You may not think of the heart as an endocrine organ, but it is. When blood volume is high, blood pressure is high, putting strain on the heart. Under these conditions, the increased venous return stretches the atria of the heart. When the atrial muscle fibers are over-stretched, they release a peptide hormone called atrial natriuretic peptide (ANP). This peptide hormone enters the circulation, and in the kidney it decreases the reabsorption of sodium. If less sodium is reabsorbed, less water is reabsorbed, and more passes into the urine. Thus ANP has the effect of lowering blood volume and therefore blood pressure.