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Chemistry

Na, K, Cl, CO2, HCO3-

Electrolytes are vital for maintaining fluid balance, nerve and muscle function, and overall homeostasis

Electrolytes

  • Definition: Ions (charged particles) that conduct electrical impulses in solution. In the body, electrolytes are essential for maintaining fluid balance, nerve and muscle function, and various other physiological processes
  • Major Electrolytes in the Body
    • Sodium (\(Na^+\))
    • Potassium (\(K^+\))
    • Chloride (\(Cl^-\))
    • Bicarbonate (\(HCO_3^-\))
      • Often measured as total \(CO_2\)
  • Location
    • Intracellular Fluid (ICF): Fluid within cells
    • Extracellular Fluid (ECF): Fluid outside cells
      • Plasma: Liquid component of blood
      • Interstitial Fluid: Fluid surrounding cells
  • Normal Ranges
    • Sodium (\(Na^+\)): 136-145 mmol/L (convential and SI units)
    • Potassium (\(K^+\)): 3.5-5.1 mmol/L (convential and SI units)
    • Chloride (\(Cl^-\)): 98-107 mmol/L (convential and SI units)
    • Total \(CO_2\): 22-33 mEq/L (convential and SI units)
    • Bicarbonate (\(HCO_3^-\)): 23-29 mmol/L (convential and SI units)
      • Calculated from blood gases
  • Functions
    • Maintaining Osmotic Pressure: Regulating fluid balance between ICF and ECF
    • Nerve and Muscle Function: Generating and transmitting electrical signals
    • pH Balance: Buffering acids and bases
    • Enzyme Function: Activating or inhibiting enzymes
    • Transport: Facilitating the transport of molecules across cell membranes

Biochemical Theory and Pathways

Sodium (\(Na^+\))

  • Location: Primary cation in the extracellular fluid (ECF)
  • Regulation
    • Kidneys: Regulate sodium reabsorption and excretion
    • Aldosterone: A hormone that increases sodium reabsorption in the kidneys
    • Antidiuretic Hormone (ADH): Regulates water reabsorption in the kidneys, affecting sodium concentration
    • Natriuretic Peptides (ANP, BNP): Promote sodium excretion in the kidneys
  • Transport
    • Sodium-Potassium Pump (\(Na^+\)/\(K^+\) ATPase): Actively transports sodium out of cells and potassium into cells, maintaining electrochemical gradients
  • Functions
    • Maintaining ECF Volume: Sodium and its associated anions (e.g., chloride) are the major determinants of ECF volume
    • Nerve and Muscle Function: Sodium influx is essential for generating action potentials in nerve and muscle cells
    • Acid-Base Balance: Sodium is exchanged for hydrogen ions in the kidneys, affecting pH balance
  • Pathophysiology of Abnormal Levels
    • Hyponatremia: Low sodium levels due to excess water retention or sodium loss
    • Hypernatremia: High sodium levels due to water deficit or sodium excess

Potassium (\(K^+\))

  • Location: Primary cation in the intracellular fluid (ICF)
  • Regulation
    • Kidneys: Regulate potassium excretion
    • Aldosterone: Increases potassium excretion in the kidneys
    • Insulin: Promotes potassium uptake into cells
    • Epinephrine: Promotes potassium uptake into cells (β2-adrenergic effect)
  • Transport
    • Sodium-Potassium Pump (\(Na^+\)/\(K^+\) ATPase): Actively transports potassium into cells and sodium out of cells
  • Functions
    • Maintaining Cell Membrane Potential: Potassium efflux is essential for maintaining the resting membrane potential of cells
    • Nerve and Muscle Function: Potassium is critical for generating action potentials in nerve and muscle cells
    • Cardiac Function: Potassium affects the rhythm and contractility of the heart
    • pH Balance: Potassium is exchanged for hydrogen ions in cells, affecting pH balance
  • Pathophysiology of Abnormal Levels
    • Hypokalemia: Low potassium levels due to potassium loss or cellular shift
    • Hyperkalemia: High potassium levels due to potassium release from cells or impaired excretion

Chloride (\(Cl^-\))

  • Location: Primary anion in the extracellular fluid (ECF)
  • Regulation
    • Kidneys: Regulate chloride reabsorption and excretion
    • Aldosterone: Indirectly affects chloride reabsorption by regulating sodium reabsorption
  • Transport
    • Chloride Channels: Facilitate chloride movement across cell membranes
    • Chloride-Bicarbonate Exchanger: Exchanges chloride for bicarbonate in red blood cells (chloride shift)
  • Functions
    • Maintaining ECF Volume and Osmotic Pressure: Chloride is a major determinant of ECF volume and osmotic pressure
    • Nerve and Muscle Function: Chloride is involved in generating inhibitory postsynaptic potentials in neurons
    • Acid-Base Balance: Chloride is exchanged for bicarbonate in red blood cells, affecting pH balance
  • Pathophysiology of Abnormal Levels
    • Hypochloremia: Low chloride levels due to chloride loss or excess water retention
    • Hyperchloremia: High chloride levels due to chloride excess or water deficit

Carbon Dioxide (\(CO_2\)) and Bicarbonate (\(HCO_3^-\))

  • Location: Primarily in the extracellular fluid (ECF)
  • Regulation
    • Lungs: Regulate carbon dioxide excretion through ventilation
    • Kidneys: Regulate bicarbonate reabsorption and excretion
  • Bicarbonate Transport
    • Carbonic Anhydrase: Catalyzes the interconversion of carbon dioxide and bicarbonate
    • Chloride-Bicarbonate Exchanger: Exchanges chloride for bicarbonate in red blood cells (chloride shift)
  • Functions
    • pH Balance: Bicarbonate is a major buffer in the blood, helping to maintain pH within the normal range
    • Carbon Dioxide Transport: Carbon dioxide is transported in the blood as bicarbonate ions
  • Pathophysiology of Abnormal Levels
    • Metabolic Acidosis: Low bicarbonate levels due to increased acid production or bicarbonate loss
    • Metabolic Alkalosis: High bicarbonate levels due to acid loss or bicarbonate excess
  • Total \(CO_2\) Measurement
    • Measures all forms of carbon dioxide in the blood, including:
      • Dissolved \(CO_2\)
      • Bicarbonate (\(HCO_3^-\))
      • Carbamino Compounds
    • Total \(CO_2\) is often used as a surrogate marker for bicarbonate in routine chemistry panels
  • Bicarbonate Calculation
    • Bicarbonate can also be directly calculated from pH and PCO2 using the Henderson-Hasselbalch equation

Key Terms

  • Electrolyte: An ion that conducts electrical impulses in solution
  • Cation: A positively charged ion (e.g., \(Na^+\), \(K^+\))
  • Anion: A negatively charged ion (e.g., \(Cl^-\), \(HCO_3^-\))
  • Intracellular Fluid (ICF): Fluid within cells
  • Extracellular Fluid (ECF): Fluid outside cells
  • Plasma: Liquid component of blood
  • Interstitial Fluid: Fluid surrounding cells
  • Osmotic Pressure: The pressure required to prevent the flow of water across a semipermeable membrane
  • Action Potential: A rapid change in electrical potential across a cell membrane
  • Electrochemical Gradient: The combined effect of electrical and chemical forces that determine the movement of ions across a cell membrane
  • Sodium-Potassium Pump (\(Na^+\)/\(K^+\) ATPase): An enzyme that actively transports sodium out of cells and potassium into cells
  • Aldosterone: A hormone that increases sodium reabsorption and potassium excretion in the kidneys
  • Antidiuretic Hormone (ADH): A hormone that regulates water reabsorption in the kidneys
  • Natriuretic Peptides (ANP, BNP): Hormones that promote sodium excretion in the kidneys
  • Carbonic Anhydrase: An enzyme that catalyzes the interconversion of carbon dioxide and bicarbonate
  • Chloride Shift: The movement of chloride ions into and out of red blood cells to maintain electrical neutrality during carbon dioxide transport
  • Hypokalemia: A condition in which there is too little potassium in the blood
  • Hyperkalemia: A condition in which there is too much potassium in the blood
  • Hypochloremia: A condition in which there is too little chloride in the blood
  • Hyperchloremia: A condition in which there is too much chloride in the blood
  • Hyponatremia: A condition in which there is too little sodium in the blood
  • Hypernatremia: A condition in which there is too much sodium in the blood
  • Hypovolemia: The medical term for a decrease in blood volume which results in decreased oxygen delivery to the body
  • Sepsis: A life-threatening condition that happens when your body’s response to an infection damages its own tissues and organs