Water and Electrolyte Basics

Water and Electrolyte Basics

• Water is the most abundant molecule in the human body
• Electrolyte composition differs in ECF and ICF
• Extracellular fluid contains predominantly sodium
• Intracellular fluid contains predominantly potassium
• Na+/K+ ATPase pump
  • Most important active transporter in cells
  • Moves 3 sodium ions out of the cell, and 2 potassium ions into the cell
  • Responsible for generating Na+ and K+ gradients across the cell membrane
  • Na+ gradient powers coupled transport of glucose and other substances
  • Consumes about 1/3 of all ATP produced in a resting body
  • If the pump stops functioning, Na+ and K+ gradients will be disrupted, which can interrupt cell signals

Water Distribution

• Water moves from intravascular space based on pressure differences
  • Hydrostatic pressure pushes fluid from vessels into interstitial space
  • Oncotic pressure, driven by albumin, holds water in the intravascular compartment
  • Osmotic pressure pulls water from low solute to high solute compartment

Osmolality

• Osmolality is a measurement of the concentration of osmotically active solutes in a solution
• Normal ECF osmolality is 275-295 mmol/kg water
• Water loss from ECF increases osmolality, causing water to move from ICF to ECF
• Osmolality can be measured directly or calculated using the formula: 2 x [Na+] + [urea] + [glucose]

Regulation of Hydration Status

• Mechanism: Kidney, Adrenal Glands, Hypothalamus, Cardiac Atria
• Stimuli: Renal perfusion, ECF tonicity, blood volume, and blood pressure
• Effects: Na+ and water excretion/retention, thirst, blood volume regulation

Osmolality vs Tonicity

• Hypothalamic osmostat regulates ADH release and thirst
• Osmostat is sensitive to small changes in plasma osmolality
• The cell membrane is selectively permeable to solutes
  • Urea and alcohol freely permeable
• Increase in plasma osmolality due to sodium increases osmotic pressure across the cell membrane, leading to water leaving the cell
• Increase in plasma osmolality due to urea does not have this effect due to free permeability of urea
• Under physiological conditions, tonicity is mainly dependent on plasma sodium concentration
• Changes in cell volume are important in the brain
  • Hypertonicity causes cerebral shrinkage and rupture of vessels
  • Hypotonicity leads to brain swelling and compression
  • The brain can adapt by altering its content of "osmolytes"

Water Depletion

• Severe water depletion causes cerebral dehydration and can lead to bleeding due to damaged blood vessels
• Cerebral shrinkage is limited in the short term by movement of ECF ions into cerebral cells
• If dehydration persists, brain cells produce osmotically active organic compounds called 'osmolytes' to adapt
• Excessive fluid replacement can cause cerebral edema due to rapid movement of water into the ICF

Osmolal Gap

• Measured osmolality and calculated osmolality are usually similar
• Normal gap is usually less than 10 mmol/kg water
• A larger gap suggests presence of unmeasured osmotically active substances, which may indicate a medical condition.