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    neutralizes the acidic chyme as it enters the small intestine.

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    Chyme

    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

    Related terms:

    BileEsophagusPepsinSphincterSecretinPylorusPeptide YYNeurotensinCholecystokininTeleost

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    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

    HCO3−

    secretion is certainly involved (Fig. 1).

    The first-feeding related investigation of intestinal

    HCO3−

    metabolism in seawater teleosts reported that rectal fluid samples contained about twofold higher

    HCO3−

    , 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

    HCO3−

    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

    HCO3−

    secretion into the lumen by

    Cl−/HCO3−

    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

    HCO3−

    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

    HCO3−

    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

    HCO3−

    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

    HCO3−

    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

    HCO3−

    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

    HCO3−

    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−/HCO3−

    exchange, as discussed in Section 2.1.3 (Fig. 1). When this was inhibited by bafilomycin, the net rates of

    HCO3−

    secretion, Cl− absorption, and water absorption all increased.

    The killifish data provide evidence that the “CO2 recycling” mechanism to enhance intestinal

    HCO3−

    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

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    chyme

    chyme, a thick semifluid mass of partially digested food and digestive secretions that is formed in the stomach and intestine during digestion. In the stomach, digestive juices are formed by the gastric glands; these secretions include the enzyme pepsin, which breaks down proteins, and hydrochloric acid. Once food is in the small intestine, it stimulates the pancreas to release fluid containing a high concentration of bicarbonate. This fluid neutralizes the highly acidic gastric juice, which would otherwise damage the membrane lining of the intestine, resulting in a duodenal ulcer. Other secretions from the pancreas, gallbladder, liver, and glands in the

    chyme

    biochemistry

    By The Editors of Encyclopaedia Britannica • Edit History

    Related Topics: digestion stomach

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    chyme, a thick semifluid mass of partially digested food and digestive secretions that is formed in the stomach and intestine during digestion. In the stomach, digestive juices are formed by the gastric glands; these secretions include the enzyme pepsin, which breaks down proteins, and hydrochloric acid. Once food is in the small intestine, it stimulates the pancreas to release fluid containing a high concentration of bicarbonate. This fluid neutralizes the highly acidic gastric juice, which would otherwise damage the membrane lining of the intestine, resulting in a duodenal ulcer. Other secretions from the pancreas, gallbladder, liver, and glands in the intestinal wall add to the total volume of chyme.

    Muscular contractions of the stomach walls help to mix food and digestive substances together in forming chyme. As particles of food become small enough, they are passed at regular intervals into the small intestine. Once in the intestine, more enzymes are added and mixing continues. When food particles are sufficiently reduced in size and composition, they are absorbed by the intestinal wall and transported to the bloodstream. Some food material is passed from the small intestine to the large intestine, or colon. In the colon, chyme is acted upon by bacteria that break down the proteins, starches, and some plant fibres not totally digested by the other organs. In both the small and the large intestine, water is normally absorbed so the chyme gradually gets thicker. As chyme passes through the stomach and intestine, it picks up cellular debris and other types of waste products. When all of the nutrients have been absorbed from chyme, the remaining waste material passes to the end of the large intestine, the sigmoid colon and rectum, to be stored as fecal matter until it is ready to be excreted from the body.

    BRITANNICA QUIZ

    Human Body: Fact or Fiction?

    How deep is your body of knowledge about the inner workings of humans? Test it with this quiz.

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    Chapter 17 Answers

    Answer key for chapter 17

    27.17

    Chapter 17 Answers

    Last Modified: Jul 14, 2017

    17.2 Introduction to the Digestive System

    Review Questions

    What is the digestive system?

    What are the three main functions of the digestive system? Define each function.

    Describe the GI tract.

    Distinguish between the upper and lower GI tracts.

    Relate the tissues in the walls of GI tract organs to the functions the organs perform.

    Identify accessory organs of digestion and their general function in digestion.

    Identify the points in the GI tract where food becomes a bolus, chyme, and feces, respectively.

    Does food pass through the pancreas? Why or why not?

    True or False. Absorption mainly occurs in the stomach.

    True or False. Some chemical digestion occurs in the mouth.

    Most chemical digestion occurs in the _____________ .

    A. Gall bladder B. Stomach C. Small intestine D. Large intestine

    a. Describe one way in which proteins are at least partially chemically digested in the digestive system.

    b. Describe one way in which carbohydrates are at least partially chemically digested in the digestive system.

    If the villi in your small intestine were damaged and could not function normally, what effect might this have on your body? Explain your reasoning.

    The esophagus is considered:

    A. An accessory organ of the digestive system

    B. Part of the upper GI tract

    C. Part of the lower GI tract

    D. The longitudinal muscle

    Review Answers

    The digestive system consists of organs that break down food, absorb its nutrients, and expel any remaining food waste.

    The three main functions of the digestive system are digestion, absorption, and elimination. Digestion is the process of breaking down food into components that the body can absorb. It includes mechanical digestion and chemical digestion. Absorption is the process of taking up nutrients from food by body fluids for circulation to the rest of the body. Elimination is the process of excreting any remaining food waste after digestion and absorption are finished.

    The GI, or gastrointestinal, tract is a long, continuous tube through which food passes as it is being digested, absorbed, or eliminated. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

    The upper GI tract consists of the mouth through the stomach. The lower GI tract consists of the small and large intestines.

    Digestion and/or absorption take place in all the organs of the GI tract. Organs of the GI tract have walls that consist of several tissue layers that enable them to carry out these functions. For example, the inner mucosa has cells that secrete digestive enzymes and other digestive substances and also cells that absorb nutrients. The muscle layer of the organs enables them to contract and relax in waves of peristalsis to move food through the GI tract.

    Accessory organs of digestion refer to the liver, gallbladder, and pancreas. They release substances needed for chemical digestion into the small intestine.

    Food becomes a bolus when it is swallowed and passes from the mouth into the pharynx. It is considered chyme when it passes from the stomach into the small intestine. Feces is produced in the large intestine and passes through the anus.

    No, food does not pass through the pancreas because it is an accessory organ of the digestive system and not part of the GI tract where food passes through.

    False True C

    Answers may vary. Sample answers.

    a. The enzyme pepsin in the stomach helps to chemically digest proteins.

    b. The enzyme amylase in the saliva starts to chemically digest carbohydrates.

    Answers may vary. Sample answer. If your villi were damaged, it would probably interfere with absorption of nutrients from food because the villi are important for absorption.

    B

    17.3 Digestion and Absorption

    Review Questions

    Define digestion. Where does it occur?

    Identify two organ systems that control the process of digestion by the digestive system.

    What is mechanical digestion? Where does it occur?

    Describe chemical digestion.

    What is the role of enzymes in chemical digestion?

    What is absorption? When does it occur?

    a. Where does most absorption occur in the digestive system?

    b. Why does most of the absorption occur in this organ and not earlier in the GI tract?

    Name two digestive enzymes found in saliva and identify which type of molecule they digest.

    a. Where is bile produced?

    b. What are some functions of bile?

    True or False. Pepsin digests cellulose.

    True or False. Glucose can be absorbed by the body without being further broken down.

    The pH of the stomach  ___________ .

    A. is neutral B. is alkaline C. is acidic

    D. depends only on what you eat

    Lymph absorbs __________ .

    A. fatty acids B. sugars C. amino acids D. vitamin C

    Review Answers

    Digestion is a form of catabolism, in which food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs in the organs of the digestive system that make up the gastrointestinal tract.

    The process of digestion by the digestive system is controlled by the endocrine system and the nervous system.

    Source : flexbooks.ck12.org

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