7th February 2024

Gas exchange

The partial pressure of gas

Dalton’s law of partial pressure states that the pressure exerted by an individual gas in a gas mixture equals the pressure the individual gas will exert alone.

Gas diffusion

Gas exchange is affected by a few factors,

• Gas
  • Solubility
  • Molecular mass
  • Concentration gradient
• Surface area
• Distance to travel during diffusion

Oxygen and carbon dioxide are exchanged between pulmonary circulation and alveoli.

Image Source: Wikimedia Commons (2018). Cross section of an alveolus and capillaries showing diffusion of gases, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/1/13/Cross_section_of_an_alveolus_and_capillaries_showing_diffusion_of_gases.svg  (Accessed: 4 January 2024).

Diffusing capacity or transfer factor of the lung 

The diffusing capacity of the lung for carbon monoxide (DLCO) is a measurement of the lung’s ability to transfer gases. It is alternatively called the transfer factor of the lung (TLCO) – and uses different units.

Carbon monoxide is the only gas used for this test as it is diffusion-limited. Here is why. Carbon monoxide has a 250 times greater affinity for Hb than oxygen. Hence, CO is rapidly bound to Hb, resulting in low plasma partial pressure of CO. There will still be a partial pressure difference between alveolar CO and plasma CO at the pulmonary end of capillary blood.

DLCO or TLCO is commonly done during pulmonary function tests. It is done by measuring the partial pressure of CO-inspired and expired. The difference between inspired and expired CO is the amount of CO that has diffused across the alveolar-capillary barrier.

Causes that decrease DLCO:

• Pulmonary fibrosis
• Pneumonia
• Pulmonary hypertension
• Chronic pulmonary emboli
• Emphysema
• Congestive cardiac failure

Causes that increase DLCO:

• Asthma
• Left-to-right shunts
• Pulmonary haemorrhage

Note: Haemoglobin count (anaemia/polycythaemia) affects DLCO. Hence, DLCO is adjusted for Hb values.

References

Diffusing Capacity. (n.d.). Retrieved Jun 7 2022, from https://fpnotebook.com/lung/lab/DfsngCpcty.htm.

Modi, P., & Cascella, M. (2022). Diffusing Capacity Of The Lungs For Carbon Monoxide. In StatPearls. StatPearls Publishing. http://www.ncbi.nlm.nih.gov/books/NBK556149/

Levitzky, M. G. (2018). Chapter 6: Diffusion of Gases and Interpretation of Pulmonary Function Tests. In Pulmonary physiology (9th ed). McGraw-Hill education.