Which of the following ion is exchanged in the stomach for H+ secretion?

The unique characteristic of gastric secretion is its high concentration of hydrochloric acid, a product of the parietal cells. As the concentration of H+ rises during secretion, that of Na+ drops in a reciprocal fashion. K+ remains relatively constant at 5-10 meq/L. Chloride concentration remains near 150 meq/L, and gastric juice maintains its isotonicity at varying secretory rates. Acid secretion at the apical membrane is accomplished by a membrane-bound H+/K+-ATPase (the proton pump); H+ is secreted into the lumen in exchange for K+. Ref: Dohey G.M., Way L.W. (2010). Chapter 23. Stomach & Duodenum. In G.M. Dohey (Ed), CURRENT Diagnosis & Treatment: Surgery, 13e.