Digestive System Handout - Part 2 PDF

TCM 115 – Basic Anatomy and Physiology DIGESTIVE (ALIMENTARY) SYSTEM – HANDOUT (part 2) Stomach The stomach is enlarged portion of digestive tube with maximal capacity to store approximately 1.5 – 2 liters of ingested food. Within its walls three layers of muscles are observed. The stomach is com...

TCM 115 – Basic Anatomy and Physiology DIGESTIVE (ALIMENTARY) SYSTEM – HANDOUT (part 2) Stomach The stomach is enlarged portion of digestive tube with maximal capacity to store approximately 1.5 – 2 liters of ingested food. Within its walls three layers of muscles are observed. The stomach is composed of four major parts: 1. Cardia 2. Fundus 3. Body 4. Pylorus a. Pyloric antrum b. Pyloric canal c. Pyloric sphincter Due to its curved shape, we recognize the greater and the lesser curvatures of the stomach. Mucosa of the stomach is thrown into numerous fold that are termed rugae. The glands of the stomach mucosa: 1. Surface mucous cells – secrete mucus 2. Mucous neck cells – secrete mucus – located deeper in the gastric pits 3. Parietal cells – secrete HCl acid and intrinsic factor (for vit. B12) 4. Chief cells – secrete pepsinogen and gastric lipase 5. G cells – secrete hormone gastrin Stomach continues creating mixing waves (instead of peristaltic waves) which mixes ingested food with gastric juice. Stomach juice dissolves food and makes it more liquid. The term used for the food contained within the stomach is “chyme”. Acidic stomach secretion will INACTIVATE salivary amylase, but will ACTIVATE lingual lipase which starts breaking down triglycerides (chat beo trung tinh) within the stomach. Proton pump – it is actually H+/K+ pump (utilizes ATP as a source of energy). Potassium ions are imported into parietal cells of pancreas and hydrogen ions are exported into the lumen of the stomach. At the same time, parietal cells allow both Cl and K ions to immediately diffuse through the apical part into the lumen. Within the cell, carbonic anhydrase enzyme catalizes reaction between water and CO2 as it follows:  CO2 + H20  H2CO3 (carbonic acid)   H2CO3  HCO3- + H+ (source of hydrogen ions)  Bicarbonate ion leaves the cell at the basal membrane and it results in a massive “alkaline tide” as it enters the circulation. Hydrogen ions will be actively exported from apical end of the cell into stomach lumen (H/K pump)  HCl produced by stomach eliminates most of the microorganisms, denatures proteins and stimulates secretion of hormones that will promote flow of bile and pancreatic juice.  Digestion of proteins also happens in the stomach as it produces enzyme called pepsin – secreted by chief cells. Pepsin requires low pH values for its maximal efficiency. It is released from chief cells in its inactive form – pepsinogen. For its activation it must make a contact with either HCl or already activated pepsin molecule (this prevents autodigestion of proteins within the chief cells).  Stomach produces also gastric lipase – an enzyme that continues digestion of lipids. Optimal pH for its activity is 5-6, and as it can’t be easily attained, pancreatic lipase will become the most important enzyme for breakdown of lipids. Regulation of gastric secretion and motility  Involves both neural and hormonal control  Occurs in three overlapping phases: cephalic, gastric and intestinal phases. Cephalic phase Other signals than just pure ingestion of food into alimentary tract are important such as: smell, vision or even thinking of food. Centers in brain and hypothalamus stimulate medulla oblongata. From the medulla, vagus nerve carries parasympathetic signals that in turn stimulate submucosal plexus to start the secretion of pepsinogen, mucus and HCl into stomach lumen. Also hormone gastrin is released into blood stream. Gastric phase When food enters stomach it stretches its walls. Sensory receptors of the stomach will ensure that gastric secretion continues as long as there is food in the stomach. Stretch receptors and pH receptors will ensure proper motility and proper acid secretion. Peristalsis will be responsible to push 10-15 ml of chyme into duodenum. Negative feedback will suppress additional acid secretion as the pH will reach certain levels and as the stomach is progressively less distended (due to its emptying). Hormonal control – when partially digested proteins raise pH value (as they act as buffers) or when the stomach is distended, parasympathetic fibers release acetylcholine. ACh stimulates G cells to release gastrin (hormone) which stimulates production of gastric juices, firms up the inferior esophageal sphincter and relaxes pyloric sphincter. Its secretion will stop when pH < 2, or when the stomach is less distended. Intestinal phase As the chyme enters the small intestine, intestinal reflexes are inhibiting the gastric phase of digestion. It slows down stomach emptying to avoid dumping large quantities of chyme into duodenum. The chyme that enters duodenum is full of carbohydrates and fatty acids and it influences the small intestine to secretes secretin and cholecystokinin (CCK). Secretin stops gastric secretion and CCK inhibits stomach emptying. Small intestine It is approximately 3 – 3.5 m long section of the gastrointestinal tract. Its primary objective is to keep passing the chyme and to absorb almost all nutrients from the chyme. Only water, minerals and water soluble vitamins will be absorbed from the lumen of the large intestine. The small intestine is composed of three sections: 1. Duodenum – 20-25 cm long 2. Jejunum – 1 m long – most vascular 3. Ileum – 2 m long Small intestine terminates at its attachment to large intestine which creates a functional ileo-cecal valve. This valve prevents retrograde passage of content back into the lumen of small intestine. Although the small intestine doesn’t offer any major surface landmarks, its mucosa is quite interesting. It is thrown into circular folds (plicae circulares) to ensure maximal contact of chyme with absorptive surface. Plicae are visible with naked eye. If additional analysis of mucosa is conducted under the microscope we would observe its fingerlike projections – termed villi whose surface cells are covered with even more submicroscopical projections called the microvilli. Small intestine is attached to posterior abdominal wall through the peritoneal layer that is called the mesentery. Small intestine physiology Mucosa of small intestine contains many glands that are composed of different types of cells such as i. Absorptive cells (cover villi) ii. Goblet cells (secrete mucus) iii. Enteroendocrine cells (secretin, CCK ) iv. Paneth cells (lysozyme and phagocytosis)  Intestinal juice 1 – 2 liters per day. pH is approximately 7.6  Enzymes (alpha-dextrinase, maltase, sucrase and lactase, aminopeptidase and dipeptidase, nucleosidase and phosphatase)  Segmentation – unique movement that enables better mix between intestinal juices and chyme Digestion of carbohydrates Salivary amylase (starch to maltose, maltotriose and alpha dextrin while in the upper GI tract.) It is inhibited by gastric juice, so that digestion of carbohydrates continues with addition of pancreatic amylase. Finally, in intestine arrival of disaccharides is requiring brush border enzymes (sucrase, lactase etc.) to create monosaccharides which could be absorbed. Digestion of proteins In stomach pepsin acts on proteins, breaking them down to peptides. Peptides are further broken down by pancreatic enzymes, trypsin, chymotrypsin etc. Finally dipeptidase splits dipeptides into amino acids that are absorbed by intestinal cells Digestion of lipids Lingual and gastric lipase start digestion of lipids. In small intestine addition of bile salts emulsifies lipids to small particles (1 micrometer) which are exposed to pancreatic lipase. Pancreatic lipase splits triglycerides into 2 fatty acids and monoglyceride. Regulation of intestinal secretion and motility  Most important are locally elicited enteric reflexes  Vasoactive intestinal polypeptide (VIP) stimulates production of intestinal juices  Stretching of small intestine is also importan for the “segmentation” process to continue  Chyme travels approximately as fast as 10 cm/ sec and it takes nearly 4 hours for chyme to reach the large intestine Absorption in small intestine It is a process which allows digested substances to pass from the lumen of intestine into blood stream or into lymphatic vessels. Most of it occurs via diffusion (high to low concentration gradient is established). Only 10% of absorption occurs in stomach or large intestine. Absorption of monosaccharides Monosacchardies (glucose, fructose and galactose) are absorbed via facilitated diffusion (fructose) or active transport (glucose and galactose) – 100% absorption Absorption of amino acids, dipeptides and tripeptides Amino acids, dipeptides and tripeptides (95-98% absorbed in small intestine) are mostly absorbed through the active transport in duodenum and jejunum. Absorbed dipeptides and tripeptides are quickly broken down to amino acids within the epithelial cells of small intestine, so that only amino acids enter circulation. Absorption of lipids Fatty acids, glycerol and monoglycerides are absorbed by simple diffusion (approximately 95% absorbed in small intestine) When fatty acids or monoglycerides combine with bile salts they form “MICELLES”. Micelles are small and they are made of fatty molecule in the center, surrounded by bile salts. Hydrophilic portion of bile salt molecules enables fatty molecules to be absorbed. Together with fats, we absorb lipid soluble vitamins: A, D, E and K Absorption of electrolytes, vitamins and water Water – there is approximately 10 l of liquid content entering intestine daily. 2.3 l is food ingestion and 7 l is gastrointestinal secretion. About 8.3 liters is absorbed in small intestine, 0.9 l in the large intestine and only about 0.1 l is lost in feces Large intestine It is approximately 1.5 m long tube which is composed of the following sections: 1. Cecum – to which appendix vermiformis is attached 2. Ascending colon – makes hepatic flexure to become 3. Transverse colon – makes splenic flexure to become 4. Descending colon 5. Sigmoid colon 6. Rectum 7. Anal canal and anal opening Three longitudinal bands of smooth muscles are visible on the large intestine’s surface. They are termed teniae colli. As they contract, they shorten the length of large intestine, and create multiple, pouch-like spaces that are termed haustrae. The peristalsis continues throughout the large intestine, but because it is carried in a different form than the peristalsis of small intestine, it is termed “mass-persitalsis”. Mechanical digestion in large intestine Ileo-cecal valve prevents back flow of chyme from small into large intestine. When chyme enters into ileum, it stimulates gastroileal reflex – enhances emptying of the ileal content and passage of chyme into large intestine Mass peristalsis – strong peristaltic wave which starts at distal half of the transverse colon. Gastro-colic reflex – initiates mass peristalsis 3-4 times per day. Chemical digestion in large intestine  Activity of bacteria that reside in large intestine  Some of B and K vitamins are absorbed in large intestine  Conversion of remaining proteins into aminoacids  Indole and skatole – odor to feces  Breakdown of bilirubin – into stercobilin – color of feces Absorption in large intestine and feces formation  Chyme spends 7 – 10 hours in large intestine and turns into semisolid feces (because of water absorption)  About 0.9 liter of fluid is absorbed in large intestine  Also some sodium , chloride and some (water-soluble) vitamins are absorbed Defecation reflex Rectum (ampulla) is a container where fecal material accumulates  receptors detect stretch of ampulla and signals are sent to sacral segement of the spinal cord  spinal cord sends motor signals via parasympathetic fibers that cause a strong contraction of the longitudinal rectal muscles. Compression of abdominal cavity is increased by contraction of diaphragm and anterior abdominal wall muscles. External anal sphincter – voluntary control Internal sphincter is involuntarily controlled.