What does a brain natriuretic peptide do?

When the walls of the atria are stretched by increased blood volume or blood pressure, the cells of the atria produce ANP (atrial natriuretic peptide). ANP's main function is to decrease the reabsorption of sodium ions (by the kidneys) in order to increase the elimination of water (in urine), and decrease blood volume and blood pressure. I'm not sure how much you've studied as far as the endocrine system goes, but it's basically the antagonist to the hormone Aldosterone.