Electrolytes.docx

Colligative Properties: Depend on the number of molecules in a solution.
- Boiling Point: Increases when non-volatile solute (e.g., NaCl) is added to the solvent.
- Freezing Point: Decreases upon adding solute.
- Osmotic Pressure: Requires pressure to stop osmosis.
  - Osmosis: Movement of solvent across a semi-permeable membrane from lower to higher concentration.
  - Factors: Fluids move from lower to higher concentration.
- Vapor Pressure: Decreases when a non-volatile solute is added.
Osmolality: Measures dissolved particles in a solution (osmoles per kg of water).
- Serum Osmolality: Expressed as milliosmoles/kg; reference range: 275-295 mOsm/kg.
Osmolality Regulation in Blood:
- Thirst Sensation: Triggered by dehydration via signals from the hypothalamus.
- Secretion of ADH (Antidiuretic Hormone): Stimulated during dehydration; acts on the kidneys.
Osmometry: Method for measuring all particles in a solution.
- Calculation formulas:
  - 1.86 Na + glucose/18 + BUN/2.8 + 9 = mOsm/kg
  - 2(Na) + glucose/20 + BUN/3 = mOsm/kg
- Normal Values:
  - BUN: 6-20 mg/dL
  - Glucose: 70-100 mg/dL
  - Na: 275-295 mOsm/kg.
Osmolal Gap: Difference between measured and calculated osmolality.
- Normal Gap: Less than 15.
  - Causes:
    - Increased production of B-hydroxybutyrate during starvation.
    - Ingestion of ethylene glycol (antifreeze) increases osmolal gap.

Serum and urine osmolality important for acid-base status.
Key Molecules: Na, Cl, glucose, and urea.
- Freezing Point Depression Osmometry: Decrease in temperature related to the number of particles present.
- Vapor Pressure Depression Osmometry: Inverse relationship between osmolality and vapor pressure.

Charged Ions:
- Cations (Positively Charged): Sodium, potassium, calcium, magnesium.
- Anions (Negatively Charged): Chloride, bicarbonate, phosphate, sulfate, organic acids, proteins.

Major Cation of ECF: 90% of extracellular fluid.
Reference Range:
- Serum/Plasma: 136-140 mEq/L
- 24-hr Urine: 40-220 mEq/L
- CSF: 136-150 mEq/L.
Changes in sodium affect plasma volume:
- Hypernatremia: Increased serum sodium leads to cell shrinkage.
- Hyponatremia: Decreased serum sodium causes cell swelling.
Regulation Factors:
- Water intake and excretion, affected by hormones (aldosterone, angiotensin II, ANP).
- Renal Handling: 60-70% filtered Na reabsorbed mostly in PCT with bicarbonate and water.

Water loss or sodium gain.
Causes:
- Gastrointestinal losses (vomiting, diarrhea).
- Excessive sweating (fever, exercise).
- Diabetes Insipidus (central and nephrogenic).
- Hyperaldosteronism.

< 145 mEq/L.
Depletional:
- Renal losses (tubular damage, RTA).
- Non-renal losses (GIT loss, excessive sweating).
Dilutional:
- SIADH leading to water retention.
- Generalized edema (CHF, cirrhosis).

Major Intracellular Cation, lesser in ECF.
Reference Range:
- Serum: 3.5 – 5 mmol/L
- Plasma: 3.5 – 4.5 mmol/L
- Newborn: 3.7 – 5.9 mmol/L.
Regulation and Clinical Significance:
- Factors affecting levels include renal handling, insulin, disturbances in potassium distribution, and medication effects.

5 mmol/L, emergency if > 6 mmol/L.
Causes:
- Increased intake (replacements).
- Cellular shifts (acidosis, chemotherapy).
- Decreased renal excretion (acute or chronic renal failure).

< 3.5 mmol/L.
Causes:
- Increased cellular uptake (alkalosis, insulin overdose).
- Renal losses (diuretics, nephritis).
- GIT losses.

Specimen Types: Serum, plasma, urine.
Methods: Colorimetric method, flame emission spectrophotometry, atomic absorption spectrophotometry, ion-selective electrode.