Why card · first principles
Why does partial pressure matter to a molecule?
Diffusion across the alveolar wall, and why the other gases do not count.
The model you brought
"Pressure pushes. Air moves over a wing because the pressure is higher on one side than the other, so oxygen should move into the blood because the pressure in the lung is higher."
The why
A molecule never feels pressure. It only bounces. Oxygen crosses the wall when an oxygen molecule happens to reach it, and more reach it from the side where oxygen's own pressure is higher, whatever else is in the mixture.
Partial pressure is our count of one gas's hits, nothing more; the nitrogen around it changes nothing, because a nitrogen molecule hitting the wall does not move an oxygen molecule. In a liquid the same count is set by solubility as well, so blood at the same oxygen pressure as the air holds fewer oxygen molecules per litre and still trades evenly with it. Over a wing, whole parcels of air move together, driven by total pressure. Across the alveolar wall nothing moves in bulk at all. The only traffic is one molecule at a time, and each gas crosses as if the others were not there.
What it unlocks
- Why the other gases do not count: in the textbook's words, "oxygen exerts a partial pressure, and nitrogen exerts another partial pressure, independent of the partial pressure of oxygen". Each gas crosses on its own difference, and the total never enters.
- Why carbon dioxide gets out on a difference of 5 mm Hg when oxygen needs 64: the rate carries a solubility term, and carbon dioxide dissolves about twenty times better in blood and in the fluid lining the alveolus, so "the relative concentrations of oxygen and carbon dioxide that diffuse across the respiratory membrane are similar".
- Why nitrogen, three quarters of the air at 569 mm Hg, changes nothing: "very little nitrogen dissolves into the blood, because the solubility of nitrogen in blood is very low". The exception is the diver breathing compressed air, whose nitrogen stands at a higher pressure and dissolves in earnest.
- Why breathing sets the count: when ventilation falls short, "the partial pressure of oxygen in the alveoli drops. Without the large difference in partial pressure between the alveoli and the blood, oxygen does not diffuse efficiently across the respiratory membrane", and the lung's answer is to send its blood to the alveoli that are ventilated.
- Where the picture you brought is still right: air reaches the alveolus by ventilation, "the movement of air into and out of the lungs", which is a bulk flow on a pressure difference. The last micron is not. There "the actual exchange of gases occurs due to simple diffusion. Energy is not required to move oxygen or carbon dioxide across membranes."
The principle in one line
"Partial pressure (Px) is the pressure of a single type of gas in a mixture of gases", the textbook says, and then the rule: "A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower. In addition, the greater the partial pressure difference between the two areas, the more rapid is the movement of gases." For the blood side it adds Henry's law, that "the concentration of gas in a liquid is directly proportional to the solubility and partial pressure of that gas", which is why the count on that side is read in pressure and not in molecules per litre. The exchange itself "occurs due to simple diffusion. Energy is not required to move oxygen or carbon dioxide across membranes", and the lung is built for it: "The respiratory membrane is highly permeable to gases; the respiratory and blood capillary membranes are very thin; and there is a large surface area throughout the lungs."
Fick's law, written out
Those three clauses are the terms of the rule that carries Fick's name. The amount of a gas that crosses a wall each minute is the wall's area divided by its thickness, times how readily that gas passes through the wall's material, times the difference in its partial pressure across it. The middle term is Krogh's permeation coefficient, the gas's diffusion coefficient in the tissue times its solubility there, and nothing about any other gas appears anywhere in the line. For the lung the review writes it as Bohr's equation, uptake equals the alveolar pressure minus the mean capillary pressure times the lung's diffusing capacity, with the membrane's share of that capacity as the coefficient times surface over thickness, the thickness taken as a harmonic mean "because the local rate of O₂ diffusion is inversely proportional to the diffusion distance". Bohr himself doubted that diffusion alone could carry the oxygen; the Kroghs' measurements settled that it does. The human terms, read off electron micrographs of seven young adults: an alveolar surface of about 130 m², a tissue barrier about 0.6 µm thick, about 1.1 µm from air to red cell, and a diffusing capacity for oxygen of about 150 to 200 mL a minute for each mm Hg of difference. The solubility inside the middle term is why carbon dioxide leaves the blood on a difference of about 5 mm Hg while oxygen enters on about 64: it dissolves about twenty times better, so, in the textbook's words, "the relative concentrations of oxygen and carbon dioxide that diffuse across the respiratory membrane are similar."