Hydration & Electrolytes

A set of practice flashcards covering electrolyte disorders, dehydration, osmolality, osmol gaps, and related physiology based on the provided lecture notes.

What are the major determinants of plasma osmolality?

Na+, Cl-, and HCO3- are the major contributors; water follows sodium. Minor contributors include glucose and urea.

What is contained ICF

primarily potassium (K+), magnesium (Mg2+), and phosphate (HPO4 2-), along with proteins and water.

What is the Osmcalc equation used in class?

Osmcalc = 2 × (Na+ + K+) + urea + glucose (an estimate of plasma osmolality from commonly measured solutes).

What is the osmolal (osm) gap and what does an elevated value indicate?

Osmol gap = measured osmolality − calculated osmolality. A gap > 10 suggests presence of an unmeasured osmotically active substance (eg, toxins like ethylene glycol, ethanol, methanol, paraldehyde, mannitol, or radiographic contrast).

Why does hypernatremia typically not increase the osmolal gap?

Because the excess Na+ is included in both the measured and calculated osmolality, so the gap remains unchanged.

Describe the three types of dehydration and how they affect osmolality.

Hypertonic dehydration: water loss > electrolyte loss (↑ osmolality); isotonic dehydration: equal loss of water and electrolytes; hypotonic dehydration: electrolyte loss > water loss (↓ osmolality).

What laboratory signs indicate dehydration?

Increased plasma proteins/albumin, highly concentrated urine (high USG), hemoconcentration (↑ PCV), and clinically evident skin tent; dehydration can be hypertonic, isotonic, or hypotonic.

What are common causes of hyponatremia and hypochloremia due to loss of NaCl-rich fluids?

Excess loss of NaCl-rich fluids via GIT (diarrhea, vomiting), kidneys, skin, or third-space losses

Decreased intake also contributes.

What additional mechanisms contribute to hypochloremia related to loss of HCl-rich fluids?

• Vomiting and reflux removal

• Sequestration (eg, LDA, duodenal/abomasal issues, pyloric blockage, ileus) can cause Cl− loss and hyponatremia; Cl− may be lost with gastric and abomasal contents depending on the situation.

What situations lead to increased total body water and dilutional hyponatremia & hypochloremia?

Edematous states (CHF, cirrhosis, nephrotic syndrome) and iatrogenic intravascular water excess with electrolyte dilution; electrolytes follow the added water.

How does sugar impact the osmolarity?

H2O shifting ICF → ECF

• hyperglycemia

• mannitol

How can dehydration lead to hypernatremia & hyperchloremia?

Dehydration reduces total body water, leading to increased serum sodium and chloride concentrations. As water is lost, the remaining electrolytes become more concentrated

What are some less common causes of hypernatremia & hyperchloremia?

• excess Na intake/retention

• inc body chloride (not common)

What causes hyperkalemia?

1. Shift from ICF → ECF

• acidemia

• muscle/tissue damage

• in vitro hemolysis

2. Inc TBK

   • dec excretion → acute renal failure, ruptured bladder, low aldosterone

   • iatrogenic

What causes hypokalemia?

1. dec intake → anorexia, poor diet

2. Shift from ECF → ICF → alkalemia

3. inc loss

   • GI, renal, burns, sweating horse

Which regulators control fluid volume and osmolality in water balance?

Fluid volume: aldosterone and atrial natriuretic peptide (ANP)

Osmolality: antidiuretic hormone (ADH/vasopressin) and thirst centers; water balance is adjusted by changing water or sodium rather than both at once.

How is water balance regulated?

• We are either adding or getting rid of sodium to either concentrate or dilute out sodium

• We are either adding or getting rid of water to either expand or contract blood volume

What forces govern water movement in the body?

1. Hydrostatic forces

2. Oncotic forces (proteins)

3. Osmotic forces (osmolality) → Na, Cl, K, urea, glucose

4. Aquaporins (ADH-regulated) in the collecting ducts facilitate water movement

What is the difference between measured osmolality and osmolcalc, and what is the osmol gap?

Measured osmolality is determined by freezing point depression → dog 300, cat 310 mOsm/kg

Osmolcalc is an estimate from major solutes (2[Na+ + K+] + urea + glucose)

Osmol gap = osmol_measured − osmolcalc; a gap >10 suggests unmeasured solutes

Inc Na leads to inc or dec ECF vol?

Inc

Dec Na leads to inc or dec ECF vol?

dec

Which substances are typically considered naturally occurring osmols in blood that affect osmolality?

Sodium, chloride, potassium, urea, and glucose are naturally occurring osmols; mannitol and ethylene glycol are not normally present but can affect the osmol gap when present.

What causes hyperosmolality?

1. inc solutes → normal gap: hypernatremia, hyperglycemia, azotemia/uremia. INC gap: ethylene glycol, mannitol, methanol, paraldehyde

2. dec ECF vlu → water loss > electrolyte loss

Laboratory evidence of dehydration

• inc plasma proteins → inc albumin

• highly concentrated USG

• hemoconcentration

• skin tent

• dehydration can be hypertonic, isotonic, hypotonic

Which analyte shows a definitive increase with dehydration and why?

Albumin (and other plasma proteins) increase due to hemoconcentration; decreases in plasma water concentrate circulating proteins.

Why does hypernatremia not cause an increased osmolal gap?

Because the excess sodium is accounted for in both measured and calculated osmolality; only exogenous osmoles (eg, ethylene glycol, mannitol) raise the gap.

Which conditions can cause an increased total body potassium?

Hypoaldosteronism, acute kidney injury, ruptured bladder (peritoneal potassium accumulation), and some cases of renal failure; in vitro hemolysis and acidosis alone do not increase total body potassium.

Which factors directly cause sodium movement in the body?

Aldosterone and atrial natriuretic peptide (ANP) directly affect sodium handling; antidiuretic hormone (ADH) regulates water, not sodium movement; thirst centers drive water intake.

Which conditions can cause hypokalemia?

Decreased intake (anorexia), shift from ECF to ICF (alkalemia), increased loss (diarrhea, GI losses, renal losses, sweating in horses), and alkalosis can contribute.

What happens to glucose measurement if the blood sample is left to clot for a long time?

Glucose decreases as blood sits in the clot (approximately 9–10% per hour); serum should be separated promptly to prevent falsely low glucose results.

Which substances, when present in the blood, increase the osmolality measured but not the calculated osmolarity, thereby raising the osmol gap?

Exogenous substances such as ethylene glycol, ethanol, methanol, paraldehyde, mannitol, and radiographic contrast medium increase the measured osmolality and raise the osmol gap.