Volume of distribution extracellular

When a small molecule is introduced into the circulation, it may attain either an intravascular, extracellular or intracellular volume of distribution( 1). The capillary walls are porous and are generally freely permeable to small molecules. Almost all compounds used as drugs are small enough to pass freely through pores in the capillary membrane, thus, in the absence of protein binding (which will be discussed below) small molecules should attain at least an extracellular volume of distribution. 

The membrane separating the extracellular space of the brain from the intravascular space is somewhat less permeable and is generally considered to be comparable to cellular membranes (with respect to permeability). Thus, many molecules such as pertech-netate ion which are distributed in the extracellular fluid, are unable to penetrate the blood brain barrier. Penetration of cellular membranes is also related to the size of the molecule, however, in this case the pore size is much smaller, being permeable to only simple ions, such as Na , Cl , etc. Due to the small size of the pores in cell membranes, it is unlikely that any Tc-complex will be able to diffuse through the pore. Thus, most of these complexes will be limited to an extracellular 

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

1) Pertechnetate, which is actively transported (as an iodide analog) into the thyroid. 

2) Hepatobiliary agents, such as 33 Tc(HlDA)2 which is actively accumulated by hepatocytes. 

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Following intravenous administration of a paramagnetic contrast medium, the zonal anatomy of the uterus is also depicted on Tl-weighted images (Fig. 3.9b). Both the endometrium and the outer myometrium are characterized by pronounced contrast enhancement while the junctional zone is of low signal intensity. This is attributed to the denser tissue and smaller extracellular volume of distribution of the contrast medium [12,16]. 

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See also in sourсe #XX -- [ ]

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