Here's what you need to know about the respiratory membrane

Once in the lungs, various elements effects oxygen's ability to cross the alveolar-capillary membrane, which is also known as the respiratory membrane. In my post "Diffusion of oxygen from air to tissues" I described how oxygen travels from the lungs to the tissues.

The ability of oxygen to cross the alveolar-capillary (respiratory) membrane depends on.

1. Rate of diffusion of oxygen across the respiratory membrane.  
2. Pulmonary Capillary Blood Volume or Flow 
3. Transit time
4. The ability of O2 to bind with Hemoglobin (Hgb)

1.  The rate of diffusion across the respiratory membrane is determined by.

• Thickness of the respiratory membrane (disease processes may damage it).  
• Normal thickness: 0.4-0.6 micrograms (sometimes as low as 0-.2 micrograms)
• Pulmonary edema, fibrosis, deposition of substances, may increase thickness
• Surface area of the respiratory membrane 
• Total capillary-alveolar surface area in normal, healthy person is 70 square meters.  
• There are over 300 million alveoli
• There is usually about 60-140 ml of blood in pulmonary capillaries
• This allows for plenty of surface area for oxygen to diffuse from alveoli to capillaries.
• Diseases like emphysema greatly reduce surface area for gas exchange to occur.
• The diffusion coefficient of gases (oxygen)
• This is essentially how soluble is the gas in water, as it has to go from alveolar air to capillary blood, a solution. 
• Solubility coefficient = concentration of dissolved gas + partial pressure (no need to memorize this)
• CO2 is 20 times more soluble than Oxygen, so CO2 diffuses 20 times more rapidly across the respiratory membrane as oxygen does.  
• The difference between alveoli (PAO2) and capillary (PcO2).  PAO2 is 104, and venous blood is 40.  This results in a pressure gradient of 64 mm Hg. 

2.  Pulmonary Capillary Blood Volume or Flow is determined by.

• Capacity of blood, especially red blood cells
• Polycythemia: Oxygen diffuses at a higher rate
• Anemia: Oxygen diffuses at a lower rate

3.  Transit Time is determined by. 

• Determined by dividing pulmonary capillary blood volume by cardiac output.  
• Normal Transit Time: 70 ml divided by 5,000 = 0.8 seconds
• This is the time available for diffusion to occur. 
• Most diffusion occurs in first 0.3 seconds of Transit Time
• This leaves 0.5 seconds, providing a large safety margin.  This explains why adequate oxygenation can still occur when a person is exercising, even thought the transit time is reduced to 2/3 of normal, or 0.1 seconds. 

4.  Capacity of binding of Oxygen with Hemoglobin is determined by.

• This is a discussion for another day, and we will not go there.

Normal Arterial Oxygen Tension can be calculated. 

• A-a Gradient:  PAO2 - PaO2
• 104-97 = 7 mm Hg
• Normal is below 15 mm Hg
• Normal range is 5-25
• Upper range may increase with age to 20 or even 30
• The following formula allows you to determine A-a Gradient adjusted for age.
• PaO2 = 102 - Age/3
• Depends on.
• Ventilation (V) 
• Perfusion (Q) 
• Shunt (Mixed venous blood)
• Is the main cause of hypoxemia (drop in PaO2) and hypercapnea (increase in PaCO2)
• V/Q Mismatching:  Oxygen is inhaled but cannot get to the blood in certain areas of the lungs.  
• Shunt: Blood doesn't come into contact with alveoli, blood is shunted away from alveoli.  No gas exchange occurs.  
• True Shunt (anatomical). Natural shunts that purposefully bypass the lungs, such as the shunt noted above whereby unoxygenated blood from bronchial veins is shunted to the pulmonary artery. 
• False Shunt (physiological).  This is where blood is supposed to come into contact with an alveoli, but this cannot happen due to a disease process.
• The best indicator of a shunt is PaO2.  This is because a small reduction in O2 results in a large reduction in PaO2 (about 7 mm Hg).  A small increase in CO2 results in a small increase in PaCO2 (less than 1 mm Hg)
• Up to the presence of a 50% shunt, Increases in FiO2 will have no effect on PaO2