Digestive System of a Pig

Digestive System of a bullshitLearning objectivesAfter you have studied this chapter, you shouldGet a unsounded under(a)standing of the porcine digestive packetDescribe the essential digestive processesUnderstand the role of the digestive organs in digestion and absorption1. gate (HNL/MSH)2. Anatomy of the digestive system (HNL)The anatomy the porcine digestive tract has been described and illustrated in detail by separates (e.g. Sisson, 1975, Moran, 1982)1 and will exactly be briefly described in the current chapter.The digestive system of the boar is fundamentally similar to all new(prenominal) mono gastric mammals, but the evolutionary ontogenesis in size and digestive cogency reflects greatly the habitual fodder. Pigs argon true omnivores but with a monstrous fraction of the diet glide path from position material. As such they have a great capability to digest enzyme degradable pelfs in the focal ratio demote of the GI tract, and a well-developed ecosystem in the ext turn backed gut to transgressly ferment and utilize fibrous material.2.1 emit and salivary glandsThe tomentum is born with 8 deciduous dentition maturation to 32 with age. The complete set of permanent teeth consists of 44 teeth with 3 couple ups of incisors, 1 pair of empennageines, 4 pair of premolars, and 3 pair of molars, which argon usually not fully acquired until 18 months of age2. The oral infernal region is lined with a innocent severalise squamous epithelium, and saliva is mainly relinquishd from 3 large glands the parotid glands, the mandibular glands, and the sublingual glands. Major lines from the parotid and mandibular glands transport saliva to the oral cavity, while the sublingual glands have multiplex openings beneath the tongue. In addition, a cast of scummy glands with a number of excretory ducts ar posit in the mouth.3 After leaving the mouth, feed enters the pharynx and oesophagus. The pharynx is long and narrow. The esophagus is m indless and c everywhereed with stratified squamous epithelium. Beneath the epithelium, a number of submucosal glands be rigid. Their section is to secrete mucin and bicarbonate, to undermine purple heart harshulated and protect the epithelium4.2.2 The patroniseThe leap out of the pigs consists of a simple compartment that is divided into 4 functionally and structurally different regions. The pars oesophagea is a non-glandular credit of the esophagus into the proper patronage. Ulceration ulcerous autodigestion of the cutaneous mucosa of the pars esophagea is a common phenomenon in swine production and develops from a analyzable interaction of dietetic particle size, gastric fluidity, dietetic carbohydrate content, comportment of gastric organisms, and environmental stress factors.Next to the pars oesophagea is the glandular cardia, which in contrast to most other species is very large and occupies approximately one third of the concentrate luminal develop. The fu ndic, or proper gastric, region is located between the cardiac and pyloric region. All three contain secretory glands located in supposed gastric pits. Structurally, they be similar, but they contain different cell types. The study airfoil of the put forward and lining of the pits are covered with dig up mucous cells, that create thick, tenacious mucous secernment to protect the epithelium over against injure from acetous and grinding activity.The gastric pits of the fundic mucosa contain HCl-producing parietal cells that are agglomerative in the spot of the gland. Distributed between these cells are mucous neck cells that maintain thin mucus and proteases. As the wholly cells of the stomach lining, mucous neck cells divide and reincarnate either down into the gland or up into the pits and differentiate into any of the mature cell types. Pepsinogen-producing school principal cells are located at the base of the fundic glands. In addition, the fundic mucosa as wel l as contain internal secernment/paracrine somatostatin producing D cells, seretonin producing EC cells, and histamine producing histamine-immunoreactive cells and mast cells (lamina propria)The cardiac glands have mucous cells that produce mucus, proteases and gastric lipase. The pyloric glands contain gastrin producing G-cells and somatostatin producing D-cells, but the dominating cells are the mucous cells. They do contain mucous neck cells that produce mucus and proteases and zymogen producing chief cells but have no parietal cells. 52.2.1 Size and capacity of the stomachIn feed pigs the pars esophagea, cardic, fundic and pyloric regions re stages well-nigh 6, 30, 44 and 20 % of the broad(a) mucosal area, respectively, while on heaviness basis the cardia represents exactly 20 % but the fundic region 56 % of occur mucosa weight.The weight of the stomach represents 0.5-0.8 % of clay weight in suckling pigs and between 1-1.3 % in exploitation pigs. In adult pigs the stomac h accounts for approximately 0.6 % of total dead body weight. The capacity range from 0.03 l in the new born to approximately 3.5 l in massacre pigs, and 5 l in adults, while under pressure the capacity under ontogenys to 8 and 12 l for slaughter and adult pigs, respectively. A number of studies have shown that the bulk of the diet can influence the subsequent capacity of the stomach. 62.3 The pancreas7The pancreas is located in proximal duodenum. The body of the pancreas separates in the two lobes with the center surrounding the portal vein. A ace pancreatic duct leaves the right lobe and enters the duodenum on a minor palpilla 12-20 cm distal to and separate from the impertinence duct entry, 20-25 cm from the pylorus.8The pancreas is a sundry(a) hormone and exocrine gland organ. The exocrine pancreas consists of the acinar cells and the duct system, representing more than 95 % of the pancreas fresh weight. The acinar cells produce and store pancreatic enzymes and inactive z ymogens, and when aroused release them into the duct system for transport to the duodenum. Water, bicarbonate and other electrolytes of pancreatic succus are produced in centroacinar cells and cells of the intercalary and intralobular ducts.The endocrine part of the pancreas is restricted to the islets of Langerhans. The islet are distributed throughout the acinar exocrine create from raw stuff and contain glucagon producing, alpha cells (15-20% of total islet cells), insulin and amylin producing beta cells (65-80%) , somatostatin producing delta cells(3-10%), pancreatic polypeptide producing PP cells (3-5%), and perchance also ghrelin producing epsilon cells (2.4 The liver and gallbladderThe porcine liver is divided into 4 principal lobes along with a thin quadrate lobe and a bobtail process. The lobes, which are the functional units, are surrounded by fine link tissue. The lobules consist of plates of hepatocytes interdigitated with hepatic sinoids, arranged radially around a substitution vein. Kupffer cells, which are specialized macrophages, along with endothelial cell line portions of the hepatic sinoids devise part of the reticuloendothelial system. Located in the peripheral interlobular connecter tissue is the portal triad the hepatic portal vein, a hepatic artery and an interlobular rancor duct, but additionally also lymphatic vessels10. Afferent blood from the portal vein and hepatic artery f down in the mouths centrally in the hepatic siniods. impudence produced by the hepatocytes drains into impertinence canaliculi formed by hepatocytes and then through ducts of Hering to the interlobular impertinence ducts in the portal triad. The interlobular insolence ducts merge into larger intrahepatic ducts, which become the extrahepatic biliary system. This includes the hepatic insolence duct, which divides into a cystic duct connected to the gallbladder, and a common bile duct connecting to the duodenum. The bile duct enters the duodenum on a study(ip) palpilla located 2-5 cm from the stomach pylorus.2.5 The comminuted intestineThe delicate intestine reconcile of the duodenum (4-4.5%), jejunum (88-91 %) and ileum (4-5 %). The proportion of duodenum in the neonate is similar to that of the adult, whereas differentiation between jejunum and ileum is not clear. Although there are distinctive morphological feature, the duodenum, jejunum and ileum share a component of common features.The small intestine consist of 4 study floors The serous membrane, the muscularis, the submucosa and the mucosa. The serosa is the outermost level of the intestinal wall. It has a squamous epithelium forming the mesentery that contains connective tissue, large blood vessels and nerves. The muscular layer contains two types of muscle fibres an outer layer of longitudal muscles and an inner layer of circular muscles, that are involved in GI motility. The submucosa is a layer of connective tissue holding together the large blood and lympha tic vessels and neural complexes. The mucosa consists of 3 sublayers the muscularis mucosa, the lamina propria and the epithelium. The muscularis mucosa consists of a longitudinal inner muscle and an outer muscle encircling the intestine and produce transient intestinal folds. The lamina propria consists of blood vessels, light lymphocytes and lymph nodes called Peyers patches, and neurons held together by connective tissue. It supports the structure and nourishes the epithelial layer. The epithelial layer consists of a single(a) layer of epithelial cells. They cover the whole luminal surface of the intestine, which is disadvantageously folded by the formation of fingerlike projections called villi, and at the base of these Crypts of Lieberkuhn, that are moat-like invaginations. there are 3 types of epithelial cells on the villus surface absorptive cells, chalice cells and enteroendocrine cells11. They all ascend from stem cells located near the base of the crypts. The entocytes migrate from the base to the tip of the villi and during migration, the enterocytes maturate. The digestive function (enzyme activity) begins as the enterocytes migrates over the elemental third of the villi. The absorptive function starts to develop as they reach the upper to midlevel and last outs to increase until they reach the top of the villi, where they are shed into the lumen. Hence, enterocytes at the surface of the villi are continuously renewed. goblet cells are secreting viscous mucus, and are interlardd among the enterocytes. Goblet cells increase in number from the proximal jejunum to the distal ileum.The formation of villi increases the mucosal surface by 10-14 fold compared to a flat surface of equate size. Furthermore, the cell-surface of the enterocytes facing the lumen has an apical membrane forming microvilli (brush-border) that further enhances absorptive surface 14-40 fold. The microvilli have important digestive enzymes and other proteins attached. They extent into a jelly-like layer of glycoprotein known as the glycocalyx that covers the apical membrane. The remaining part of the enterocyte plasma membrane is called the basolateral membrane, referring to the base and stead of the cell.The length of villi increases from the duodenum to the mid-jejunum and then decreases again towards the terminal ileum. This reflects the respective(a) functions of the different segments of the small intestine.Crypts also vary in size and composition along the intestine. They are deepest in the proximal small intestine (duodenum and jejunum) and shorter distally in the ileum. Paneth cells are located at adjacent to stem cells at the base of the crypts12. Their exact function is unknown but due to the presence of lysozymes and defensins they most likely contribute to maintenance of the gastrointestinal barrier.While the duodenum is the aim where digesta leaving the stomach is mixed with discriminations from the intestine, liver and pancreas, th e jejunum is the main site of absorption. Brunner glands, which are located in the submucosa on the part above the sphincter of Oddi13, produce bicarbonate containing alkaline secernment, which protect the duodenum from the acidic content of chyme, provide an alkaline condition for the intestinal enzymes and lubricate the intestinal walls.2.5.1 Size and capacity of the small intestineAt birth the small intestine is about 2 m long and has a capacity of 72 ml. At wean it has more than tripled its length (6.6 m) and has a 9-fold as high capacity (660 ml). The small intestine of fully grown pigs is 16-21 m, weighs 2-2.5 kg and has a capacity of about 20 l. While the small intestine accounts for approximately 4-5 % during the suckling period, it decreases to 1.5 % when ambit slaughter weight.2.6 The large intestineThe pig has a relatively short caecum and a long colon, consisting of an ascending, transverse and descending colon.14 The caecum is a cylindrical blind sac located at the p roximal end of the colon. The cecum, the ascending and transverse colon and the proximal portion of the descending colon are arranged in a series of centrifugal and receptive coils known as the handbuild colon. The caecum and proximal part of the spiral colon has longitudinal muscular bands resulting in a series pouches (haustra)15. The rectum is engraft in alter and is dilated to form ampulla recti just beforehand ending at the anus.The mucosa of the large intestine has no villi, but columnar epithelial cells with microvilli formed into straight tubular crypts. Numerous goblet cells secreting sulphated carbohydrate-protein complex intersperse the columnar cells to lubricate the colon. The rectum has a simple structure with columnar cells and only few goblet cells.2.6.1 Size and capacity of the large intestineDuring the suckling period the large intestine is small From a weight of 10 g and a length of 0.8 m and with a capacity of 40 ml at birth to 36 g, 1.2 m and a capacity of 1 00 ml at 20 d of age. This corresponds approximately to 1.2 % of body weight. After weaning and during the growing period it grows dramatically (2-2.5 % of body weight) and increases its weight to 1.3 kg and length to 5 m at 100 kg with a capacity of approximately 10 l. Adult pigs have a large intestine weighing about 2.8 kg, a length of 7.5 m and a capacity of 25 l.3. Function of the digestive organs 3.1 salivary secretion (HNL)Saliva contains a mixture of water (99 %), inorganic salts, mucins, a-amylase. In addition, to serve some protection against diseases, it also contains lysozyme, which breaks down the polysaccharide walls of many a(prenominal) kinds of bacteria and immunoglobulin A, which play a critical role in mucosal immunity. Saliva moistens the pabulum, lubricates the esophagus, and initiates the digestion of amylum. However, the activity of salivary a-amylase is low, and although secreted in the oral cavity, starch digestion is not believed to be of denary importan ce here, as the time naughtyigued in the mouth is too short. Some digestion whitethorn on the other hand take place in the proximal part of the stomach prior to acidification with gastric juice. 16 The volume and duration of salivary secretion varies in response to external cognitive or sensory stimuli (cephalic stimulation) and physical and/or chemical stimulation in the mouth. Volume and total activity increases with increase feeding level. However as the ratio of total salivary amylase to total pancreatic amylase is only about 1250,000 in the postprandial course17 (0-5 h after feeding), salivary a-amylase may be considered insignificant from a quantitative point of view.3.2 gastric secretion (MSH) gastric juice is a clear and slightly viscous fluid. The major constituents in gastric juice are shown in Table 1.Triglyceride digestionHCl is secreted by the parietal cells. However, HCl is not produced within the parietal cell because it would destroy the cell. Both H+ and Cl- are in bloodsuckingly transported from the parietal cell into the stomach lumen. Hydrogen ions are generated from the dissociation of carbonic acid that is produced by the enzyme carbonic anhydrase acting upon CO2 and H2O. H+ is then transported to the stomach lumen though a proton pump (H+/K+-ATPase). As hydrogen ions are secreted bicarbonate anions accumulate in the cell. To counterbalance this accumulation HCO3- is interchange for Cl- at the basolateral membrane. The K+ cations that accumulate within the cells are released back into the lumen in combination with Cl- anions.HCl plays two important roles in gastric juice. Firstly, it facilitates the protein digestion. HCl denaturates dietetic protein, which results in exposure of peptide bonds to proteolytic enzymes. In addition, HCl activates pepsinogen to pepsin and provides a medium of low pH that ensures the optimal activity of the enzyme. Secondly, the low pH provides a non-specific falsifying chemical mechanism because it i nhibits microorganisms from proliferating in the gastric lumen and cause damage to the gastrointestinal tract.Four types of proteases have been found in the gastric juice of pigs (Table 1). They are all secreted as inactive zymogens (proenzymes that are activated in the lumen) to repeal self-digestion of the cells. The zymogens are activated in the lumen at an acidic pH below 5 or by active pepsin A. Pepsin A is the dominant gastric protease in adult pigs followed by gastricsin. They have starchy proteolytic activity at pH 2-3. Pepsin digests approximately 10-15% of dietary protein before it is inactivated in the small intestine18. In suckling piglets, chymosin is the predominant protease. It has steadfast take out clotting activity at pH around 6. draw clotting is important in suckling animals it prolongs the passage time of milk along the gastrointestinal tract and enables the thorough digestion and absorption of milk nutrients. unconnected from pepsinogen, the chief cells o f the cardiac region of the pig stomach also secrete minor amounts of gastric lipase. This enzyme hydrolyses medium- and long-chain triglycerides and plays a role in the hydrolysis of triglycerides in the stomach of the young pig.A layer of protecting mucus covers the mucosal surface of the stomach. This layer protects the stomach epithelium from the acid conditions and grinding activity present in the lumen. Mucin secreted by the mucous neck cells of the gastric glands constitutes a major component of the viscous mucus layer.3.2.1 Regulation of gastric secretionGastric acid secretion is regulated by gastrin, histamine, and acetylcholine that stimulates while somatostatin inhibits acid secretion.Gastrin is produced by G cells in the antral mucosa. The production and release of gastrin is stirred up by pabulum compounds mainly small peptides and amino acids and by nervous reflexes activated by gastric distension when food enters the stomach. Gastrin is secreted into the blood be adrift and acts on the parietal cells via a G receptor. Histamine is an amplifying substance in acid secretion. Histamine is produced by local anesthetic mast cells and enterochromaffin-like cells and acts on parietal cells in a paracrine fashion. Acetylcholine is a neural sender produced by cholinergic neuraon. Acetylcholine is released as response to activation of stretch receptors19. The secretion of hydrochloric acid is most efficient when all three regulators are present. Gastric acid secretion is harborled by a feed back mechanism. When pH is 3 or below20 acid secretion diminishes and gastrin release is blocked. The acidity prevents amines from spread out into G cells and activate hormone secretion. Furthermore, acid in the lumen causes D cells to release somatostatin. Somatostatin inhibits the parietal cells from secreting acid and G cells from releasing gastrin.The regulatory mechanisms that control pepsinogen secretion are much less researched but it is generally believe d that the pepsinogen secretion is under same regulatory influences as acid secretion.The gastric secretory activity can be divided into three phases cephalic, gastric, and intestinal. The anticipation of food stimulates gastric acid secretion. This is controlled by the central nervous system and is called the cephalic phase. The cephalic phase lasts for minutes and prepares the stomach for the entry of food. The gastric phase begins when food enters the stomach. It lasts for hours and accounts for two thirds of the gastric secretions. During the gastric phase acid and pepsinogen secretion is increased. When digesta enters the duodenum the intestinal phase initiates. This phase functions to decrease gastric motility and to centralise the secretion of gastric acid and pepsinogen. The intestinal phase lasts for hours.3.3 Pancreatic exocrine secretion (MSH)The primary function of the exocrine pancreas is 1) to provide digestive enzymes for the digestion of the major nutrients and 2) t o neutralize the acidic chyme entering the duodenum from the stomach to allow the pancreatic enzymes to function. The pancreatic juice is a clear, discolorless liquid that contains salts, bicarbonate, and enzymes. The acini, the functional part of the exocrine pancreas, are composed of acinar cells, that synthesize and secrete the digestive enzymes and ductal cells where fluids and electrolytes originate from.The main regulatory pathways that control exocrine pancreatic secretion are the hormones secretin and cholesystokinin (CCK) and nervous stimulation.Acinar, centroacinar, and duct cells have receptors for secretin, CCK, and acetylcholine. When these binding sites are occupied the cells are stimulated to secrete, however, maximal secretion is observed when all receptors are occupied. Secretin is secreted by the endocrine S cells in the mucosa of the proximal small intestine. Secretin is released in response to acid or fatty acids in the duodenal lumen and it stimulates release o f bicarbonate by pancreatic duct cells. CCK is released into the blood stream in response to the presence of animo acids, peptides, and fatty acids in the duodenal lumen. CCK is secreted by I cells in the proximal small intestine and it stimulates the secretion of digestive enzymes by the acinar cells. Acetylcholine, released by nerve endings near the pancreatic cells, stimulates secretion. The neurons are stimulated to release acetylcholine by impulses from the enteric nerve system or through the vagus nerve. The sight and smell of food induces vagal responses lede to pancreatic secretion21. This is the cephalic phase of pancreatic secretion analogous to the cephalic phase of gastric secretion described previously. Distension of the stomach also causes a vagovagal reflex stimulating pancreatic secretion, which is the gastric phase of pancreatic secretion. When digesta enters the duodenum it evokes a large increase in the rate of pancreatic secretion and the intestinal phase involv es both endocrine as well as neuronal stimuli. The distention of the duodenum produces enteric nerve impulses that lead to the release of acetylcholine. The endocrine (hormonal) part of the intestinal phase occurs in response to the chemical stimulation, digestion products of protein and fat stimulates the release of CCK and the low pH of the digesta stimulates the release of secretin.The exocrine pancreatic secretion is controlled by a feed back mechanism. Diversion of pancreatic juice from the duodenum increases pancreatic secretion. It has been suggested that trypsin is the main component in this feed back ordinance as reintroduction of pancreatic juice or infusion of trypsin but not amylase into the duodenum markedly decreased pancreatic secretion. Furthermore ingestion of raw soybeans containing trypsin inhibitor increases pancreatic secretion. There is strong evidence that this feed back standard is connect with the release of CCK. Enterostatin, a pentapeptide released from procolipase when it is activated by trypsin in the duodenal lumen, may play a role in the feed back mechanism as well. Intraduodenal infusion of enterostatin hs been shown to inhibit pancreatic enzyme secretion.3.3.1 a-amylasePancreatic -amylase hydrolyses starch (from plant sources) and glycogen (from animal sources). Starch is composed of amylose, a linear polymer of glucose that is connect by -1,4 glycosidic bonds and amylopectin, a branched polymer of glucose, that contains both -1,4 glycosidic bonds and -1,6 glycosidic bonds. -amylase cleaves the interior -1,4 glycosidic bonds of starch. During the lifetime of the enzyme-substrate complex amylase hydrolyzes starch by multiple attacks through cleavage of several bonds. The major products of starch hydrolysis are maltose, isomaltose, maltotriose, sugars composed of two or three glucose units, and -limit dextrins, polysaccharides of 5 to 10 glucose residues containing both -1,4 and -1,6 glycosidic bonds.3.3.2 LipasesPancreatic j uice contains three lipolytic enzymes lipase, phospholipase A2, and carboxyl ester hydrolase, and a protein cofactor, colipase. Lipase is secreted as a fully active enzyme and is the most important enzyme in the digestion of fat. Lipase hydrolyses triglycerides the most abundant lipid in the diet and the products are free fatty acids and monoglycerides. Lipase is strongly inhibited by bile salts in the duodenum and the protein cofactor colipase is the only agent known to counteract this inhibition. Colipase is secreted as a zymogen, procolipase, which requires cleavage by trypsin to become active. Phospholipase A2 splits fatty acids from phospholipids. It is secreted as an inactive zymogen that requires activation by trypsin. Carboxyl ester hydrolase, also known as carboxyl ester lipase and cholesterol ester hydrolase, has an outstandingly broad substrate specificity, it hydrolyses mono-, di-, and triglycerides, cholesterol and retinol esters and lysophosphatidylglycerols. However, the main physiological function probably is to hydrolyse retinol and cholesterol esters.3.3.3 proteasesThe major proteolytic enzymes secreted by the exocrine pancreas are listed in Table 1. All proteolytic enzymes are secreted as inactive zymogens to protect the gland from autodigestion.The activation of the proteolytic enzymes is initiated by the activation of trypsin by enterokinase, an intestinal brush-border enzyme. Trypsin then activates all other zymogens as well as trypsinogen. Trypsin is an endopeptidase meaning that it breaks proteins at internal points along the amino acid chain, it specifically cleaves peptide bonds on the carboxyl side of basic amino acids (lysine and arginine). The catalytic activity of chymotrypsin is directed towards peptide bonds involving the carboxyl groups of tyrosine, tryptophan, phenylalanine and leucine. Elastase cleaves on the carboxyl side of aliphatic amino acids (alanine, leucine, isoleucine, valine, and genus Glycine). The carboxypeptidas es are zinc-containing metalloenzymes. They are exopeptidases meaning that they remove a single amino acid from the carboxyl-terminal end of proteins and peptides.3.3.4 Pancreatic secretion and dietary compositionThe enzymatic composition of the pancreatic juice has been shown to be dependent on the dietary composition.3.4 Bile secretion (HNL)The bile has pH of 7.4-7.9 and contains bile salts, phospholipids, cholesterol (summing up to a total lipid content of 0.6-0.7 %), sodium, potassium, chloride, bicarbonate, mucus and bile pigments, of which the latter are endogenous waste products. Bilirubin is a major end product of red blood cell overturn produced by Kupffer cells and transported to hepatocytes for conjugation. The meld bilirubin is secreted in the bile responsible for its grand/yellow colour. In the intestine conjugated bilirubin is converted by the microflora to urobilinogen, then to urobilin and stercobilin22 and finally excreted by defaecation, giving faeces its charac teristic brownness colour. Some urobilinogen is reabsorbed and excreted by the kidney as urobilin, which is responsible for the yellow colour of urine.Both bile acids and phospholipids play an important role in digestive function, and the molar ratio of total phospholipid to total bile salts is 110.123. Bile salts are conjugated bile acids, and their function is to aid emulsification and absorption of lipids. The bile acids in porcine bile are mainly conjugated with glycine but also some taurine (6.5 %). Chenodeoxycholic acid (CDCA), found in the form of 31.3 molar % glyco-CDCA and 3% taurine-CDCA is de novo synthesized from cholesterol by the hepatocytes. Hyocholic acid (HCA) in the form of 12.6 % glyco-HCA is produced by hydroxylation of CDCA. Reduction of HCA by the microflora of the intestine leads to formation of hyodeoxycholic acid (HDCA), which in bile is found as 48.2 % glyco-HDCA and 3.5 % tauro-HDCA . In contrast to humans, pig bile contains very little cholic acid(CA), f ound as glyco-CA (1.3 %). When excreted to the intestine conjugated bile acids are deconjugated and converted by the microflora in the distal small intestine. A majority of the bile acids are reabsorbed in the distal small intestine and transported to the liver via the portal vein. Along with de novo synthesized bile acids they are reconjugated and again excreted in bile. This phenomenon is termed entero-hepatic circulation, and is a mechanism to cope with the demand of bile acids, which by far exceeds the capacity for production. The phospholipids of porcine bile is entirely in the form of phosphatidyl choline, dominated by the 160-182 diacyl forms (59.6 %), followed by 160-181 (18.4 %) and 180-182 (15.9 %). 24The bile secretion from the hepatocytes is constant, but bile is only released to the intestine, when needed for lipid digestion. Hence, when little or no food is present in the duodenum, the anatomical sphincter of Oddi is closed and bile is diverted from the bile duct to th e gall bladder, where the bile is concentrated. When food, particularly fat-rich food, enters the duodenum, the Spincter of Oddi is relaxed and the gall bladder contracts by a combination of neural and hormonal factors. Gut endocrine cells are stimulated to release CCK, while neurale receptors located at the Spincter of Oddi in conjuction with the intramural rete coordinates the bile duct and bladder peristalsis.In bile duct cannulated pigs, where the Sphincter of Oddi is not controlling bile break away, the total bile flow over 24 hours has previously been measured to be 38 and 46 ml/kg in 60 and 45 kg pigs, respectively. Using reentrant cannulation of the bile duct, which allow gallbladder storage of bile and regulation of flow by the Sphincter of Oddi, it was found that a traditional European pig diet induced a bile 24-h bile flow of 48 ml/kg, while a semi-synthetic diet based on starch, sucrose, casein, maize oil and cellulose led to a flow of 30 ml/kg. Measurement of bile flo w by cannulation of the common bile duct and re-entrant cannulation of the proximal duodenum to reintroduce bile at the same rate of excretion resulted in flows of 35 ml/kg for 43 kg pigs fed a wheat-fish meal-casein diet and 59 ml/kg when a similar diet was supplemented with 40 % wheat bran. Hence, the bile flow is influence by the diet. Increasing fat content of the diet from 2 to 10 % induce a dramatic increase in bile acid secretion along with a moderate increase in phospholipid and cholesterol output. A further increase in fat content to 20 % of the diet does not lead to further increase in bile acid flow, while phospholipid and cholesterol output continue to increase. Lipid composition also influences the bile output. While degree of colour does not appear to influence the rate of bile acid and phospholipid secretion, the secretion of cholesterol is increased.253.5 Small intestinal digestion and absorption (MSH)3.5.1 Digestion of carbohydratesThe luminal phase of carbohydrate digestion applies only to starches and the enzyme involved is -amylase secreted from the pancreas. Starch hydrolysis products (maltose, isomaltose, maltotriose, and -limit dextrins) and dietary disaccharides (sucrose and lactose) are digested in the membranous phase by digestive enzymes that are a structural part of the intestinal surface membrane.Four different oligo