Question

ANF is secreted by _________ and its function is __________
A. Kidney; raising blood pressure
B. Atria; lowering blood pressure and blood volume
C. Liver; increasing metabolism
D. Lungs; increasing the rate of respiration and gaseous exchange

Answer 1

Hint: ANF stands for the Atrial natriuretic factor. It is a natriuretic peptide hormone encoded by the NPPA gene. It is 28 amino acid polypeptide. It plays a crucial role in maintaining the blood pressure thus also contributing to the blood volume indirectly. It predominantly acts on the kidneys.

Complete answer:
ANF is produced by the heart particularly, the atrium. ANF is secreted by the atrium in response to decreased blood pressure. It acts by decreasing vascular resistance and reducing blood volume. Its major function is to increase GFR and increase the excretion of sodium.
When the blood pressure decreases, that is when the body is in a hypervolemic state, the level of ANP increases. ANP is synthesized in the heart cells in an inactivated form as preAMP. This is activated to pro ANP and ultimately to ANP. Its normal value ranges from 40 - 60 pg/ml.
When the blood volume increases, it leads to the stretching of the atrial walls. This triggers the release of ANP. ANP acts on the vessels of the kidneys to increase the GFR. By acting on the renal tubules, it also increases sodium excretion. Along with sodium, water follows. This decreases the blood volume and brings it back to normal. Normalizing the blood volume corrects the blood pressure.
So, the correct answer is option B.

Note: ANP is found to be elevated in heart conditions like cardiac failure. In heart failure, the stress on the heart increases. This triggers an ANP release to reduce stress. There are specific types of ANP receptors on the kidneys to which the ANP binds. ANP is a strong vasodilator. ANP works in contrast to the RAAS ( Renin-angiotensin) mechanism