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Why does low $\ce{Na+}$ levels cause hypotension (low blood pressure). Alternatively, why does excessive intake of $\ce{Na+}$ cause hypertension or high blood pressure?

From what I understand, there are two different mechanisms at play. Renin secretion from kidneys occur during low salt levels in blood and hence, under low sodium concentration, high amount of Renin secretion from kidneys should actually raise the blood pressure, via the Renin-Angiotensin loop, constricting the arteries. Under high sodium concentrations, low renin secretion should be accompanied by low pressure. If the osmotic entry of water into blood raising the blood volume and thus pressure, plays a role here, will not, on increase of blood volume, decreased Vasopressin increase urinary output and thus control the blood volume, not allowing it to cause any body changes? Why isn't the first effect (renin) significant here as compared to the second (osmotic entry).

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Renin angiotensin is for regaining homeostasis. If you have lost salts, blood pressure and blood volume will decrease due to loss of fluids. But the renin angiotensin mechanism will try to undo this by stimulating uptake of Na and water.But still, they can not fully compensate for loss of Na , INTAKE of Na is NECESSARY. –  biogirl Dec 6 '13 at 9:18
By the way, the main stimulus for triggering renin angiotensin is Blood volume and pressure rather than Osmolarity of blood. In conditions like hemorrhage where blood loss occurs but there is no change in osmolarity, Renin-Angiotensin will still respond ( but ADH(vasopressin) will not as ADH responds only to change in osmolarity.) –  biogirl Dec 6 '13 at 9:20
@biogirl Thank you. This helped to a considerable extent. :) –  Satwik Pasani Dec 11 '13 at 15:06
I think it will help you a lot if you read these topics from Tortora and Derrickson or other physiology books. –  biogirl Dec 11 '13 at 15:46
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