In man, as you know him on Earth, this is accomplished by the lungs. The venous blood, charged with its carbonic acid and its waste products, needing a renewal of oxygen and a removal of the carbonic acid, is pumped by the heart through the lungs. These by their construction present an immense amount of internal surface covered by a vascular network, through which the blood flows in innumerable minute streamlets.
In respiration, the inhaled air is separated from the blood only by an extraordinarily thin membrane—less than 1/20,000 of an inch in thickness. Through this membrane the blood absorbs oxygen from the air, giving in return to the air its noxious carbonic acid gas.
Such is the basic process in you Earth-men. In the case, let us say, of your Earth-fishes breathing your water, there is little fundamental difference. The blood in their gills is brought practically into contact with a steadily moving stream of water. But fishes do not get their oxygen from the water in some mysterious fashion. Did you think they did? They get the oxygen, not from water, but from air—the air that is held in solution in the water.
But for two things, you on Earth could breathe your water. First, your lung passages are too minute to receive a substance so heavy, so unvolatile, let me say, as is the water of Earth. Secondly, there is not proportionally enough air in your water.
Both these conditions were different on my meteor.
This water on my meteor was very different from water as you know it. I have already said it was light and thin. To be exact, I estimate that on your Earth it would have a specific gravity of no more than .18, placing your water at 1.00.
In your sea-water a normally fleshy man will float with a small margin to spare. This water on my meteor was not saline; but more than that, Nona and I stood submerged in it with hardly any perceptible feeling of buoyancy.
Let me make my point still clearer. The low specific gravity of this water compared to yours was principally caused by the large amount of air it held in solution. It was, in a word, highly aerified to an extent proportionally eleven times more than is your average water on Earth. For this reason, my lungs needed but one-eleventh the amount of it from which to secure the necessary oxygen.
On Earth, your normal respiration varies widely; sixteen to twenty times per minute for a healthy adult at rest might be taken as a fair average. I was breathing this water at approximately eighty respirations per minute.