Digestive System PDF

Digestive System Digestion: Digestion is the process by which food is broken down both chemically and mechanically in the gastrointestinal tract (GIT) and is converted into absorbable form enzymatically. Therefore, the digestive system is composed of all the organs and glands associated with the dig...

Digestive System Digestion: Digestion is the process by which food is broken down both chemically and mechanically in the gastrointestinal tract (GIT) and is converted into absorbable form enzymatically. Therefore, the digestive system is composed of all the organs and glands associated with the digestion of food. However, these organs can be subdivided into 2 groups- 1. The gastrointestinal tract (GIT) or alimentary canal 2. The accessory digestive organs Processes/ Functions of the Digestive System: Ingestion takes place when materials enter the digestive tract through the mouth. Mechanical processing is crushing and shearing that makes materials easier to propel along the digestive tract. It also increases their surface area, making them more susceptible to attack by enzymes. Digestion refers to the chemical breakdown of food into small organic fragments suitable for absorption by the digestive epithelium. Large & complex food molecules i.e. proteins, polysaccharides or triglycerides are broken down by digestive enzymes into simple amino acids, sugars & fatty acids so that the digestive epithelium can absorb them for distribution to body cells. Secretion is the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by glandular organs. Absorption is the movement of organic molecules, electrolytes (inorganic ions), vitamins, and water across the digestive epithelium and into the interstitial fluid of the digestive tract. Excretion is the removal of waste products from body fluid. The digestive tract and glandular organs discharge waste products in secretions that enter the lumen of the tract. Most of these waste products mix with the indigestible residue of the digestive process and then leave the body. The process called defecation or egestion, ejects materials from the digestive tract, eliminating them as feces. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 1 Parts of the Digestive System: 1. The Gastrointestinal Tract a) Oral cavity (mouth) 2. The Accessory Digestive Organs b) Pharynx (throat) a) Teeth c) Esophagus b) Tongue d) Stomach c) Various glandular organs such as the salivary glands, liver and e) Small intestine pancreas f) Large intestine Layers of the Digestive Tract: The four major layers of the digestive tract are: The Mucosa The Muscularis Externa The Submucosa The Serosa PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 2 The Mucosa: The mucosa is the innermost layer of the gastrointestinal tract. This layer comes in direct contact with food. The mucosa is made up of: Epithelium: Innermost layer, responsible for most digestive, absorptive & secretory process. Lamina Propria: A layer of loose connective tissue, containing many blood & lymphatic vessels. The Submucosa: The submucosa is a layer of dense irregular connective tissue that binds the mucosa. It contains many blood vessels, lymphatic vessels and nerves. It also contains exocrine glands (mucus secretion). The Muscularis Externa: The muscularis externa consists of an inner circular layer & a longitudinal outer muscular layer. These layers help in segment contraction and peristalsis. The circular muscle layer prevents food form travelling backward. The longitudinal layer shortens the tract. The Serosa: A serous membrane known as the serosa covers the muscularis externa along most portions of the digestive tract. There is no serosa covering the muscularis externa of the oral cavity, pharynx and esophagus. Instead, a dense network of collagen fibers firmly attaches the digestive tract to adjacent structures. This fibrous sheath is called an adventitia. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 3 Movement of Digestive Materials: Peristalsis: The muscularis externa propels materials from one portion of the digestive tract to another by contractions known as peristalsis. Peristalsis consists of waves of muscular contractions that move a bolus (soft rounded ball of digestive contents) along the length of the digestive tract. Segmentation: Most areas of the small intestine and some portions of the large intestine undergo cycles of contraction that churn and fragment the chyme, mixing the contents with intestinal secretions. This activity, called segmentation, does not follow a set pattern. For this reason, segmentation does not push materials along the tract in any one direction. *Secretion and composition of digestive juices. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 4  Carbohydrate Digestion Carbohydrates in the Diet: Only three major sources of carbohydrates that exist in the normal human diet. They are- Sucrose (which is the disaccharide known as cane sugar) Lactose (which is a disaccharide in milk) and Starches (which are large polysaccharides) Digestion of Carbohydrates in the Mouth and Stomach: When food is chewed, it is mixed with saliva, which contains the enzyme ptyalin (an α- amylase) secreted mainly by the parotid glands. This enzyme hydrolyzes starch into the disaccharide maltose and other small polymers of glucose that contain three to nine glucose molecules. The food remains in the mouth only a short time, so that probably not more than 5% of all the starches will have become hydrolyzed by the time the food is swallowed. Digestion of Carbohydrates in the Small Intestine: Digestion by pancreatic amylase: Pancreatic secretion contains a large quantity of α-amylase that is almost identical in its function with the α-amylase of saliva but is several times as powerful. Therefore, within 15 to 30 minutes after the chyme empties from the stomach into the duodenum and mixes with pancreatic juice, virtually all the starches are almost totally converted into maltose. Hydrolysis of disaccharides and small glucose polymers into monosaccharide by intestinal enzymes: The villi of the small intestine contain four enzymes - lactase, sucrase, maltase, and α-dextrinase - which are capable of splitting the disaccharides lactose, sucrose and maltose as well as the other small glucose polymers into their constituent monosaccharide. Thus, the final products of carbohydrates digestion are all monosaccharides. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 5 Glucose represents more than 80% of the final products of carbohydrate digestion, and galactose and fructose each seldom represent more than 10% of the products of carbohydrate digestion. Carbohydrate Absorption: Essentially all the carbohydrates in the food are absorbed in the form of monosaccharides. Only a small fraction is absorbed as disaccharides. The intestinal epithelium then absorbs the monosaccharides by facilitated diffusion and cotransport mechanisms. Both methods involve a carrier protein. Facilitated diffusion and cotransport differ in three major ways: 1. Facilitated diffusion moves only one molecule or ion through the plasma membrane, whereas cotransport moves more than one molecule or ion through the membrane at the same time. 2. Facilitated diffusion does not require ATP. Cotransport by itself does not consume ATP, but the cell must often expend ATP to preserve homeostasis. 3. Facilitated diffusion does not take place if there is an opposing concentration gradient for the particular molecule or ion. By contrast, cotransport can take place despite an opposing concentration gradient for one of the transported substances. For example, cells lining the small intestine continue to absorb glucose when glucose concentrations inside the cells are much higher than they are in the intestinal contents. The cotransport system that takes up glucose also brings sodium ions into the cell. Glucose cotransport is an example of sodium linked cotransport. The simple sugars that are transported into the cell diffuse through the cytoplasm. They then reach the interstitial fluid by facilitated diffusion. Lipid Digestion Fats in the Diet: The most important & abundant dietary lipids are triglycerides. They consist of three fatty acids attached to a single molecule of glycerol. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 6 Digestion of Fats in the Intestine: A small quantity of triglycerides is digested in the stomach by lingual lipase produced form the glands of tongue (less than 10% and generally unimportant). Maximum fat digestion occurs in the small intestine. The first step in fat digestion is to break the fat globules into smaller sizes so that the water- soluble digestive enzymes can act on the globule surfaces. This process is called emulsification of the fat. Bile contains a large quantity of bile salts as well as the phospholipids lecithin, both of which, but especially the lecithin is extremely important for the emulsification of fat. Digestion of triglycerides by pancreatic lipase: The most important enzyme for the digestion of triglycerides is pancreatic lipase, present in enormous quantities in pancreatic juice. Role of bile salts in accelerating fat digestion formation of micelles: The hydrolysis of triglycerides is a highly reversible process; therefore, accumulation of monoglycerides and free fatty acids blocks further digestion. The bile salts play an additional important role in removing the monoglycerides and free fatty acids by forming a complex called micelles. Micelles are small aggregates (4-8 nm in diameter) of mixed lipids and bile acids. When a micelle contacts the intestinal epithelium, the lipids diffuse across the plasma membrane and enter the cytoplasm. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 7 Lipid Absorption: The major products of lipid digestion fatty acids and 2-monoglycerides (also in micelle form) can enter the epithelial cells of intestine by simple diffusion across the plasma membrane. At the endoplasmic reticulum of the intestinal cells, the fatty acids & monoglycerides are again converted into triglycerides. These triglycerides, in association with absorbed steroids, phospholipids, and fat-soluble vitamins, are then coated with proteins. The resulting complexes are known as chylomicrons. The intestinal cells then secrete the chylomicrons into interstitial fluid by exocytosis. Transport of Lipids into the Circulation: Instead of being absorbed directly into capillary blood, chylomicrons are transported first into the lymphatic vessel of the intestine. They finally enter the bloodstream. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 8  Protein Digestion Proteins in the Diet: The dietary proteins are chemical long chains of amino acids bound together by peptide linkages. Digestion of Proteins in the Stomach: The acidic content of the stomach also provides the proper environment for the activity of pepsin, the proteolytic enzyme secreted in an inactive form by chief cells of the stomach. Pepsin works effectively at a pH of 1.5–2.0. It breaks the peptide bonds within a polypeptide chain. Pepsin only initiates the process of protein digestion, usually providing only 10 to 20% of the total protein digestion to convert the protein to proteoses, and a few polypeptides. Digestion of Proteins by Pancreatic Secretions: Immediately on entering the small intestine, the partial protein breakdown products are attacked by the major proteolytic pancreatic enzymes i.e. trypsin, chymotrypsin, carboxypolypeptidase and proelastase. Both trypsin and chymotrypsin can split protein molecules into small polypeptides. Carboxypolypeptidase then cleaves individual amino acids from the carboxyl ends of the polypeptides. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 9  Proelastase gives rise to elastase that in turn digests the elastin fibers that hold meats together. Only a small percentage of the proteins are digested all the way to their constituent amino acids by the pancreatic juices. Most remain as dipeptides, tripeptides, and some even larger. Digestion by peptidases in the enterocytes that line the small intestinal villi: Two types of peptidase enzymes are especially important, aminopolypeptidase & several dipeptidases. They split the remaining larger polypeptides into tripeptides, dipeptides and amino acids. All the amino acids, dipeptides and tripeptides are then easily transported through the microvillar membrane to the interior of the enterocyte. Finally, inside the cytosol of the enterocyte are multiple other peptidases that are specific for the remaining types of linkages between the amino acids. Within minutes virtually all the last dipeptides and tripeptides are digested to the final stage of single amino acids. More than 99% of the final protein digestive products are absorbed in the form of amino acids, with only rare absorption of peptide and very, very rare absorption of whole protein molecules. These amino acids, as well as those produced by the pancreatic enzymes, are absorbed through both facilitated diffusion and co-transport mechanisms. PHRM 7201/201: Human Physiology I Nigar Sultana Tithi, Senior Lecturer, DOP, EWU Page 10