The physiologic power of fluid circulation and implications for fluid therapy.

Water is the most prevalent and ubiquitous substance in the body, an essential solvent for all chemical processes and a prerequisite for solute and gas transport. It is reasonable, therefore, that multiple systems and intricate interdependent regulatory mechanisms have evolved to maintain water balance within 1%-2% of body requirements. The overlapping complexity of body fluid regulatory mechanisms provides ample opportunity for healthy animals to tolerate and recover from substantial fluctuations in water volume deficits or excesses, provided that water balance is restored before tissue oxygen and nutrient delivery become irreversibly compromised. Intravenous fluid therapy can restore tissue perfusion in hypovolemic patients, provided the reestablishment of vascular volume and systemic hemodynamic variables are effective in remedying microcirculatory derangements. Inappropriately administered IV fluid, however, results in fluid imbalance, electrolyte and acid-base disorders, fluid accumulation, impaired healing, prolonged recovery, and unfavorable outcomes, especially in patients that are fluid non-responsive or intolerant. Revised and alternative interpretations of circulatory physiology and IV fluid disposition (e.g., volume kinetics) continue to emerge, providing insights on how IV fluids are delivered to avoid adverse effects (e.g., fluid accumulation syndrome) and improve outcomes. Fluid choice, dosage, and the timing of fluid administration are species- and context-dependent and should be individualized based on macro-micro hemodynamic-interstitial-lymphatic concordance, disease-associated changes in fluid disposition, and an understanding of fluid volume kinetics.