Yesterday's post was on the organs inside the mantle cavity of a snail, including its lung. A quick search today of the net netted a useful open-access paper by John Maina on comparative respiratory morphology, that is, gills and lungs and what have you (Anatomical Record 261:25, 2000 pdf). Most of the information in this paper is also available in older publications, but it's good to have it all in one place in an authoritative review. This way one can cite one recent paper instead of 5 older ones.
No tissues or cells specialized in extracting oxygen from water or air have evolved. In other words, unlike, say liver cells or nerve cells, there are no specialized "lung cells" or "gill cells". This is probably because there is no need for such cells. Extraction of oxygen from an external medium is only a matter of oxygen molecules diffusing thru cell membranes and cytoplasm until they reach the point where they are needed or a circulatory system where they get bound to carrier molecules and transported elsewhere.
On the other hand, organs specialized in extracting oxygen have evolved. These are primarily gills and lungs, but there is also the tracheal system of insects.
In the simplest case (A), there is no organ specialized for gas exchange; oxygen simply diffuses in thru the cell membrane and carbon dioxide diffuses out. This is seen in very small animals that have large surface area to volume ratios; since the oxygen diffusing thru the cell surfaces is sufficient for all metabolic needs, specialized organs are unnecessary. As the body volume increases, however, the relative surface area decreases and specialized organs that concentrate blood vessels (vascularization) in pockets of increased surface area, achieved by folding or filaments, become necessary to extract sufficient oxygen from the surrounding medium. There are 2 primary groups of anatomies specializing in gas exchange: 1. organs formed as evaginations (B), e.g., gills; 2. organs formed as invaginations (C), e.g., lungs.
3 fundamental types of gas exchangers. (Inspired by Fig. 8 in Maina, 2000.)
Of course, even in organs of the type A or B, the underlying mechanism of oxygen intake is identical to that in case A: oxygen diffuses in thru the cell membranes. In addition, there are animals, including some snails, that have both gills and lungs.
To be continued...