Electrolytes (CC2)

• Electrolytes


  Ions capable of carrying an electric charge, based on how the ion migrates in an electric field
• Electrolyte functions


  • Volume and osmotic regulation (Na, Cl, K)
  • Myocardial rhythm and contractility (K, Mg, C)
  • Cofactor in enzyme activation (eg. Mg, Ca, Zn)
  • Regulation of adenosine triphosphatase (ATPase) ion pumps (Mg)
  • Acid base balance (HCO₃, K, Cl)
  • Blood coagulation (Ca, Mg)
  • Neuromuscular excitability (K, Ca, Mg)
  • Production and use of ATP from glucose
• Anions


  Negatively charged ions that move toward the anode
• Cations


  Positively charged ions that migrate toward the cathode
• Water


  40/75% of total body weight, women have lower average water content than men
• Water


  • Solvent for all processes in the human body, transports nutrients to cells, removes waste product by way of urine, acts as the body's coolant by way sweating
• Water compartments


  • Intracellular Fluid (ICF)
  • Extracellular Fluid (ECF)
• Intracellular Fluid (ICF)

  Fluid inside the cells, accounts for about two thirds of total body water
• Extracellular Fluid (ECF)


  Accounts for the other one third of total body water, can be subdivided in the intravascular extracellular fluid (plasma)
• Normal plasma is about 93% water, the remaining volume occupied by lipids and proteins
• Active transport


  Mechanism that requires energy to move ions, because most biologic membranes are freely permeable to water but not to ions or proteins
• Diffusion


  Passive movement of ions across a membrane, depends on the size and charge of the ion being transported and on the nature of the membrane through which it is passing
• Osmolality

  Physical property of a solution based on the concentration of solutes (expressed as millimoles) per kilogram of solvent (w/w)
• Osmolarity


  Reported in milliosmoles per liter
• Increased osmolality of blood

  Stimulates thirst sensation and antidiuretic hormone (ADH) secretion by the hypothalamus
• Thirst sensation


  Leads to consuming more fluids, increasing the water content of the ECF, diluting the elevated solute (Na) levels, and decreasing the osmolality of the plasma
• ADH (vasopressin)


  Secreted by posterior pituitary gland, acts on the cells of the collecting ducts in the kidneys to increase water reabsorption
• Decreased osmolality


  Turns off ADH secretion as water is conserved
• Osmolality regulation


  Affects the sodium concentration in plasma, as sodium and its associated anions account for approximately 90% of the osmotic activity in plasma
• Normal plasma osmolality is 275-295 mOsm/kg of plasma H²O
• Osmoreceptors in the hypothalamus respond quickly to small changes in osmolality, a 1-2% increase causes a 4-fold increase in the circulating concentration of ADH and a 1-2% decrease shuts off ADH production
• Excess water intake (polydipsia)

  Lowers plasma osmolality, suppressing both ADH and thirst
• Water deficit


  Increases plasma osmolality, activating both ADH secretion and thirst
• Osmotic stimulation of thirst progressively diminishes in people older than age 60, increasing the risk of dehydration in older patients with illness and diminished mental status
• Blood volume regulation


  • Sodium and water are interrelated, the renin-angiotensin-aldosterone system responds to decreased blood volume, atrial natriuretic peptide (ANP) promotes sodium excretion in the kidney, volume receptors stimulate ADH release to conserve water, glomerular filtration rate increases with volume expansion and decreases with volume depletion
• Osmolal gap

  The difference between the measured osmolality and the calculated osmolality, indirectly indicates the presence of osmotically active substances other than Na, urea, or glucose
• Reference ranges for osmolality

  • Serum: 275–295 mOsm/kg
  • Urine (24-h): 300–900 mOsm/kg
  • Urine/serum ratio: 1.0–3.0
  • Random urine: 50–1200 mOsm/kg
  • Osmolal gap: 5–10 mOsm/kg
• Sodium


  The most abundant cation in the ECF, representing 90% of all extracellular cations, and largely determines the osmolality of the plasma
• Normal plasma osmolality is approximately 295 mmol/L
• Na+,K+-ATPase ion pump


  Moves three Na+ ions out of the cell in exchange for two K+ ions moving into the cell as ATP is converted to ADP
• Processes regulating plasma Na+ concentration

  • Intake of water in response to thirst, as stimulated or suppressed by plasma osmolality
  • Excretion of water, largely affected by AVP release in response to changes in either blood volume or osmolality
  • Blood volume status, which affects Na+ excretion through aldosterone, angiotensin II, and ANP
• Normally, 60% to 75% of filtered Na is reabsorbed in the proximal tubule, with electroneutrality maintained by either Cl- reabsorption or hydrogen ion (H ion) secretion
• Hyponatremia


  Serum/plasma levels less than 133 mmol/L, one of the most common electrolyte disorders in hospitalized and nonhospitalized patients
• Causes of hyponatremia


  • Increased sodium loss (hypoadrenalism, potassium deficiency, diuretic use, ketonuria, salt-losing nephropathy, prolonged vomiting or diarrhea, severe burns)
  • Increased water retention (renal failure, nephrotic syndrome, hepatic cirrhosis, congestive heart failure)
  • Water imbalance (excess water intake, SIADH)
• Classification of hyponatremia by osmolality

  • With low osmolality (increased sodium loss, increased water retention)
  • With normal osmolality (increased nonsodium cations, lithium excess, increased globulins, severe hyperkalemia, severe hypermagnesemia, severe hypercalcemia, pseudohyponatremia, hyperlipidemia, hyperproteinemia, pseudohyperkalemia)
  • With high osmolality (hyperglycemia)
• Symptoms of hyponatremia


  Between 125 and 130 mmol/L, primarily gastrointestinal (GI), more severe neuropsychiatric symptoms below 125 mmol/L, level below 120 mmol/L for 48 hours or less is a medical emergency
• Treatment of hyponatremia


  Directed at correcting the condition that caused either water loss or Na loss in excess of water loss, considering the onset and severity, includes fluid restrictions, hypertonic saline, and pharmacologic agents, correcting too rapidly can cause cerebral myelinolysis and too slowly can cause cerebral edema
• Hypernatremia


  Increased serum Na concentration, results from excess loss of water relative to Na loss, decreased water intake, or increased Na intake or retention
• Causes of hypernatremia


  • Excess water loss (diabetes insipidus, renal tubular disorder, prolonged diarrhea, profuse sweating, severe burns)
  • Decreased water intake (older persons, infants, mental impairment)
• Hyponatremia


  Onset and severity are considered in treatment