Biochemical Characteristics of Represented Tissues
Digestion is a complex process which is controlled by several factors. pH plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx, and esophagus; pH is weakly acidic and is typically about 6.8. Saliva controls pH in this region of the digestive tract.
There are three pairs of large salivary glands - the parotid, submandibular, and sublingual glands. There are also numerous smaller glands that are scattered through out the oral cavity.
These glands produce serous saliva. Saliva contains a mixture of enzymes such as salivary amylase, maltase, and lysozyme. The enzymes in saliva help to convert starch into maltose which is then converted partially to glucose by the maltase. Lysozyme helps prevent overgrowth of oral microbial populations by lysing bacteria. Although the parotid glands are the largest pair of salivary glands, only 25% of saliva is produced here.
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It produces mixed serous and mucous secretions. Although these glands are much smaller than parotid glands the submandibular glands produce more than 70% of saliva.
The secretion produced is mainly mucous; however it is categorized as a mixed gland. Approximately 5% of saliva entering the oral cavity is produced by these glands.
Minor Salivary Glands:
These glands are located throughout the oral cavity within the lamina propria of the oral mucosa. The other salivary glands are encapsulated by connective tissue but minor salivary glands are only surrounded by it. This gland is usually composed of a number of acini that connects into a tiny lobule.
Von Ebner's Glands:
Von Ebner's glands are glands found in circumvallate papillae of the tongue and are an essential component of taste. They secrete a serous fluid that contains lingual lipase that begins lipid hydrolysis. The secretions from these glands also helps flush material from the moat to enable the taste buds to respond rapidly to changing stimuli.
The majority of hormones and enzymes produced by the GI tract are synthesized by cells in the mucosa of the stomach and small intestine. The enzymes help digest food while the hormones stimulate digestive juices and cause organ movement.
Secretions of the Stomach
The stomach stores and helps continue the digestion of food. The average adult produces 2-3 L of gastric juice every 24 hours. Gastric juice contains the following components:
Mucus is secreted by goblet cells of the surface epithelium and mucus neck cells, especially in the pyloric antrum. The alkaline mucus of the stomach is a thick, sticky mucopolysaccharide. It is secreted with bicarbonate ions, which are exchanged for chloride ions by the epithelial cells. It plays an important role in the protection of the stomach against its acid contents. Mucus forms a water-soluble gel that adheres to the surface of the stomach lumen. It reduces the flow of hydrogen ions and acts as a barrier to pepsin. Although pepsin can degrade mucus, the bicarbonate secretions increase the pH and make the enzyme less active.
Pepsin is secreted from the chief cells in the gastric pits in the form of its precursor, pepsinogen. Pepsinogen is activated by the acidic condition id the stomach as HCl cleaves off nine amino acids residues off pepsinogen to convert it into its active counterpart pepsin. Pepsin is a proteolytic enzyme that hydrolyzes internal peptide bonds in proteins and polypeptides and hence breaking them down into amino acids.
Gastric lipase is an enzyme that acts on triglycerides to produce fatty acids and glycerol. It is useful in facilitating subsequent hydrolysis by pancreatic lipases, but is of little physiological importance except in pancreatic insufficiency.
HCl is produced by the parietal cells. It’s secretion is stimulated by histamine, acetylcholine, and gastrin.
The rate of HCl secretion depends on:
• The amount of buffering provided by the resting juice, ingested food and drink, and the alkaline secretion of the pyloric glands, duodenum, pancreases, and bile.
• Gastric motility.
• The rate of gastric emptying
• The amount of diffusion back into the mucosa
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The pH of the contents of the stomach after feeding is normally about 2-3. The strong acid content of the stomach provides many benefits:
• It helps denature proteins for later digestion in the small intestines.
• Provides non-specific immunity by retarding or eliminating various pathogens.
• Inhibits the breakdown of carbohydrates.
• Provides an optimum pH for protein digestion by enabling the activation of pepsin from pepsinogen.
• Stimulates the flow of bile and pancreatic juice.
Intrinsic factor is made in the parietal cells of the stomach; it is a glycoprotein vital for the absorption of vitamin B12 in the terminal ileum. Without intrinsic factor, vitamin B12 is digested in the intestine and not absorbed. R protein in the saliva protects vitamin B12 until it reaches the stomach.
Ghrelin is produced by cells lining the fundus of the stomach in the absence of food in the digestive system and stimulates appetite.
Chymosin: is a protease that infants also secrete. It coagulates milk allowing it to be retained longer in the stomach
Secretions of the Small intestine
In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralize the acidic conditions from the stomach. Also the pancreatic duct empties into the duodenum, adding bicarbonate to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline, creating a pH of about 8.5, thus enabling absorption in a mild alkaline in the environment.
• Enzyme secretions and bile (from the gall bladder) initiate or continue the breakdown of carbohydrates (to monosaccharides), proteins (to amino acids), and lipids (to glyceride units); pH about 7-8, is about 4-7 meters long
• Chyme from the stomach, pancreatic and hepatic secretions, as well as water and certain electrolytes are then absorbed in the small intestines, most of which occurs in the ileum
• The intestine absorptive cells, called enterocytes, have disaccharidases and dipeptidases bound to their apical epithelium membranes
• The enzymes are located in the glycocalyces, a network of polysaccharides that extend from the cellular surface of the microvilli
• Mucus is produced by the goblet cells which are most numerous on the villi. The mucus lubricates the mucosal surface and protects it from trauma as particles of food pass through it.
• Numerous gastrointestinal hormones.
Hormone Gastrointestinal source Signal for release Action
CCK Enteroendocrine (AUPD) cells in upper intestine Peptides and amino acids and fats, elevated serum calcium levels in duodenum Contracts gall bladder and causes secretion of alkaline enzymatic pancreatic juice. It inhibits gastric emptying, acts as a satiety hormone, stimulates glucagon secretion and contracts the pyloric
Secretin S cells in upper small intestine Acid and products of fat digestion in duodenum Secretin stimulates production of pepsinogen It also increases secretion of bicarbonate ions by the pancreases and biliary tract, decreases acid secretion, contracts the pyloric sphincter and augments CCK’s production of pancreatic secretions.
Gastrin G cells in antrum Peptides and amino acids, distentions, vagal stimulation, blood-borne calcium and adrenaline Stimulates acid and pepsinogen secretion, increases gastric motility, and stimulates insulin and glucagon secretion after protein meal. Inhibits gastric emptying.
Somatostatin D cells of pancreatic islets, intestinal cells Glucose, amino acids, free fatty acids, glucagon and β-adrenergic and cholinergic neurotransmitters Inhibits secretion of insulin, glucagon, acid, pepsin, gastrin, secretin and intestinal and intestinal juices, decreases gastric, duodenal and gall bladder motility.
Peptide YY Neuroendocrine cells in the ileum and colon Food especially fat in digestive system Regulated food intake by slowing gastric emptying; hence, increasing efficiency of digestion and nutrient absorption after meal.
Glucose-dependent insulinotropic peptide (GIP) K cells of the duodenum and the jejunum Protein, fat and carbohydrates. Induces insulin secretion. Affects fatty acid metabolism through stimulation of lipoprotein lipase activity in adipocytes.
Transport and secretions of the Large intestine
Food is not broken down any further in this stage of digestion. The large intestine simply absorbs vitamins that are created by the bacteria inhabiting the colon. It also absorbs water and compacts feces, and stores fecal matter in the rectum until eliminated through the anus and thus is responsible for passing along solid waste.
Secretion of mucus:
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The colonic mucosa has many goblets cells in its crypts and surface epithelium which secrete mucus in response to mechanical irritation of the mucosa. Contents in the large intestine become increasingly solids as water is reabsorbed on the way through hence mucus lubricates the colon thus preventing trauma.
Transport of urea and electrolytes:
The exchange of sodium, potassium, hydrogen and bicarbonate ions result in a net potential difference across the colonic mucosa. It is this potential difference that allows the passage of potassium ions across the tight junctions from the paracellular space into the lumen accounts for the potassium rich secretions found in the colon. Urea synthesis is greater than its excretion by about 20%. The excess is secreted into the colon for metabolism by bacteria and the products are then absorbed. Metabolism occurs near the mucosa, rather than in the lumen. The ammonium and bicarbonate ions produced are converted into ammonia, carbon dioxide and water. These freely diffuse across the mucosal epithelium into the circulation. The ammonia transported to the liver for synthesis of amino acids.
Water absorption in the colon:
The main function of the colon is absorption of water from the contents passing through it. The exchange of sodium and chloride ions creates an ion gradient which regulates osmosis.
Digestion and absorption of different components of the diet.
Fats are not water soluble and thus its digestion and absorption is more complex than of other substances. Fat in the diet is principally in the form of triglycerides which are composed of a glycerol backbone and three fatty acids. Fat is also commonly ingested as cholesterol and phospholipids. Fat assimilation begins in the stomach, and is released in small portions into the duodenum. Except for short-chain fatty acids, there is no absorption of fat from the stomach. CCK slows gastric motility and emptying when fat is in the small intestine. CCK also stimulates pancreas to secrete lipase and causes contraction of the gallbladder. Bile salts excreted by the gall bladder and released into the duodenum emulsify fat hence increasing surface area of lipids in preparation for their enzymatic hydrolysis.
Enzymes from four sources are involved in the digestion of dietary lipids. These include food-bearing lipases, lingual lipase, gastric lipase and pancreatic lipase.
Lipase breaks down the fat into monoglycerides and fatty acids. Monoglycerides, free fatty acids, bile acids, phospholipids, and cholesterol form a complex called micelles. Most monoglycerides and fatty acids passively diffuse the plasma membrane of enterocytes and are transported into the endoplasmic reticulum to synthesize triglycerides.
Instead they are absorbed into the walls of the intestine villi and reassembled again into triglycerides. The triglycerides are coated with cholesterol and protein to form chylomicron. Within the villi, the chylomicron enters the lacteal, which merges into larger lymphatic vessels. It is transported via the lymphatic system and the thoracic duct and finally emptied into the bloodstream via the left subclavian vein. At this point the chylomicrons can transport the triglycerides to where they are needed.
• o NPC1L1 transports cholesterol into jejunal enterocytes where it becomes esterified
• Triglycerides are then packaged along with cholesterol, lipoprotein and accessory proteins into chylomicrons, which releases its products through exocytosis into the surrounding cellular space and drains into the lymphatic system
The products of fat digestion, being lipids, diffuse across the lipid membrane of the brush border of the small intestine. Difference components are absorbed at different rates. Free fatty acids diffuse across rapidly, cholesterol more slowly. The micelles, therefore, become more concentrated in cholesterol as they move along the small intestine. Under normal conditions, most dietary fat is absorbed before the contents reach the end of the jejenum. The surface of the normal small intestine is convoluted. An unstirred layer is present on the surface, through which micelles must pass before dietary fat can be absorbed by the epithelial cells. Once inside the epithelial cells, lipid is taken into the smooth endoplasmic reticulum where much of it is re-esterified. Some lipid is also synthesized into the epithelial cells. Dietary and synthesized lipids are then incorporated into chylomicrons and, provided β-lipoprotein is present, the chylomicrons are exocytosed into lateral intercellular spaces to enter the lacteals. Having reached the lymphatic system, they travel up the thoracic duct and enter the venous circulation.
Absorption of fat soluble vitamins A, D, E and K depend in the absorption of fat and any conditions in which fat digestion and absorption is decreased will eventually lead to a deficiency of these vitamins.
Carbohydrates provide most of the calories in the average diet and majority in the form of starch. Starch contains α -1, 4 and α -1, 6 linkages: the former are hydrolyzed by the α-amylase in saliva and in pancreatic secretions.
This produces maltose, maltotriose and α-limit dextrans. Starch is, therefore, partially digested in the mouth and its digestion continues in the duodenum.
Other enzymes further break down starch into glucose and galactose, which are actively up taken. Carbohydrates are only absorbed as monosaccharides through the gut mucosa. The Monosaccharide glucose and galactose are co-transported, along with sodium, across the apical membrane by sodium-dependent glucose co-transporters (SGLT1). The transporters bind sodium first, and then induce a conformational change allowing the binding of monosaccharide molecules. These Monosaccharides are then transported out of enterocytes by an additional co-transporter (GLUT-2) into the basolateral membrane, which then diffuse into fenestrated capillary blood within the villi of the small intestines.
Another monosaccharide, fructose, is also produced and is taken by sodium-independent mechanism. Fructose is transported by facilitated diffusion by GLUT-5 which is a carrier protein located in the apical membrane of the enterocytes.
Dietary proteins are rarely absorbed without first digestion into di- or tripeptides and amino acids.
• Peptide bonds have already begun to be cleaved by the protease pepsin, which is secreted and activated in the stomach, by the time the proteins reach the small intestines
• Once in the duodenum, protein also interacts with proteases secreted from the pancreas (i.e. trypsin and carboxypeptidases)
• Microvilli of the small intestines possess integral membrane bound proteases which hydrolyze luminal peptides to free amino acids and small peptides suitable for absorption
• Mechanistically similar absorption to monosaccharides; utilize a sodium-dependent amino acid co-transporter (multiple types for basic, acidic, or neutral molecules)
• Specific transporters do exist to move amino acids into the blood stream from the enterocyte, as opposed to depending on a concentration gradient mechanism