Chyme

Nahrungs-aufnahme, /. reception or absorption of food, -bedarf, m. food requirement, -brei, m. chyme, -dotter, m. (Biol.) food yoljc, deutoplasm, -fliissigkeit, /. nutritive liquid chyle, -kanal, m. alimentary canal, -milch, /. (Physiol.) chyle. 

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... 

The design of a proper delivery system requires a knowledge of the G.I.tract (i). The nature of the gastric acidic and enzymatic medium has been elucidated. More recently Davis (2) and Harris (3) have studied the rate of emptying of the stomach. Dressman (4) has clinically followed pH variations in both the empty stomach and after a meal finding that sinusoidal pH reductions occur during mastication of solid food, whereas the duodenum maintains a relatively constant pH during introduction of the chyme. 

In humans, the large intestine usually receives about 500 mL of fluid-like food material (chyme) per day. As... 

Food is stored in the body of the stomach, which may expand to hold as much as 11 of chyme. As food enters the stomach, it undergoes a reflex relaxation referred to as receptive relaxation. It enhances the ability of the stomach to accommodate an increase in volume with only a small increase in stomach pressure. The fundus does not typically store food because it is located above the esophageal opening into the stomach. Instead, it usually contains a pocket of gas. 

The major gastric factor that affects motility and the rate of emptying is the volume of chyme in the stomach. As the volume of chyme increases, the wall of the stomach becomes distended and mechanoreceptors are stimulated. This elicits reflexes that enhance gastric motility by way of the intrinsic and vagus nerves. The release of the hormone gastrin from the antral region of the stomach further contributes to enhanced motility. 

The most important factors that regulate gastric motility and the rate of emptying of the stomach involve the volume and chemical composition of chyme in the duodenum. Receptors in the duodenum are sensitive to ... 

As the volume of the chyme in the duodenum increases, it causes distension of the duodenal wall and stimulation of mechanoreceptors. This receptor stimulation elicits reflex inhibition of gastric motility mediated through the intrinsic and vagus nerves. Distension also causes release of gastric inhibitory peptide from the duodenum, which contributes to inhibition of gastric contractions. 

Duodenal receptors are also sensitive to the chemical composition of chyme and are able to detect the presence of lipids, excess hydrogen ion, and hyperosmotic chyme. These conditions also elicit the enterogastric reflex and release of the enterogastrones in order to decrease the rate of gastric emptying. 

Pancreatic juice neutralizes the acidic chyme entering the duodenum from the stomach. Neutralization not only prevents damage to the duodenal mucosa, but also creates a neutral or slightly alkaline environment optimal for the function of pancreatic enzymes. The pancreas also secretes several enzymes involved in the digestion of carbohydrates, proteins, and lipids. 

Most pancreatic secretion takes place during the intestinal phase. The intestinal hormone secretin stimulates release of a large volume of pancreatic juice with a high concentration of bicarbonate ion. Secretin is released in response to acidic chyme in the duodenum (maximal release at pH < 3.0). The intestinal hormone cholecystokinin is released in response to the presence of the products of protein and lipid digestion. Cholecystokinin then stimulates the release of digestive enzymes from the pancreas. 

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 large intestine typically receives 500 to 1500 ml of chyme per day from the small intestine. As discussed, most digestion and absorption have... 

The colon s absorption of most of the water and salt from the chyme results in this "drying" or concentrating process. As a result, only about 100 ml of water is lost through this route daily. The remaining contents, now referred to as feces, are "stored" in the large intestine until it can be eliminated by way of defecation. 

A second form of motility in the large intestine is mass movement. Three or four times per day, typically after a meal, a strong propulsive contraction occurs that moves a substantial bolus of chyme forward toward the distal portion of the colon. Mass movements may result in the sudden distension of the rectum that elicits the defecation reflex. 

For the most part, the resting pH of the stomach is nearer to 2 than 1 however, during feeding the meal causes a transient rise in pH to 4—5 depending on the volume and nature of the meal consumed. The fundus undergoes receptive relaxation to allow the proximal stomach to accommodate the food mass in the distal stomach, the food is triturated to form chyme, which is ejected into the duodenum in spurts of 2-5 mL. The division of function causes significant inhomogeneity in... 

By the end of the small intestine, deposition is almost complete and there is no need for intestinal secretions to aid assimilation. The principal role of the colon is to resorb water and reclaim sodium, which it does very efficiently. In fact, for every 2 L of water entering the colon, the residual water in the stools will be <200 mL. The environment becomes problematic for delivery past the hepatic flexure as the lack of water will restrict dispersion and dissolution. The flow of chyme from the ileum to the colon in healthy human beings is 1-2 L h 1. 

The absorption efficiency of the different carotenoids is variable. For example, (3-cryptoxanthin has been reported to have higher absorption efficiency than a-cryptoxanthin in rats (Breithaupt and others 2007). Carotenoids must be liberated from the food before they can be absorbed by intestinal cells (Faulks and Southon 2005). Mechanical disruption of the food by mastication, ingestion, and mixing leads to carotenoid liberation (Guyton and Hall 2001). The enzymatic and acid-mediated hydrolysis of carbohydrates, lipids, and proteins (chemical breaking of the food) also contributes to carotenoids liberation from the food matrix (Faulks and Southon 2005). Once released, carotenoids must be dissolved in oil droplets, which are emulsified with the aqueous components of the chyme. When these oil droplets are mixed with bile in the small intestine, their size is reduced, facilitating the hydrolytic processing of lipids by the pancreatic enzymes (Pasquier and others 1996 Furr and Clark 1997 ... 

There is no solid evidence that relates human aging and reduction of carotenoid absorption. In some studies, old people have shown a lower (3-carotene absorption than that of young people (Madani and others 1989), whereas the opposite has also been reported by other studies (Sugarman and others 1991). The absorption of lipid-soluble substances, including carotenoids, is affected by any disease related to the digestion and absorption of fats (West and Castenmiller 1998). Inadequate production of lipase and bile as well as an inadequate neutralization of the chyme in the duodenum affect carotenoid bioavailability (Guyton and Hall 2001). 

Digestion Conditions. Peptic conditions were not emphasized since calcium solubility is high at the peptic stage (Table I) and chyme release to the duodenum is more dependent on particle size than completeness of digestion (23>24). 

Hydrodynamics of the upper GI tract are characterized by 1) the kinetics of gastric emptying, and 2) the small intestinal transit and the flow rate of intestinal fluid (chyme). Gastric... 

Figure 16 Variability (time dependency) of differential GI flow rates (DFR) in the small intestine of Labradors. VR represents the cumulative volume of chyme collected at midgut following oral administration of 200 mL glucose solution 20% (I) and 200 mL NaCl 0.9% (J). Source From Ref. 10.

Influence of Osmolality on Intestinal Transit and on Chyme Volume Available for Dissolution... 

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