Fluid physiology part 1: Volume and distribution of water and its major solutes between plasma, the interstitium and intracellular fluid

Abstract

This chapter focuses on the compartmentalization of body water and its solutes, which is essential for understanding the physiology of body fluid spaces. The modern view of body fluid circulation is an actively pumped double circulation of extracellular fluid that enables solutes to be transferred to and from the intracellular fluid. The different factors that determine fluid flux across cell membrane and microvascular permeability barriers will be discussed, including hydrostatic pressure differences and solute concentration gradients. The regulation of total body water volume and body sodium is also discussed, as well as the clinical relevance of non-osmotic sodium storage capacity in the interstitium. The importance of balancing body potassium and sodium is highlighted, which depends on an adequate availability of magnesium. Additionally, this chapter emphasizes the major contributors to plasma osmolality and the danger of rapid extracellular fluid osmolality changes. However, an alternative model of body water response to intravenous infusions is proposed based on evidence from surgical practice, suggesting that adaptive mechanisms exist to stabilize intracellular volume in the face of excessive fluid infusions. Overall, this chapter provides a comprehensive overview of the distribution and regulation of body water and its solutes, providing important new insights into fluid physiology.