Hello burna:
Is Gas exchange at the Lungs Really a Silly Question?
Well, judging from the number of times this question has been asked, it cannot be concluded that it is silly. It certainly is not unique.
We are fortunate that exchange in the lungs is rapid. We looked at this during altitude decompressions at NASA and found that, even when the venous system has many gas bubbles [Spencer-Johanson Grade IV], gas exchange was not measurably hindered. [Norfleet W, M Davis, and MR Powell. Absence of pulmonary dead space changes during hypobaric decompressions with high Spencer precordial Doppler grades. Undersea Biomed. Res. (Suppl.) 1992]
It is a very rare incident when a diver experiences shortness of breath during a decompression and a bad sign too, I might add. When the lungs are blocked by bubbles, a problem known as the chokes arises. [It is believed than bubbles are the cause although not proved.] Cigarette smoke makes a person cough uncontrollably, even in smokers. This is called Behnkes Sign.
Are Tissue Halftimes Real?
They certainly are, but possible not for the reasons given in diving. All molecules enter and leave tissues of the body at varying rates. These rates are measured by pharmacologists and are of value in designing medicines for a given purpose.
A problem arises in diving since different halftimes are ascribed to different blood flows in the tissues. As pointed out decades ago, a short deep dive [fast tissue] with inadequate decompression will give you DCS pain, e.g., in the knee. Likewise, a moderately long dive would give you pain in the knees. A saturation dive that loads only very long halftimes can again result in pain in the knee. The different halftime tissues cannot be in the same place.
We have a situation where the bookkeeping algorithm has difficulty translating into physical reality. Halftimes are of value in the decompression algorithm, but less so in explanations of decompression Pathophysiology. Followers of Ask Dr Deco over the past decade will note that an explanation such as, we can ascribe the problem to exceeding the limit in the 40-minute tissue is never given. The explanations are given in terms of gas phase dynamics.
Respiration
Your original question asked, Does respiration rate effect off-gassing? Actually it does. However, the physiological definition of respiration concerns the lungs and the circulatory system. The biochemical definition is one of cellular respiration oxygen utilization by the living cell. The latter will affect the flow of blood to the tissue.
We wish to minimize blood flow during the tissue nitrogen uptake portion of a dive. [Hibernation is good, but maybe not practical.] We wish to maximize the nitrogen elimination during the decompression part of the dive. Running very fast would do this, but it would generate too many tissue gas nuclei and leave us in a very bad position.
So, different blood flows give different halftimes and this leads to table calculations using different, multiple halftimes.
Dr Deco :doctor: