RESPIRATORY PRESSURES
1. Define partial pressure.
2. Why is a jugular vein blood sample not representative of whole body blood?
3. What are the gases of the atmosphere and what is the approximate percentage composition of each?
4.
Why does the composition of atmospheric air differ from that of alveolar air?Solutes and solvents diffuse from an area of their higher concentration to an area of their lower concentration, and so do gases. The concentrations of gases are usually expressed as pressures. It occasionally helps to think in terms of concentration instead of pressure when determining the diffusion of a single gas within a mixture of gases.
Partial Pressure
Usually, gas pressure is considered in terms of total pressure, regardless of whether it is a single gas or a mixture of gases. When considering the equilibrium of two gas mixtures separated by a permeable membrane, however, it is necessary to consider each gas in the mixture separately in terms of its contribution to the total pressure. The term partial pressure is therefore used. It is defined as the pressure exerted by a particular gas in a mixture of gases. The sum of the partial pressures of the gases within a mixture equals the total pressure. The physiologic notation for partial pressure is P. Specific gases are noted by their chemical symbol. Accordingly, the partial pressure of oxygen in a gas mixture is denoted by PO2. The partial pressure of oxygen in arterial blood and venous blood is given by PaO2 and,PvO2, respectively, where the particularization of arterial and venous blood is noted by the subscripts a and v.
Arterial and Venous Blood Partial Pressure
Because oxygen is consumed and carbon dioxide is produced by cells, it is expected that venous blood (blood returning to the lungs after its service to the cells) will have a higher PCO2 and a lower PO2 than arterial blood (blood that has been replenished by the lungs and is on its way to the cells).
Arterial blood taken from one part of the body will have approximately the same gas content as arterial blood from another part of the body because none of it has reached capillary systems where the exchange (loss of O2 and gain of CO2) takes place. Venous blood from different parts of the body may vary, however, because of different metabolisms associated with the function of the body part. A more active location would consume more O2 and produce more CO2 than less active locations. Because of these differences, the jugular vein blood may not be representative of whole body venous blood (i.e., blood from the right atrium).Atmospheric Air versus Alveolar Air
The total pressure of one atmosphere (1 atm) of air under conditions of standard temperature and pressure is 760 mm Hg. The appropriate composition of dry atmospheric air (and corresponding partial pressures) is as follows: 21.0% O2 (PO2; about 159 mm Hg); 0.03% CO2 (PCO2; about 0.23 mm Hg); 79.0% N2 (Pn2; about 600 mm Hg). The total pressure is approximately 760 mm Hg. CO2 is almost absent in the atmospheric air. This explains the effective diffusion gradient for CO2 from the body (where it is produced) to the air around us. Note that this is the composition of dry air. Any amount of humidification is represented by a water vapor partial pressure value (PH2O). Its presence would cause a dilution of the other gases, and thus their partial pressures would be lowered to maintain the total pressure at 760 mm Hg.
It might be supposed that the composition of alveolar air is the same as atmospheric air because it represents the transfer of air from one place to another. However, the ventilation process does not evacuate the alveoli completely with each breath, but rather it is a gradual replenishment and evacuation. The approximate composition of alveolar air, measured in partial pressure, is as follows (dry atmospheric air partial pressures are in parentheses): PO2 = 104 mm Hg (159); PCO2 = 40 mm Hg (0.23); PN2 = 569 mm Hg (600); PH2O =.47 mm Hg (0.00).
The differences from atmospheric air are apparent. The total pressure of alveolar air is equal to 760 mm Hg, and all of its components are diluted by water vapor, which is equal to 47 mm Hg. A PH2O of.47 mm Hg represents 100% humidification of alveolar air at body temperature (37 °C for humans). In addition, the PO2 is lower and the PCO2 is higher than their respective atmospheric pressures because oxygen is continually diffusing from alveolar air to the tissues (where it is used) and Co2 is continually diffusing from the tissues (where it is produced) to the alveolar air (where it is expelled). The PN2 of alveolar air is lower than its value in atmospheric air primarily because of its dilution by water vapor.
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