Final answer:
The heart secretes atrial natriuretic peptide, a hormone that helps regulate sodium reabsorption in the kidneys and lowers blood volume and pressure.
Step-by-step explanation:
The heart functions as a secondary endocrine organ because it secretes the hormone atrial natriuretic peptide (ANP), which plays a role in the regulation of sodium reabsorption by the kidneys. ANP works by reducing blood volume, blood pressure, and sodium concentration in the blood. This hormone is released in response to increased blood volume and stretches in the walls of the atria of the heart, leading to a reduction in the reabsorption of sodium in the kidneys, thereby promoting the excretion of sodium and water, which ultimately lowers blood pressure.