Chyme

The amount of chyme in the model is monitored with a pressure sensor (e) and kept on a pre-set level through the absorption of water with a pump in the dialysis circuit.

From: Designing Functional Foods , 2009

Carbon Dioxide

Chris M. Wood , in Fish Physiology, 2019

2.2.3 Intestine

Chyme entering the intestine from the stomach through the pyloric sphincter is usually neutralized quickly, though at high feeding rates, acidic chyme may persist in the anterior intestine for some time (e.g., Usher et al., 1990). Commercial food pellets are acidic when hydrated (pH   =   5.0–6.0), and even in the agastric killifish, acidic chyme was present in the anterior intestine at 1–3   h postfeeding, but was neutralized by 12–24   h (Wood et al., 2010). As for fasted animals, the potential role of the pancreas in this neutralization remains unknown, but intestinal HCO 3 − secretion is certainly involved (Fig. 1).

The first-feeding related investigation of intestinal HCO 3 − metabolism in seawater teleosts reported that rectal fluid samples contained about twofold higher HCO 3 − , measured as titratable base, in starved versus fed rainbow trout (Wilson et al., 1996), but secretion and excretion rates were not measured. However a later study in the same species, using in vitro gut sac preparations, documented a clear stimulation of intestinal HCO 3 − secretion following feeding, varying from four- to sevenfold in various sections of the intestine (Bucking et al., 2009). In the toadfish, Taylor and Grosell (2006b) recorded an approximate doubling of [HCO3], together with a marked fall in [Cl−], in the intestinal fluids of the toadfish at 24–48   h postfeeding, and speculated that this reflected a stimulation of HCO 3 − secretion into the lumen by Cl − / HCO 3 − exchange. A similar suggestion was made based on the composition of intestinal fluid samples of the European flounder after feeding (Taylor et al., 2007). In the toadfish, this hypothesis was later confirmed by use of an Ussing chamber system, revealing a 1.5-fold stimulation of the rate of HCO 3 − secretion by the anterior intestine that persisted for 48   h postfeeding. This far outlasted a 6-h doubling of the O2 consumption rate of the intestinal tissue, such that there was increased reliance on basolateral HCO 3 − uptake relative to endogenous CO2 generation as substrate sources (Taylor and Grosell, 2009). Given the documented importance of water [Ca2   +] in controlling drinking rate and intestinal HCO 3 − secretion rate (Section 2.1.6), Taylor and Grosell (2006b) investigated the impact of feeding toadfish with two different diets (squid   =   low [Ca2   +] and sardines   =   high [Ca2   +]) that differed more than 300-fold in their Ca2   + content. However, the differences in postprandial intestinal fluid composition were surprisingly modest, suggesting that other factors may come into play when fish both eat and drink.

The neuroendocrine, mechanical, and/or chemical signals for elevating intestinal HCO 3 − secretion after feeding have yet to be identified, though low pH in the chyme is thought to be an important stimulus (Holmgren and Olsson, 2011); this is an important area for future investigation. However, there are at least three obvious functional benefits. The first is to help neutralize the greater HCl secretion coming into the tract from the stomach, and indirectly coupled to this, the second is to help clear the postprandial "alkaline tide" in the systemic blood stream (Bucking et al., 2009) caused by this elevated gastric HCl secretion (Fig. 1; Section 3.3). The third is to promote intestinal water absorption at a time when the osmotic gradient opposing this process will be greater due to the organic and inorganic osmolytes originating from the food (Bucking et al., 2011; Taylor and Grosell, 2006a,b).

The agastric seawater-acclimated killifish presents an interesting contrast (Wood et al., 2010). The HCO 3 − in the intestinal fluids was markedly depressed at 1–3   h after feeding, and simply returned to the fasting level (which was quite low ~   16   mmol   L−   1; Fig. 2A) at 12–24   h. In gut sac preparations, the net rate of HCO 3 − secretion was also depressed after feeding, even though Cl− and water absorption were elevated. The mystery was solved by the identification of an H+ pumping mechanism (vH+  ATPase) that was active at this time, running in parallel to Cl − / HCO 3 − exchange, as discussed in Section 2.1.3 (Fig. 1). When this was inhibited by bafilomycin, the net rates of HCO 3 − secretion, Cl− absorption, and water absorption all increased.

The killifish data provide evidence that the "CO2 recycling" mechanism to enhance intestinal HCO 3 − secretion (Fig. 1; Section 2.1.3) may become more important after feeding in marine teleosts. In killifish gut sacs, PCO2 elevation was greatest at 1–3   h postfeeding, and was reduced by bafilomycin and by fasting (Wood et al., 2010). Calculated PCO2 in intestinal chyme samples from live killifish were approximately 78   torr at 1–3   h and 22   torr at 12–24   h postfeeding, and similarly declined with fasting (Fig. 2B). As the stomach is absent, elevated chyme PCO2 obviously cannot originate from gastric HCl action on ingested food. Calculated intestinal PCO2 was also elevated after feeding (22   torr at 24   h versus 3   torr in fasting animals) in the gastric gulf toadfish (Taylor and Grosell, 2006b). Our recent direct PCO2 measurements in the anterior intestine of the gastric lemon sole show that PCO2 rises to over 50   torr after a meal, with similar values in other segments (E.H. Jung, J. Eom, and C.M. Wood, unpublished). All these observations suggest an increased role for "CO2 recycling" after feeding. The algivorous Magadi tilapia (Alcolapia grahami) provides an extreme example. This species has a highly acidic stomach (pH   =   3.5) in order to digest the cell walls of cyanobacteria (Bergman et al., 2003). However, it lives in a bizarre, highly alkaline but salty environment (pH   =   10, titration alkalinity HCO 3 −   >   250   mmol   L−   1, osmolality ~   60% seawater values) and must drink the medium for osmoregulation (Wood et al., 2002). This species has evolved a unique anatomical shunting system so that drinking of the highly alkaline water can bypass the highly acidic stomach, avoiding potentially fatal CO2 generation (Bergman et al., 2003). Nevertheless, HCO 3 − (measured by titration) in the chyme of the anterior intestine was 157   mmol   L−   1 in naturally feeding animals (Fig. 2A) and calculated PCO2 was greater than 700   torr (Fig. 2B), the highest values ever recorded for a teleost fish.

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Gastrointestinal Digestion and Absorption

J. Keller , in Encyclopedia of Biological Chemistry (Second Edition), 2013

Intestinal pH

The chyme that reaches the duodenal bulb in small portions usually has an acidic pH. Both, the pancreatic duct and the common bile duct lead into the proximal duodenum at the Papilla of Vater, and, mainly due to bicarbonate secretion from the pancreas and to a far lesser extent from biliary secretions or from the intestinal mucosa itself, the pH in the descending duodenum is maintained at a considerably higher level. The pH of aspirates from the distal duodenum of healthy subjects is 6–7 in the fasting state. Following ingestion of a normal meal, duodenal pH is around 6 early postprandially, drops toward 5–5.5 during the second and third postprandial hour, and reaches preprandial values at the end of the digestive period. The high duodenal pH is a prerequisite for efficient digestion of complex carbohydrates, proteins, and lipids because all pancreatic enzymes have their pH optima in the alkaline range. Lipase in particular is even irreversibly destroyed at pH levels below 4.

Due to impaired accessibility of the more distal parts of the human small intestine, pH profiles of the jejunum and the ileum are less well established. However, it has been shown that pH increases toward pH 7–8 and is more stable in the distal small intestine.

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INTEGRATED FUNCTION AND CONTROL OF THE GUT | Gut Motility

A. Gräns , C. Olsson , in Encyclopedia of Fish Physiology, 2011

Local Contractions

Transport of food (chyme) in the gut is a discontinuous process. To improve digestion, the chyme needs to be mixed with enzymes. To facilitate absorption, chyme has to be in close contact with the epithelial cells. Hence, propagating movements are interrupted by local, standing contractions in the postprandial state.

The propulsion rate varies along the gut, with muscular sphincters function acting as dividers between different regions (see also GUT ANATOMY AND MORPHOLOGY | Gut Anatomy). Both propulsion rate and the state of the sphincters are controlled by an orchestra of subconscious signals. The sphincters are normally tonically contracted, keeping them closed, and open in the presence of an appropriate stimulus. For example, gastric emptying (the process where stomach contents are released into the intestine) normally occurs in smaller portions at a rate adjusted so that the food can be handled by the small intestine. The rate is ultimately determined by the state of the pyloric sphincter situated between the distal part of the stomach and the proximal intestine. If the sphincter is contracted, very little chyme is released while relaxation of the muscle results in opening of the sphincter.

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Improving in vitro simulation of the stomach and intestines

Venema K. , ... Minekus M. , in Designing Functional Foods, 2009

Composition of the chyme

The composition of the chyme is influenced by the transit of the meal, secretion of digestive fluids, and absorption of nutrients and water. None of the models described above simulates these combined aspects and obtains a physiological chyme composition for the different digestive stages in time.

Enzyme, bile and electrolyte concentrations are not physiological (Boisen and Eggum, 1991). Especially the single enzyme methods are limited in their use since enzymes usually work together to digest a meal (Savoie, 1994). Pancreatin or intestinal fluid is relatively cheap and contains a mixture of relevant enzymes. A disadvantage of these preparations is that their composition is not well defined, with a batch-to-batch variation of enzyme activities. Also, pancreatin contains a considerable amount of nonenzyme material. Generally, the gastric pH profile after ingestion of the meal is not simulated, which may result in an unrealistic exposure to peptic and acidic conditions.

None of the colonic models described so far includes removal of metabolites and water (Rumney and Rowland, 1992). This results in lower microbial density than found in the colonic content in vivo.

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Chemical Regulation of Feeding, Digestion and Metabolism

David O. Norris Ph.D. , James A. Carr Ph.D. , in Vertebrate Endocrinology (Fifth Edition), 2013

1 Secretin

The presence of acidic chyme (pH less than 4) from the stomach directly stimulates the S cell in the duodenal mucosa to release the peptide secretin into the blood. Although H+ are the primary stimulus, secretin release also is stimulated by bile salts, fatty acids, sodium oleate, and several herbal extracts. Secretin stimulates the pancreas to secrete basic juice (rich in HCO3 –) and helps to neutralize the acidity of the chyme that has entered the small intestine. Although secretin levels in the blood do not increase following ingestion of a meal, the action of secretin on the exocrine pancreas is potentiated by another intestinal peptide hormone, CCK, that also increases in the blood following ingestion of a meal (see ahead).

Originally it was believed that secretin also was responsible for stimulating secretion of the digestive enzymes normally present in the pancreatic juice, including the proteases chymotrypsin and trypsin, pancreatic lipase, pancreatic amylase, and nucleases for DNA and RNA. After 40 years of controversy following the demonstration of secretin, it was confirmed finally by Harper and Raper (1943) that purified secretin stimulates secretion of pancreatic fluid that is rich in sodium bicarbonate but poor in digestive enzymes. Secretion of the digestive enzymes was attributable to a second duodenal peptide that was found to contaminate some secretin preparations. Because zymogen granules represent vesicles of stored enzyme within the acinar (exocrine) pancreatic cell, the peptide that caused extrusion of zymogen granules from pancreatic acinar cells initially was called pancreozymin (pancreas–zymogen). It was postulated that the release of pancreozymin into the blood in response to the presence of peptides and amino acids in the chyme is due to direct actions of these molecules on pancreozymin-producing cells, similar to the action of H+ on the S cell. Sometimes the term secretagogue is applied to substances present in food, substances secreted from the mucosa into the gut lumen, or products of digestion that induce gastric or intestinal secretions. Pancreozymin later turned out to be another intestinal peptide hormone previously named for a different function (see ahead).

Secretin consists of 27 amino acids and chemically is related to several other peptides of the PACAP (pituitary adenylate cyclase-activating polypeptide) family (see Chapter 4), several of which are involved in digestion. Secretin has been isolated from several mammalian species and the primary sequence appears to be conserved (Figure 12-14), although mammalian secretins differ markedly from avian secretin. Receptors for secretin are G-protein linked, and secretin apparently operates through production of a cAMP second messenger to stimulate pancreatic HCO3 – secretion (see Chapter 3).

FIGURE 12-14. Comparison of secretins from mammals and the chicken.

Mammalian secretins are very conservative whereas more than half of the amino acids are different in the bird. See Appendix C for an explanation of the letters coding for individual amino acids.

 (Adapted with permission from Leiter, A.B. et al., in "Gut Peptides" (J.H. Walsh and G.J. Dockray, Eds.), Raven, New York, 1994, pp. 147–173.)

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Integrative Functions of the Enteric Nervous System

Jackie D. Wood , in Physiology of the Gastrointestinal Tract (Fifth Edition), 2012

22.2.7 Gastric Emptying

The orderly delivery of gastric chyme to the duodenum at a rate that does not overload the digestive and absorptive functions of the small intestine is another function that requires neural integration of gastric motility. Integrative neural control compensates for minute-to-minute variations in the volume, composition, and physical state of the duodenal contents by adjusting the rate of delivery into the duodenum. This is necessary because the intraluminal milieu of the small intestine is different from that of the stomach and undiluted gastric contents have a composition that is poorly tolerated by the duodenum. Neural control of gastric emptying automatically adjusts the delivery of gastric chyme to an optimal rate for the small intestine and guards against overloading the small intestinal mechanisms for the neutralization of acid, dilution to iso-osmolality, and enzymatic digestion of the foodstuff.

Some of the moment-to-moment neural control of the rate of gastric emptying involves feedback regulation of the gastric reservoir. An example is the powerful actions of lipids in the duodenum to slow gastric emptying. In this case CCK released from enteroendocrine cells in the intestinal mucosa act to stimulate CCK receptors on vagal gastric afferents, thereby initiating vagovagal reflex relaxation of the reservoir.

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Carbohydrate Digestion

Larry R. Engelking , in Textbook of Veterinary Physiological Chemistry (Third Edition), 2015

Luminal Phase (Pancreatic α-Amylase)

The entry of partially digested acidic chyme into the duodenum stimulates specialized mucosal cells to release two important polypeptide hormones into blood; secretin (from duodenal S cells), and cholecystokinin (CCK, from duodenal I cells). These hormones then stimulate exocrine pancreatic secretions into the duodenal lumen containing NaHCO3 (needed to neutralize acidic chyme), and digestive enzymes (including α-amylase). Both salivary and pancreatic α-amylase (which are similar enzymes), continue internal starch, glycogen, and dextrin digestion in a favorable neutral duodenal pH environment (i.e., pH 7). Polysaccharides are digested to a mixture of dextrins and isomaltose (which contain all of the α-1,6 branch-point linkages), as well as maltose and maltotriose ( Fig. 38-1 ). Most salivary and pancreatic α-amylase is destroyed by trypsin activity in lower portions of the intestinal tract, although some amylase activity may be present in feces.

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Gastrointestinal Toxicology

J.-M. Sauer , in Comprehensive Toxicology, 2010

10.03.2.4.2 Colonic motility

Whereas small bowel mechanical activities propel and mix the chyme, the major colonic mechanical function is storage of chyme, usually for several days with the chyme held mainly in the cecum ( Johnson 1994, 1997). The benefit of this prolonged storage is absorption of salt and water, amounting to 5–10% of that which occurred in the small bowel during several hours. As chyme is propelled to the ileocecal junction by peristaltic waves, the sphincter relaxes transiently, thereby allowing the chyme to enter the cecum. Then the sphincter closes abruptly and tightly to prevent retropulsion of cecal contents back into the ileum.

An important structural feature of the colon contributes to its storage function. The walls of the cecal, ascending, transverse, and descending portions of the colon lack a continuous layer of longitudinal smooth muscle. Therefore, peristalsis cannot take place in the proximal 85% of the colon. Since there is a continuous layer of circular muscle, rhythmic segmentation is unaffected and is the prevalent mechanical event in the colon. Because rhythmic segmentation prompts retropulsion and does not cause aboral propulsion of the chyme from the cecum, the colonic storage function depends upon this mechanical activity.

Nevertheless, sooner or later, absorption of salt and water from cecal chyme is complete and converts the liquid into semisolid feces. This waste material has to be propelled to the rectosigmoid portion of the colon in advance of defecation. The motor event responsible for this transit is termed 'mass movement.' Mass movement occurs once or twice daily and usually following a meal. The most distal portion of the cecal fecal mass is sequestered, is squeezed up the ascending colon, and then passes along the hepatic flexure, the transverse colon, the splenic flexure, and the descending colon. This colonic fecal transit covers 75   cm of large bowel in about 15   min.

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The Digestive System

Bruce M. Carlson MD, PhD , in The Human Body, 2019

Digestion and Absorption in the Large Intestine

For all practical purposes, digestion is completed by the time chyme enters the large intestine. Absorption of organic molecules is usually limited to minor amounts of small lipid fragments, including vitamins, that either pass through the small intestine or may be produced by intestinal bacteria. The colonic mucosa is lined with cells that are adapted for absorption of water and ions or for the production of mucus (goblet cells, see Fig. 12.24).

The major absorptive activities of the large intestine involve inorganic ions and water. Na+ is actively absorbed from colonic fluid and is largely replaced by K+. similarly, Cl− is replaced by HCO 3 − . If metabolically necessary, the adrenal hormone, aldosterone, can assist the colonic epithelium in further increasing the absorption of Na+ until virtually none is lost from the body. The replacement of Na+ by K+ in the large intestine leaves the body susceptible to K+ deficiency after prolonged bouts of severe diarrhea. Eating foods, like bananas, which are rich in K+, is a good source of replenishment in cases of diarrhea.

Of the roughly 1500   ml of water that enters the colon from the small intestine, 1400   ml are absorbed into the colonic wall (see Fig. 12.32). As in other areas of the intestine, water leaves the colon by following an osmotic gradient, and the chyme entering the colon is hypo-osmotic.

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Neurophysiologic Mechanisms of Human Large Intestinal Motility

Adil E. Bharucha , Simon J.H. Brookes , in Physiology of the Gastrointestinal Tract (Fifth Edition), 2012

36.6.2 The Colon as a Storage Organ

Under basal conditions, the healthy colon receives approximately 1500 ml of chyme over 24 hours, absorbing all but 100  ml of fluid and 1mEq of sodium and chloride, which are lost in the feces. 348 Colonic absorptive capacity can increase to 5–6   L and 800–1000   mEq of sodium and chloride daily when challenged by larger fluid loads entering the cecum, as long as there is a slow infusion rate (i.e., 1–2   ml/minute). Since the work of Cannon (1902), the proximal colon has been recognized to be the primary site responsible for storage, mixing, and absorption of water and electrolytes. 90 While the rectosigmoid colon functions primarily as a conduit, it can also participate in this compensatory absorptive response. 201 For 25 years, secretory and absorptive processes were believed to be segregated to crypt and surface epithelial cells, respectively. It is now recognized that absorptive mechanisms are constitutively expressed in crypt epithelial cells; secretion is regulated by one or more neurohumoral agonists released from lamina propria cells, including myofibroblasts. 407 When the colon is perfused with a plasma-like solution, water, sodium, and chloride are absorbed, while potassium and bicarbonate are secreted into the colon. 395 Absorption of sodium and secretion of bicarbonate in the colon are active processes occurring against an electrochemical gradient. There are several different active (transcellular) processes for absorbing sodium, and these show considerable segmental heterogeneity in the human colon. The regional differentiation of colonic mucosal absorption is also demonstrated by regional effects of glucocorticoids and mineralocorticoids on sodium and water fluxes. For example, in the distal colon, epithelial Na+, K+, ATPase is activated by mineralocorticoids. 52 On the other hand, the Na+/H+ exchange is activated in proximal colonic epithelium by the mineralocorticoid, aldosterone. 100 Specific channels are involved in water transport across surfaces and epithelia. These water channels, or aquaporins (AQP), are a diverse family of proteins, of which AQP8 is expressed preferentially in colonic epithelium and small intestinal villous tip cells.

Potassium is absorbed and secreted by active processes; it is unclear if chloride is absorbed by an active process. In contrast to the small intestine, glucose and amino acids are not absorbed in the colon. Colonic conservation of sodium is vital to fluid and electrolyte balance, particularly during dehydration, when it is enhanced by aldosterone. 51 Patients with ileostomies are susceptible to dehydration, particularly when placed on a low-sodium diet or during an intercurrent illness. In addition to glucocorticoids and mineralocorticoids (aldosterone), other factors enhancing active sodium transport include somatostatin, α2-adrenergic agents, and short chain fatty acids. Clonidine mimics the effects of adrenergic innervation by stimulating α2-receptors on colonocytes. In contrast, stimulation of mucosal muscarinic cholinergic receptors inhibits active NaCl absorption and stimulates active chloride secretion.

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