Small intestine lumen

Dietary fats, libers, and other carotenoids have been reported to interfere with carotenoid bioaccessibility. It is clear that by their presence in the gut, lipids create an environment in favor of hydrophobic compounds such as carotenoids. When arriving in the small intestinal lumen, dietary fats stimulate bile flow from the gallbladder and therefore enhance the micelle formation, which in turn could facilitate the emulsification of carotenoids into lipid micelles. Without micelle formation, carotenoids are poorly absorbed a minimum of 3 g of fat in meal is necessary for an efficient absorption of carotenoids, except for lutein esters that require higher amounts of fat. ... 

Ingested protein is digested in a stepwise fashion in the stomach, small intestinal lumen, and small intestinal mucosal cells (Chapter 12). Peptides formed in the intestinal lumen are absorbed into the mucosal cells and degraded to free amino acids. The outflow of amino acids to the portal vein does not reflect the amino acid composition of the ingested protein. Thus, alanine levels increase two-to fourfold, and glutamine, glutamate, and aspartate are absent. These changes arise from amino acid interconversions within the intestinal cell. 

Table 3. Degradation or minimal synthesis of amino acids for protein accretion as absolute or percentage of amounts entering the portal vein from the small-intestinal lumen in fed young pigsJ... 

Sodium SGLTl -dependent unidirectionai transporter Small intestine and kidney Active uptake of glucose from lumen of intestine and reabsorption of glucose in proximal tubule of kidney against a concentration gradient... 

Drugs taken orally are slow to act. Most are absorbed in the small intestine where the villi, which penetrate into the lumen, present a large surface area. Unfortunately in order to pass through the gut wall into the bloodstream the drug has to become dissolved in its cell s membranes and to achieve this it needs to be lipid-soluble. 

In this model, no attempt is made to reproduce the in vivo physiochemical conditions occurring in the lumen of the human small intestine during digestion. This cell culture model only provides information about the intestinal absorption and metabolism processes of the compound. Using this cell culture system in con-... 

The food, now in a liquid form known as chyme, passes through the pyloric sphincter into the duodenum, where stomach acid is neutralized. There is wide variation in lengths of the components of the small intestine (i.e., duodenum, jejunum, and ileum) between individuals (Table 98-1). Most absorption of digested carbohydrate and protein occurs within the jejunum. Most fat absorption occurs within the jejunum and ileum. In the small bowel, breakdown of macronutrients (i.e., carbohydrate, protein, and fat) occurs both within the lumen of the gut and at the intestinal mucosal membrane surface. The absorptive units on the intestinal mucosal membrane are infoldings known as... 

Fig. 4 Diagrammatic sketch of the small intestine illustrating the projection of the villi into the lumen (left) and the anatomic features of a single villus (right). (Modified from Ref. 7.)...

The surface area in the luminal side of the small intestine per unit length of the serosal (blood) side is enormous in the proximal jejunum, and steadily decreases (to about 20% of the starting value [62]) in the distal portions of the small intestine. The surface area is increased threefold [69] by ridges oriented circumferentially around the lumen. Similar folds are found in all segments of the GIT, except the mouth and esophagus [66]. Further 4—10-fold expansion [62,69] of the surface is produced by the villi structures, shown schematically in Fig. 2.4. The layer of epithelial cells lining the villi structures separate the lumen from the circulatory system. Epithelial cells are made in the crypt folds of the villi, and take about... 

The in vivo environment of the GIT is characterized by a pH gradient the pH value is constant at 7.4 in the receiving compartment (blood), and varying in the donor compartment (lumen) from 5 to 8 from the start to the end of the small intestine. In contrast, the BBB has a constant iso-pH 7.4. Modeling the two environments requires proper pH adjustment in the in vitro model, as indicated in Table 7.22. 

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccharidases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Water and electrolytes. Each day in an average adult, about 5.51 of food and fluids move from the stomach to the small intestine as chyme. An additional 3.5 1 of pancreatic and intestinal secretions produce a total of 9 1 of material in the lumen. Most of this (>7.5 1) is absorbed from the small intestine. The absorption of nutrient molecules, which takes place primarily in the duodenum and jejunum, creates an osmotic gradient for the passive absorption of water. Sodium may be absorbed passively or actively. Passive absorption occurs when the electrochemical gradient favors the movement of Na+ between the absorptive cells through "leaky" tight junctions. Sodium is actively absorbed by way of transporters in the absorptive cell membrane. One type of transporter carries a Na+ ion and a Cl ion into the cell. Another carries a Na+ ion, a K+ ion, and two Cl ions into the cell. 

The gut wall within the small intestine is particularly well adapted for its role as an absorptive surface. Absorption rate is proportional to the area of the surface that is available for absorption. Thus, the internal surface of the small intestine is folded towards the lumen of the gut. This folding increases the surface area of the gut by approximately 3-fold. In this area, the gut wall is covered with many fingerlike projections called villi, and these provide a further 10-fold increase in surface area. In addition, the gut wall epithelial cells are polarized such that on the luminal surface there are millions of microvilli providing a further 20-fold increase in surface area for absorption. In all, these surface area modifications provide an absorptive area which is some 600-fold higher than would be provided by a simple cylinder. Thus, the estimated surface area of the human gut is approximately 200 m2 [1],... 

---