Sodium Retaining Hormones

Sodium depletion, by reducing plasma volume and renal perfusion, stimulates the production of renin (from the kidneys), which generates angiotensin in circulation. This hormone is a vasoconstrictor (so protects blood pressure), stimulates thirst (so helps to restore ECF volume), and, above all, stimulates sodium retention both directly (renally) and indirectly (by stimulating adrenal secretion of aldosterone); it thus reduces the sodium concentration of urine, feces, saliva, and sweat, but not milk.

Indices of aldosterone secretion (reduced sodium or potassium concentration in urine, feces, etc.) are often taken as evidence of sodium depletion or inadequate sodium intake, but the following points apply:

1. Aldosterone secretion is also stimulated directly by hyperkalemia (elevated plasma K+) and promotes potassium excretion.

2. Such interpretations involve a subjective judgement concerning adequate or excessive sodium intake. Because physiologists and clinicians were traditionally more concerned with sodium depletion as well as its consequences and the defenses against it, elevated aldosterone secretion was readily seen as a warning signal. However, if sodium intakes associated with increased aldos-terone have no other harmful effects, and especially if excess sodium intakes cause concern, low levels of aldosterone secretion might equally indicate excessive salt intake.

While sodium reabsorption in the distal nephron, influenced by aldosterone, is particularly important because it can produce sodium-free urine and promote potassium loss, the great majority of renal sodium reabsorption occurs elsewhere; about 25% in the loop of Henle and most in the proximal tubule. The loop is also a main site of magnesium reabsorption, hence the tendency for loop diuretics to cause hypomagnesemia.

The factors controlling proximal reabsorption are incompletely understood but their effect is clear: proximal reabsorption of sodium increases or decreases according to the need to enhance or diminish plasma volume. Since the fluid in the proximal tubule is similar to plasma, having been formed from it by glomerular filtration, it has the ideal composition for this purpose.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

Get My Free Ebook


Post a comment