The ability of oxygen to cross the alveolar-capillary (respiratory) membrane depends on.
- Rate of diffusion of oxygen across the respiratory membrane.
- Pulmonary Capillary Blood Volume or Flow
- Transit time
- 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