Final answer:
The hormone ANP is released by the heart in response to increased blood volume. It acts on the kidneys to reduce the reabsorption of sodium, causing sodium and water to be excreted in the urine. This leads to a reduction in blood volume, blood pressure, and sodium concentration.
Step-by-step explanation:
The heart possesses endocrine cells in the walls of the atria that are specialized cardiac muscle cells. These cells release the hormone atrial natriuretic peptide (ANP) in response to increased blood volume. High blood volume causes the cells to be stretched, resulting in hormone release. ANP acts on the kidneys to reduce the reabsorption of Na+, causing Na+ and water to be excreted in the urine.
ANP also reduces the amounts of renin released by the kidneys and aldosterone released by the adrenal cortex, further preventing the retention of water. In this way, ANP causes a reduction in blood volume and blood pressure, and reduces the concentration of Na+ in the blood.