Human and Mammalian Respiration
Facts:-
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. (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 (i.e. 7% of the total volume) that are inhaled is absorbed by the lungs and the remaining oxygen is exhaled along with all the nitrogen and some ejected carbon dioxide.
Theory:-
In terms of the currently accepted kinetic atomic theory of gases:-
1) 99.9% of the volume of atmospheric gas is empty space, which 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 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, the oxygen atoms are green and the red is the lung tissue which is covered by a blue liquid surfactant.
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 25-30% of the available oxygen in the space of, in some cases, less than one second?
However let us for the moment ignore the observed slow diffusion of gases, and assuming that somehow oxygen atoms/molecules in sufficient quantities collide with the surfactant covering the inner walls of the lungs, and consider this 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 mass or velocity.
In this case, how is it possible that the lung tissue is capable of identifying the different characteristics of these elemental gases during the instantaneous collisions they have with it, so that the nitrogen molecules (presumably) rebound from the surface of the surfactant 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 are repelled (and in doing so necessarily generate a force of pressure on the internal surfaces of the lungs), while the collisions of oxygen, of a similar mass and often identical velocities, are not perfectly elastic and are accordingly (by some inexplicable 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 would be able to differentiate between ‘kinetic’ molecules of oxygen and those of nitrogen in instantaneous collisions is beyond imagination.
Facts:-
The internal surfaces of a typical pair of human lungs are estimated to contain between 300 and 500 million alveoli, i.e. “tiny air sacs in the lungs which allow for rapid gaseous exchange”. It has been calculated that the internal surfaces of these sacs amount to a total area of around 80M2, and that 1mm2 of lung tissue contains around 170 alveoli.
The tidal volume in respiration is about 0.5 litre, in other words in one breathing cycle of between 2-4 seconds this volume of the atmosphere is inhaled and exhaled. Most of this volume of gas is drawn into the alveolar sacs on inhalation, due to the expansive actions of the diaphragm and the chest muscles, which volume is then immediately exhaled, i.e. there is no pause between inhalation and exhalation.
The process of exhalation is the opposite, where the muscles of the rib cage and the diaphragm contract, forcing the expulsion of all the nitrogen, together with the oxygen that has not been absorbed into the blood and with some carbon dioxide that has been expelled from it.
This process is indicated in the diagram below, where the flow of air inhaled into the sacs is indicated :-
The internal surface area of the alveoli totals 80M2 and, as air is composed of a continuous, and homogeneous, distribution of 78% nitrogen and 21% oxygen atoms, this volume of oxygen is the proportion of atoms that would be in direct contact with this, relatively huge, surface area, which would clearly allow sufficient time for the observed 1/3rd of the available oxygen atoms, i.e. 7% of the total volume inhaled, to react with and be absorbed into the surfactant, as in the second diagram which represents the internal surface of an alveolar sac, and the transference of oxygen into the blood vessels in the underlying tissue.
