Fluid and Electrolyte Balance: Homeostasis, Solutions, and Disorders

Homeostasis

Process that maintains stable internal conditions—fluids, electrolytes, temperature.

60-40-20 Rule

Total body water ~60% body weight; ICF = 40%, ECF = 20%.

Intracellular Fluid (ICF)

Fluid inside cells; rich in K+, Mg2+, phosphate, proteins.

Extracellular Fluid (ECF)

Fluid outside cells; includes interstitial and intravascular compartments.

Osmosis

Water movement from low solute to high solute concentration.

Osmotic Pressure

Pulling force that draws water toward solutes (solvent movement into area of higher solute).

Hydrostatic Pressure

Pushing force of fluid against vessel walls that pushes fluid out into interstitial space.

Oncotic Pressure

Colloid osmotic pressure from proteins (albumin) that pulls fluid back into capillaries.

Isotonic Solution

Equal solute concentration as plasma; no major fluid shifts (0.9% NaCl, LR).

Hypotonic Solution

Lower solute outside → water moves into cells → cells swell (0.45% NaCl).

Hypertonic Solution

Higher solute outside → water moves out of cells → cells shrink (3% NaCl).

Fluid Volume Excess (Hypervolemia)

Too much intravascular fluid → edema, JVD, crackles, HTN.

Causes of Fluid Excess

Heart failure, renal failure, excessive IV fluids, high sodium intake.

Fluid Volume Deficit (Hypovolemia)

Low intravascular volume → hypotension, tachycardia, dry mucous membranes.

Causes of Fluid Deficit

Vomiting, diarrhea, hemorrhage, burns, excessive sweating, diabetes insipidus.

Daily Fluid Intake

About 2200-2700 mL/day for adults (typical average).

Thirst (Hypothalamus)

Osmoreceptors in hypothalamus detect increased osmolality and trigger thirst.

ADH (Posterior Pituitary)

Promotes water reabsorption in kidneys; secreted when osmolality is high.

Aldosterone (Adrenal Cortex)

Promotes Na+ and water reabsorption and K+ excretion.

RAAS (Renin-Angiotensin-Aldosterone System)

Activated when perfusion is low; raises BP by vasoconstriction and aldosterone release.

ANP (Atrial Natriuretic Peptide)

Released from atria when stretched; promotes Na+ excretion and lowers blood volume.

Edema

Excess fluid in interstitial space due to increased hydrostatic pressure, decreased oncotic pressure, or capillary leak.

Pitting Edema

When pressure leaves indent in skin; graded by severity.

Urine Specific Gravity

Normal ~1.005-1.030; high in dehydration, low in overhydration.

BUN/Creatinine (Dehydration clue)

Elevated BUN with normal creatinine suggests dehydration/volume depletion.

Daily Weights

Best objective measure of fluid status (same time, same clothes).