THE ENDOCRINE FUNCTION OF THE HEART
The cells of the atria of the heart contain secretory granules that secrete a hormone called atrial natri-uretic peptide (ANP) and is released when theplasma sodium level is increased and/or the volume of the extracellular fluid is increased. The target or-gan of this hormone is the kidney, where it promotes sodium excretion (the opposite effect of aldosterone), lowers the blood pressure, and reduces the sensitivity of smooth muscles of blood vessels to vasoconstrictor substances. It also reduces the secretion of vaso-pressin. ANP secretion is increased directly by stretch of the atrial wall.