In fluid dynamics and hydrometry, the<unk> , also volume<unk> <unk>  and<unk>  of fluid<unk> , is the volume of fluid which passes through a given surface per unit time (for example cubic meters per second [m3 s-1] in SI units, or cubic feet per second [cu ft/s]). It is usually represented by the symbol Q.<unk>  should not be confused with<unk>  flux, represented by the symbol q, with units of m3/(m2 s), that is, m s-1.  The integration of a flux over an area gives the<unk> .<unk>  is also linked to viscosity.
Given an area A, and a fluid<unk> ing through it with uniform velocity v with an angle θ away from the perpendicular to A, the<unk> <unk>  is:
In the special case where the<unk>  is perpendicular to the area A, that is, θ = 0, the<unk>  is:
If the velocity of the fluid through the area is non-uniform (or if the area is non-planar) then the<unk>  of fluid<unk>  can be calculated by means of a surface integral:
where dS is a differential surface described by:
with n the unit surface normal and dA the differential magnitude of the area.
If a surface S encloses a volume V, the divergence theorem states that the<unk>  of fluid<unk>  through the surface is the integral of the divergence of the velocity vector field v on that volume:
Volumetric flow rate
Pressure gradient 
Ventilation/perfusion ratio
Electrochemical gradient
Stroke volume
Oxygen permeability
Cardiac output
Alveolar-arterial gradient