Human Respiration
In the context of the diffusion of gases let us now consider how the process of human (and other mammalian) respiration functions in a kinetic atmosphere.
If observed phenomena are combined with kinetic theory assumptions we can make the following statements: –
Observed Phenomena and Characteristics
1) The atmosphere is made up of 78 percent nitrogen and 21% oxygen.
2) The time between inhalation and exhalation for humans is from one second up to four seconds. (This is dependent on age and on the degree of physical exertion at the time.)
3) Between 25 and 30% of the oxygen content of the gases that are inhaled is absorbed by the lungs and the remaining oxygen is exhaled along with all the nitrogen.
Kinetic Theory Assumptions
1) Given that 99.9% of the volume of atmospheric gas is empty space then this means that matter, in the form of atoms or molecules, take up 0.1% of the volume of the gas, which, in turn, means that oxygen molecules take up 0.02% of the volume of the air that we inhale.
2) The average kinetic velocities of both oxygen and nitrogen molecules are in the region of 4-500 metres per second.
3) The molecules of oxygen and nitrogen are traveling at these velocities and colliding with each other and with the tissue matter (i.e. molecules) that comprise the internal surfaces of the lungs and in doing so maintain an atmospheric level of pressure on these surfaces.
The simple diagram below shows atmospheric kinetic gases in the close proximity of lung tissue.
On this basis the first question is that, in view of the low volumetric concentration of oxygen molecules and the proven slow diffusion of gases:-
How can the lungs absorb about 25% of the available oxygen in the space of, in some cases, less than one second?
With the conditions as outlined above this does not seem possible. However let us for the moment ignore the observed slow diffusion of gases, and assume that somehow oxygen atoms/molecules in sufficient quantities collide with the inner walls of the lungs (or rather with its constituent molecules) and consider the second question in terms of the kinetic atomic theory of gases.
This is that, as the relative atomic masses of oxygen and nitrogen molecules are very similar at about 16 and 14 respectively and their average velocities are also similar (which means that in many instances their velocities would be identical) these molecules cannot therefore be identified by any difference in velocity:-
In this case, how is it possible that the lung tissue is capable of identifying the different characteristics of these gases in the instantaneous collisions they have with it, so that the nitrogen molecules presumably rebound from the molecules of the lung tissue while the oxygen molecules are absorbed?
Thus a perfectly elastic collision is allowed between the molecules of the lung and the nitrogen molecules so that they rebound (and in doing so generate the observed force of atmospheric pressure on the internal surfaces of the lungs), while the collisions of oxygen, of a similar mass and often identical velocities, are not elastic and these molecules are accordingly by some means instantly absorbed.
Note that any absorption of nitrogen into the blood is very dangerous, a small quantity can cause what deep sea divers call the ‘bends’.
The human body is a wonderful instrument, with the period between sensory stimulation and physical reaction occurring within fractions of a second, but as to how the lung tissue is able to differentiate between ‘kinetic’ molecules of oxygen and those of nitrogen in instantaneous collisions is beyond imagination.
Roger Munday