Partial Pressures (mmHg) of Gases in the Respiratory Tract

One atmosphere of pressure at sea level is equivalent to 1 Barr or 760 mmHg pressure.  That means that if we add up the average partial pressures contributed by all of the gases that make up our atmosphere, they will exert a combined pressure of 760 mmHg (1 Bar) at sea level.  This is represented by the partial pressures listed under "ambient air" column (often referred to as atmospheric gas concentrations) in the table below.  Note that the partial pressures of gases making up the atmosphere total to 760 mmHg.  The partial gas pressures total 760 mmHg in all of the gaseous compartments of the body.  For example, the partial pressures of the inspired and expired gases will total 760 mmHg and the partial pressures of the alveolar gases will total 760 mmHg.

The Table Above (A):  This is probably a simplified version of the gas pressures table you studied in anatomy class... notably, the table above does not include partial pressures of expiratory or blood gases.  But let's start with this simplified version to orient ourselves.

First (Table A), notice that as soon as you breathe in ambient (atmospheric) air, your air passages saturate the "inspired air" with water vapor!  Humidification of incoming air is one of the roles of the air conducting passages you studied in anatomy.  Humidification of incoming air helps prevent ventilation from drying out the living tissues within the exposed surfaces of the lungs.  Whether we look at air in the airways or in the alveoli, the air is saturated with water (that is H₂O vapor has increased from an average of 5 mmHg in ambient air to 47 mmHg - this higher value represents 100% saturation/relative humidity, or all the water the air can possibly hold at body temperature).

Second (Table A), notice that as air is inspired and we saturate the air with water vapor, the water vapor displaces (or dilutes, if you prefer) some of the O₂ and N₂ present in the inspired air (that is because water vapor takes up space) and so the partial pressures of both the N₂ and O₂ in the inspired air drop from their partial pressures in ambient air.

Third (Table A), notice that there is almost no CO₂ in the air we breathe (see ambient).  Very little gas exchange occurs before we reach the respiratory bronchioles, and the bulk of gas exchange does not begin until we reach the alveoli, so our inspired air sample does not contain much CO₂ either.  In the alveoli, 40mmHg CO₂ diffused from the blood stream (pulmonary capillaries) into the alveolar air.  Intuitively, we know that this CO₂ came from the bloodstream.  Remember, blood is carried to the lungs in order to rid (reduce) the blood of CO₂ and H⁺ (acid).

Fourth (Table A), notice that the partial pressure of O₂ drops in the alveolar air!  Obviously the O₂ went somewhere!  We intuitively know that some of the O₂ diffused into the bloodstream (pulmonary capillaries) and that is why the partial pressure of O₂ dropped in the alveoli.  Remember, blood is carried to the lungs to pick up O₂ from the alveoli.  So now the only problem with the chart above is that it does not have all of the information covered in anatomy class!!!  We need to add the partial pressures of these gases in expired air and in "oxygenated blood" (blood returning to the left side of the heart in the pulmonary veins or traveling to the body in the systemic arteries) and in "partially de-oxygenated blood" (blood returning to the heart in the systemic venous circulation or traveling to the arteries for re-oxygenation in the pulmonary arteries)!  In the table below, we add the partial pressures in the "expired air" as well as in the systemic arterial and systemic venous blood and tissues!

The Table Below (Table B):  The table below is flipped around as, while working on labs, some students have mentioned that they prefer reading data listed in columns, while others have stated a preference for data in rows, and we want to keep everyone happy!  Table B presents partial gas pressures (in mmHg) just as in the Table A, above.

First (Table B), note that all of the information that was contained in Table A is also found in Table B... but there is additional important information added to Table B! 

Second (Table B), notice that the first column, labeled "site of sample" indicates where the sample was collected!

Third (Table B), notice that the values in Table B are labeled following convention.  That means that the values provided in the table and labeled as "venous" are those typically measured in systemic venous blood... however, it is understood that the systemic "venous" values are nearly identical to the values measured in pulmonary arterial blood on the way to the lungs for elimination of CO₂ and for re-oxygenation... and so there is no need to repeat pulmonary arterial values in the table!  Similarly, the values labeled "arterial" are those typically measured in systemic arteries.  As the partial gas pressures in systemic arteries are nearly identical to the partial gas pressures found in pulmonary veins carrying oxygenated blood back to the heart, there is not need to repeat partial pressures of gases in pulmonary venous blood in the table.  The greatest disparity between values presented in the table occurs between systemic arterial and pulmonary venous blood... systemic arterial blood actually has slightly less O₂ tension than that of pulmonary venous blood (about 95 mmHg versus 100 mmHg), for reasons outlined in earlier notes in this section.  It may be of interest to note that some pocket gas analyzers are calibrated slightly high to give readings for arterial blood gases close to 100 mmHg... as 100 mmHg is simply easy to remember as a normal value.

There are additional copies of this table as you scroll down the page, so that you don't have to be scrolling long distances up and down the page as you refer to values in the discussion!

The Table Above (B): 

First (Table B), logic dictates that the concentration of CO₂ must be higher in the pulmonary arterial blood arriving at the lung for elimination of CO₂ (marked venous... as in systemic venous... which is equivalent to pulmonary arterial), than the alveolar CO₂, and of course it is!  If this was not the case, CO₂ would not diffuse from blood to the alveoli down the natural concentration gradient that exists for CO₂!  For the same reason, O₂ must be higher in the alveoli than in the pulmonary arterial blood arriving at the lung for re-oxygenation (marked venous... as in systemic venous... which is equivalent to pulmonary arterial), and of course, it is!

Second (Table B), note that CO₂ in the tissues is higher than in any other compartment... this gradient guarantees that the CO₂, generated in the tissues, will be carried away in circulation!  By the same token, the O₂ tension in cells is lower than in any other compartment... this gradient guarantees that O₂ will be delivered from circulation to cells; assuming, of course, that the lungs are being ventilated and the tissues are adequately perfused!

Third (Table B), note that there is a big difference in O₂ concentrations between "oxygenated" and "partially deoxygenated blood" (95mmHg - 40 mmHg = 55 mmHg)!  The difference in CO₂ concentrations is much less (40 mmHg - 46 mmHg = -6mmHg)!  Thus, the concentration gradient for delivery of O₂ to the tissues is several fold greater than the concentration gradient for removal of CO₂ from the tissues!  As a point of physical interest, this large difference in the steepness of the gradients required to move O₂ into cells and to move CO₂ out of cells can be attributed to the fact that CO₂ is many times more diffusible in aqueous solution than is O₂!  Recall that CO₂ is about 20 times more diffusible in aqueous solution than is O₂.  Remember, carbonated beverages can be carbonated under relatively low pressures!  Oxygenation of tissues independent of hemoglobin, for example, in a hyperbaric chamber, takes much higher pressures (2-3 atmospheres)!

Fourth (Table B), note that, at rest, there typically exists a very stable gas composition in the alveoli.  The alveolar gas is in contact with the pulmonary capillary which is receiving mixed venous blood with low O₂ (PO₂ = 40 mmHg) which rapidly takes up O₂ to a PO₂ of 100 mmHg. Then, we see a very small decrease in PO₂ in the left ventricle, large arteries and arterioles and then a large decrease in the capillaries closer to average tissue values (30 mmHg) such that mixed venous blood has a PO₂ of 40 mmHg.  The PO₂ in cells may be as low as 1 mmHg or so low that it cannot be measured by conventional techniques (ie. inside cells).

PCO₂ changes are essentially opposite to changes in PO₂.  PCO₂ is highest in venous blood (note in the pulmonary circulation this means in the pulmonary trunk and pulmonary arteries) being delivered to the lungs from the right ventricle.  In the pulmonary capillaries, PCO₂ decreases to about 40 mmHg which does not really increase until the blood enters the tissue capillaries where it picks up newly generated CO₂.

Again, bronchial arteries supply blood to the airways and this blood is dumped into the pulmonary veins which causes a decrease in the PO₂ in the pulmonary vein (this is why arterial PO₂ is shown as 5 mmHg less than alveolar PO₂). Also, the Thebesian veins of the heart provide some blood to the left side of the heart which also decreases the PO₂ in the blood a bit before it leaves the left ventricle for the body.

Finally, note that pCO₂ in expired air is greater than in inspired air but is less than in alveolar air!  As you exhale, the portion of the alveolar air leaving the parenchyma of the lung mixes with air in the air conducting pathways!  Recall that gas exchange does not occur in the air conducting pathways.  Thus, as you exhale, alveolar air with a lot of CO₂ mixes with air in the conducting pathways that does not contain CO₂.  Further, recall that resting tidal volume is approximately 500 mL but that the air conducting pathways contain approximately 125 mL (anatomic dead space).  The net effect is that we see that expired air has slightly less CO₂ than alveolar air and expired air has slightly more O₂ than alveolar air!  Once again, the reason is that CO₂ from the alveoli is diluted in the air in the conducting pathways on the way out.  Similarly, the lower O₂ alveolar air is mixed with higher O₂ air pathway air on the way out!  For this reason, if you want to collect a good sample of alveolar air, you need to get your subject to breathe out as forcefully as they possibly can and collect a small sample of air at the very tail end of the expiratory reserve volume; this very last bit of air that they can possible exhale will most closely reflect the partial pressures of gases in the alveoli!

Figure: Simple Gas Exchange via Diffusion (With Ventilation & Perfusion)

Gas exchange requires gas diffusion form the alveolar spaces across the alveolar epithelium and the interstitial space and capillary endothelium to the RBC. Thickened alveolar walls (eg. pneumonia, pulmonary edema) create problems with gas diffusion and the human being has problems breathing.

Here is a fun and interesting exercise!  Save the pictures below using your right mouse button and selecting "save image as" or "save picture as" in the pop up menu.  Then, using any graphics package you have (it might be, for example, Windows "Paint," which you all have and can find by left clicking Start ---> then sliding the mouse up to Programs ---> then sliding the mouse around to Accessories ---> then sliding over to Paint and left clicking on Paint!)  Then open the file in your graphics program and label each of the bars with the appropriate compartment name... the very last compartment on the right in each of the diagrams below is "ICF" or "cell."  "ICF" is one compartment past "tissue," ... tissue is ostensibly the same as "ISF."  The red diagram shows concentrations of O₂ and the blue diagram shows concentrations of CO₂; the diagrams represent the gradients for delivery of O₂ to the cells and removal of CO₂ from the cells.