1. Intracellular
2. Extracellular
3. Extracellular is further divided into interstitial and intravascular
4. Transcellular

1. Two thirds is in the ICF, one third is in ECF
2. Of the ECF, 25% is intravascular and 75% is interstitial

1. The dextrose of D5 is metabolized, and it is distributed throughout all fluid compartments (that is, it is distributed in the same ratio that the rest of the body's fluids are distributed)
2. 100% of NS is in the ECF (0% in the ICF). It follows normal distribution for ECF (75% interstitial, 25% intravascular)

1. Intracellular vs. extracellular distribution is governed by osmotic forces
2. Intravascular vs interstitial distribution is governed by hydrostatic and oncotic (colloid) pressure

1. Plasma permeability increases, resulting in leakage of albumin from the intravascular to the interstitial space
2. Plasma oncotic pressure reduces, promoting movement of fluid from the intravascular to the interstitial space (third spacing)

1. Intestinal obstruction
2. Severe pancreatitis
3. Crush injuries
4. Bleeding
5. Peritonitis
6. Obstruction of a major venous system

1. Abnormalities in renal excretion
2. Excessive losses from GI tract (vomiting, GI suctioning, diarrhea)

Isotonic hyponatremia occurs with excess plasma proteins or lipids (pseudohyponatremia)

1. Hypervolemic hyponatremia: CHF, renal failure, cirrhosis (body can't remove excess fluid)
2. Euvolemic hyponatremia: SIADH, psychogenic polydipsia (inadequate solute intake given heavy losses)
3. Hypovolemic hyponatremic: diuresis, heavy GI losses (deficit of fluid and sodium)

1. Hypervolemic: edema
2. Euvolemic: normal pulse, BP, skin turgor, no edema
3. Hypovolemic: tachycardia, decreased BP, decreased skin turgor

1. Hypervolemic: sodium and water restriction
2. Euvolemic: water restriction
3. Hypovolemic: volume expansion

1. Patients release excessive amounts of anti-diuretic hormone (ADH)
2. Patients have normal sodium intake, but retain excessive amounts of water due to excessive ADH release

1. Hypervolemic hypernatremia: excessive saline administration or mineralcorticoid excess
2. Euvolemic hypernatremia: diabetes insipidus or insensible extrarenal losses
3. Hypovolemic hypernatremia: diuretics, glycosuria, profuse sweating/diarrhea

1. Hypervolemic hypernatremia: diuretics, water replacement, dialysis
2. Euvolemic hypernatremia: water replacement
3. Hypovolemic hypernatremia: volume expansion w/ hypotonic saline

1. Losses via urine or stool
2. Metabolic alkalosis
3. Increased insulin
4. Increased catecholamines (epinephrine)
5. Medications

1. Oral supplementation is safer and generally preferred
2. IV supplementation is reserved for treatment of severe hypokalemia or when the GI tract precludes use of oral supplements

1. Use of D5 as a diluent should be avoided due to possibility of worsening hypokalemia
2. Hypomagnesemia should be corrected because it may result in refractory hypokalemia

1. A relatively large decrease in total body potassium is needed to effect a small decrease in serum levels
2. A small excess of total body potassium will elicit a sharp increase in serum potassium

1. Renal insufficiency
2. Metabolic acidosis
3. Tissue catabolism
4. Pseudohyperkalemia (hemolysis)

1. An elevated base excess indicates metabolic alkalosis
2. A decreased base excess represents metabolic acidosis