Episode Details

Fluid Compartments & Homeostasis

• Distribution: Water is 50%–60% of adult weight (varies with fat). ICF holds 2/3 of water; ECF holds 1/3 (interstitial fluid/plasma). Weight change is the best fluid shift indicator (1 kg = 1 L).
• Capillary Exchange: Governed by hydrostatic (pushes out) and plasma oncotic (albumin pulls in) pressures. Edema occurs if hydrostatic pressure rises, oncotic falls, or lymphatics are blocked. Third-spacing traps nonfunctional fluid.

Regulatory Mechanisms

• Hormones: Dehydration triggers thirst and pituitary ADH, raising renal water reabsorption. Aldosterone promotes sodium retention and potassium excretion. Atrial stretch releases natriuretic peptides (ANP/BNP) to excrete sodium/water.

Electrolytes & Clinical Imbalances

• Sodium (136–145 mEq/L): Governs ECF osmolality (280–295 mOsm/kg) and impacts CNS. Hypernatremia (>145 mEq/L) causes cell shrinkage; rapid correction risks cerebral edema. Hyponatremia (<136 mEq/L) causes cell swelling; rapid correction risks osmotic demyelination.
• Potassium (3.5–5.0 mEq/L): Dictates membrane potentials. Hyperkalemia (>5.0 mEq/L) causes peaked T waves, wide QRS, and arrest. Treatment: stop intake, diuretics/binders, shift K+ with insulin/beta-agonists, and stabilize heart with IV calcium. Hypokalemia (<3.5 mEq/L) causes flat T waves, U waves, and weakness. IV KCl must be diluted, infused slowly (<10 mEq/hr), and never pushed.
• Divalent Cations: Calcium (9.0–10.5 mg/dL) is regulated by PTH and calcitonin. Hypercalcemia sedates nerves/muscles; hypocalcemia causes tetany with positive Chvostek/Trousseau signs. Magnesium (1.3–2.1 mEq/L) is vital for ATP. Hypomagnesemia (<1.3 mEq/L) resembles hypocalcemia, causing hyperactive reflexes and torsades.

Acid-Base Balance (pH 7.35–7.45)

Regulated by buffers, lung CO2 excretion, and renal bicarbonate/H+ control:

1. Respiratory Acidosis: Carbonic acid excess from hypoventilation (CO2 retention).
2. Respiratory Alkalosis: Carbonic acid deficit from hyperventilation (CO2 depletion).
3. Metabolic Acidosis: Bicarbonate deficit or acid buildup; anion gap is 8–12 mmol/L.
4. Metabolic Alkalosis: Bicarbonate excess or acid loss (vomiting/NG suction).

Crystalloid Solutions

• Tonicity Effects: Hypotonic (e.g., 0.45% NaCl) dilutes ECF, swelling cells to treat hypernatremia. Isotonic (e.g., 0.9% NaCl, Lactated Ringer's) expands ECF volume without shifts. Hypertonic (e.g., 3.0% NaCl) draws water out of cells to treat hyponatremia.