Capillary network

Transport. Wood is composed of a complex capillary network through which transport occurs by capillarity, pressure permeability, and diffusion. A detailed study of the effect of capillary stmcture on the three transport mechanisms is given in Stamm (13). 

These reaetions are due to the presence of immune eomplexes either in the eireulation or extravascular space. The complexes may localize in capillary networks (lungs, kidney, joints) where, together with complement and polymorphs, they may produce extensive tissue damage. Two main types of reactions fall into this group. 

Fig. 2.11 (a) Dissection of VNC — Mouse Lemur (Microcebus murinus). Cl-C3 = Para-septal cartilage bars LV = ventral vein NC = arterioles/capillary network SV = dorsal vein and VNw = ventral wall (from Schilling, 1970). Vomeronasal complex in murine Rodents. Comparison of LS with TS in Rat (b) LS (horizontal). VV = vascular sinus arrow = venous diverticulum = VN lumen and NE = neuroepithelium (from Larriva-Sahd, 1994). (c) TS (coronal). G = glands RFE = non-sensory epithelium (from Mendoza, 1993). 

Although most drugs are absorbed from the intestine by the blood capillary network in the villi, they can also be taken up by the lymphatic system (an integral and necessary part of the vascular system, the function of which is to collect extra tissue fluid and return it to the vascular compartment), particularly by M cells that reside in the Peyer s patch regions of the intestine. Peyer s patches have also been implicated in the regulation of the secretory immune response. Wachsmann et al. [277] reported that an antigenic material encapsulated within a liposome, when administered perorally, is taken up by these M cells and exhibited better saliva and serum IgA (primary and secondary)... 

Figure 2.4 Schematic of the villi fingers covered by a monolayer of epithelial cells, separating the lumen from the blood capillary network [63,69]. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]...

Capillaries are the site of exchange between blood and the interstitial fluid surrounding tissue cells. Tissues with a higher metabolic rate have a more extensive capillary network, that is, a greater number of capillaries per unit area. Because of extensive branching of these vessels, the cells of the body are typically within 20 pm of the nearest capillary. Consequently, the distance that substances must travel between blood and the cells is minimized. Capillaries are permeable to water and small water-soluble substances, such as glucose, amino acids, lactic acid, and urea, and impermeable to proteins. 

The normal coronary system consists of large epicardial or surface vessels (Rj) that offer little resistance to myocardial flow and intramyocardial arteries and arterioles (R2), which branch into a dense capillary network to supply basal blood flow (Fig. 11-1). Under normal circumstances, the resistance in R2 is much greater than that in Rj. Myocardial blood flow is inversely related to arteriolar resistance and directly related to the coronary driving pressure. 

Takayama S, McDonald JC, Ostuni E, Liang MN, Kenis PJA, Ismagilov RF, Whitesides GM (1999) Patterning cells and their environments using multiple laminar fluid flows in capillary networks. Proc Natl Acad Sci USA 96 5545-5548... 

The ratios of these four pressures alter at different areas of the capillary network so that net fluid movement into or out of the capillary can also change as shown below. 

The density of cerebral capillaries, especially in the cortical grey matter, is very high with mean distances of 40 /xm. The capillary network has a total length of 600-650 km, the mean velocity of the blood flow is below 0.1 cm/s, and the luminal surface extends to 15-30 m2. Thus the blood-brain barrier represents an important surface for potential drug delivery besides gut (30CM100 m2), lung (70-120 m2), or skin (1.8 m2) [24-26, 33-37],... 

The glomerular capillary network lined with fenestrated endothelia is surrounded by the glomerular basement membrane and visceral epithelial cells (podocytes). Podocytes cover the outer aspect of the glomerular basement membrane with their... 

The distribution of drugs depends on both the physicochemical properties of the drug molecules and the composition of tissue membranes. These factors can either result in a uniform or uneven distribution of dmgs into the various body compartments and fluids. In the extreme, distribution may tend toward an accumulation of drugs in particular tissues or to an almost complete exclusion of the drag from a particular compartment in a defined length of time. One unique compartment that has to be considered in this respect is the brain, which is separated from the capillary system of the blood by the blood-brain barrier, whose membrane has a special structure. It consists of a cerebral capillary network formed by a capillary endothelium that consists of a cell layer with continuous compact intercellular junctions. It has no pores, but special cells, astrocytes, which support the stability of the tissues, are situated at the bases of the endothelial membrane separating the brain and CSF from the blood. The astrocytes form an envelope around the capillaries. 

It is interesting that the adult human lung has an enormous gas tissue interface, approximately 90 m2, 70 m2 alveolar space. This large surface, together with the blood capillary network surface of 140 m2, with its continuous and profuse blood flow, offers an extremely rapid and efficient medium for the absorption of chemicals from the air, into the alveolar portion of the lungs, and into the bloodstream. 

In the concentration time curve the first pass of the bolus appears after the pre-contrast baseline (Fig. 6.4a). After the first pass, concentration does not return to zero, but remains increased (Fig. 6.4b). This effect is due to contrast agent molecules remaining within the capillary network and to second pass effects. One way to avoid these effects is to fit a gamma-variate function to the measured values of contrast agent concentration (Belliveau et al. 1991 Thompson et al. 1964) (Fig. 6.4c) ... 

Brain perfusion refers to the microcirculation of the brain (see Chap. 6). Microcirculation comprises the blood circulation in capillary networks and the exchange of oxygen and nutrients between the blood and the brain tissue. The effectiveness of brain perfusion depends on blood pressure, blood velocity, characteristics of the capillary network, capillary wall permeability, and diffusion rates of oxygen and nutrients. In the healthy brain, perfusion is symmetrical, and is substantially higher in the CM than in the WM. It has been approximated that the CBF is about 80 ml/100 g/min in the CM, and 20 ml/100 g/ min in the WM. Brain perfusion is usually quantified in terms of ml/100 g (cerebral blood volume,... 

The electrolyte has a particular structure. A mixture of LiA102 and alkali carbonates (typically >50 vol%) is hot pressed (about 5000 psi) at temperatures slightly below the melting point of the carbonate salts. In this way, a porous matrix support material of ceramic particles (LiA102) is formed that contains a capillary network filled with molten electrolyte. The ceramic material in the electrolyte structure represents a mechanical resistance which does not participate in the electrical or electrochemical processes. The prepared electrolyte has a thickness of 1-2 mm, and it is very difficult to produce it in large shapes. 

Sublingual Placement under the tongue allows the drug to diffuse into the capillary network and therefore to enter the systemic circulation directly. Administration of an agent by this route has the advantage that the drug bypasses the intestine and liver and is not inactivated by metabolism. 

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