Norris 8 Disorders of Fluid, Electrolyte, and Acid–Base Balance

What are the two major body fluid compartments?

Intracellular fluid (ICF) and extracellular fluid (ECF).

What fraction of total body water is inside cells (ICF) vs outside (ECF) in a healthy adult?

ICF about two-thirds; ECF about one-third of body water.

Which electrolyte is the most abundant intracellularly?

Potassium (K+).

What is the Na+/K+-ATPase pump and its function?

Pumps 3 Na+ out and 2 K+ in, using ATP; maintains cell volume and electrochemical gradients.

How does water cross cell membranes?

Through aquaporin channels (water channels).

Differentiate diffusion and osmosis.

Diffusion: movement of solutes along a concentration gradient. Osmosis: movement of water across a semi-permeable membrane toward higher solute concentration.

Define tonicity.

The effect of a solution’s effective osmotic pressure on cell size; isotonic, hypotonic, or hypertonic solutions.

What is an effective osmole vs an ineffective osmole?

Effective osmoles cannot cross the cell membrane and drive water movement; ineffective osmoles can cross and do not sustain tonicity (e.g., urea).

How is total body water distributed in adults (TBW, ICF, ECF)?

TBW ~60% of body weight; ICF ~40% of body weight; ECF ~20% (plasma ~4–5%, interstitial ~15%).

Name the four forces governing capillary–interstitial fluid exchange.

Capillary filtration pressure, capillary colloidal osmotic pressure, interstitial hydrostatic pressure, tissue colloidal osmotic pressure.

What is edema and what causes it?

Palpable swelling from increased interstitial fluid; caused by increased capillary pressure, decreased capillary oncotic pressure, increased capillary permeability, or lymphatic obstruction.

What is third-spacing?

Fluid sequestration in transcellular spaces (e.g., pericardial, pleural, peritoneal cavities) not readily exchangeable with the rest of the ECF.

What is third-space fluid commonly called when in serous cavities?

Effusion or ascites (depending on location).

What system primarily regulates body sodium and water balance?

The effective circulating volume ( monitored by baroreceptors); interacts with RAAS, ADH, ANP, and the sympathetic nervous system.

What is the effective circulating volume (ECV)?

The perfused vascular bed; low ECV triggers Na+/water retention, high ECV triggers decreased retention.

Which hormones regulate renal Na+ reabsorption and water excretion?

RAAS (renin–angiotensin–aldosterone system), ADH, and ANP; modulated by baroreceptors and SNS.

How does ADH affect water reabsorption in the kidney?

ADH increases water reabsorption by inserting aquaporin-2 channels in the collecting ducts.

DI vs SIADH: brief distinction?

Diabetes insipidus: ADH deficiency or renal insensitivity causing polyuria/polydipsia. SIADH: excessive ADH causing water retention and hyponatremia.

What triggers thirst and ADH release?

ECF osmolality changes and effective circulating volume changes; angiotensin II stimulates thirst as a backup; ADH responds to osmolality and volume.

Where is the majority of body potassium located?

Intracellular; 98% of body potassium is inside cells.

How is extracellular potassium regulated and eliminated?

Renal secretion in the late distal and cortical collecting tubules; aldosterone promotes K+ excretion; shifts between ICF and ECF also part of regulation.

What are common causes of hypokalemia?

Inadequate intake; renal losses (diuretics, hyperaldosteronism); GI losses (vomiting, diarrhea); transcellular shifts (alkalosis, insulin); magnesium deficiency.

What are common causes of hyperkalemia?

Decreased renal elimination (renal failure, certain meds); rapid IV potassium infusion; transcellular shifts (acidosis, tissue breakdown); ACE inhibitors/ARBs; potassium-sparing diuretics.

Where is calcium stored and what forms exist in the extracellular fluid?

99% in bone; remaining in ECF as ionized Ca2+ (~50%), protein-bound (~40%), and complexed (~10%); ionized Ca2+ is the active form.

What is hypoparathyroidism and its manifestations?

Deficient PTH; hypocalcemia and hyperphosphatemia; tetany, paresthesias, prolonged QT; Chvostek and Trousseau signs.

What is hyperparathyroidism and its consequences?

Overproduction of PTH; hypercalcemia; bone resorption; kidney stones; CKD-related bone disease (CKD–MBD).

How is phosphate balance regulated and what role does PTH play?

Phosphorus mainly in bone; filtered and reabsorbed by NPT2 in kidney; PTH inhibits NPT2, increasing phosphate excretion; phosphatonins (FGF23, sFRP4) also regulate.

What is hypophosphatemia: causes and manifestations?

Decreased intestinal absorption, transcellular shifts, and increased renal losses; neural and hematologic manifestations; treat with phosphate replacement carefully.

What is hyperphosphatemia: common causes and effects?

Usually decreased renal elimination (CKD); leads to hypocalcemia and CKD–MBD; treated with phosphate binders and dialysis in CKD.

Magnesium: distribution and roles; what happens in hypomagnesemia and hypermagnesemia?

Mostly intracellular; 1% extracellular; cofactor in many enzymes; hypomagnesemia can cause hypocalcemia and hypokalemia; hypermagnesemia from renal failure; treat with Mg replacement; severe cases may need calcium or dialysis.

What are the three major buffer systems for pH regulation?

Bicarbonate buffer system, proteins, and the transcellular H+/K+ exchange system.

What are volatile vs nonvolatile acids and how are they handled?

Volatile acids (CO2/H2CO3) are eliminated by lungs; nonvolatile (fixed) acids like sulfuric, phosphoric acids are buffered by buffers and excreted by kidneys.

What is the Henderson–Hasselbalch equation used to calculate pH in the bicarbonate system?

pH = pKa + log([HCO3−]/(0.03 × PCO2)); for bicarbonate system, pH = 6.1 + log([HCO3−]/(0.03 × PCO2)).

What is the normal serum osmolality range?

About 275–295 mOsm/kg.

What is osmolar gap and what does it indicate?

Osmolar gap = measured osmolality − calculated osmolality; a gap >10 mOsm suggests unmeasured osmotically active substances (e.g., ethanol, methanol, acetone, mannitol).

Name the three buffering systems in the renal tubules.

Phosphate buffer system (HPO4^2−/H2PO4−) and ammonia buffer system (NH3/NH4+); both generate new HCO3−.

What are the three lines of defense against acid–base disturbances and their order?

1) Chemical buffers (immediate), 2) lungs (rapid, adjust CO2), 3) kidneys (hours to days, adjust H+ and HCO3−).

What distinguishes metabolic acidosis from metabolic alkalosis?

Metabolic acidosis: decreased HCO3− and pH; metabolic alkalosis: increased HCO3− and pH; both have renal/respiratory compensations.

What distinguishes respiratory acidosis from respiratory alkalosis?

Respiratory acidosis: increased PCO2 and decreased pH; respiratory alkalosis: decreased PCO2 and increased pH; compensations by kidneys (HCO3−) or buffers.

What is base excess and base deficit?

Base excess indicates metabolic alkalosis; base deficit indicates metabolic acidosis; reflect non-respiratory acid–base balance.

How is the anion gap calculated and what does it indicate?

AG = Na+ − (Cl− + HCO3−); normal ~8–16 mEq/L; increased AG suggests unmeasured anions (MUDPILES) in high–anionic-gap acidosis; albumin levels affect the interpretation.

Why does the bone buffer during acute acidosis matter clinically?

Bone can release alkaline substances (e.g., NaHCO3, CaCO3) to buffer acids; buffering capacity can be up to ~40% of acute buffering; chronic acidosis risks bone demineralization and kidney stones.

What is the role of calcitonin in calcium balance?

Calcitonin helps remove calcium from the extracellular circulation; acts on bone and kidney to reduce serum calcium (less central than PTH/vitamin D).

What are the signs and risks of severe hyponatremia in the brain?

CNS symptoms: confusion, seizures, coma due to brain swelling from intracellular water gain; rapid correction risks osmotic demyelination.

What is isotonic fluid volume deficit and isotonic fluid volume excess?

Deficit: decreased ECF with proportionate Na and water loss; Excess: isotonic expansion of ECF with increased interstitial and vascular volumes; both are isotonic changes.

How does the RAAS respond to decreased effective circulating volume?

Renin release → angiotensin II → aldosterone; increased Na+ and water reabsorption; efferent arteriolar constriction to preserve GFR.

How does sodium concentration relate to ECF osmolality?

Sodium largely determines ECF osmolality; changes in Na+ are typically accompanied by water movements to maintain osmolality.

What is the approximate daily obligatory urine output that maintains metabolic waste excretion?

Approximately 300–500 mL/day (obligatory urine output); kidneys continue urine production even with fluid withholding.