What an immense quantity! And pray how much of carbonic acid gas do we expel from our lungs at each expiration?
MRS. B.
The quantity of air which we take into our lungs at each inspiration, is about 40 cubic inches, which contain a little less than 10 cubic inches of oxygen; and of those 10 inches, one-eighth is converted into carbonic acid gas on passing once through the lungs[*], a change which is sufficient to prevent air which has only been breathed once from suffering a taper to burn in it.
CAROLINE.
Pray, how does the air come in contact with the blood in the lungs?
MRS. B.
I cannot answer this question without entering into an explanation of the nature and structure of the lungs. You recollect that the venous blood, on being expelled from the right ventricle, enters the lungs to go through what we may call the lesser circulation; the large trunk or vessel that conveys it branches out, at its entrance into the lungs, into an infinite number of very fine ramifications. The windpipe, which conveys the air from the mouth into the lungs, likewise spreads out into a corresponding number of air vessels, which follow the same course as the blood vessels, forming millions of very minute air-cells. These two sets of vessels are so interwoven as to form a sort of net-work, connected into a kind of spongy mass, in which every particle of blood must necessarily come in contact with a particle of air.
CAROLINE.
But since the blood and the air are contained in different vessels, how can they come into contact?
MRS. B.