The function of SLC9A3 (solute carrier family 9 member A3, ENSG00000066230) is as follows. Plasma membrane Na(+)/H(+) antiporter (PubMed:18829453, PubMed:26358773, PubMed:35613257). Exchanges intracellular H(+) ions for extracellular Na(+) in 1:1 stoichiometry, playing a key role in salt and fluid absorption and pH homeostasis (By similarity). Major apical Na(+)/H(+) exchanger in kidney and intestine playing an important role in renal and intestine Na(+) absorption and blood pressure regulation (PubMed:24622516, PubMed:26358773). {ECO:0000250|UniProtKB:G3X939, ECO:0000269|PubMed:18829453, ECO:0000269|PubMed:24622516, ECO:0000269|PubMed:26358773, ECO:0000269|PubMed:35613257}.