The fluid balance in the body is closely linked to the presence and amount of sodium ion in the blood.
When we eat salty foods, our blood sodium increases, which causes increased blood tonicity. Nerve centers of the hypothalamus, the headquarters centers detect this increase in tone and produce the thirsty feeling.
If you drink water, it will dilute your blood, lowering your tonicity to normal levels. Blood volume, however, increases, a situation that must be corrected immediately so that there is no increase in blood pressure. Restoring blood volume to its normal level is achieved by decreasing ADH production, which results in greater elimination of water in the urine.
The amount of sodium in your blood is controlled by aldosterone hormone, secreted by the adrenal (adrenal) gland cortex. When the amount of sodium in your blood goes down, aldosterone secretion increases. This hormone acts on the distal tubules and the collecting tubules, stimulating sodium reabsorption from the glomerular filtrate.
The secretion of the hormone aldosterone, in turn, is regulated by renina and angiotensin. If blood pressure or sodium concentration decreases, the kidneys release renin into the blood.
Renin is an enzyme that catalyzes the formation of a blood protein called angiotensin, which causes the caliber of blood vessels to decrease. There is thus an increase in blood pressure, which stimulates aldosterone secretion. This, in turn, leads to increased reabsorption of sodium by the kidneys.