Water and Electrolytes Balance Quiz

Water and Electrolytes Balance

• Water is the most abundant molecule in the human body.
• The electrolyte compositions of the extracellular fluid (ECF) and intracellular fluid (ICF) are different.
• The extracellular space is predominantly made up of sodium.
• The intracellular space is primarily composed of potassium.

Na/K ATPase pump

• Most widespread and physiologically important active transporter in cells.
• Moves three sodium ions (Na+) out of the cell and two potassium ions (K+) into the cell with each ATP hydrolysis cycle.
• Responsible for generating the typical Na+ and K+ gradients found across the cell membrane.
• The Na+ gradient is used to power coupled transport of glucose and many other substances.
• In a body at rest, the activity of the Na+/K+-ATPase consumes about a third of all ATP.
• If the Na/K-ATPase stops working, the concentration gradients of Na+ and K+ on the inside and outside of the cell may be affected.
• This can interrupt cell signals.

Water Distribution

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

Osmolality

• Osmolality is a physical property of a solution based on the concentration of osmotically active solutes.
• Normal ECF osmolality is in the range of 275–295 mmol/kg water.
• Water loss from the ECF increases osmolality, resulting in the movement of water from the ICF to the ECF.
• Osmolality can be directly measured or calculated using the formula: 2x [Na+] + [urea] + [glucose].

Regulation of Hydration Status

• Glomerular Filtration Rate (GFR): Regulated by the kidney. Permits Na+ and water excretion.
• Aldosterone: Secreted by the adrenal glands. Increases renal Na+ and water retention in response to decreased renal perfusion.
• Antidiuretic Hormone (ADH): Secreted by the hypothalamus. Increases pure water retention in response to increased ECF tonicity and decreased blood volume.
• Atrial Natriuretic Factor (ANF): Released by cardiac atria. Increases renal Na+ and water excretion in response to increased blood volume.

Osmolality vs. Tonicity

• The hypothalamic osmostat controls both ADH release and the sensation of thirst.
• The hypothalamic osmostat is acutely sensitive to small changes in plasma osmolality.
• The cell membrane is selectively permeable to various solutes.
• Urea and alcohol are freely permeable.
• An increase in plasma osmolality due to sodium implies an increase in osmotic pressure across the cell membrane and withdraws water from the cell to equalize osmolalities.
• An increase in plasma osmolality due to urea does not have this effect because of the free permeability of urea between the ICF and ECF.
• Effective osmolality or tonicity under physiological conditions is primarily dependent on plasma sodium concentration.
• Changes in cell volume are particularly important in the case of the brain.
• Cerebral dehydration due to hypertonicity causes osmolar imbalance, leading to extracellular movement of water and cerebral shrinkage, which can rupture vessels.
• Hypertonicity also leads to intracellular movement of water and cerebral swelling (oedema), which causes compression.
• The brain can adapt by altering the content of “osmolytes”.

Blood Volume

• Severe water depletion causes cerebral dehydration, leading to cerebral bleeding through damage to blood vessels.
• In the short term: cerebral shrinkage is limited by the movement of extracellular ions into cerebral cells, resulting in an osmotic shift of water.
• If dehydration persists, brain cells adapt by synthesizing osmotically active organic compounds (“osmolytes”).
• Excessive fluid replacement may cause cerebral oedema because of rapid intracellular movement of water.

Osmolal Gap

• Measured osmolality (in mmol/kg of water) and calculated osmolarity (in mmol/L of solution) are normally very similar.
• The normal gap is <10 mmol/kg.