Sodium & Water
This ion is the primary determinant of extracellular fluid volume
Sodium
This fluid compartment makes up about two-thirds of total body water.
Intracellular Fluid (ICF)
A lab reports plasma osmolality of 290 mOsm/kg. Define osmolality and explain what this number represents.
Osmolality is the number of dissolved particles per kg of water; 290 mOsm/kg reflects the total solute concentration in plasma, including sodium, glucose, and urea.
In the nephron, this segment is permeable to water but not solutes, allowing water to leave and concentrate the tubular fluid.
The descending limb of the loop of Henle concentrates tubular fluid as water moves into the medullary interstitium.
What is the karyotype for Klinefelter Syndrome?
XXY
A patient has been vomiting for several days, resulting in fluid loss. Their blood pressure remains normal. Explain why extracellular fluid volume is decreased, but effective circulating volume may still appear normal.
ECF volume decreases due to total fluid loss, but effective circulating volume (ECV) is preserved because cardiac output and perfusion remain adequate.
A 70-kg patient has total body water of 42 L. Approximately 27 L is located inside cells. Which ion primarily maintains this compartment?
Potassium maintains intracellular fluid by balancing osmotic forces across cell membranes.
This property cannot be directly measured in the lab but can be inferred using laboratory values
Tonicity
This segment actively reabsorbs Na⁺, K⁺, and Cl⁻ but is impermeable to water, helping to generate the medullary osmotic gradient.
The thick ascending limb (TAL) of the loop of Henle creates a hypertonic medulla by pumping solutes out without water movement.
What's the classic translocation for APL?
t(15;17)
This hormone increases sodium reabsorption in the distal nephron and collecting duct.
Aldosterone
In a 70-kg patient, this is the approximate total body water.
42 liters
These three substances are the major contributors to plasma osmolality.
Sodium, Glucose, urea (BUN)
The vasa recta preserves the medullary osmotic gradient. Explain how it does this.
Blood flows in a countercurrent pattern, allowing solutes and water to move without dissipating the gradient, maintaining the kidney’s ability to concentrate urine.
What are the four components of Tetralogy of Fallot?
(1) ventricular septal defect (VSD)
(2) pulmonary stenosis (RVOT obstruction)
(3) overriding aorta
(4) right ventricular hypertrophy (RVH)
A patient receives 0.9% normal saline for volume resuscitation. Explain how this fluid affects intracellular and extracellular volumes.
Normal saline is isotonic, so it expands the extracellular fluid without causing water to shift into or out of cells
This force helps determine fluid movement between capillaries and interstitial space.
Starling Forces
Urea is elevated in a patient. Explain why this increases plasma osmolality but does not change cell volume.
Urea diffuses freely across cell membranes, so it raises measured osmolality but does not create a sustained osmotic gradient or water shift—making it an ineffective osmole.
A patient’s plasma osmolality rises to 320 mOsm/kg. Explain how the countercurrent mechanism and ADH respond.
High osmolality triggers ADH release, increasing water reabsorption in collecting ducts, while the medullary gradient from the loop of Henle allows urine to be concentrated to conserve water.
What virulence factor of TB inhibits neutrophils and activated macrophages?
Cord factor
A patient with heart failure presents with edema. Their extracellular fluid is increased, but they are hypotensive. Explain why the kidneys retain sodium in this case.
ECF is high, but ECV is low due to reduced perfusion, so the kidneys activate sodium-retaining mechanisms like RAAS to restore effective circulating volume.
A patient receives 5% dextrose without insulin. Explain where the water and solute distribute.
Glucose remains largely extracellular because it cannot enter cells without insulin; water distributes in extracellular compartments, but minimal enters cells
A patient presents with very high blood sugar but no insulin activity. Water shifts out of cells into the bloodstream, increasing plasma tonicity. In this scenario, glucose is acting as what type of osmole?
Conditional osmole
A patient with low plasma osmolality (250 mOsm/kg) but significant volume depletion (>10% ECV loss) requires water retention. Explain how the kidney prioritizes volume over tonicity.
Baroreceptors detect low circulating volume, overriding osmotic control; ADH is released despite hypotonic plasma to conserve water and maintain perfusion, which may worsen hyponatremia.
What is the embryological dysfunction in Congenital High Airway Obstruction Syndrome (CHAOS)?
Failure of recanalization of the larynx/ laryngeal webs