In fluid mechanics, inertance is a measure of the pressure difference in a fluid required to cause a unit change in the rate of change of volumetric flow-rate with time. The base SI units of inertance are kg m−4 or Pa s2 m−3 and the usual symbol is I. The inertance of a tube is given by: where The pressure difference is related to the change in flow-rate by the equation: where This equation assumes constant density, that the acceleration is uniform, and that the flow is fully developed "plug flow". This precludes sharp bends, water hammer, and so on. To some, it may appear counterintuitive that an increase in cross-sectional area of a tube reduces the inertance of the tube. However, for the same mass flow-rate, a lower cross-sectional area implies a higher fluid velocity and therefore a higher pressure difference to accelerate the fluid. In respiratory physiology, inertance (of air) is measured in cmH2O s2 L−1. Using small-signal analysis, an inertance can be represented as a fluid reactance (cf. electrical reactance) through the relation: where