Structure of the gastrointestinal tract

Over the entire length of the large and small intestines and the stomach, the brush border has a uniform coating (3 nm thick) of mucopolysaccharides which consist of multi-branched polymeric chains. This coating layer appears to act as a mechanical barrier to bacteria, cells or food particles, or as a filter. Whatever its function, the weakly acidic, sulfated mucopolysaccharides influence the charge on the cell membrane and complicate the explanations of absorption. 

The goblet cells of the epithelium form mucus secretions are stored in granule form in the apical cell region and are liquefied on contact with water to form mucus, which is composed of protein and carbohydrate. 

The large intestine is concerned primarily with the absorption of water and the secretion of mucus to aid the intestinal contents to slide down the intestinal tube. Villi are therefore completely absent from the large intestine, but there are deep crypts distributed over its surface. 

Differences in the absorptive areas and volumes of gut contents in different animals are important when comparing experimental results on dmg absorption in various species. The human small intestine has a calculated active surface area of approximately 100 m. No analogous calculations are available for the most commonly used laboratory animals, although the surface area of the small intestine of the rat is estimated to be 700 cm.  

Passive transport, carrier-mediated transport and specialised transport 

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The small-intestinal transit flow is a fundamental process for all gastrointestinal absorption phenomena. However, the structure of the gastrointestinal tract is highly complex and it is practically impossible to explicitly write and solve the equations of motion for the drug flow. Instead, numerical computer simulation techniques that incorporate the heterogeneous features of the gastrointestinal wall structure and of the drug flow are used in this section to characterize the intestinal transit process in humans. 

Irritable bowel syndrome (IBS) is a disorder of the gastrointestinal tract that interferes with the normal functions of the colon. At various points in the past, IBS has been referred to as mucous colitis, spastic colon, irritable colon, or nervous stomach. IBS is generally described as afunctional disorder rather than a disease per se. A functional disorder involves symptoms that cannot be attributed to a specific injury, infection, or other physical problem. A functional disorder occurs because of altered physiologic processes rather than structural or biochemical defects and may be subject to nervous system influence. IBS is associated with frequent fluctuation in symptoms, loss of productivity, and decreased quality of life. Although IBS has been referred to as functional bowel disease, true functional bowel disease may be more indicative of widespread gastrointestinal involvement including (but not limited to) the colon. 

Apart from influencing the absorption of foreign compounds, the environment of the gastrointestinal tract may also affect the compound itself, making it more or less toxic. For example, gut bacteria may enzymically alter the compound, and the pH of the tract may affect its chemical structure. 

What is to be emphasized here is the fact that the per cent dose incorporation into the y- plus 0-globulin fractions of the plasma proteins of the hepatectomized rat is not measurably different from that noted in the eviscerated surviving rat. In other words, the concomitant presence of the gastrointestinal tract, pancreas, spleen, and mesentery structures has not grossly altered the qualitative or quantitative pat-... 

Facilitated diffusion is very similar to passive diffusion with the difference that transfer across membranes is assisted by the participation of carrier proteins embedded in the membrane bilayer. Again, the direction of passage will be from the side of the membrane with high concentration of a chemical to the side with low concentration this also occurs without energy expenditure by the cell. Such a process is somewhat specific in the sense that it applies to molecules that are able to bind to a carrier protein. Absorption of nutrients such as glucose and amino acids across the epithelial membrane of the gastrointestinal tract occurs by facilitated diffusion. Since a finite number of carriers are available for transport, the process is saturable at high concentrations of the transported molecules and competition for transport may occur between molecules of similar structure. 

The epithelial layer is in immediate contact with the lumen of the gastrointestinal tract. The lamina propria, which functions as a structural support for the epithelial layer, is situated on the basolateral side of the epithelial layer. The lamina propria contains lymph vessels, smooth muscle cells, nerves, and blood vessels, which nourish the epithelium. The muscularis mucosa makes up the deepest layer, which is thought to be involved in contractility.8 A more detailed description of the forces that hold together the epithelial layer is provided below. 

Campylobacter i2iS polar flagella, which are essential for motility. It has been shown that the motility imparted by the flagella is required for colonization of the gastrointestinal tract and invasion of intestinal epithelial cells in vitro The Campylobacter flagellar filament is composed of two flagellin structural proteins FlaA... 

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