Natriuretic peptides ANP and BNP are small molecules and are the group of naturally-occuring substances that act in the body to oppose the activity of the renin-angiotensin-aldosterone (RAA) system. They serve as counter-regulatory hormones and are secreted in response to the increased atrial and ventricular stretching that occurs in secondary increased blood volume. Natrecor (nesiritide) is the proprietary name for the IV formulation of human B-type natriuretic peptide (hBNP).