Body Fluids and Compartments

• Body fluids, including water, electrolytes, and glucose, are administered via IV fluids.
• These fluids are essential for maintaining physiological balance in various clinical scenarios.

Body Water Distribution

• Total body water (TBW) varies from 45-75% depending on age and gender.
• Younger adult females: 50-55% water.
• Younger adult males: ~60% water.
• TBW is highest in infancy and decreases with age.
• Brain tissue is about 85% water, skeletal muscle is 75%, bone is 25%, and adipose tissue is 10-20%.
• Gender differences in TBW relate to differences in skeletal muscle and adipose tissue.
• Newborn infants can be up to 75% water due to low body fat and bone mass.
• TBW simplifies to 50-60% of total body mass.

Body Fluid Compartments

• Body water is divided into intracellular fluid (ICF) and extracellular fluid (ECF).
• ICF is divided into trillions of compartments i.e. the cells.
• The cell membrane separates ICF and ECF.
• ECF comprises all fluid outside cell membranes: plasma, interstitial fluid, and transcellular fluid.
  • Plasma is the fluid within blood vessels; plasma concentrations is about 20% of ECF.
  • Interstitial fluid surrounds the cells in tissues; it is about 75-80% of ECF.
  • Transcellular fluid is in spaces like the pleural cavity, CSF, joints, eyes, and GI tract secretions; it is about 5-7% of ECF, a lubricant and sometimes a transport role.
  • It has similar solute composition to interstitial fluid.

Compartment Composition

• Water within ICF and ECF contains solutes:
  • Electrolytes like chemical compounds that dissociate into ions and conducting electrical current.
  • Glucose
  • Dissolved gasses like oxygen, carbon dioxide
  • Proteins
  • Lipids
  • Products of metabolism like urea and creatinine

Osmolarity and Osmolality

• Osmolarity represents total solute concentration per liter of solution (mOsm/L).
• Osmolality represents total solute particles per kilogram of water (mOsm/kg).
• Normal Osmolarity is ~300 mOsmol/L in all compartments.
• Osmolality and osmolarity are interchangeable.
• Sudden changes in osmolarity can shift water between compartments.
  • Na+ and Cl- are primary determinants of ECF osmolarity.
  • A decrease in ECF [Na+] will shift water into the ICF, causing cells to swell.
  • Neuronal swelling in the skull can compress brain blood vessels.

Tonicity Defined

• Tonicity is the effect of solution osmolarity has on cell shape by altering the cell's internal water volume.
• Tonicity is dependent on:
  • Solute concentration
  • Cell membrane solute permeability
  • Isotonic Solutions.
  • Isotonic solutions have the same solute concentration and osmolarity as the ICF.
  • Cells in isotonic solutions like 0.9% saline retain their shape; water doesn't move in or out.
  • Hypertonic Solutions.
  • Hypertonic solutions have higher solute concentrations than ICF.
  • They cause cells to shrink as water exits.
  • Hypotonic Solutions.
  • Hypotonic solutions have lower solute concentrations than ICF.
  • Water enters the cells, causing them to swell.
  • 5% dextrose IV fluids are functionally hypotonic because glucose is rapidly absorbed, leaving only water.

Resting Membrane Potential

• Electrolyte ratios, specifically K+ ratios of ECF to ICF on each of the cell membrane must be kept constant in order to maintain the resting membrane potential (RMP).
  • RMP is -50 to -90mV, the cytoplasm is negative to the outside.
  • The cell membrane is permeable to K+ and impermeable to proteins.
• RMP maintenance is crucial for neural, muscle, and cardiac cell function, to prevent unwanted activity, overactivity, or inactivity.
  • K+ imbalance can disrupt cardiac myocyte coordination, leading to cardiac arrest.
• Maintaining correct ECF electrolyte balance is important as an intern during electrolyte measurement and fluid prescription.

Electrolyte Composition

• Intracellular and extracellular compartments differ in electrolyte composition.

Interstitial and Plasma Fluid

• Interstitial fluid and plasma are similar in fluid
• High in protein concentration in plasma, capillary walls restrict larger molecules.

Key Electrolyte Distribution

• Na+ and K+ balances differ between ECF and ICF.
  • Na+ is dominant in ECF, K+ is dominant in ICF.
  • Cl- and HPO4- switch roles as major anions, though proteins in ICF play a significant role also.

ICF vs ECF Composition