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-aldosterone system and ANP/BNP are two pathways involved in water and sodium homeostasis in body through the renal system. Renin -Angiotensin-Aldosterone system activates in respose to the decrease in Blood pressure and decrease in Glomerular filtration rate.Baroreceptors are present in the afferent arterioles detect the lowering in blood pressure ,which triggers the Juxta-glomerular cells to release renin. Renin act on the angiotensinogen produced by liver ,converting it into Angiotensin -I.On conversion Angiotensin -I reach lungs, where Angiotensinogen converting enzyme is released , it convert Angiotensin -I into Angiotensinogen -II.Angiotensinogen-II Act on three sites:

• On Blood vessels - Causes vasoconstriction, increases blood pressure.
• On Adrenal cortex- Stimulate the release of aldosterone ,aldosterone act on collecting tubules and promote reabsorption of Na+ and water from filtrate into interstitium
• On Posterior Pitutary gland-Stimulate the secretion of ADH(Vasopressin) ,which act on collecting tubules and promote reabsorption of water from filtrate into interstitium.

On other hand ANP/BNP activates in response to increase in Blood pressure .On overstimulation of sympathetic system or due to cardiac distension ANP/BNP are released from the heart walls .It act in blood vessels and stimulates vasidialation and,it also act on the collecting tubules in kidney and increase Na+ secretion into filtrate and along with Na+ ,water is also secretes from tubules decreasing the Plasma volume and maintaining the Blood pressure .Thus in this way Both Renin-Angiotensin-Aldosterone system and ANP/BNP pathway works antagonistic to each other and help in maintaining the Na+ and water homeostasis in body.