How does fluid move between the intracellular and interstitial compartments?
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There are three major fluid compartments; intravascular, interstitial, and intracellular. Fluid movement from the intravascular to interstitial and intracellular compartments occurs in the capillaries. A capillary “membrane,” which consists of the endothelial glycocalyx, endothelial cells, and the subendothelial cell matrix, separates the capillary intravascular space from the interstitial fluid compartment. This capillary “membrane” is freely permeable to water and small-molecular-weight particles such as electrolytes, glucose, acetate, lactate, gluconate, and bicarbonate. Gases such as oxygen and carbon dioxide diffuse freely through this membrane, following their concentration gradient, to enter or exit the intravascular compartment. Show
The interstitial compartment is the space between the capillaries and the cells. Fluids support the matrix and cells within the interstitial space. The intracellular compartment is separated from the interstitial space by a cell membrane. This membrane is freely permeable to water but not to small- or large-molecular-weight particles. Any particle movement between the interstitium and the cell must occur through some transport mechanism (eg, channel, ion pump, carrier mechanism). Fluids are in a constant state of flux across the capillary endothelial membrane, through the interstitium, and into and out of the cell. The amount of fluid that moves across the capillary “membrane” depends on a number of factors, including capillary colloid oncotic pressure (COP), hydrostatic pressure, and permeability, which is dictated by factors such as the endothelial glycocalyx layer (EGL) and pore sizes between the cells. The natural particles in blood that create COP are proteins: primarily albumin but also globulins, fibrinogen, and others. The hydrostatic pressure within the capillary is the pressure forcing outward on the capillary membrane generated by the blood pressure and cardiac output. Fluid moves into the interstitial space when intravascular hydrostatic pressure is increased over COP, when membrane pore size increases, the EGL is disrupted, or when intravascular COP becomes lower than interstitial COP. The EGL is now known to play an important role in controlling fluid and other molecule (eg, albumin) transport across the capillary layer, and the oncotic pressure of the glycocalyx plays a larger role than the oncotic pressure of the interstitium; various disease processes and therapy (such as IV fluid administration) can significantly disrupt the EGL, resulting in altered transcapillary movement. \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\) How fluid moves through compartments depends on several variables described by Starling’s equation. Learning Objectives
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Fluid MovementExtracellular fluid is separated among the various compartments of the body by membranes. These membranes are hydrophobic and repel water; however, there a few ways that fluids can move between body compartments. There are small gaps in membranes, such as the tight junctions, that allow fluids and some of their contents to pass through membranes by way of pressure gradients. Formation of Interstitial FluidHydrostatic pressure is generated by the contractions of the heart during systole. It pushes water out of the small tight junctions in the capillaries. The water potential is created due to the ability of the small solutes to pass through the walls of capillaries. This buildup of solutes induces osmosis. The water passes from a high concentration (of water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach an equilibrium. The osmotic pressure drives water back into the vessels. Because the blood in the capillaries is constantly flowing, equilibrium is never reached. The balance between the two forces differs at different points on the capillaries. At the arterial end of a vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement favors water and other solutes being passed into the tissue fluid. At the venous end, the osmotic pressure is greater, so the net movement favors substances being passed back into the capillary. This difference is created by the direction of the flow of blood and the imbalance in solutes created by the net movement of water that favors the tissue fluid. Removal of Interstitial FluidThe lymphatic system plays a part in the transport of tissue fluid by preventing the buildup of tissue fluid that surrounds the cells in the tissue. Tissue fluid passes into the surrounding lymph vessels and eventually rejoins the blood. Sometimes the removal of tissue fluid does not function correctly and there is a buildup, which is called edema. Edema is responsible for the swelling that occurs during inflammation, and in certain diseases where the lymphatic drainage pathways are obstructed. Starling Equation The Starling model: Note the concentration of interstitial solutes (orange) increases proportionally to the distance from the arteriole. Capillary permeability can be increased by the release of certain cytokines, anaphylatoxins, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.) that are released by cells during inflammation. The Starling equation defines the forces across a semipermeable membrane to calculate the net flux. The solution to the equation is known as the net filtration or net fluid movement. If positive, fluid will tend to leave the capillary (filtration). If negative, fluid will tend to enter the capillary (absorption). This equation has a number of important physiologic implications, especially when disease processes grossly alter one or more of the variables. Capillary dynamics: Oncotic pressure exerted by the proteins in blood plasma tends to pull water into the circulatory system. This is a diagram of the Starling model. Note how the concentration of interstitial solutes increases proportionally to the distance from the arteriole. According to Starling’s equation, the movement of fluid depends on six variables:
The Starling Equation is mathematically described as Flux=Kf[(Pc-Pi)-σ (πz-πi)]. LICENSES AND ATTRIBUTIONS CC LICENSED CONTENT, SHARED PREVIOUSLY
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25.2D: Movement of Fluid Among Compartments is shared under a CC BY-SA license and was authored, remixed, and/or curated by LibreTexts. How does fluid move between intravascular and interstitial space?Hydrostatic Forces
The hydrostatic pressure in the intravascular space (Pc) is the principle force driving water and electrolytes out of the capillary into the interstitial space.
How does water move between plasma and ICF?When the osmolarity in the ECF rises compared to ICF, water moves by osmosis from the ICF into the ECF.
What determines the distribution of fluid between intracellular and extracellular compartments?Distribution of fluid between intracellular and extracellular compartments is determined by the concentration of Na+, chloride. Electrolytes (Cl–), and other electrolytes.
What separates intracellular and interstitial fluid?The intracellular compartment is separated from the interstitial space by a cell membrane.
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