📌 Professor’s Notes: Quick Revision
High-yield revision notes for rapid review before university examinations.
- Alveolar ventilation is the volume of fresh air reaching the alveoli each minute for gas exchange. Efficient oxygen uptake and carbon dioxide elimination depend on adequate alveolar ventilation, an intact alveolar-capillary membrane, sufficient pulmonary blood flow, and a favorable diffusion gradient.
- Minute ventilation is the total volume of air inspired or expired per minute and is calculated as:
- Minute ventilation = Tidal volume × Respiratory rate
- Normal value: 5–10 L/min (approximately 6 L/min at rest with a tidal volume of 500 mL and respiratory rate of 12/min).
- Dead space is the portion of the respiratory tract where ventilation occurs without gas exchange.
- Anatomical dead space: Conducting airways only; normal value 150 mL (≈2 mL/kg).
- Alveolar dead space: Ventilated alveoli with little or no perfusion.
- Physiological dead space = Anatomical dead space + Alveolar dead space. In healthy individuals, physiological dead space is almost equal to anatomical dead space. Increased dead space reduces gas exchange efficiency.
- Measurement of dead space:
- Radford formula estimates anatomical dead space.
- Single-breath carbon dioxide method (Bohr equation) measures physiological dead space.
- Single-breath nitrogen washout test measures anatomical dead space and ventilation distribution.
- Alveolar ventilation is calculated as:
- (Tidal volume − Dead space) × Respiratory rate
- Normal value: 4.2 L/min (500 − 150 mL × 12/min).
- Deep, slow breathing improves alveolar ventilation more effectively than rapid, shallow breathing. Increased dead space or shallow breathing causes hypoxemia and hypercapnia. Pranayama improves alveolar ventilation by increasing tidal volume.
- Hypoventilation causes reduced alveolar ventilation, leading to hypoxemia and hypercapnia. Common causes include chronic obstructive pulmonary disease, severe asthma, respiratory center depression, and neuromuscular disorders. Hyperventilation exceeds metabolic demand and lowers arterial carbon dioxide, producing respiratory alkalosis.
- Gas exchange occurs by diffusion across the alveolar-capillary membrane, which consists of alveolar epithelium, basement membranes, interstitium, capillary endothelium, plasma, erythrocyte membrane, and hemoglobin. The membrane is only 0.2–0.5 μm thick, allowing rapid diffusion.
- Factors affecting diffusion include:
- Partial pressure gradient (most important)
- Surface area available for diffusion
- Membrane thickness
- Gas solubility (carbon dioxide diffuses about 20 times faster than oxygen)
- Molecular weight (Graham’s law) and Fick’s law. Thickened membranes in pulmonary fibrosis or edema impair oxygen diffusion.
- Pulmonary capillary blood flow also influences gas transfer. Oxygen transfer is normally flow-limited, whereas carbon monoxide transfer is diffusion-limited and is therefore used to measure diffusing capacity. Diffusing capacity reflects the efficiency of gas transfer and decreases in diseases affecting the alveolar-capillary membrane or with reduced hemoglobin concentration.
📝 Practice MCQs
🎙️ Frequently Asked Viva Questions
High-yield viva questions with concise answers for rapid university examination preparation.
Q1. Why is alveolar ventilation more important than minute ventilation?
Answer: Because only alveolar ventilation delivers fresh air to the alveoli for oxygen uptake and carbon dioxide removal. Dead space ventilation does not participate in gas exchange.
Q2. What is physiological dead space, and how does it differ from anatomical dead space?
Answer: Physiological dead space equals anatomical dead space plus alveolar dead space. In healthy lungs they are nearly equal, but physiological dead space increases in diseases with poor pulmonary perfusion.
Q3. Why does an increase in dead space reduce gas exchange?
Answer: More inspired air remains in non-exchanging airways, so less fresh air reaches the alveoli, reducing oxygen uptake and carbon dioxide elimination.
Q4. Which method measures physiological dead space?
Answer: Bohr’s single-breath carbon dioxide method measures physiological dead space using the difference between arterial and mixed expired carbon dioxide.
Q5. Why is rapid, shallow breathing inefficient?
Answer: A larger proportion of each breath remains in dead space, so alveolar ventilation falls despite normal minute ventilation, causing hypoxemia and hypercapnia.
Q6. Why is deep, slow breathing more effective than rapid breathing?
Answer: It increases tidal volume, allowing more fresh air to reach the alveoli while reducing the relative effect of dead space ventilation.
Q7. Why does pranayama improve alveolar ventilation?
Answer: It increases tidal volume and decreases the proportion of each breath occupied by dead space, improving pulmonary ventilation and gas exchange.
Q8. How is alveolar ventilation related to arterial PCO₂?
Answer: They are inversely related. Reduced alveolar ventilation raises arterial PCO₂, while increased alveolar ventilation lowers arterial PCO₂.
Q9. Why does gas exchange occur by diffusion instead of bulk flow?
Answer: Air reaches alveoli by bulk flow, but oxygen and carbon dioxide cross the alveolar-capillary membrane by diffusion along their partial pressure gradients.
Q10. Why is the alveolar-capillary membrane ideal for gas exchange?
Answer: It is extremely thin (about 0.2–0.5 μm), providing a very short diffusion distance for rapid oxygen and carbon dioxide exchange.
Q11. Which factors most influence diffusion of gases across the alveolar-capillary membrane?
Answer: Partial pressure gradient, membrane surface area, membrane thickness, gas solubility, diffusion coefficient, and pulmonary capillary blood flow.
Q12. Why does carbon dioxide diffuse faster than oxygen?
Answer: Carbon dioxide is about 20 times more soluble than oxygen in biological fluids, so it diffuses much more rapidly despite a smaller pressure gradient.
Q13. Why does pulmonary fibrosis mainly cause hypoxemia first?
Answer: Fibrosis thickens the respiratory membrane, impairing oxygen diffusion. Carbon dioxide diffusion is preserved initially because it is much more soluble.
Q14. Differentiate flow-limited and diffusion-limited gas transfer with one example each.
Answer: Flow-limited: uptake depends on blood flow (nitrous oxide). Diffusion-limited: uptake depends on membrane diffusion (carbon monoxide).
Q15. Why is carbon monoxide used to measure lung diffusing capacity?
Answer: It binds strongly to hemoglobin, keeping its blood partial pressure very low. Therefore, its uptake depends almost entirely on diffusion across the alveolar-capillary membrane.
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