Overview of Disorders of Fluid Volume
(See also Water and Sodium Balance.)
Because sodium is the major osmotically active ion in the extracellular fluid (ECF), total body sodium content determines ECF volume. Deficiency or excess of total body sodium content causes ECF volume depletion or volume overload. Serum sodium concentration does not necessarily reflect total body sodium.
Dietary intake and renal excretion regulate total body sodium content. When total sodium content and ECF volume are low, the kidneys increase sodium conservation. When total sodium content and ECF volume are high, sodium excretion (natriuresis) increases so that volume decreases.
Renal sodium excretion can be adjusted widely to match sodium intake. Renal sodium excretion requires delivery of sodium to the kidneys and so depends on renal blood flow and GFR. Thus, inadequate sodium excretion may be secondary to decreased renal blood flow, as in chronic kidney disease or heart failure.
The renin-angiotensin-aldosterone axis is the main regulatory mechanism of renal sodium excretion. In volume-depleted states, GFR and sodium delivery to the distal nephrons decreases, causing release of renin. Renin cleaves angiotensinogen (renin substrate) to form angiotensin I. ACE then cleaves angiotensin I to angiotensin II. Angiotensin II does the following:
Increases sodium retention by decreasing the filtered load of sodium and enhancing proximal tubular sodium reabsorption
Increases BP (has pressor activity)
Directly impairs water excretion
Stimulates the adrenal cortex to secrete aldosterone, which increases sodium reabsorption via multiple renal mechanisms
Angiotensin I can also be transformed to angiotensin III, which stimulates aldosterone release as much as angiotensin II but has much less pressor activity. Aldosterone release is also stimulated by hyperkalemia.
Several other natriuretic factors have been identified, including atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and a C-type natriuretic peptide (CNP).
ANP is secreted by cardiac atrial tissue. Concentration increases in response to ECF volume overload (eg, heart failure, chronic kidney disease, cirrhosis with ascites) and primary aldosteronism and in some patients with primary hypertension. Decreases have occurred in the subset of patients with nephrotic syndrome who have presumed ECF volume contraction. High concentrations increase sodium excretion and increase GFR even when BP is low.
BNP is synthesized mainly in the atria and left ventricle and has similar triggers and effects to ANP. BNP assays are readily available. High BNP concentration is used to diagnose volume overload.
CNP, in contrast to ANP and BNP, is primarily vasodilatory.
Sodium depletion requires inadequate sodium intake plus abnormal losses from the skin, GI tract, or kidneys (defective renal sodium conservation). Defective renal sodium conservation may be caused by primary renal disease, adrenal insufficiency, or diuretic therapy.