Biochemical Investigation of Fluid & Electrolyte Balance PDF

Biochemical investigation of fluid and electrolyte balance Dr Azghar Ali 22/01/2025  Introduction The body is continuously adjusting as fluids and electrolytes move in and out of cells to maintain a nearly perfect balance. Even a slight sh...

Biochemical investigation of fluid and electrolyte balance Dr Azghar Ali 22/01/2025  Introduction The body is continuously adjusting as fluids and electrolytes move in and out of cells to maintain a nearly perfect balance. Even a slight shift in this balance can have significant effects on various body systems. To maintain homeostasis, the body carefully regulates water and electrolyte distribution, as well as acid-base balance. This regulation results from the complex interaction of cellular membrane forces, organ activities, and the actions of hormones both locally and systemically. Distribution of body fluid Intracellular and Extracellular Compartments Fluid and Solute Movement: Occurs within the body through mechanisms Osmotic Hydrostatic Osmosis pressure pressure Active Diffusion Filtration transport  Extracellular and Intracellular Fluids compostion Intracellular and extracellular fluids are separated by semi-permeable membranes, which gives them different ionic compositions. Intracellular Compartment: - Dominant cations: K⁺ (potassium) and Mg²⁺ (magnesium). - Dominant anions: phosphates and sulfates Extracellular Compartment: - Main cations: Na⁺ (sodium). - Main anions: Cl⁻ (chloride). Donnan Equilibrium: This is an ionic equilibrium between two solutions separated by a semi-permeable membrane Σ of anions = Σ of cations in each compartment  Extracellular and Intracellular Fluids compostion In practice, we measure at the plasma level: Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻, and proteins, but not Ph⁻, SO₄²⁻, or RCOO⁻ (the anion gap is approximately 10 mEq/L, and its widening holds significant clinical and prognostic value). The urinary ionogram mainly involves measuring Na⁺, K⁺, and the Na⁺/K⁺ ratio, but these parameters can vary widely. Osmolality=2 [Na(+)]+glucose (mg/dL)/18+BUN (mg/dL)/2.8 = 275-295mOsm/kg of water. Is also the simplest and best formula to calculate plasma osmolality. The concentration of only effective osmoles evaluates effective osmolality or tonicity as: Effective osmolality=2 [Na(+)]+glucose/18 = 275-295mOsm/kg of water. *Blood Urea Nitrogen  Water Balance  Water electrolyte Balance The average adult maintains a fluid balance of 2,500 ml of fluid intake corresponding to 2,500 ml of fluid output. Elimination of fluids: This takes place mainly through the kidneys in the form of urine, but also through the skin (perspiration), the gastrointestinal tract (faeces) and the lungs when we breathe. 60% of daily fluid production is in the form of urine, with a normal production of around 1,500 ml per day when fluid intake is sufficient. Decreased urine production is an early sign of dehydration or kidney dysfunction. 40% of daily fluid production is lost through the skin, gastrointestinal tract and lungs, which cannot be measured.  Fluid and Electrolyte Regulation Water Balance Mechanisms: Controlled by ADH, thirst, and the Renin- Angiotensin-Aldosterone System (RAAS). Thirst signals are triggered by increased sodium levels and serum osmolality, prompting fluid intake. Osmoreceptors located in the hypothalamus, they detect increased serum osmolality and release ADH to retain fluid while also stimulating thirst. Effective thirst response requires mental and physical ability, accessible fluids, and physical strength to drink. Impaired thirst perception, especially in older adults, increases the risk of dehydration. The average adult consumes around 2,500 mL of fluids daily, with increased needs during conditions like fever, vomiting, diarrhea, or bleeding.  Renin- Angiotensin- Aldosterone System  Renin-Angiotensin-Aldosterone System "Aldosterone saves salt" and "Water follows salt" Regulates fluid output and blood pressure. Decreased blood pressure, often due to fluid loss, stimulates the kidneys to release renin. Renin acts on angiotensinogen from the liver, converting it to angiotensin I, which is further converted to angiotensin II. Actions of Angiotensin II: - Causes vasoconstriction to improve blood flow to vital organs. - Stimulates the adrenal cortex to release aldosterone. Aldosterone: A steroid hormone that promotes sodium reabsorption by the kidneys, leading to increased serum osmolality. Increased serum osmolality causes fluid to move into the intravascular compartment to balance solute particles. The increased fluid volume in the blood raises blood pressure.  Fluid and Electrolyte Regulation The Kallikrein-Kallidin System (vasodilatory and natriuretic action). Catecholamines and Prostaglandins (redistribution of renal blood flow).  Biochemical investigation of fluid and electrolyte balance I. Sample Collection: Dry Tube: For serum testing. Lithium Heparinate: For plasma testing. Avoid collecting hemolyzed samples, especially for potassium (K⁺) measurements. EDTA: For complete blood count (CBC) and hematocrit (Hte) analysis.  Biochemical investigation of fluid and electrolyte balance II. Testing Methods: A) Hematocrit (Hte): - Represents the volume ratio of red blood cells (RBC) to total blood, used to assess plasma volume. - Reference Values: - Men: 40 – 52% - Women: 37 – 46%  Biochemical investigation of fluid and electrolyte balance B) Blood Ionogram: Na+ and K+: Selective Electrodes +++: Measures the potential difference between two interfaces of a selective membrane for the ion being measured. Flame Photometry Colorimetric method Cl-: Selective Electrodes Other Methods: Coulometry (Chloridometer); Mercurometric Method; Microtitration.  Biochemical investigation of fluid and electrolyte balance: Reference or Usual Values Blood: Urine: Cerebrospinal Fluid (CSF): Na+: 50 to 250 mmol/24h Na+: 130 to 142 mmol/L Na+: 137 to 145 mmol/L Adaptation: 0 to 400 mmol/24h K+: 3 to 4 mmol/L K+: 3.5 to 5 mmol/L K+: 25 to 150 mmol/24h Cl-: 125 to 130 mmol/L Cl-: 110 to 260 mmol/24h Cl-: 95 to 105 mmol/L Note: The urinary Na+/K+ ratio is >1 in a normal individual. Pathological variations fluid and electrolyte Classification and causes of hypernatremia  Hypokalemia Heavy Fluid Loss Inadequate Too much water (NasoGastric suction, Drugs (laxatives, consumption of intake (dilutes the vomiting, diarrhea, diuretics, Potassium (Nil Per potassium) wound drainage, corticosteroids) os, anorexia) sweating) Cushing’s Syndrome (during this condition the adrenal glands produce excessive amounts of cortisol (if cortisol levels are excessive enough, they will start to affect the action of the Na+/K+ pump which will have properties like aldosterone and cause the body to retain sodium/water but waste potassium).  Pathological variations fluid and electrolyte Chloride levels tend to follow sodium movements. A) Plasma Chloride: Decreased Chloride Levels (Hypochloremia): Salt loss (e.g., due to excessive sweating, vomiting, or diuretics) Low-salt diet Severe diarrhea Addison's disease (adrenal insufficiency) Increased Chloride Levels (Hyperchloremia): Acidosis, especially of renal origin, as chloride levels often vary inversely with bicarbonate levels (compensatory mechanisms).  Pathological variations fluid and electrolyte B) Chloride in Cerebrospinal Fluid (CSF): Decreased chloride levels may indicate tuberculous meningitis. C) Chloride in Sweat: Normal Range (NR): 50 mmol/L Pathological Value: >50 mmol/L, highly suggestive of cystic fibrosis (CF). Cystic fibrosis is associated with significantly elevated sweat chloride levels due to defective chloride channel function (CFTR mutation).  Regulatory Homeostasis Mechanisms Clinical Biochemical Relevance investigations