= Respiratory dead space
{title2=$V_D$}
\b[Respiratory dead space] describes ventilated gas volume that does not contribute to the gas exchange represented by the chosen measurement. <Anatomical dead space> concerns conducting <airways>; <alveolar dead space> concerns ventilation of alveolar regions with ineffective exchange relative to their <perfusion>. The definition and estimator must be stated before the number is interpreted.<Pulmonary Function Testing by Warren M. Gold and Laura L. Koth>[The Bohr Equation for Respiratory Dead Space; Physiologic Dead Space]{ref}
A two-compartment <carbon dioxide> balance illustrates the gas-exchange estimate. Suppose inspired gas contains negligible carbon dioxide, a dead-space fraction contributes none, and the remaining expired volume carries representative alveolar fraction $F_A$. For mixed expired fraction $F_E$,
$$
F_EV_T=F_A(V_T-V_D),\qquad
\frac{V_D}{V_T}=1-\frac{F_E}{F_A}.
$$
Stipulated $F_A=0.05$ and $F_E=0.035$ give a dead-space fraction 0.30. With <tidal volume> 0.50 L, the corresponding effective dead-space volume is 0.15 L. This is a dilution-model result, not proof that a conducting tube of exactly that physical volume exists.
The alveolar quantity in the equation is important. Replacing it with an arterial carbon-dioxide measurement adds physiological assumptions and can reflect more than conducting-airway volume. The technical account distinguishes the alveolar balance from its arterial substitution.<Pulmonary Function Testing by Warren M. Gold and Laura L. Koth>[Physiologic Dead Space, assumptions underlying substitution of arterial for alveolar carbon dioxide]{ref}
An original comparison makes the distinction concrete. Two modeled systems each have <anatomical dead space> 0.15 L. In one, all remaining inspired gas reaches effectively exchanging regions. In the other, an additional 0.10 L per breath goes to a ventilated region that does not exchange with perfusing <blood>. Their effective dead-space volumes are 0.15 and 0.25 L despite the same conducting geometry.
The label therefore belongs to a specified physical or physiological model. A measured dead-space fraction, a geometrical airway volume and an estimate of wasted ventilation are related observations, rather than automatically interchangeable measurements.
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