Capillary wall pores

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

In the case of multiparticle blockage, as the suspension flows through the medium, the capillary walls of the pores are gradually covered by a uniform layer of particles. This particle layer continues to build up due to mechanical impaction, particle interception and physical adsorption of particles. As the process continues, the available flow area of the pores decreases. Denoting as the ratio of accumulated cake on the inside pore walls to the volume of filtrate recovered, and applying the Hagen-Poiseuille equation, the rate of filtration (per unit area of filter medium) at the start of the process is ... 

Like the walls of other capillaries, the glomerular capillary wall consists of a single layer of endothelial cells. However, these cells are specialized in that they are fenestrated. The presence of large pores in these capillaries makes them 100 times more permeable than the typical capillary. These pores are too small, however, to permit the passage of blood cells through them. 

Most substances, lipid soluble or not, cross the capillary wall at rates that are extremely rapid in comparison with their rates of passage across other body membranes. In fact, the supply of most drugs to the various tissues is limited by blood flow rather than by restraint imposed by the capillary wall. This bulk flow of liquid occurs through intercellular pores and is the major mechanism of passage of drugs across most capillary endothelial membranes, with the exception of those in the CNS. 

Several factors, including molecular size, charge, and shape, influence the glomerular filtration of large molecules. The restricted passage of macromolecules can be thought of as a consequence of the presence of a glomerular capillary wall barrier with uniform pores. 

The concept of a pore potential is generally accepted in gas adsorption theory to account for capillary condensation at pressures well below the expected values. Gregg and Sing ° described the intensification of the attractive forces acting on adsorbate molecules by overlapping fields from the pore wall. Adamson has pointed out that evidence exists for changes induced in liquids by capillary walls over distances in the order of a micron. The Polanyi potential theory postulates that molecules can fall into the potential field at the surface of a solid, a phenomenon which would be greatly enhanced in a narrow pore. 

Fig. 16.3. Scanning electron micrographs of cross-sections of a MIP-filled capillary column. The super-porous morphology of the polymer monolith can be seen. Micrometre-sized globular units of macroporous MIP surrounded by interconnecting super-pores (left). A superpore of about 7 pm in width (above, right). Covalent attachments of the MIP to the capillary wall (below, right). Reprinted from [39] Copyright (1997), with permission from American Chemical Society.

T = pore radius, R = universal gas constant, T = absolute temperature, ce = contact angte between the liquid and capillary walls. 

The second important consequence of protein binding is related to the fact that only free dmg is able to cross the pores of the capillary endothelium. At equilibrium, levels of free dmg on both sides of the capillary wall are equal. Albumin levels in most sites are considerably less than those in semm, so there is... 

The aminoglycosides are well absorbed following i.m. and s.c. administration because of large pores in the capillary walls of the subcutaneous tissues and muscle and the bioavailability approaches 100%. Aminoglycosides are absorbed poorly after p.o. administration but enough may be absorbed in animals with enteritis to result in drug residues in food-producing animals. 

The role of adsorption kinetics and the diffusion of surfactants is especially important in controlling capillary impregnation. According to studies by N.N. Churaev, the solution impregnating the capillary quickly loses its dissolved surfactant due to adsorption of the latter on capillary walls, so the rate of impregnation may be limited by the diffusional transport of surfactant from the bulk of the solution to the menisci in the pores. 

The pore size distribution can be obtained from capillary pressure measurements or mercury porosimetry. The capillary pressure is related to the specific free energies of the interface between fluids and between the fluid and the capillary wall. At mechanical equilibrium, the surface free energy between the fluids is a minimum. The equilibrium condition is expressed by the Laplace equation ... 

The capillary wall is composed of a single layer of endothelial cells about 1 /tm thick. Lipid soluble substances (e.g., O2) can diffuse across the entire wall surface, whereas water soluble substances are restricted to small aqueous pathways equivalent to cyhndrical pores 8 to 9 nm in diameter (e.g., glucose in most capillaries in capillaries with tight junctions and few fenestrations (brain, testes), glucose is predominantly transported). Total pore area is about 0.1% of the surface area of a capillary. The permeability of the capillary wall to a particular substance depends upon the relative size of the substance and the pore ( restricted diffusion). The efficiency of diffusive exchange can be increased by increasing the number of perfused capillaries (e.g., heart and muscle tissue from rest to exercise), since this increases the surface area available for exchange and decreases the distances across which molecules must diffuse. 

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. 

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