Tissue system, capillary

Consequently, bioavailability depends on the route of administration as well as the drug s ability to cross membrane barriers. Once in the systemic circulation, further distribution into peripheral tissues may also be important in allowing the drug to reach the target site. Many drugs must eventually leave the systemic capillaries and enter other cells. Thus, drugs have to move across cell membranes and tissue barriers to get into the body and be distributed within the body. In this section, the ability of these membranes to affect absorption and distribution of drugs is discussed. 

A vast amount of work is being done to describe quantitatively the - transport of anabolites and metabolites in the microcirculation of the brain. The classical geometry for the capillaries and tissue has been the Krogh capillary-tissue cylinder. Recent investigations have considered other geometrical configurations to determine if other models derived for multicapillary systems are more descriptive than the single capillary structure used here. 

Process Model. To simplify computational problems and to allow simultaneous examination of physiological control phenomena, it was necessary to approximate the actual distributed parameter capillary-tissue system with lumped parameter models. 

In this study a simulation of such processes for various arrangements of capillaries was performed to attain a better understanding of oxygen supply conditions in the brain cortex. Oxygen tension values were calculated for steady-state conditions by numerical solution of partial differential equations for different capillary-tissue systems. The analysis helps to explain the experimental results obtained from the microstructure of rat... 

To simulate the histological complexity of capillary-tissue arrangements in a better way and to develop an alternative concept to the Krogh model, a three-dimensional network system is suggested here, providing for differences in directions and flow velocities within the branches of the network (Figure 1). 

The speed of reaction within the erythrocytes is about 13 000 times faster than in the plasma. This accounts both for the rapid uptake of carbon dioxide by the blood during the short transit time (about one sec.) of blood through the systemic capillary and for the rapid release of carbon dioxide from the blood to the alveolar gas during the transit of the blood through the pulmonary capillaries. Without carbonic anhydrase, carbon dioxide partial pressures in the venous blood, and hence in the tissues would be much higher. Indeed when... 

Histamine is a substance present in various tissues of die body, such as die heart, lungs, gastric mucosa, and skin (Pig. 36-1). The highest concentration of histamine is found in die basophil (a type of white blood cell) and mast cells diat are found near capillaries. Histamine is produced in response to injury. It acts on areas such as die vascular system and smooth muscle, producing dilatation of arterioles and an increased permeability of capillaries and venules. Dilatation of die arterioles results in localized redness. An increase in die permeability of... 

The pathophysiologic mechanisms of portal hypertension and of cirrhosis itself are entwined with the mechanisms of ascites (Fig. 19-3). Cirrhotic changes and the subsequent decrease in synthetic function lead to a decrease in production of albumin (hypoalbuminemia). Albumin is the major intravascular protein involved in maintaining oncotic pressure in the vascular system low serum albumin levels and increased capillary permeability allow fluid to leak from the vascular space into body tissues. This can result in peripheral edema, ascites, and fluid in the pulmonary system. The obstruction of hepatic sinusoids and... 

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)... 

The movement of substances between the blood and the extracellular fluid surrounding the cells in most tissues of the body occurs very readily. This exchange takes place at the level of the capillaries, the smallest blood vessels in the cardiovascular system whose walls are formed by a single layer of endothelial cells. Lipid-soluble substances are able to move across this layer of endothelial cells at any point because they can move directly through the plasma membrane by passing between the phospholipid molecules of the bilayer. The movement of water-soluble substances is limited to the multiple pores found between the cells however, it also takes place rapidly and efficiently. 

Figure 15.1 The circulatory system. Arteries carry blood away from the heart. The smallest arterial vessels, the arterioles, are composed mainly of smooth muscle and are the major resistance vessels in the circuit. The capillaries are the site of exchange between blood and tissues. Veins carry blood back toward the heart. The small veins are the major compliance vessels in the circuit and, under resting conditions, contain 64% of the blood volume.

Introduction of a water-soluble ionic substance into the vascular system results in an increase in the number of particles in the bloodstream as the contrast substance dissolves. The body possesses several internal regulation systems and, when perturbed by an injection, attempts to restore the concentrations of substances in the blood to their normal or preinjection levels. To re-equilibrate the system, water from the cells of surrounding body tissue moves into the blood plasma through capillary membranes. This transfer of water is an example of osmosis, the diffusion of a solvent (water) through a semipermeable membrane (the blood vessels) into a more concentrated solution (the blood) to equalize the concentrations on both sides of the membrane. To accommodate the increase in... 

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