Question

Compare & Contrast the activation pathways and the effects of the Renin-Angiotensin-Aldosterone System (RAAS) and ANP/BNP on sodium and water homeostasis.

Answer 1

Renin Angiotensin system is activated when there is

1. Low blood pressure.

2. Low ECF volume.

3. Low plasma sodium.

These factors stimulate release of renin form juxtaglomerular cells. Renin cause activation of angiotensinogen into angiotensin 1 and then to angiotensin 2. Angiotensin 2 increases aldosterone secretion and cause vasoconstriction which ultimately increases blood volume and plasma sodium level.

ANP (atrial natruretic peptide) and brain natriuretic peptide(BNP) are both secreted from cardiac cells in ventricle. These are secreted when their is increased blood volume in ventricles leading to stretching of ventricular walls.

These two hormones acts on renal tubules to eliminate excessive sodium. Along with sodium, water is also excreted leading to decrease in plasma volume.