Atrial Natriuretic Factor

The human body maintains a very fine-tuned balance of fluids and electrolytes through osmoregulation. One important component of this system is atrial natriuretic factor (also called atrial natriuretic peptide, "ANP"). This hormone plays a fundamental role in regulating blood volume and pressure. 

1.0Atrial Natriuretic Factor

Atrial natriuretic factor is a type of polypeptide hormone that is produced and released from specific cells located in the atria of the heart. These cells act as sensors that detect an increase in blood volume and pressure. When the atria stretch due to excess blood volume, they secrete ANF into the circulation.

2.0Mechanism of Action: How ANF Lowers Blood Pressure

ANF works through several mechanisms to decrease blood volume and pressure, acting as a direct counterbalance to the RAAS. Its primary functions include:

• Vasodilation: ANF relaxes smooth muscle in blood vessels, particularly arterioles, leading to vasodilation. This widens the blood vessels, which in turn reduces peripheral resistance and lowers blood pressure.
• Sodium and Water Excretion: ANF increases the glomerular filtration rate (GFR) and inhibits sodium reabsorption in the renal tubules, leading to increased excretion of sodium (Na+) and water in the urine. This process is called natriuresis (excretion of sodium) and diuresis (excretion of water), hence the name "natriuretic factor."
• Inhibition of Renin-Angiotensin-Aldosterone System (RAAS): ANF directly inhibits the release of renin from the juxtaglomerular apparatus (JGA) in the kidneys. Suppressing the RAAS, it prevents the formation of angiotensin II and the secretion of aldosterone. This is a crucial negative feedback loop, as both angiotensin II and aldosterone act to increase blood volume and pressure.

3.0The Counter-Regulatory Role of ANF

Atrial natriuretic factor, or ANF, is an important hormone in cardiovascular homeostasis. When blood pressure increases, ANF is released to bring blood pressure down. Similarly, when blood pressure decreases, the renin angiotensin system, or RAAS, is activated to bring blood pressure up. ANF and RAAS work antagonistically to keep blood pressure and volume in a slim, healthy range. Structure of Atrial Natriuretic Factor

• Peptide hormone: ANF is a 28-amino-acid polypeptide.
• Produced by atrial myocytes: It is primarily synthesised and stored in the granules of atrial cardiac muscle cells.
• Released in response to stretching: Stretching of the atrial walls due to increased blood volume or pressure stimulates the release of ANF.

4.0Synthesis and Secretion

• Stimulus: Increased blood pressure, increased blood volume, or atrial stretch. 
• Origin: Secreted by the right atrium of the heart. 
• Storage: Stored in secretory granules and kick-started when needed. 
• Degradation: Quickly cleared from circulation through enzymatic degradation and renal excretion.

5.0Physiological Roles of Atrial Natriuretic Factor

1. Blood Pressure Regulation – Maintains blood pressure within normal limits by promoting vasodilation and natriuresis.
2. Volume Homeostasis – Reduces extracellular fluid volume during hypervolemic states.
3. Electrolyte Balance – Helps in sodium and water excretion.
4. Cardiovascular Protection – Prevents heart overload and reduces hypertension risk.
5. Interaction with Other Hormones – Opposes the effects of aldosterone, angiotensin II, and ADH.