ diffusion)
Membrane thickness (d) (↑ d = ↓ diffusion)
Partial pressure difference (ΔP) (↑ ΔP = ↑ diffusion)
Gas solubility and molecular weight (MW)
Why is oxygen diffusion efficiency highest at end-inspiration?
At end-inspiration, alveoli are fully expanded, increasing surface area and reducing membrane thickness, improving diffusion efficiency.
How do alveolar size and membrane thickness affect diffusion across the lungs?
Larger alveoli → More surface area → Better diffusion
Thicker alveolar membranes (e.g., fibrosis) → Slower diffusion
Define anatomical dead space and physiological dead space. How do they differ?
Anatomical dead space: Air in conducting airways (trachea, bronchi) that does not reach alveoli.
Physiological dead space: Anatomical dead space + alveoli that are ventilated but not perfused (e.g., pulmonary embolism).
If a person breathes in 500 mL of air, why does only 350 mL reach the alveoli for gas exchange?
The first 150 mL of each breath remains in the conducting airways (dead space) and does not reach alveoli.
What is the equation for alveolar ventilation (AV), and why is it more efficient to increase TV rather than RR?
AV = (TV – DS) × RR. Increasing TV is more efficient because deeper breaths reduce the proportion of air lost in dead space.
How does Fowler’s N₂ washout method estimate anatomical dead space?
The subject inhales 100% O₂, and exhaled nitrogen (N₂) concentration is analyzed. A sharp transition in N₂ concentration indicates anatomical dead space volume.
How does Bohr’s expired CO₂ method estimate physiological dead space?
Measures exhaled CO₂ concentration. If some alveoli are not perfused, exhaled CO₂ will be lower than expected, indicating physiological dead space.
Why does a pulmonary embolism cause a discrepancy between the Fowler and Bohr methods?
A pulmonary embolism blocks perfusion to some alveoli:
Fowler’s method (N₂ washout) shows normal anatomical dead space.
Bohr’s method shows increased physiological dead space (because less CO₂ is exhaled).
AKA Bohr would detect but fowler would not
What does it mean when oxygen transport is perfusion-limited under normal conditions?
O₂ uptake is dependent on blood flow, not diffusion rate. The blood gets fully oxygenated before leaving the pulmonary capillary.
Using the train analogy, explain the difference between perfusion-limited and diffusion-limited gas exchange.
Perfusion-limited: The train (blood flow) moves at normal speed, and all boxes (O₂) are loaded.
Diffusion-limited: Workers (diffusion) are slow, so not all boxes (O₂) are loaded before the train leaves.
In what conditions does oxygen transport become diffusion-limited instead of perfusion-limited?
Pulmonary fibrosis (thicker alveolar membrane).
High altitude (lower PAO₂ reduces O₂ diffusion).
Exercise (faster blood flow reduces transit time).
How does pulmonary vascular resistance (PVR) change with lung volume?
Lowest at functional residual capacity (FRC). Increases when lung volume is too high (alveolar vessels compressed) or too low (extra-alveolar vessels collapse).
What happens to extra-alveolar and alveolar vessels during inspiration and expiration?
Inspiration → Extra-alveolar vessels expand (↓ resistance), alveolar vessels get crushed (↑ resistance).
Expiration → Extra-alveolar vessels collapse (↑ resistance), alveolar vessels open (↓ resistance).
Why does exercise lower pulmonary resistance?
Pulmonary artery pressure (PAP) increases, leading to capillary recruitment and distension, reducing overall resistance.
What is an ideal V/Q ratio, and why is it important?
V/Q = 1 (ventilation matches perfusion), ensuring optimal gas exchange.
What happens to ventilation and perfusion in a pulmonary embolism vs. an airway obstruction?
Pulmonary embolism (high V/Q): Ventilation is normal, but perfusion is reduced.
Airway obstruction (low V/Q): Perfusion is normal, but ventilation is reduced.
Describe the gravity effect on pulmonary perfusion and the differences in Zones 1, 2, and 3 of the lung.
Zone 1 (Apex): PA > Ppa > Ppv → Capillaries collapse (dead space).
Zone 2 (Middle): Ppa > PA > Ppv → Some capillaries open.
Zone 3 (Base): Ppa > Ppv > PA → Full perfusion, best gas exchange.
How does hypoxic pulmonary vasoconstriction help improve V/Q matching?
Low O₂ levels cause pulmonary vasoconstriction, redirecting blood to well-ventilated alveoli.
How is the respiratory quotient (RQ) calculated, and what does it represent?
RQ = VCO₂ / VO₂. It represents the ratio of CO₂ produced to O₂ consumed, depending on the fuel source.
Why does a high-fat diet lead to a lower RQ compared to a high-carb diet?
Fats require more O₂ for metabolism, leading to RQ = 0.7, while carbs require less O₂, leading to RQ = 1.0.
