Hypercapnia has these interrelated effects:ġ. Effects of increased P aCO 2Įlevated P aCO 2 is called hypercapnia, or sometimes hypercarbia. The value at the end of a normal “tidal” breath, the “end tidal” value P ETCO 2, is a sample of mixed P ACO 2. PCO 2 at the mouth and nose, shown as a function of time in the graph, varies from zero during inspiration to a maximum during expiration. The blood that leaves the lungs, usually equilibrated with alveolar gas, becomes the systemic arterial blood after its passage through the heart, with CO 2 partial pressure P aCO 2. The CO 2 partial pressures discussed.īlood in the lungs is exposed to alveolar gas with P ACO 2. Although P aCO 2, P ACO 2 and P ETCO 2 are not identical because of regional and breathing cycle variations within the lungs, for the purposes of this article, they are equivalent. Thus, the end tidal partial pressure of CO 2 (P ETCO 2) approximates P ACO 2. In normal lungs, alveolar CO 2 partial pressure (P ACO 2) (Figure 2) approximately equals P aCO 2, as is discussed later in this article Finally, the gas exhaled at the end of a normal (“tidal”) breath, called “end tidal gas”, comes from the alveoli. The partial pressure of CO 2 in arterial blood (P aCO 2) is the value the body needs to control. Which partial pressures are important for whole-body CO 2 balance? At equilibrium, one molecule of A leaves the liquid for each molecule of A that enters it, and the partial pressure of A in the liquid is defined as the partial pPressure of A in the gas in equilibrium with it. The partial pressure of A in the cylinder is thus half the total pressure, as indicated by the pressure gauge on the hypothetical second cylinder from which all molecules other than gas A have been removed.ī) The space above a liquid contains molecules of gas A, and gas A is soluble in the liquid. Illustrations of the concept of partial pressure a) in gases and b) in liquids.Ī) Half of the molecules in the first gas cylinder are “gas A”. Although the number of molecules of gas A dissolved in a volume of the liquid depends also on the solubility of A in the liquid, it is useful to express the amount of dissolved gas A as the partial pressure of A in the gas at equilibrium with the liquid. The direction and rate of gas transfer of A across a liquid – gas surface surface depends on the partial pressure difference, gas to liquid. (Figure 1b) In other words, the gas and liquid are at equilibrium. After “long enough” with a gas containing gas A in contact with the liquid, the rate at which molecules of A leave the liquid phase matches the rate at which they enter it. The molecules in the liquid are also in motion, and molecules of gas reaching the surface again are able to leave the liquid. If one of those “walls” is a liquid surface, soluble gas molecules will dissolve in the liquid. The molecules in a gas, always in motion, exert pressure by bumping into the walls of their container. The meaning of partial pressure of a gas in a liquid is less obvious. (For example, the partial pressure of A in a gas containing 65% of A at a total pressure of 2 bar is 0.65 x 2 = 1.3 bar.) This also applies to containers at ambient pressure, for example, the lungs. Numerically it is the product of total pressure and the fraction of the gas that consists of molecules A. (Figure 1a) (We assume an ideal gas, that is to say, we consider interactions among gas molecules to be insignificant.) Partial pressure of A indicates the number of molecules of A per volume. The concept of partial pressure of any gas, say “gas A”, in a cylinder of mixed gas is relatively straightforward: it is the pressure that would remain if all gas molecules that are not “gas A” were removed. However, the use of breathing gear is an unusual condition that is rarely covered in physiology courses. (Contamination with carbon monoxide is an entirely different problem.) Effects of elevated CO 2 partial pressure in the bloodĬO 2 usually influences breathing so that the body maintains a healthy arterial CO 2 partial pressure (P aCO 2) of approximately 40 Torr (40 mm Hg, 5.3 kPa) even when inspired gas contains a low concentration of CO 2. A look at the physiology of CO 2 shows, though, that the danger of high CO 2 in diving is real and important. Since submariners tolerate inspired CO 2 levels that are higher than the current limits for diving gear, one could be forgiven for suspecting a marketing ploy by any manufacturer touting benefits of lower inspired CO 2. Some current standards 1,2 permit up to 20 mBar (2 kPa, 2% SEV) as the average inspired CO 2 partial pressure, while some research, for example, work by the authors of this article, 3,4 indicates that amount to be dangerous. The acceptable level of inspired carbon dioxide (CO 2) in diving gear is a controversial topic.

Scuba diving carbon monoxide symptoms