Movement of water across microvascular walls is dependent upon pressures acting upon both sides of the microvessel. Starling first described this principle in 1896, by proposing that plasma was retained in the circulation by the osmotic pressure exerted by plasma proteins (oncotic, or colloid osmotic pressure), which opposes the outward hydrostatic pressure. In addition, the hydrostatic and oncotic pressures of the extravascular compartment (interstitium) affect net water movement, which is governed by the difference between hydrostatic and oncotic pressure gradients (intravascular minus extravascular). This balance, referred to as "Starling equilibrium," is illustrated in Figure 1. The modern form of the Starling equation for fluid filtration is also shown in Figure 1. It includes hydraulic conductivity (Lp), defined in the introduction, and a second coefficient, the osmotic reflection coefficient (o, a dimensionless coefficient ranging from 0 to 1), an index of the selectivity of the microvascular barrier to an osmotically active solute. In the majority of the circulation, small solutes such as NaCl have o approaching 0, whereas in vivo measures of o to albumin under physiologic conditions typically range between 0.8 (heart) and 0.99 (skeletal muscle). Based on this equation, positive values of Jv denote filtration, whereas negative values denote water reabsorption into the microvessels.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.