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Overview of Disorders of Fluid Volume


James L. Lewis III

, MD, Brookwood Baptist Health and Saint Vincent’s Ascension Health, Birmingham

Reviewed/Revised Apr 2022 | Modified Sep 2022
Topic Resources

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.

Renin-angiotensin-aldosterone axis

The renin-angiotensin-aldosterone axis is the main regulatory mechanism of renal sodium excretion. In volume-depleted states, GFR and chloride delivery to the distal nephrons decreases, causing release of renin. Renin cleaves angiotensinogen (renin substrate) to form angiotensin I. Angiotensin-converting enzyme (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 blood pressure (has pressor activity)

  • Increases thirst

  • 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 Hyperkalemia Hyperkalemia is a serum potassium concentration > 5.5 mEq/L (> 5.5 mmol/L), usually resulting from decreased renal potassium excretion or abnormal movement of potassium out of cells. There... read more .

Other natriuretic factors

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 Heart Failure (HF) Heart failure (HF) is a syndrome of ventricular dysfunction. Left ventricular (LV) failure causes shortness of breath and fatigue, and right ventricular (RV) failure causes peripheral and abdominal... read more Heart Failure (HF) , chronic kidney disease Chronic Kidney Disease Chronic kidney disease (CKD) is long-standing, progressive deterioration of renal function. Symptoms develop slowly and in advanced stages include anorexia, nausea, vomiting, stomatitis, dysgeusia... read more Chronic Kidney Disease , cirrhosis Cirrhosis Cirrhosis is a late stage of hepatic fibrosis that has resulted in widespread distortion of normal hepatic architecture. Cirrhosis is characterized by regenerative nodules surrounded by dense... read more with ascites) and primary aldosteronism Primary Aldosteronism Primary aldosteronism is a disorder caused by autonomous production of aldosterone by the adrenal cortex (due to hyperplasia, adenoma, or carcinoma). Symptoms and signs include episodic weakness... read more and in some patients with primary hypertension Hypertension Hypertension is sustained elevation of resting systolic blood pressure (≥ 130 mm Hg), diastolic blood pressure (≥ 80 mm Hg), or both. Hypertension with no known cause (primary; formerly, essential... read more 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 blood pressure 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 and excess

Sodium depletion requires inadequate sodium intake plus abnormal losses from the skin, gastrointestinal tract, or kidneys (defective renal sodium conservation). Defective renal sodium conservation may be caused by primary renal disease, adrenal insufficiency Secondary Adrenal Insufficiency Secondary adrenal insufficiency is adrenal hypofunction due to a lack of adrenocorticotropic hormone (ACTH). Symptoms are the same as for Addison disease and include fatigue, weakness, weight... read more , or diuretic therapy.

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