This equation computes the secretion rate, `T_x` of some substance, x.

This equation assumes the substance is only secreted and not re-absorbed.

Inputs to this equation are:

• GFR - the Glomerular Riltration Rate (see GFR)
• `P_x` - concentration of substance x in the plasma
• `U_x` - concentration of substance x in the plasma
• `V_U` - volumetric flow of urine

Notes

If a substance (the solute) is only secreted and not re-absorbed, the secretion rate (mass secretion rate) of the substance can be computed as the difference between the excretion rate of the substance and the amount filtered per unit time of the substance, where:

mass filtration rate = `GFR * P_x`

and

excretion rate = `U_x * V_U`