Your body secretes H+ and HCO3- through the kidneys. They combine to form H2CO3 (carbonic acid). An enzyme, carbonic anhydrase, splits the carbonic acid into H2O and CO2 in the renal tubular lumen and this water and carbon dioxide enter the renal tubular cell passively. This is because carbon dioxide is lipid soluble and can easily pass through the cell membrane. Water can also move easily across the cell membrane.

Once the H2O and CO2 are inside the renal tubular cell, carbonic anyhydrase acts on them again to form carbonic acid. This carbonic acid splits into H+ and HCO3- again. Now the H+ ion is excreted from the renal tubular cell through a ion exchanger known as the Na-H exchanger where the H+ is exchanged for Na+ in the renal tubular lumen.

Thus, the same H+ ion is used for reabsorption of HCO3- and Na+.

I hope this helps.