water and sodium balance

Body Fluid Compartments

• 60% of the human body is water
• Water and sodium balance is crucial
• Two compartments: intracellular fluid (ICF) and extracellular fluid (ECF)
• ECF includes blood and interstitial fluid
• Water follows sodium
• Sodium loss leads to water loss and dehydration
• Kidneys regulate ECF volume

Changes in Water and Sodium Amounts

• Loss or gain of 5 liters of water affects individual compartments
• Sodium loss (along with water loss) causes dehydration
• Consequences of fluid loss in ICF include cell dysfunction (lethargy, confusion, coma)
• Consequences of fluid loss in ECF include dehydration and blood loss

Osmolality

• Measures solute particles per unit of solvent, irrespective of solute identity
• Concentrated solutions have high osmolality; dilute solutions have low osmolality
• ICF and ECF osmolality are maintained equal by water movement
• Increased blood osmolality stimulates ADH (antidiuretic hormone) secretion
• Measured vs. calculated osmolality used for clinical analysis

Calculation of Osmolality

• Derived from sodium, blood urea nitrogen (BUN), and glucose
• Simplest formula: Serum osmolality (mmol/Kg) = 2 x serum sodium (mmol/L)
• When serum urea and glucose are outside reference ranges, use a more complex formula
• Osmolal gap (measured minus calculated): A gap less than 10 mOsm/Kg is usually normal, while larger gaps may suggest other issues
• Osmometers measure serum and urine osmolality

Osmolality Clinical Significance

• Reference ranges: serum 275–295 mOsm/kg, urine 300–900 mOsm/kg
• Homeostasis: Maintaining balance in the body
• Diseases, medications, and toxins can impact osmolality values
• Significant changes (<240 mOsm/Kg or >320 mOsm/Kg) need immediate medical intervention

Electrolytes

• Na+, K+, Cl−, and HCO3− are major electrolytes
• Function:
  • Na+: major extracellular cation
  • K+: major intracellular cation
• Electrolytes create osmotic pressure within and outside of cells
• Kidneys reabsorb/excrete water and electrolytes
• Loss of Na+ / K+ with water loss leads to constant electrolyte concentration

Electrolyte Regulation

• Water and sodium levels vary, but homeostasis maintains balance
• ECF volume must be maintained for survival
• ADH (antidiuretic hormone, AVP) maintains constant water levels
• Hypothalamus senses changes in osmolality in the ECF
• High osmolality stimulates ADH release; low osmolality inhibits ADH release

Sodium and Its Physiological Regulation

• Sources: primarily table salt and found in extracellular fluid (ECF)
• Total body sodium generally stays constant.
• Sodium regulation: two main hormones
  • Renin-angiotensin-aldosterone system (RAAS): triggered by decreased ECF volume
  • Atrial natriuretic peptide (ANP): triggered by increased ECF volume

Effects of Low Sodium (Hyponatremia)

• General causes: water retention and sodium depletion
• If sodium is lost, water is also lost
• Decrease in ECF volume may occur
• Hyponatremia with hypovolemia strongly suggests sodium depletion

Assessment and Management of Hyponatremia

• Assess excess water or insufficient sodium in the body
• Common symptoms: nonspecific (lethargy, headache, confusion, dizziness)
• No history of fluid loss suggests water retention
• Decreased ECF and blood volume may be a factor
• Sodium depletion may also be present

Clients with Edema

• Causes: heart failure or hypoalbuminemia and increase in aldosterone
• Increased ECF volume
• Treatment depends on volume status
• Hypovolemic clients (sodium depleted): treatment varies and often needs specific considerations
• Normovolemic clients (retaining H2O): further investigation varies based on clinical presentation for appropriate treatment
• Edematous clients (retaining Na+ and H2O): diuretics may help as appropriate to treat condition

Effects of High Sodium (Hypernatremia)

• Hypernatremia is commonly caused by water loss
• Clinical characteristics: this is associated with normal sodium, sodium depletion and sodium gain

Fluid Changes and Potassium Imbalances

• Hypokalemia:
  • Clinical characteristics: increased losses and redistribution to cells
  • Causes: history (vomiting, diarrhea, diuretics, metabolic alkalosis).
  • Consequences result in symptoms in excitable tissues (muscle weakness, hyporeflexia, cardiac arrhythmias).
  • Treatment: oral and IV potassium
• Hyperkalemia:
  • Potassium balance is tightly controlled; 98% is intracellular
  • Causes include decreased excretion, hypoaldosteronism, metabolic acidosis, and dietary intake.
  • Consequences: cardiac arrest is severe
  • Treatment: insulin (with glucose), dialysis for refractory cases.

Investigation of Renal Function

• Renal functions: electrolyte balance, fluid balance, waste product removal -Glomerular function: measured by glomerular filtration rate (GFR). Reduced GFR associated with disease progression -Tubular function: measures kidney's ability to concentrate urine Urine osmolality is commonly looked at to assess water reabsorption.

Specific Tubular Defects - Kidney Stones

• Common cause of obstruction in the urinary tract
• Types of stones: calcium phosphate, magnesium ammonium phosphate, oxalate, uric acid, cystine

Acute Kidney Injury (AKI)

• Rapid decline in kidney function over hours or days
• Causes: Prerenal, Postrenal, and Renal
• Clinical Biochemistry Patterns: Serum creatinine, urine output, estimated glomerular filtration rate (eGFR)

Chronic Kidney Disease (CKD)

• Gradual and irreversible loss of kidney function over months or years
• Clinical Biochemistry Patterns: Serum creatinine, urine output, estimated GFR.

Kidney Dysfunction

• Azotemia (lab finding): elevated nitrogen-containing compounds in the blood indicative of impaired kidney function
• Uremia (clinical syndrome): systemic effects of kidney failure due to waste product buildup

Renal Function and Abnormalities

• BUN (Blood Urea Nitrogen):
  • Elevated levels suggest kidney dysfunction or dehydration
  • Low levels may indicate liver dysfunction
• Creatinine:
  • Elevated levels indicate kidney dysfunction or muscle disorders
  • Low levels may indicate muscle atrophy
• Uric Acid:
  • Elevated levels suggest kidney dysfunction, excess purine consumption (leading potentially to gout)
  • Low levels may indicate kidney or liver disease

Gout

• Clinical syndrome characterized by hyperuricemia and recurrent acute arthritis
• Chemical basis: accumulation of uric acid crystals in joints and tissues
• Crystallization of uric acid in joints due to breakdown of purines.
• Clinical relevance: sudden, severe pain in joints. Potential triggers include purine-rich foods (meat). Treatment includes medications and lifestyle changes.

Urinalysis

• Diagnostic tool for multiple myeloma and nephrotic syndrome
• Multiple myeloma: presence of Bence Jones proteins (indicative of plasma cell tumors).
• Nephrotic syndrome: characterized by massive proteinuria, hypoalbuminemia, and fluid retention, indicative of glomerular damage.