= Tidal volume
{title2=$V_T$}
\b[Tidal volume], $V_T$, is the volume of gas inhaled or exhaled during one respiratory cycle. It is a per-breath increment, distinct from the gas remaining at <functional residual capacity> or the maximum <total lung capacity>.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Definitions and subdivisions of lung volume; Figure 1]{ref}
A flow measurement becomes a volume by integration. If $Q(t)$ is positive during inspiration, then
$$
V_T=\int_{\rm inspiration}Q(t)\,dt.
$$
A stipulated triangular inspiratory flow pulse lasting 1 s with peak 1 L/s has area $\tfrac12\times1\times1=0.5$ L. Integrating a signed flow over an entire balanced breath instead gives approximately zero, because expiration cancels inspiration; that signed net transfer is not <tidal volume>.
Multiplying by <respiratory rate> $f$ gives <minute ventilation>, $\dot V_E=fV_T$. Two constructed patterns, 0.5 L at 12 breaths/min and 0.25 L at 24 breaths/min, both move 6 L/min. They need not deliver the same gas exchange. In an ideal fixed-<respiratory dead space> model,
$$
\dot V_A=f(V_T-V_D),
$$
where $V_D$ is the per-breath volume not participating in the modeled exchange. For stipulated $V_D=0.15$ L, the two patterns give <alveolar ventilation> 4.2 and 2.4 L/min. This comparison assumes each breath exceeds the modeled dead-space volume; it is an accounting example, not an individual breathing prescription.
A <spirometry> or flow trace therefore needs an identified interval and sign convention before its integral is interpreted. The illustration places a quiet <tidal volume> within a full lung-volume excursion: it shows increments on a constructed absolute scale rather than claiming that ordinary spirometry directly observes the residual offset.
\Image[/lung-volume-subdivisions.png]
{title=Lung capacities share endpoints but measure different excursions}
{description=Original illustrative spirogram and volume ledger, with residual volume 1.4 L, functional residual capacity 2.5 L, tidal volume 0.5 L and total lung capacity 6.0 L. The absolute scale is stipulated; a spirometer alone measures changes rather than its residual-volume offset. Values are not clinical reference limits.}
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