Why is this? And why the heck do we have to eat so much salt?
Well, there is this thing called the
It's a big deal. It takes place mainly in your kidneys and adrenal cortex, and affects your blood pressure directly. Lets break down the three components:
1. Renin is a protein (enzyme) released by special kidney cells when you have decreased salt (sodium levels) or low blood volume, and stimulates the formation of Angiotensin in blood and tissues.
2. Angiotensin then undergoes a series of reactions that convert it to Angiotensin II (AII in the diagram below), which in turn stimulates the release of Aldosterone from the adrenal cortex.
3. Aldosterone is the main mineralocorticoid (steroid hormones that balance electrolytes) hormone secreted by the adrenal cortex, the principal biological activity of which is the regulation of electrolyte and water balance by promoting the retention of sodium (and, therefore, of water) and the excretion of potassium. In plain English - Aldosterone helps your kidneys retain salt, and therefor retain fluids, helping to keep blood volume and blood pressure up.
Here is a diagram of the entire system.