 Tidal volume

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.^[ref]

A flow measurement becomes a volume by integration. If $Q(t)$ is positive during inspiration, thenA stipulated triangular inspiratory flow pulse lasting 1 s with peak 1 L/s has area $\frac{1}{2}\times 1\times 1=0.5$ 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, ${\dot{V}}_E=f{V}_T$. 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,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.