Answer:
The atria of the heart secrets ANP (Atrial natriuretic peptide).
Explanation:
Atrial natriuretic peptide (ANP) is a hormone released by the atria of the heart. When strain sensors in the atrium wall detect higher blood pressure, for example due to an increased blood volume, more ANP is released. ANP has a direct effect on sodium absorption in the kidneys. It ensures that less sodium is resorbed in the collecting tube of nephrones. It therefore increases sodium secretion. Because of this lower resorption, the blood volume eventually decreases due to a decreased osmosis. In the case of a lower blood pressure in the atria, therefore, less ANP will be released, so that more sodium is resorbed and the blood volume increases. In addition, ANP also has a dilating effect on the afferent arteriole of the kidney. With low blood pressure there will therefore be less dilatation of this arteriole. This gives more resistance and will ensure that less blood goes to the kidneys. This will also increase the blood volume.
