The second type of dead space is physiological dead space. Often in diseased lungs, some alveoli are perfused but not ventilated. The total volume of gas in each breath that does not participate in gas exchange is called the physiological dead space. This volume includes the anatomical dead space and the dead space secondary to perfused but unventilated alveoli. The physiological dead space is always at least as large as the anatomical dead space, and in the presence of disease, it may be considerably larger.
Both anatomical and physiological dead space can be measured, but they are not measured routinely in the course of patient care.
Composition of Air
Inspiration brings ambient or atmospheric air to the alveoli, where O2 is taken up and CO2 is excreted. Ambient air is a
In individuals with certain types of chronic obstructive pulmonary disease (COPD), such as emphysema, physiological dead space is increased. If dead space doubles, tidal volume must increase in order to maintain the same level of alveolar ventilation. If tidal volume is 500 mL and VD/VT is 0.25, then
If VD increases to 250 mL in this example, tidal volume (VT) must increase to 625 mL to maintain a normal alveolar ventilation (i.e., VA = 375 mL): gas mixture composed of N2 and O2, with minute quantities of CO2, argon, and inert gases. The composition of this gas mixture can be described in terms of either gas fractions or the corresponding partial pressure.
Because ambient air is a gas, the gas laws can be applied, from which two important principles arise. The first is that when the components are viewed in terms of gas fractions (F), the sum of the individual gas fractions must equal one:
Equation 23.6 1 0. = FN2 + FO2 + Fargon and other gases
It follows, then, that the sum of the partial pressures (in millimeters of mercury) of a gas, also known as the gas tension (in torr), must be equal to the total pressure. Thus at sea level, where atmospheric pressure (also known as barometric pressure [Pb]) is 760 mm Hg, the partial pressures of the gases in air are as follows:
Equation 23.7
Three important gas laws govern ambient air and alveolar ventilation. According to Boyle’s law, when temperature is constant, pressure (P) and volume (V) are inversely related; that is,
Boyle’s law is used in the measurement of lung volumes (see
Fig. 21.4 ). Dalton’s law is that the partial pressure of a gas in a gas mixture is the pressure that the gas would exert if it occupied the total volume of the mixture in the absence of the other components.
Eq. 23.7 is an example of how Dalton’s law is used in the lung. According to Henry’s law, the concentration of a gas dissolved in a liquid is proportional to its partial pressure.
The second important principle is that the partial pressure of a gas (Pgas) is equal to the fraction of that gas in the gas mixture (Fgas) multiplied by the atmospheric (barometric) pressure:
Equation 23.8
Ambient air is composed of approximately 21% O2 and 79% N2. Therefore, the partial pressure of O2 in inspired ambient air (PO2) is calculated as follows:
Equation 23.9 where (FiO2) is the fraction of oxygen in inspired air. The partial pressure of O2, or oxygen tension, in ambient air at the mouth at the start of inspiration is therefore 159 mm Hg, or 159 torr. The O2 tension at the mouth can be altered in one of two ways: by changing the fraction of O2 in inspired air (FiO2) or by changing barometric pressure. Thus ambient O2 tension can be increased through the administration of supplemental O2 and is decreased at high altitude.