1. On which factors does adequate blood oxygenation depend on?

• Normal ventilation of the lung: this is determined by a normal respiratory drive and a functionally normal respiratory apparatus (includes the brain, chest wall, airways and lung parenchyma)

• Adequate diffusion of respiratory gasses across the alveolar wall

• Matching of ventilation and perfusion

• 2. By what process do the respiratory gases pass through the various anatomic barriers to pass into the blood?

• Through diffusion.

• 3. Which physical law determines diffusion across membranes?

• Fick's law of diffusion: This states that the amount of gas diffusing per unit time (i.e. the rate of diffusion) is inversely proportional to the thickness of the barrier and directly proportional to the surface area of the barrier.

• 4. What are the anatomic layers that respiratory gases have to pass through to reach the haemoglobin molecule in the red cells?

• The fluid lining the alveoli: gases initially dissolve in this before proceeding to the next layer

• Alveolar epithelium and through its basement membrane

• Interstitial space: which also contains fluid

• Basement membrane of capillary endothelium

• Capillary endothelium

• Plasma

• Red cell membrane

5. What is shunt?

This refers to venous blood that passes to the systemic circulation without first being oxygenated in the lungs.

6. Is this always pathological?

No, under normal circumstances, 1–2% of the CO bypasses the alveoli. This is called the anatomic shunt.

7. Where are the sites for normal anatomic shunt?

• The bronchial circulation

• Cardiac Thebesian veins: that drain coronary venous blood directly into the left side of the heart

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