Physiology Chapter 15- Digestive System
What are bile pigments?
Substances formed from the heme portion of hemoglobin when old or damaged erythrocytes are broken down in the spleen and liver. The predominant bile pigment is bilirubin, which is extracted from the blood by hepatocytes and actively secreted into the bile. Bilirubin is yellow and contributes to the color of bile.
What is the function of HCO3-?
The HCO3- , like that from the pancreas, helps neutralize acid from the stomach, whereas the bile salts solubilize dietary fat. These fats would otherwise be insoluble in water, and their solubilization increases the rates at which they are digested and absorbed.
What does the emulsification of fat require?
(1) mechanical disruption of the large lipid droplets into smaller droplets and (2) an emulsifying agent, which acts to prevent the smaller droplets from reaggregating back into large droplets. The mechanical disruption is provided by the motility of the GI tract, occurring in the lower portion of the stomach and in the small intestine, which grinds and mixes the luminal contents. Phospholipids in food, along with phospholipids and bile salts secreted in the bile, provide the emulsifying agents.
What are the 2 types of neural-reflex arcs?
(1) short reflexes from receptors through the nerve plexuses to effector cells all within the GI tract; and (2) long reflexes from receptors in the tract to the CNS by way of afferent nerves, and back to the nerve plexuses and effector cells by way of autonomic nerves.
Figure 15.26- Microscopic Appearance of the Liver
(a) Hepatic lobules are the functional units of the liver. (b) A small section of the liver showing the location of bile canaliculi and ducts with respect to blood and liver cells (hepatocytes). The hepatic portal veins communicate with the hepatic sinusoids and bring absorbed substances to the liver from the small intestines. Hepatocytes take up and process nutrients and other factors from the hepatic sinusoids. Bile (green) is formed by uptake by hepatocytes of bile salts and secretion into bile canaliculi. Finally, central veins, located at the center of each lobule, drain blood from the lobules into the systemic venous circulation.
What is the function of the antrum of the stomach?
-Mixing and grinding Secretes: 1.) Mucus 2.) Pepsinogen 3.) Gastrin
What are the 3 sections of the small intestine?
1.) Duodenum 2.) Jejunum 3.) Ileum The duodenum receives chyme from the stomach, the jejunum is the middle section, and the ileum is the terminal region from which chyme enters the large intestine.
What are the 4 neurohormal inputs to parietal cells?
1.) Gastrin- stimulates 2.) Histamine- stimulates 3.) ACh- stimulates 4.) Somatostatin- inhibits
Figure 15.9- The Two Regions of the Stomach: Body and Antrum
1.) Body- upper portion of stomach (includes fundus at very top) 2.) Antrum- lower portion of stomach At the junction between the antrum and the small intestine is a ring of contractile smooth muscle called the pyloric sphincter
What are 4 types of macromolecules ingested into the body?
1.) Carbohydrates 2.) Proteins 3.) Triglycerides 4.) Nucleic Acids (DNA and RNA)
What are the 3 phases of gastrointestinal control?
1.) Cephalic Phase 2.) Gastric Phase 3.) Intestinal Phase
Figure 15.3 -The Four Major Processes of the Gastrointestinal Tract: Secretion, Digestion, Absorption and Motility
1.) Digestion- mouth, esophagus, stomach 2.) Absorption- starts a little in the stomach but majority occurs in the small intestine 3.) Secretion- occurs between the small intestine and blood flow 4.) Excretion- The colon strictly absorbs and starts the process of excretion -Excretion occurs in the rectum Motility- contractions of smooth muscle, peristalsis
What are the 6 major functions of the Digestive System?
1.) Ingestion of foods and liquids containing nutrients 2.) Digestion of large molecules in ingested food into absorbable molecular forms 3.) Absorption of nutrients from the gut into the internal environment 4.) Metabolic transformation of fuel molecules and detoxification of foreign substances (liver) 5.) Elimination of small amounts of metabolic end products secreted by the liver 6.) Carrying out a variety of immune functions, including production of antibodies and fighting infectious microorganisms not destroyed by the acidity of the stomach
What are the 2 types of digestion?
1.) Mechanical 2.) Chemical
What are the 6 diseases that result from the GI tract?
1.) Ulcers 2.) Vomiting 3.) Gallstones 4.) Lactose Intolerance 5.) Constipation and Diarrhea 6.) Inflammatory Bowel Disease
What are the 4 layers of the GI tract wall?
1.) mucosa 2.) submucosa 3.) muscularis externa 4.) serosa
What are the 4 best-understood GI hormones?
1.) secretin 2.) cholecystokinin (CCK) 3.) gastrin 4.) glucose-dependent insulinotropic peptide (GIP)
Figure 15.7- Functional Anatomy of the Mouth, Pharynx, and Esophagus
3 glands stimulated when you salivate- sublingual, submandibular, parotid
Why is the duodenum most likely to get gastric ulcers?
3 secretions into the duodenum: 1.) Hydrocloric acid- from the stomach 2.) Pancreatic juices- flows into duodenum through the pancreatic duct 3.) Bile- gall bladder Why it is most likely to get gastric ulcers Duodenum contains epithelial tissue on the inside, smooth muscle, connective tissue
Figure 15.10- A Gastric Gland in the Body of the Stomach
A gastric gland secretes- acid, intrinsic factor, pepsinogen
Figure 15.16- Slow-Wave Oscillations in the Membrane Potential of Gastric Smooth Muscle Fibers Trigger Bursts of Action Potentials When Threshold Potential Is Reached at the Wave Peak
Action potentials may be generated at the peak of the slow-wave cycle if threshold is reached (Figure 15.16), causing larger contractions. The number of action potentials fired with each wave determines the strength of the muscular contraction. Therefore, whereas the frequency of contraction is determined by the intrinsic basic electrical rhythm and remains essentially constant, the force of contraction—and, consequently, the amount of gastric emptying per contraction—is determined by neural and hormonal input to the antral smooth muscle.
What is the Migrating Myoelectrical Complex?
After most of a meal has been absorbed, the segmenting contractions cease and are replaced by a pattern of peristaltic activity known as the migrating myoelectrical complex (MMC). Beginning in the lower portion of the stomach, repeated waves of peristaltic activity travel about 2 feet along the small intestine and then die out. The next MMC starts slightly farther down the small intestine so that peristaltic activity slowly migrates down the small intestine, taking about 2 hours to reach the large intestine. The MMC moves any undigested material still remaining in the small intestine into the large intestine and also prevents bacteria from remaining in the small intestine long enough to grow and multiply excessively.
Table 15.2 -Properties of Gastrointestinal Hormones (1)
All 4 GI hormones are peptide hormones All are produced in the small intestine EXCEPT for gastrin Gastrin is produced in the antrum of the stomach
What are the products of protein digestion?
Amino acids
Figure 15.30- Protein Digestion and Peptide and Amino Acid Absorption in the Small Intestine
Amino acids moved into the blood stream Proteins and peptides are digested in the lumen of the intestine to small peptides and amino acids. Small peptides can be absorbed by cotransport with H+ into the cytosol where they are catabolized to amino acids by peptidases. Small peptides in the lumen are also catabolized to amino acids by peptidases located on the apical (brush border) membrane. Amino acids are absorbed into the cytosol by cotransport with Na+. Amino acids then cross the basolateral membrane by facilitated diffusion via many different specific amino acid transporters (only one is shown in the figure for simplicity). Amino acids then diffuse into the blood from the interstitial fluid through capillary pores. The energy for these processes is provided primarily by Na+/K+-ATPase pumps on the basolateral membrane. Also remember that protein digestion begins in the acidic environment of the stomach.
What enzyme breaks down carbohydrates?
Amylase (amyl= starch)
What is an ulcer?
An ulcer is an erosion of the lining of the gastrointestinal wall
Figure 15.35- The Hepatic Portal System: A Main Gastrointestinal System Absorption Pathway
Capillaries from the stomach, pancreas, small intestine, and large intestine drain into the hepatic portal vein, which branches to form capillaries again within the liver. Liver capillaries drain into the hepatic veins and the inferior vena cava.
What does bile contain?
Bile contains HCO3- , cholesterol, phospholipids, bile pigments, a number of organic wastes, and a group of substances collectively termed bile salts.
Why is the stomach an energy storage device?
Bolus sits in the stomach for hours- body feeds off of nutrients- why we don't need to eat for hours
Figure 15.6 -Long and Short Neural-Reflex Pathways Activated by Stimuli in the Gastrointestinal Tract
Central nervous system stimulates long reflexes Gastrointestinal walls stimulate short reflexes
Table 15.4- Control of HCI Secretion During a Meal
Cephalic phase and gastric phase increase HCl secretion Intestinal phase decreases HCl secretion
Figure 15.13- Cephalic and Gastric Phases Controlling Acid Secretion By the Stomach
Cephalic phase stimuli travel to the brain which stimulates enteric neural activity Negative feedback from HCl to gastrin sectretion Negative feedback from Somatostatin sectretion to parietal cells
When/where does chemical digestion occur?
Chemical- location in the stomach -Breaks down enzymes -Denaturation- break down of proteins and nucleic acids
Figure 15.14- Conversion of Pepsinogen to Pepsin in the Lumen of the Stomach
Chief cell- secretes pepsinogen Parietal cell- secretes HCl and intrinsic factor HCl from the parietal cell converts pepsinogen to pepsin
How does motility of the large intestine occur?
Contractions of the circular smooth muscle in the large intestine produce a segmentation motion with a rhythm considerably slower (one every 30 minutes) than that in the small intestine. Three to four times a day, generally following a meal, a wave of intense contraction known as a mass movement spreads rapidly over the transverse segment of the large intestine toward the rectum.
How are gastrointestinal processes regulated?
Control mechanisms of the gastrointestinal system are governed by the volume and composition of the luminal contents, rather than by the nutritional state of the body. This means that the body is designed to absorb all the nutrients that are ingested, whether or not the body really needs them to function.
What is the first step of digestion?
Digestion begins with chewing in the mouth, where large pieces of food are broken up into smaller particles, mixed with secretions of the salivary glands, and formed into a rounded mass referred to as a bolus
What occurs during enterohepatic circulation?
During the digestion of a fatty meal, most of the bile salts entering the intestinal tract via the bile are absorbed by specific Na+ -coupled transporters in the ileum (the last segment of the small intestine). The absorbed bile salts are returned via the portal vein to the liver, where they are once again secreted into the bile. Uptake of bile salts from portal blood into hepatocytes is driven by secondary active transport coupled to Na+. A small amount (5%) of the bile salts escapes this recycling and is lost in the feces, but the liver synthesizes new bile salts from cholesterol to replace it. During the digestion of a meal, the entire bile salt content of the body may be recycled several times via the enterohepatic circulation.
What division of the nervous system regulates gastrointestinal processes?
Enteric nervous system The GI tract has its own local neural control, a division of the autonomic nervous system known as the enteric nervous system, comprised of the two nerve plexuses: the myenteric and submucosal.
What is the function of the enzymes secreted by the pancreas?
Enzymes secreted by the pancreas digest fat, polysaccharides, proteins, and nucleic acids to fatty acids and monoglycerides, sugars, amino acids, and nucleotides, respectively.
What are the functions of the liver?
Exocrine secretions from the liver enter the small intestine and are essential for normal digestion. One of major function of the liver in relation to the physiology of the gastrointestinal tract is to secrete bile.
What pathway do fat soluble vitamins follow?
Fat-soluble vitamins (A, D, E, and K) follow the pathway for fat absorption like other lipids. Don't want to take in too many fat soluble vitamins! Surplus of vitamin K- blood clots, jaundice
Figure 15.18- Specializations of the Wall of the Small Intestine That Increase Surface Area
Figure 15.18 Specializations of the wall of the small intestine that increase surface area. (a) Circular folds formed from the mucosa and submucosa increase surface area. (b) Surface area is further increased by villi formed from the mucosa. (c) Structure of a villus—epithelial microvilli further increase surface area. Outer layer- serosa- rugae (folds) covered by intestinal villi= increase surface area and increase efficiency of absorption Arteries= red, veins= blue, green=lymph, yellow= nervous, purple= caplillary
Figure 15.19- Longitudinal Section Showing Microvilli on the Surface of Intestinal Epithelial Cells Facing the Lumen of the Small Intestine
Figure 15.19 Longitudinal section showing microvilli on the surface of intestinal epithelial cells facing the lumen of the small intestine. The microvilli form a brush border. Magnification approximately 16,000X.
Figure 15.27- Enterohepatic Circulation of Bile Salts
Figure 15.27 Enterohepatic circulation of bile salts. Bile salts are secreted into bile (green) and enter the duodenum through the common bile duct. Bile salts are reabsorbed from the lumen of the ileum into hepatic portal blood (red arrows). The liver (hepatocytes) reclaims bile salts from hepatic portal blood. The hepatic portal vein drains blood from the entire intestine, not just the ileum as shown here for simplicity. The break in the intestine indicates that only a portion of the intestine is shown.
Figure 15.17- Intestinal Phase Pathways Inhibiting Gastric Emptying (Enterogastric Reflex)
Gastric emptying is inhibited by distension of the duodenum, the presence of fat, high acidity (low pH), or hypertonic solutions in the lumen of the duodenum. This reflex is referred to as the enterogastric reflex Figure 15.17 Intestinal phase pathways inhibiting gastric emptying (enterogastric reflex). Gastric acid and pepsinogen secretion are regulated similarly.
What occurs during the intestinal phase?
Initiated by stimuli in the small intestine including distension, acidity, osmolarity, and various digestive products that stimulate responses that are mediated by both short and long neural reflexes and by the hormones secretin, CCK, and GIP, which are secreted by enteroendocrine cells of the small intestine.
What occurs during the cephalic phase?
Initiated when sensory receptors in the head are stimulated by sight, smell, taste, and chewing. Various emotional states can also initiate this phase. The efferent pathways for these reflexes are primarily mediated by parasympathetic neurons carried in the vagus nerves that activate neurons in the gastrointestinal nerve plexuses, which in turn affect secretory and contractile activity.
What enzyme breaks down triglycerides (fats)?
Lipase
What causes ulcers?
Many factors, including genetic susceptibility, drugs, alcohol, bile salts, and an excessive secretion of acid and pepsin, may contribute to ulcer formation. One major factor, however, is the presence of a bacterium, Helicobacter pylori, that is present in the stomachs of many patients with ulcers or gastritis (inflammation of the stomach walls).
When/where does mechanical digestion occur?
Mechanical (shredding of food) happens first- aids chemical digestion- breaks down food and increases surface area for chemical digestion to occur Mechanical- location in the mouth with small amount of chemical digestion
What are the products of triglyceride digestion?
Monoglyceride plus 2 free fatty acids
What are the products of carbohydrate digestion?
Monosaccharides glucose, fructose, galactose
Figure 15.24- Hormonal Regulation of Pancreatic HCO3- Secretion
Negative feedback from neutralization of intestinal acid within the small intestine to the increase of secretin secretion in the small intestine
Figure 15.25- Hormonal Regulation of Pancreatic Enzyme Secretion
Negative feedback from the increase in digestion and absorption of fats and proteins to the increase in CCK secretion of the small intestine
Table 15.5- Pancreatic Enzymes
Nucleic Acid enzymes- example, GMOs- technically not harmful to digestive system- because we break it down to the monomers that no longer have information within them
Figure 15.4 -Average Amounts of Solids and Fluid Ingested, Secreted, Absorbed, and Excreted from the Gastrointestinal Tract Daily
Of the approximately 8 L of fluid entering the tract each day, as much as 99% is absorbed; only about 100-200 mL is normally lost in the feces. This small amount of fluid loss represents only 4% of the total fluids lost from the body each day. Most fluid loss is via the kidneys and respiratory system. Almost all the ions in the fluids that are secreted into the GI tract are also reabsorbed into the blood. Moreover, the secreted digestive enzymes are themselves digested, and the resulting amino acids are absorbed into the blood.
What secretes hydrochloric acid (HCl)?
Parietal cells of the gastric glands
Figure 15.11- Secretion of Hydrochloric Acid by Parietal Cells
Parietal cells transport HCO3- to capillaries from the basolateral membrane Transport H+, Cl- and K+ to the stomach lumen from the apical membrane Transport HCl to the stomach
What enzyme breaks down proteins?
Peptidases Pepsin, Trypsin, Chymotrypsin
How are proteins digested in the body?
Proteins are broken down to peptide fragments in the stomach by pepsin, and in the small intestine by the enzymes trypsin and chymotrypsin, the major proteases secreted by the pancreas. These peptide fragments are further digested to free amino acids by carboxypeptidases from the pancreas and aminopeptidases, located on the apical membranes of the small-intestine epithelial cells. The free amino acids then enter the epithelial cells by secondary active transport coupled to Na+. Short chains of two or three amino acids are also absorbed by a secondary active transport coupled to the H+ gradient.
Figure 15.15- Peristaltic Waves Passing Over the Stomach Force a Small Amount of Luminal Into the DuoMaterialdenum
Sphincters allow one way direction of movement Muscular movements travel in a wavelike fashion in one direction along the length of a part of the tract, a process called peristalsis, or peristaltic waves
What is Enterohepatic Circulation?
Recycling pathway from the liver to the intestine and back to the liver
How does a perforated gastric ulcer occur?
Result of body being stimulated by food but not consuming food Occurs when you do not have enough mucus to protect the tissue layer
What is jaundice?
Result of too much bilirubin in the blood- eyes, skin, finger nails turn yellow Can result from hepatitis, disfunction of the liver, drinking too much alcohol
What is the function of the body of the stomach?
Secretes: 1.) Mucus 2.) Pepsinogen 3.) HCl
What is chyme?
Secretion of hydrochloric acid by cells in the wall of the stomach creates a highly acidic gastric (adjective for "stomach") environment. The mixture of ingested food particles and gastric secretions is referred to as chyme.
Figure 15.29- Carbohydrate Digestion and Absorption in the Small Intestine
Starches (polysaccharides) and ingested small sugars (disaccharides) are metabolized to simple sugars (monosaccharides) by enzymes from the pancreas and on the apical membrane (brush border). Fructose is absorbed into the cell by facilitated diffusion via a glucose transporter (GLUT). Glucose and galactose are absorbed into the cell by cotransport with Na+ via sodium-glucose cotransporters (SGLTs). Monosachharides are then absorbed across the basolateral membrane into the interstitial fluid by facilitated diffusion (GLUTs; shown for simplicity as a single type) and diffuse into the blood. The energy required for absorption is provided primarily by Na+/K+-ATPase pumps on the basolateral membrane. Movement occurs into a capillary bed
What is emulsification?
The breakdown of a large fat droplet into smaller droplets Because pancreatic lipase is a water-soluble enzyme, its digestive action in the small intestine can take place only at the surface of a lipid droplet. Therefore, if most of the ingested fat remained in large lipid droplets, the rate of triglyceride digestion would be very slow because of the small surface-area-to-volume ratio of these big fat droplets. The rate of digestion is, however, substantially increased by division of the large lipid droplets into many very small droplets, each about 1 mm in diameter, thereby increasing their surface area and accessibility to lipase action
What is segmentation?
The chyme in the lumen of a contracting segment is forced both forward and backward in the intestine. This rhythmic contraction and relaxation of the intestine, known as segmentation, produces a continuous division and subdivision of the intestinal contents, thoroughly mixing the chyme in the lumen and bringing it into contact with the intestinal wall. Figure 15.36- Segmentation contractions in a portion of the small intestine in which segments of the intestines contract and relax in a rhythmic pattern but do not undergo peristalsis. This is the rhythm encountered during a meal. Dotted lines are reference points to visualize the same sites along the length of the intestine. As contractions occur at the next site, the chyme is broken up more and more and pushed back and forth, mixing the luminal contents.
How do pancreatic secretions occur?
The enzymes are secreted from lobules called acini at the end of the pancreatic duct system; the cells are thus referred to as acinar cells. HCO3- is secreted by the epithelial cells lining the pancreatic ducts. The high acidity of the chyme coming from the stomach would inactivate the pancreatic enzymes in the small intestine if the acid were not neutralized by the HCO3- in the pancreatic fluid.
Figure 15.21- Structure of the Pancreas
The exocrine portion secretes enzymes (acinar cells) and HCO3− (duct cells) into the pancreatic ducts. The endocrine portion secretes insulin, glucagon, and other hormones into the blood.
Figure 15.37- The Segments of the Large Intestine
The first portion of the large intestine is the cecum. A sphincter between the ileum and the cecum is called the ileocecal valve (or ileocecal sphincter) and is composed primarily of circular smooth muscle innervated by sympathetic nerves. The circular muscle contracts with distension of the colon and limits the movement of colonic contents backward into the ileum. This prevents bacteria from the large intestine from colonizing the final part of the small intestine. The colon consists of three relatively straight segments—the ascending, transverse, and descending portions.
How do hormones regulate gastrointestinal processes?
The hormones that control the gastrointestinal system are secreted mainly by enteroendocrine cells scattered throughout the epithelium of the stomach and small intestine. One surface of each endocrine cell is exposed to the lumen of the GI tract. At this surface, various chemical substances in the chyme stimulate the cell to secrete its hormones from the opposite side of the cell into the blood. The gastrointestinal hormones reach their target cells via the circulation.
What is the anatomy of the small intestine?
The large surface area provided by the morphology of the small intestine allows for the highly efficient digestion and absorption of nutrients. One of the specialized anatomical structures are the circular folds (surface area specializations of the mucosa and submucosa) that are covered with fingerlike projections called villi.
Figure 15.1- Anatomy of the Digestive System
The liver overlies the gallbladder and a portion of the stomach, and the stomach overlies part of the pancreas. The position of the trachea is shown for orientation; it is NOT part of the digestive system.
Figure 15.20- Liver, Gallbladder, Pancreas, and Associated Ducts Connecting to the Small Intestine
The liver, a large organ located in the upper-right portion of the abdomen, secretes bile into small ducts that join to form the common hepatic duct. Between meals, secreted bile is stored in the gallbladder, a small sac underneath the liver that branches from the common hepatic duct. During a meal, the smooth muscles in the gallbladder wall are stimulated to contract, causing a concentrated bile solution to flow down the common bile duct (an extension of the common hepatic duct) and be injected through the sphincter of Oddi into the duodenum.
What is the function of the motility of the small intestine?
The motility of the small intestine, brought about by the smooth muscles in its walls, (1) mixes the luminal contents with the various secretions, (2) brings the contents into contact with the epithelial surface where absorption takes place, and (3) slowly advances the luminal material toward the large intestine. Because most substances are absorbed in the small intestine, only small quantities of water, ions, and undigested material pass on to the large intestine.
What is the function of the pharynx and esophagus?
The next segments of the tract, the pharynx and esophagus, do not contribute significantly to digestion, but the muscles in the walls of these segments participate in the swallowing reflex that moves ingested food and drink to the stomach
What are the functions of the Pancreas?
The pancreas has both exocrine and endocrine functions. The exocrine portion of the pancreas secretes HCO3- and a number of digestive enzymes into the duodenum.
Figure 15.22- Ion-Transport Pathways in Pancreatic Duct Cells
The pancreatic duct cells secrete HCO3− (produced from CO2 and water) into the duct lumen via an apical membrane Cl−/HCO3− exchanger, while the H+ produced is exchanged for extracellular Na+ on the basolateral side of the cell (Figure 15.22). The H+ enters the pancreatic capillaries to eventually meet up in venous blood containing the HCO3− produced by the stomach during the generation of luminal H+ Figure 15.22 Ion-transport pathways in pancreatic duct cells. (CFTR = cystic fibrosis transmembrane conductance regulator)
What is the digestion and absorption process in the large intestine?
The primary absorptive process in the large intestine is the active transport of Na+ from lumen to extracellular fluid, with the accompanying osmotic absorption of water. There is normally a net movement of K+ from blood into the large intestine lumen. There is also a net movement of HCO3- into the lumen coupled to Cl- absorption from the lumen. Some undigested polysaccharides (fiber) are converted to short-chain fatty acids by bacteria in the large intestine and are absorbed into the blood. Recent evidence suggests that these fatty acids may have important functions in immunity, cardiovascular health, and neural function. These bacteria also produce small amounts of vitamins, especially vitamin K, for absorption into the blood.
How are carbohydrates digested in the body?
The products of both salivary and pancreatic amylase are the disaccharide maltose and a mixture of short, branched chains of glucose molecules. These products, along with ingested sucrose and lactose, are broken down into monosaccharides—glucose, galactose, and fructose—by enzymes located on the apical membranes of the small-intestine epithelial cells (brush border). These monosaccharides are then transported across the intestinal epithelium into the blood. Most ingested carbohydrates are digested and absorbed within the first 20% of the small intestine.
What are the secretions of the large intestine?
The secretions of the large intestine are scanty, lack digestive enzymes, and consist mostly of mucus and fluid containing HCO3- and K+.
How does defecation occur in the rectum?
The sudden distension of the walls of the rectum initiates the neutrally-mediated defecation reflex. The reflex response consists of a contraction of the rectum and relaxation of the internal anal sphincter but contraction of the external anal sphincter (initially) and increased motility in the sigmoid colon. Eventually, a pressure is reached in the rectum that triggers reflex relaxation of the external anal sphincter, allowing the feces to be expelled.
What is responsible for producing gastric peristaltic waves?
Their rate (approximately three per minute) is generated by pacemaker cells in the longitudinal smooth muscle layer. These smooth muscle cells undergo spontaneous depolarization-repolarization cycles (slow waves) known as the basic electrical rhythm of the stomach. These slow waves are conducted through gap junctions along the stomach's longitudinal muscle layer and also induce similar slow waves in the overlying circular muscle layer.
How do the neurons of the enteric nervous system interact?
These neurons either synapse with other neurons within a given plexus or end near smooth muscles, glands, and epithelial cells. In addition, efferent neurons from both the sympathetic and parasympathetic branches of the autonomic nervous system enter the intestinal tract and synapse with neurons in both plexuses (myenteric and submucosal)
How are trigylcerides digested in the body?
Triglyceride digestion occurs to a very limited extent in the mouth and stomach, but it predominantly occurs in the small intestine. The major digestive enzyme in this process is pancreatic lipase, which catalyzes the splitting of bonds linking fatty acids to the first and third carbon atoms of glycerol, producing two free fatty acids and a monoglyceride as products
Figure 15.23- Activation of Pancreatic Enzyme Precursors in the Small Intestine
Trypsin is also a proteolytic enzyme; once activated, it activates the other pancreatic zymogens by splitting off peptide fragments (Figure 15.23). This activating function is in addition to the function of trypsin in digesting ingested protein. The nonproteolytic enzymes secreted by the pancreas (e.g., amylase and lipase) are released in fully active form.
How do ulcers form?
Ulcer formation involves breaking the mucosal barrier and exposing the underlying tissue to the corrosive action of acid and pepsin, but it is not always clear what produces the initial damage to the barrier.
Where can ulcers occur?
Ulcers can occur not only in the stomach but also in the lower part of the esophagus and in the duodenum. Duodenal ulcers are about 10 times more frequent than gastric ulcers, affecting about 10% of the U.S. population. If the ulcer is deep enough it can affect the blood vessels of the underlying tissue and result in bleeding into the gastrointestinal lumen. On occasion the ulcer may penetrate the entire wall, resulting in leakage of luminal contents into the abdominal cavity.
Figure 15.38a- Endoscopy Procedure
Video endoscopy of the upper GI tract. The physician passes the endoscope through the mouth (or nose) down the esophagus, through the stomach, and into the duodenum. A light source at the tip of the endoscope illuminates the mucosa. The tip also has a miniature video chip, which transmits images up the endoscope to a video recorder. Local treatments can be applied and small tissue samples (biopsies) can be taken with the endoscope. Earlier versions of this device used fiber-optic technology.
What is the function of villi?
Villi increase surface area and contain blood vessels and lacteals, which function in the absorption of nutrients. Additionally, the surface of each villus is covered with a layer of epithelial cells whose surface membranes form small projections called microvilli (singular, microvillus; also known collectively as a brush border).
Figure 15.8- Movements of Food Through the Pharynx and Upper Esophagus During Swallowing
Voluntary control up to certain point- only tongue, jaw, cheek muscles When it gets to the pharynx it becomes involuntary- smooth muscle of the esophagus
How are water and minerals absorbed?
Water and minerals are absorbed by diffusion primarily within the small intestine. The absorption of Na+, Cl-, and K+ as well as Ca(2+) and iron are important to maintain physiological processes.
What pathway do water soluble vitamins follow?
Water-soluble vitamins are absorbed by diffusion or mediated transport, except for vitamin B12, which must first bind to a transport protein known as intrinsic factor. Intrinsic factor with bound vitamin B12 then binds to specific sites on the epithelial cells in the lower portion of the ileum, where vitamin B12 is absorbed by endocytosis.
What occurs during the gastric phase?
Within the stomach, distension, acidity, amino acids, and peptides formed during the partial digestion of ingested protein stimulate responses that are mediated by short and long neural reflexes and by release of the hormone gastrin.
What is a bolus?
chewed food mixed with saliva When it reaches the stomach it's called chyme
What does too much alcohol intake result in?
cirrhosis of the liver- Chronic liver damage from a variety of causes leading to scarring and liver failure.
What is the sphincter of Oddi?
opens and closes common bile duct entrance into the duodenum Shared between the liver, gall bladder and pancreas
