Overhydration and Fluid Volume Excess

Fluid volume excess (FVE), also called hypervolemia or overhydration, occurs when the body retains more fluid than it can excrete, leading to expansion of the extracellular fluid compartment. The regulation of fluid balance depends on three interconnected mechanisms: the renin-angiotensin-aldosterone system (RAAS), antidiuretic hormone (ADH) from the posterior pituitary, and atrial natriuretic peptide (ANP) from the cardiac atria. In normal physiology, when blood volume increases, stretch receptors in the atrial walls detect the increased pressure and release ANP, which promotes sodium and water excretion by the kidneys. Simultaneously, the RAAS is suppressed, reducing aldosterone-mediated sodium reabsorption. ADH secretion decreases, reducing water reabsorption in the renal collecting ducts. However, when these regulatory mechanisms fail (as in heart failure, liver cirrhosis, or renal failure) or are overwhelmed (as with excessive IV fluid administration or excessive sodium intake), fluid accumulates in the vascular and interstitial spaces. The excess fluid increases hydrostatic pressure within the capillaries, pushing fluid out of the vascular space and into the interstitial tissues (edema). When hydrostatic pressure exceeds oncotic pressure in the pulmonary capillaries, fluid moves into the alveolar...