In this post… The renal regulation of bicarbonate excretion The pathogenesis of proximal RTA The pathophysiology of vomiting How is renal bicarbonate excretion regulated? With respect to bicarbonate reabsorption, there are two key parameters to consider: the Tm = maximal rate of tubular HCO3 reabsoption and the renal bicarbonate threshold = the serum [HCO3] at which the filtered load exceeds Tm, and bicarbonate begins to appear in the urine.