Nitrogen circulation and desorbtion

[DrSteve]

This question was raised at a recent DM class on diving physics:
We know that nitrogen dissolves in the blood and body tissues. We know that nitrogen gets exhaled after gas exchange in the lungs due to changes in the partial pressure of N2. How does the dissolved gas get transported to the lungs? Is it simply a matter of time where eventually all the absorbed gas will pass through the lungs?

I recall that CO2 gets converted into carbonate species for transport purposes. Is there a similar mechanism for N2 transport? Come to think of it, what advantage is there to converting the CO2 to carbonate to be transported to the lungs and exhaled as CO2? Why can't the solvated gas just float through the blood like N2 and be desorbed that way?

Thanks!

 

[TSandM]

Nitrogen is carried simply as dissolved gas. It has a fair lipid solubility, and diffuses across cell membranes fairly readily. Whether it is going into or out of any tissue of fluid is simply determined by gradients. So when you descend, and the partial pressure of nitrogen in the gas you're breathing is higher than that in the blood and tissues, the diffusion is into the body. As you ascend, the pressure in the alveoli (air sacs) drops, gas diffuses out of the blood and into the alveoli, and the blood then circulates to the tissues and the nitrogen in the tissues diffuses into the blood. It's all passive. All blood passes through the lungs on each circuit of the body, so the nitrogen from all tissues is brought to the lungs for exchange.

CO2 reacts with water to form carbonic acid. This reaction occurs whenever the two are in contact, but is markedly accelerated within the body by the enzyme, carbonic anhydrase. The advantage to having the CO2 in the form of bicarbonate ion is that bicarbonate is the major component of the body's critical buffer system which keeps the blood pH constant.

 

[Dr Deco]

Hello Dr Steve:

Carbon dioxide is transported as a compound because it reacts with water. This is the same as reactions between water and, for example, the gasses hydrogen chloride or ammonia.

Nitrogen does not form such compounds; thus, it is simply surrounded by water molecules.

Buffering is also and important function of carbon dioxide as mentioned by TS and M.

Dr Deco :doctor:

The next class in Decompression Physiology for 2006 is September 16 – 17. :1book: http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[ae3753]

The next class in Decompression Physiology for 2006 is September 16 – 17. :1book: http://wrigley.usc.edu/hyperbaric/advdeco.htm

Woah, that's cool. If I wasn't head to Mexico, I'd be there.

 

[DrSteve]

Thank you for the input guys. The carbonic anhydrase explains a few things to me as well.