1. Key Fluids to Replace: Composition and Loss Mechanisms The body loses two primary categories of fluids, each with unique compositions and loss pathways. Understanding their differences is foundational to targeted replacement.
1.1 Water (Pure Solvent Fluid) Main Components: Primarily H₂O, with minimal electrolytes or solutes. It constitutes the largest portion of body fluid (about 50-60% of adult body weight) and is distributed in intracellular fluid (ICF, inside cells) and extracellular fluid (ECF, outside cells, including blood plasma and interstitial fluid). Common Loss Pathways: Insensible loss: Occurs without awareness, accounting for 400-600 mL of daily fluid loss in adults. This includes water vapor exhaled during breathing (200-300 mL/day) and transepidermal water loss (water evaporating through the skin, 200-300 mL/day), which is not associated with sweat glands.Sensible loss: Observable losses, such as urination (1000-1500 mL/day, the primary route for excreting waste like urea) and mild sweating during daily activities (100-200 mL/day).