The extracellular fluid (ECF) is composed of interstitial fluid and plasma. In the body, the interstitial fluid accounts for three-fourths of the ECF, which amounts to 10.5 liters. On the other hand, plasma constitutes one-fourth of the ECF, totaling 3.5 liters. The relationship between the volumes of these different fluid compartments is illustrated in Figure 2.2. An example of fluid accumulation in the peritoneal cavity, known as ascites, occurs in individuals with liver disease.

Water moves between the intracellular fluid (ICF) and ECF compartments through the plasma membranes of cells. Simultaneously, water also moves between the vascular (plasma) and interstitial compartments across the walls of capillaries. It is important to note that the pathways and driving forces for water movement differ across cell membranes compared to capillary walls.

The movement of water between the ICF and ECF compartments, across cell membranes, takes place via aquaporins present in the plasma membrane. This phenomenon is explained in Chapter 1. The driving force behind this water movement is an osmotic pressure difference. The osmotic pressure in both the ICF and ECF is influenced by the molecules and ions present in these fluids. To simplify, these substances can be categorized as low molecular weight molecules (e.g., glucose) and ions (e.g., Na+), as well as macromolecules (e.g., proteins). The osmotic pressures of both the ICF and ECF range from 280 to 295 mOsm/kg H2O. In the ECF, the osmotic pressure primarily stems from low-molecular-weight molecules and ions, while proteins contribute only 1 to 2 mOsm/kg H2O. The molecules and ions that contribute to the osmotic pressure within the cell are not as well understood, but they also include low-molecular-weight molecules (e.g., glucose), ions (e.g., Na+), and macromolecules (e.g., proteins). The fact that cell volume remains constant when ECF osmolality is constant indicates that the osmotic pressure inside the cells is equal to that of the ECF. Any osmotic pressure difference would result in cell swelling or shrinking, as explained in the "Nonisotonic Cell Volume Regulation" section.

Moving on to the movement of water between the vascular (plasma) compartment and the interstitial fluid compartment, this occurs across the capillary wall. The amount and mechanism of water movement vary depending on the type of capillary. For instance, in the capillary sinusoids of the liver, there are significant gaps between endothelial cells (discontinuous capillary), allowing easy passage of water, plasma components, and some cellular elements across the wall. In other capillaries, endothelial cells have fenestrations ranging from 80 to 100 nm in diameter (e.g., in the kidneys). These fenestrations enable the movement of all plasma components, with only cellular elements of blood being unable to pass through. Some capillaries, such as those in the brain, create a relatively tight barrier to water, small molecules, and ions. In these cases, water movement occurs through small pores on the endothelial cell surface or through clefts between adjacent endothelial cells. These pores and clefts allow the passage of water and molecules smaller than 4 nm. Additionally, a small amount of water traverses the capillary wall through pinocytosis by endothelial cells.
Interstitial fluid 3/4 of ECF 10.5 L
Plasma 1/4 of ECF 3.5 L •Fig. 2.2 Relationship Between the Volumes of the Various Body Fluid Compartments. Theactualvaluesshownareforanindividualweighing70kg.(ModifiedfromLevyMN,KoeppenBM,StantonBA.Berne & Levy’s Principles of Physiology.4thed.St.Louis:Mosby;2006.) example is the accumulation of fluid in the peritoneal cavity (ascites) of individuals with liver disease.
Movement of Water Between Body Fluid Compartments
Fig. 2.2 , water moves between the ICF and ECF compartments across the plasma membranes of cells, and it moves between the vascular (plasma) and interstitial compartments across capillary walls. The pathways and driving forces for this water movement are different across cell membranes, in comparison to the capillary walls.
Movement of water between the ICF and ECF compartments, across cell membranes, occurs through aquaporins expressed in the plasma membrane (see
Chapter 1).
The driving force for this water movement is an osmotic pressure difference. The osmotic pressure of both the ICF and ECF is determined by the molecules/ions present in these fluids. For simplicity, these can be divided into (1) molecules of low molecular weight (e.g., glucose) and ions (e.g., Na+) and (2) macromolecules (e.g., proteins). The osmotic pressures of both the ICF and ECF are in the range of 280 to 295 mOsm/kg H2O. For the ECF, the low-molecular-weight molecules and ions account for nearly all of this pressure because the osmotic pressure contributed by proteins is only 1 to 2 mOsm/kg H2O. The molecules/ions contributing to the osmotic pressure within the cell are less well understood, but they also include low-molecular-weight molecules (e.g., glucose), ions (e.g., Na+), and macromolecules (e.g., proteins). The fact that cell volume remains constant when ECF osmolality is constant means that the osmotic pressure inside the cells is equal to that of the ECF. If an osmotic pressure difference did exist, the cells would either swell or shrink, as described in the section “Nonisotonic Cell Volume Regulation.”
Movement of water between the vascular (plasma) compartment and the interstitial fluid compartment occurs across the capillary wall. The amount of water that moves across the capillary wall, and the mechanism of the water movement varies depending on the capillary. For example, in the capillary sinusoids of the liver, endothelial cells are often separated by large gaps (discontinuous capillary). As a result, water and all components of the plasma (and some cellular elements) can pass easily across the wall. Other capillaries are lined by endothelial cells that contain fenestrations that are up to 80 to 100 nm in diameter (e.g., in the kidneys). These fenestrations allow all components of the plasma (only cellular elements of blood cannot pass through the fenestrations) to move across the capillary wall. Some capillaries (e.g., in the brain) form a relatively tight barrier to water and small molecules and ions, and water movement occurs through small pores on the endothelial cell surface or through clefts between adjacent endothelial cells. These pores and clefts allow water and molecules smaller than 4 nm to pass. In addition, a small amount of water traverses the capillary wall via pinocytosis by endothelial cells.