Chpt23_Digestive PDF

CHAPTER TWENTY-THREE The Digestive System 23.1 Overview of the Digestive System A. The digestive system consists of a muscular tube called the alimentary canal or gastrointestinal tract (GI) tract and a variety of accessory organs. 1. Alimentary tract – th...

CHAPTER TWENTY-THREE The Digestive System 23.1 Overview of the Digestive System A. The digestive system consists of a muscular tube called the alimentary canal or gastrointestinal tract (GI) tract and a variety of accessory organs. 1. Alimentary tract – the muscular tube, also known as the GI tract, which begins with the mouth and continues through the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine which opens to the exterior through the anus. 2. Accessory organs – organs that include the salivary glands, liver, gallbladder, and pancreas, situated along the GI tract that produce secretions to assist in digesting and absorbing nutrients. 3. The digestive system works with other systems to support tissues that have no direct connection to the outside environment and no other means to obtain nutrients. B. Histology - the organs of the GI tract have distinctive structural and functional characteristics, but all share an underlying pattern of histological organization, though variation does exist within some organs. 1. Mucosa – the innermost lining containing the mucous membrane of epithelial tissue supported by a basement membrane and thin layer of muscle tissue. a. Epithelium - varies according to location within the GI tract. I. (Non-keratinized) stratified squamous epithelium is found in the oral cavity, pharynx, the upper portions of the esophagus, and rectum. II. Simple columnar epithelium is found within the lower portions of the esophagus, the stomach, small intestine, and most of the large intestine. i. Specialized mucus producing cells types vary too. ii. Enteroendocrine cells for hormone release vary also. b. Lamina propria – a basement membrane composed of areolar connective tissue containg blood vessels, sensory nerve endings, lymphatic system structures, and mucous glands. c. Muscularis mucosae – smooth muscle that creates various folds on the epithelial surface of the GI tract such as rugae, plicae circularis, and villi to increase surface area. 2. Submucosa – a layer of dense irregular connective tissue that contains large blood vessels, lymphatic vessels, the submucosal plexus, and in some regions, exocrine glands that secrete enzymes and buffers into the lumen of the GI tract. 3. Muscularis – essential in the mechanical processing and propulsion of food through the GI tract, but the muscle tissue type and structure will vary according to location within the GI tract. 1  a. Predominately composed of two layers of smooth muscle, an inner circular layer and an outer longitudinal layer, for involuntary control. b. The mouth, pharynx, anterior esophagus and external anal sphincter are made up of skeletal muscle for voluntary control of ingestion and defecation. c. The stomach has a third oblique layer to aid in churning while the colon has the outer longitudinal layer modified into a band of muscle known as tenia coli. 4. Serosa – the double layer serous membrane covering many of the organs in the abdominal cavity; those outside have a dense layer of collagen called the adventitia. C. Innervation. 1. Intrinsic innervation of much of the alimentary canal is provided by the enteric nervous system, which are grouped into two plexuses. a. The myenteric plexus (plexus of Auerbach) lies in the muscularis layer of the and is responsible for motility, especially the rhythm and force of the contractions. b. The submucosal plexus (plexus of Meissner) lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food. 2. Extrinsic innervation is also provided by the autonomic nervous system, with the sympathetic nervous system restricts activity while the parasympathetic system stimulates it. D. Blood supply. 1. The arterial supply has two functions. a. Transport absorbed nutrients to the liver. b. Supply the organs of the alimentary canal with the nutrients and oxygen needed to drive their cellular processes. 2. The venous supply transports absorbed nutrients via the hepatic portal vein to the liver before returning to systemic circulation. E. The Peritoneum. 1. A double layer serosal membrane that holds abdominal organs in place. 2. In certain areas the serosa pulls away from the surface of the organs to create five major peritoneal folds. a. Greater omentum – apron-like structure that lies superficial to the small intestine and transverse colon; a site of fat deposition in people who are overweight. b. Falciform ligament – anchors the liver to the anterior abdominal wall and inferior border of the diaphragm. c. Lesser omentum – suspends the stomach from the inferior border of the liver; provides a pathway for structures connecting to the liver. d. Mesentery – vertical band of tissue anterior to the lumbar vertebrae and anchoring all of the small intestine except the initial portion (the duodenum). e. Mesocolon – attaches two portions of the large intestine (the transverse and sigmoid colon) to the posterior abdominal wall. 2  3. Some posterior abdominal organs are outside of the peritoneum and thus referred to as retroperitoneal. 23.2 Digestive System Processes and Regulation A. Digestive Processes 1. Ingestion – occurs when foods and beverages enter the alimentary canal via the mouth. 2. Mechanical digestion – physical breakdown of food into smaller pieces, increasing surface area without changing the chemical nature of the food. Examples include mastication or chewing in the mouth, churning of food into chyme in the stomach, and segmentation in the small intestine. 3. Propulsion – movement of food through the alimentary canal by swallowing and peristalsis, alternating waves of contraction and relaxation. 4. Chemical digestion – the chemical and enzymatic breakdown of foods into small organic molecules that can be absorbed such as carbohydrates, proteins and lipids. 5. Absorption – movement of small organic molecules and other nutrients from the lumen of the GI tract into the interstitial fluids, blood, or lymph. 6. Defecation – removal of undigested and unabsorbed materials from the body as feces. B. Regulation 1. A variety of sensors, including mechanoreceptors, chemoreceptors, and osmoreceptors, help regulate digestive functioning along with the enteric nervous system’s myenteric and submucosal plexuses discussed in section 23.1. 2. A variety of hormones are involved and are further discussed with their respective organs; gastrin from the stomach in section 23.4, as well as secretin, cholecystokinin, and gastric inhibitory peptide from the small intestine in section 23.5. 23.3 The Mouth, Pharynx, and Esophagus (Upper GI tract) A. Mouth – also known as the oral cavity is lined with non-keratinized stratified squamous. 1. The anterior and lateral borders of the oral cavity a. Labia – also known as lips, are thin layers of skin that overlay the orbicularis oris muscle and can be distinguished by their reddish color. A superior and inferior labial frenulum attaches the lips to the gums. b. Cheeks – formed by the buccinator muscles. c. Oral vestibule - area bounded by the lips and cheeks externally and internally by the gums and teeth. 2. The superior boundary of the oral cavity (or roof) a. Hard palate - composed of the palatine process of the maxilla and the palatine bones. b. Soft palate - composed of skeletal muscle tissue. 3. The posterior boundary of the oral cavity a. Uvula – the flap of tissue that drops from the soft palate and helps to prevent food from entering the oropharynx prematurely. b. Palatine tonsils – lie on either side of the oral cavity in the palatoglossal and palatopharyngeal arches called the fauces. B. Tongue – composed of skeletal muscle, covered in stratified squamous epithelia, and anchored in the oral cavity by the lingual frenulum; it functions to manipulate food into a bolus, helps form words, and serve as a sensory organ for gustation. 3  1. The superior surface of the tongue is covered in four types of epithelial projections called papillae. 2. Three of the four, fungiform, foliate, and circumvallate contain taste buds while filiform papillae provide texture and tactile sensation. 3. Lingual lipase is secreted by lingual glands to aid in triglyceride digestion, but is only activated upon entry into the stomach. C. Salivary glands – exocrine glands that produce and secrete saliva, a mixture of water, mucus, buffers, lysozymes, and enzymes. 1. Functions a. Moistens and lubricates the mouth and food. b. Inhibits microbial entry. c. Dissolves food chemicals for taste receptor cells. d. Initiates chemical digestion of complex carbohydrates by salivary amylase. 2. The Major Salivary glands a. Submandibular glands - releases a thicker mixture of both salivary amylase and mucin via submandibular ducts. b. Parotid glands – releases a watery solution rich in salivary amylase via parotid ducts. c. Sublingual glands – releases the thickest saliva, primarily mucus and a little salivary amylase via the sublingual ducts. 3. Salivation or the production of saliva is regulated by the autonomic nervous system; under parasympathetic stimuli it continually flows, but under sympathetic stimuli it’s reduced. D. Teeth – or dentes break food into smaller pieces. 1. Types of teeth a. Your first dentition or set of teeth are the 20 deciduous or baby teeth that start to erupt through the gums during the first few months of life. b. Your second dentition are your 32 permanent or adult teeth (32), classified into 4 different types. I. Incisors – the 8 (4 on top/4 on bottom) at the front of the mouth are for biting. II. Cuspids – the 4 (4 on top/4 on bottom) on either side of the incisors with a pointed cusp to tear and pierce are commonly called canines. III. Premolars – 8 (4 on top/4 on bottom) on either side of the cuspids with rounded cusps for mashing and crushing can also be called bicuspids or as 1st and 2nd premolars. IV. Molars – 12 (6 on top/6 on bottom) have several pointed cusps for crushing with the 3rd set referred to as wisdom teeth. 2. Anatomy of a Tooth a. Composed of highly mineralized, bone-like material called dentin with a pulp cavity inside and enamel, the hardest material in the body, covering the crown or exposed surface. b. Below the gingivae or gums is the root of the tooth, bound in the alveolar processes of the jaw bones by a hardened substance known as cementum and attached to a periodontal ligament. E. Pharynx – more commonly called the throat is composed on skeletal muscle for swallowing and can be divided into three regions 4  1. Nasopharynx – the superior portion of the pharynx, located between the internal nares of the nasal cavity and the soft palate, is lined with pseudostratified columnar epithelium is involved in breathing and speech only; the uvula serves to block it off with swallowing. 2. Oropharynx – extends between the soft palate to the level of the hyoid bone and is composed of stratified squamous epithelium to accommodate the movement of food and protect against abrasion. 3. Laryngopharynx – includes the portion of the pharynx between the hyoid bone and the entrance to the larynx and esophagus and is lined with stratified squamous epithelium to accommodate the movement of food; the epiglottis serves to block of the larynx with swallowing. F. Esophagus – a hollow muscular tube that runs from the pharynx to the stomach, passing through the esophageal hiatus in the diaphragm. 1. The upper esophageal sphincter prevents backflow of food into the oral cavity while the lower esophageal sphincter, also known as the gastroesophageal or even cardiac sphincter, prevents backflow of stomach contents into the esophagus. a. The muscularis layer which changes from purely skeletal muscle in the upper portion, to a mix of skeletal and smooth in the middle, to purely smooth by the bottom portion uses peristalsis to propel the bolus into the stomach. b. The mucosa is composed of (non-keratinized) stratified squamous with mucus secreting goblet cells to further protect against abrasion and aid in swallowing. 2. Failure of the lower esophageal sphincter to close can cause reflux, also known as heartburn or gastroesophageal reflux disease (GERD). G. Deglutition – also known as swallowing is a 3-phase process that moves the bolus from the oral cavity into the pharynx and then into the esophagus 1. Voluntary phase – also known as buccal or oral phase is the voluntary push by the tongue into the oropharynx once chewing has ceased. 2. Pharyngeal phase – when the uvula moves up to close the nasopharynx while the epiglottis seals off the larynx, a momentary apnea or cessation of breathing, and the upper esophageal sphincter relaxes. 3. Esophageal phase – peristaltic contractions propel the bolus toward the stomach, stimulating the mucus glands in the esophagus and relaxing the lower esophageal sphincter. 23.4 The Stomach (Lower GI tract) A. Stomach – a muscular, expandable, J-shaped organ that converts the bolus into chyme. 1. Structurally it can be divided into four distinct regions. a. Cardia – also known as the cardiac region is the area where the esophagus empties into the stomach. Fundus – the dome-shaped portion at the top of the stomach that lies superior to the cardia. b. Body – the largest region of the stomach, it functions as a mixing bowl for ingested food and gastric secretions. c. Pylorus – the funnel-shape region that has a large pyloric antrum tapering to the pyloric canal which uses the pyloric sphincter to regulate the flow of chyme out of the stomach and into the duodenum. 5  2. Rugae or folds in the mucosa allow for distension and stretch by flattening out with filling. B. Histology 1. Resembles the majority of the alimentary canal with simple columnar epithelia forming the mucus cell rich mucosa layer, but it adds a third layer of smooth muscle to the innermost muscularis layer, known as the oblique layer to aid in churning and mixing the chyme with enzymes. 2. Gastric pits – shallow depressions that cover the mucosal lining to allow secretions for the gastric glands to reach the lumen. 3. Gastric glands – composed of four special cell types, primarily located in the fundus and body, that secrete most of the stomach juices used for gastric digestion. a. Parietal cells – secrete hydrochloric acid (HCl) for activating the pepsinogen and creating an inhospitable environment for bacteria as well as intrinsic factor which is important in the absorption of vitamin B12. b. Chief cells - secretes the inactive enzyme pepsinogen, which when activated by HCl, forms pepsin that can then begin the process of protein digestion. c. Mucus neck cells – distinct from the goblet cells on the mucosal surface, their role is not currently known. d. Enteroendocrine cells – produce a variety of regulatory digestive hormones into the lamina propria. I. Gastrin – released by G cells, increases stomach motility and churning. II. Histamine – stimulates release of HCl from parietal cells III. Somatostatin – inhibits stomach motility and emptying. IV. Ghrelin – regulates food intake by stimulating hunger and satiety. C. Gastric Secretion occurs in three phases, controlled by the coordination of both the autonomic nervous system and hormones. 1. Cephalic phase – occurs before the bolus enters the stomach to prepare it for digestion and can be initiated by taste, smell, sight and/or even thoughts and mention of food. a. A conditioned reflex that can be enhanced by a preference or desire for food, but also inhibited by depression or illness that results in the sensation of a lost appetite. b. It also plays a major role in all forms of food advertising. 2. Gastric phase – initiated by the entry of the bolus into the stomach and controlled locally by gastric hormones and neural response a. Distension triggers the parasympathetic nervous system to release gastric juices. b. A raising pH and detection of proteins causes gastrin release that then triggers HCl release to lower the pH back and enable pepsinogen activation. c. Decreased pH on the other hand inhibits HCl release and increases mucus secretion to protect the stomach from mucosal erosion that can lead to ulcers as well as relaxation of the lower esophageal sphincter that can lead to heartburn/GERD. 6  3. Intestinal phase – initiated as chyme is delivered into the duodenum and has both an excitatory and inhibitory reaction a. Detection of food stimulates intestinal gastrin secretion which stimulates increased (stomach) gastrin release. b. Distension from the food closes the pyloric sphincter to maintain small amounts to enter the duodenum slowly. D. Mucosal Barrier – protects the stomach from self-digestion by pepsin. 1. A bicarbonate-rich mucus secretion is produced by mucosal surface cells physically separates the acidic chyme from the epithelial layer. 2. Tight junctions between the epithelial cells protect against chyme leakage into surrounding tissues. 3. The epithelial cells undergo mitosis about every 3 – 6 days. E. Digestive Functions 1. Mechanical digestion a. Mixing waves utilize the third muscularis layer to toss food, with increasing force, to better mix with the gastric juices and physically break against the stomach walls. b. Appropriately sized contents experience a rhythmic push to the pyloric sphincter, a process known as gastric emptying, that is influenced by the intestinal phase so that the chyme entering the duodenum is delivered in small amounts only when the duodenum is ready to process it. 2. Chemical digestion a. Focuses primarily on pepsin’s breakdown of protein, aided by the mechanical activities of the stomach. b. Food may reside in the fundus as it waits for its addition to the mixing action, allowing salivary amylase activity to continue digesting carbohydrates. c. Lingual lipase will be activated by the acidic conditions and can begin working on triglycerides, but without an emulsifier there is minimal progress. 3. Additional roles a. Minimal absorption by things like alcohol and aspirin does occur. b. Production and secretion of intrinsic factor, essential for intestinal vitamin B 12 absorption that supports erythropoiesis and neuronal functioning. 23.5. The Small and Large Intestines (lower GI tract) A. Small Intestine – primary digestive organ of the body that has a specifically modified mucosal layer to maximize absorption. 1. Structurally it can be divided into three regions. a. Duodenum is the first region I. Receives chyme from the pyloric sphincter as well as accessory secretions from the gallbladder, liver, and pancreas II. The hepatopancreatic ampulla is where the common bile duct and main pancreatic duct enter the duodenum through a structure known as the 7  major duodenal papilla, under the control of the heptopancreatic sphincter, also known as the sphincter of Oddi). b. Jejunum and ileum are the middle and last region respectively with the former ending at the ileocecal sphincter (valve) to regulate release into the large intestine. c. The autonomic nervous system extrinsically regulate functioning, while the superior mesenteric artery and vein handle blood supply, and the hepatic portal vein transports all of the absorbed nutrients to the liver. 2. Histology has the same four basic layers, but the mucosa and submucosa layers have additional adaptions suited to the primary role played by the small intestine in terms of digestion and absorption. a. 3 unique structural adaptations exponentially increase surface area. I. Circular folds, also known as plicae circularis run the entire length of the small intestine and involve projections of the submucosa, creating deep folds in the mucosa. II. Villi are an additional layer of projections along the circular folds surface, each of which contains an artery, vein, and lacteal or specialized lymphatic capillary. III. Microvilli are finger-like extensions on the apical surface of the epithelial cells themselves that are so tiny and numerous that they collectively look like a bristle brush and earn the nickname brush border. b. Intestinal glands, also called crypts of Lieberkuhn, found in-between the villi and produce intestinal juice, a slightly alkaline, mucus solution. c. The submucosa of the duodenum is rich in mucus secreting duodenal glands, also known as Brunner’s glands, to help buffer against the acidic nature of chyme. d. Intestinal MALT is found throughout the lamina propia to prevent bacteria from entering the blood supply, with large collections known as Peyer’s Patches in the ileum. 3. Mechanical digestion in the Small Intestine a. Segmentation moves chyme forward and back as it is propelled forward to combine it with digestive juices while also pushing it against the mucosal walls at the contracted segments. b. Migrating motility complex systematically contracts regions of the small intestine starting at the duodenum incrementally under the influence of motilin until the ileum is reached at the process restarts at the duodenum again. c. The gastroileal reflex utilizes input from stomach motility and gastrin to increase segmentation and motility in the ileum while also relaxing the ileocecal sphincter/valve to allow chyme to enter the large intestine. 4. Chemical digestion in the Small Intestine 8  a. Carbohydrates, initiated in the mouth and proteins, initiated in the stomach complete digestion in the small intestine due to intestinal and pancreatic secretions. b. Lipid digestion begins as bile emulsifies the large molecule to enhance pancreatic lipase activity. c. Brush border enzymes on the mucosal surface of the lumen enable additional digestion at the microvilli which distinguishes the absorptive ability of the small intestine from the stomach. d. The entire process requires slow, small amounts of chyme delivery to maintain the osmotic gradient in the lumen as well as regulate the pH to a slightly alkaline level. B. Large Intestine – stores and concentrates fecal material by reabsorbing water as well as absorbing any remaining nutrients and synthesized vitamins before eliminating the residual contents as feces. 1. Structurally it can be divided into four distinct regions. a. Cecum – dead-end sac inferior to the ileocecal sphincter/valve that houses many residential bacteria and has the appendix projecting off medially. I. The appendix is a collection of lymphatic tissue that secured in place by the mesoappendix or mesentery of the ileum discussed in 23.1. II. Considered to be vestigial or no longer serving a purpose, it is may also be of benefit following intestinal illness as it serves as a reservoir for residential bacteria that can repopulate the large intestine. III. Bacterial infection can result in appendicitis or inflammation of the appendix and almost always requires an appendectomy or surgical removal of the organ. b. Colon – subdivided into four regions. I. From the ileocecal valve it turns superiorly as the ascending colon until the right colic flexure at the base of the liver where it turns medially to become the transverse colon until reaching the spleen at the left colic flexure where it turns inferiorly to become the descending colon. II. The descending colon turns medially as it enters the pelvic cavity to become the sigmoid colon that makes a serpentine bend to the midline where the rectum is located. c. Rectum – expandable portion of the large intestine for temporary storage of feces and has distal folds known as rectal valves that allow the release of gas while retaining the feces. d. Anal canal – opens to the outside of the body and regulated by an internal anal sphincter of smooth muscle and an external anal sphincter of skeletal muscle. 2. Histology sees distinct variation from the small intestine to support its focus on water absorption and fecal excretion. 9  a. Mucosal layer of simple columnar enterocytes, except for the stratified squamous cells in the anus, that are specialized for absorption of water, salt, and bacterially synthesized vitamins rather than enzyme secretion. b. Numerous goblet cells secrete mucus to aid in fecal propulsion and protection from acidic conditions created by the residential bacteria. c. No circular folds or villi though the longitudinal muscle of colon’s muscularis layer is modified as a three-banded strip of muscle known as the teniae coli that pulls the colon into repeating pouches known as haustra to allow expansion and elongation. I. Hanging from the teniae coli are fat filled pouches of the peritoneum, known as epiploic appendages. II. Not to be confused with polyps or that are intestinal pouches bulging inside the colon or diverticula that are intestinal pouches bulging outside the colon. 3. Bacterial Flora are the normal intestinal bacteria that reside in the large intestine. a. Most are nonpathogenic and serve either mutualistic and aid in processing of nutrients or synthesis of vitamins or commensal and cause no harm so long as they don’t migrate out. b. Nonresidential bacteria obtained with ingestion is usually destroyed by multiple physical barriers throughout the GI tract including lysozymes, defensins, acidic conditions, and protein digesting enzymes. I. Notable bacterial products include vitamin K synthesis which is necessary for blood clotting. II. Dendritic cells can penetrate the large intestines tight junctions, triggered by bacterial cell walls, to sample the flagged bacteria and present them to residential T-cells for assessment and trigger an IgA mediated response if necessary. 4. Digestive Functions a. Digestion of the nutrients is no longer necessary and so the focus is on movement of the fecal matter. I. Damage or disease may result in the need to temporarily or permanently remove some or all of the large intestine, a procedure known as a colectomy. II. The remaining region can be sutured to the anus or else it can be brought through the abdominal wall; a procedure known as a colostomy and have the feces collect in a colostomy bag. b. Mechanical digestion combines three types of movement. I. Haustral contractions – slow moving process stimulated by the presence of material in the colon to propel feces from one haustra to the next, aided by the teniae coli, mixing the food residue and helping to absorb water. 10  II. Peristalsis III. Mass movement – push feces through the transverse colon, compacting it in the rectum. IV. Gastrocolic reflex – is stimulated by distension of the stomach and digestion in the small intestine to increase motility, which is further aided by fiber that traps water to keep feces softer and adds bulk for stronger colonic contractions. c. Chemical digestion I. Solely the work of the bacterial flora as only mucus is secreted. II. Bacteria due break down some remaining carbohydrates, known as saccharolytic fermentation which produces multiple types of flatus or gas, that when produced excessively can lead to flatulence. 5. Absorption, Feces Formation, and Defecation a. The small intestine absorbs about 90% of the water secreted in the digestive process with the large intestine responsible for the last 10%. b. Undigested and unabsorbed semisolid material is known as feces and can include sloughed off cells and bacteria. c. Defecation begins with mass movements pressing feces into the rectum so that it distends and relaxes the internal anal sphincter. I. The brain voluntarily controls activity of the external anal sphincter to defecate or not. II. Valsalva’s maneuver occurs when you voluntarily contract your diaphragm and abdominal muscles to assist the elimination process. III. Delaying defecation leads to increased water absorption and further compaction of the feces that can make it more difficult to pass, potentially leading to constipation. IV. Alternatively, feces that moves too quickly through the large intestines has excess amounts of water that make the feces more watery, known as diarrhea and can lead to dehydration. 23.6 Accessory Organs of the Digestive Tract A. The Liver - largest internal organ in the body is a major metabolic organ, but is digestively responsible for the production of bile, an important emulsifier. 1. Structure a. Consists of a left lobe and a larger right lobe, divided by the falciform ligament which is a remnant of the umbilical cord, and secured to the stomach by the lessor omentum, both discussed in 23.1. b. Some consider the right lobe to also be delineated inferiorly into the quadrate and caudate lobe. c. Hepatic circulation includes a hepatic artery and vein for delivering oxygen and removing wastes and stored nutrients, but also a hepatic portal vein that brings all absorbed nutrients as well as drugs and toxins to the liver for processing. 11  2. Histology a. Hepatocytes or liver cells synthesize bile, process molecules delivered by the hepatic portal vein. I. Arranged as hepatic lobules, a hexagonal shape surrounding a central vein that drains all secretions and products from the cells. II. Hepatic sinusoids, fenestrated capillaries from the hepatic artery and hepatic portal vein, running between the cells give easy access to the delivered molecules. III. Portal triads consisting of a bile duct, hepatic artery, and hepatic portal vein are situated at the corners of each lobule. b. Reticuloendothelial cells or Kupffer cells are fixed macrophages that remove debris such as dead blood cells, liver cells, and bacteria. 3. Bile – enables emulsification or disruption of the lipid molecules in chyme as it enters the small intestine. a. A slightly alkaline mixture of water, bile sales, bile pigments, and phospholipids. I. Bile salts are actually reclaimed by the enterohepatic circulation system in the ileum by the hepatic portal vein to be recycled. II. One of the main pigments is bilirubin, a waste product of red blood cell breakdown by the spleen that give bile its greenish-yellow color. III. The amphipathic phospholipids interact with the lipid molecules as well as the water molecules in chyme to break large molecules up and increase surface area for lipases to work. b. Collected into bile canaliculi between hepatocytes, the eventually converge into left and right hepatic ducts from their respective lobes to leave the liver together as the common hepatic duct. I. The common hepatic duct will merge with the cystic duct from the gallbladder to form the common bile duct that enters the duodenum. II. Bile production never stops and follows a similar path to access the gallbladder for storage, however, under secretin signaling from the duodenum, production will increase. B. The Pancreas – organ with both exocrine and endocrine function tucked into the duodenum. 1. Clusters of acini cells surround pancreatic ducts that collect enzyme-rich pancreatic juice into the main pancreatic duct and smaller accessory duct (duct of Santorini) for convergence with the common bile duct. 2. Small collections of endocrine cells form the islets of Langerhans and produce the hormones pancreatic polypeptide, insulin, glucagon, and somatostatin. 3. Pancreatic Juice a. Mostly water, digestive enzymes, and bicarbonate. b. Buffers acidic chyme and inactivates pepsin so that pancreatic enzymes can begin digesting fats, as well as continue digestion of carbohydrates and proteins. 12  c. Protein digesting enzymes are released in an inactive form that utilize a cascading activation that starts with enteropeptidase to protect the pancreas, whereas the other enzymes are secreted as is. 4. Pancreatic secretion is under the control of hormones and the parasympathetic nervous system. a. Duodenal entry of acidic chyme stimulates secretin release to trigger bicarbonate release. b. Presence of fats and proteins stimulate CCK release to trigger pancreatic enzyme release and enhance secretin. c. Vagal nerve signaling via the parasympathetic nervous system also triggers pancreatic secretion. C. The Gallbladder – stores and concentrates bile for release into the duodenum. 1. Divided into three regions, the wide fundus, the medial body, and the narrowed neck. 2. CCK controls the release into the cystic duct that can flow into the common hepatic duct. 23.7 Chemical Digestion and Absorption: A Closer Look A. Chemical Digestion – is the breakdown of large food molecules into small enough units to be absorbed by the intestinal villi without destroying their chemical makeup (table 23.8). 1. Carbohydrate digestion a. Should be 40 – 60% of your caloric intake. b. Starts in the mouth with salivary amylase, is halted in the stomach, and restarts to finish in the small intestine with pancreatic amylase for large polysaccharides. c. Brush border enzymes like sucrase, maltase, and lactase in the mucosal cells of the small intestine finish off small disaccharides and monosaccharides. 2. Protein digestion a. Should be 15 – 20% of your caloric intake. b. Starts in the stomach with pepsin and finishes in the small intestines with chymotrypsin and trypsin for large proteins and polypeptides. c. Brush border enzymes in the mucosal cells of the small intestine finish off amino acids and dipeptides with amino peptidase and dipeptidase respectively. 3. Lipid digestion a. Should be about 35% of your caloric intake b. Essentially only occurs in the stomach due to the presence of pancreatic lipase that splits triglycerides into fatty acids and glycerol. c. Lingual and gastric lipase are secreted in their respective organs, but without the assistance of bile the functionality is limited. 4. Nucleic Acid digestion a. Present in all foods. b. Pancreatic nucleases deoxyribonuclease and ribonuclease breakdown DNA and RNA respectively, in the small intestine 13  c. Brush border mucosal cells also secrete nucleosidase and phosphatase to further digest nucleotides. B. Absorption – the movement of molecules from the GI tract into the intestinal mucosal cells for transport into the capillary blood or lacteals in the villi (table 23.10) 1. May use active transport, passive diffusion, facilitated diffusion, co-transport, or endocytosis. 2. Water-soluble nutrients will be transported directly into the blood for transport to the liver via the hepatic portal vein. a. All carbohydrates are absorbed as monosaccharides except for the undigestible polysaccharides commonly referred to as fiber which will be excreted as feces. b. Proteins are mostly absorbed as amino acids, those that are absorbed as dipeptides and tripeptides are quickly broken down into amino acids in the mucosal cells. c. All components of a nucleotide are absorbed. d. All minerals, whether from food or GI secretions are absorbed regardless of need except for iron and calcium which depends on individual physiology. I. Since iron is toxic in large amounts to cells, it must be bound to proteins for storage as well as transport II. Calcium self-regulates is absorption, in conjunction with parathyroid hormone and vitamin D. e. All of the B vitamins as well as vitamin C are water-soluble and absorbed, with unnecessary amounts secreted for release in urine. f. Most of the water is reabsorbed, with the majority occurring in the small intestine though the speed at which the GI tract processes all of the digestive processed can cause that to vary. 3. Lipid-soluble nutrients will be packaged as chylomicrons before moving into the lacteals for transport by lymphatic vessels into venous circulation. a. Bile’s presence speeds up absorption while also I. Small fatty acids can actually follow the route of monosaccharides and proteins. II. Larger fatty acids and monoglycerides are initially incased in a micelle or water-soluble pouch for absorption into the mucosal cells. III. Once inside they are reassembled into triglycerides for packaging into a chylomicron whose large size permits them into the lacteals only. IV. Once in the blood supply, lipoprotein lipase will slowly breakdown triglycerides within the chylomicrons into free fatty acids that can enter the cells. b. Vitamins A, D, E, and K are all lipid-soluble and can only be absorbed with dietary lipids that they are packaged into the micelles with. C. Disorders of the Digestive System 1. Mouth related 14  a. Loss of appetite – may be due to diminished receptivity of taste buds. b. Dental caries – also known as cavities are due to erosion in the enamel, often resulting from the action of residential bacteria in the mouth; dental plaques are usually the first sign but can be minimized with regular brushing and flossing. c. Gingivitis – inflammation of the gums usually caused by plaque buildup that left untreated can cause erosion and separation of the gums from the teeth and lead to periodontal disease where the root becomes damaged and eventual tooth loss can occur. 2. Digestive organs a. Peritonitis – inflammation of the peritoneum usually coinciding with any other disorder that causes GI contents, bacteria, and/or blood to fill the peritoneum. b. Acid reflux – failure of the lower esophageal sphincter to prevent the backflow of chyme into the esophagus; also called heart burn or gastroesophageal reflux disorder (GERD) if it is s a chronic condition. c. Hernia – occurs when an abdominal organ protrudes through the abdominal wall; hiatal hernias specifically refer to protrusion through the esophageal hiatus. d. Ulcers – deterioration in the wall of digestive organs, typically caused by Helicobacter pylori infection. e. Lactose intolerance – insufficient levels of lactase enzyme necessary to digest lactose lead to digestion in the large intestine by bacteria that results in increased gas production and bloating and cramping as a result. f. Appendicitis – inflammation of the appendix as a result of a blockage trapping infectious bacteria within the lumen. g. Colorectal cancer – often resulting from mucosal growths known as polyps; regular colonoscopies, recommended over the age of 50 are best preventative measure to identify and remove. h. Hemorrhoids – varicose veins of the rectum and anus i. Diverticulitis – inflamed herniation in the wall of the large intestines. j. Constipation – hard and difficult to pass stool typically due to slow movement through the digestive tract or not enough fiber. 3. Accessory organs a. Mumps – swollen parotid glands as a result of a virus (myxovirus) that cause swelling of the neck and jaw; generally rare due to vaccine efforts b. Pancreatitis – inflammation of the pancreas from either drugs, toxins, bacterial or viral infections, or blockages. c. Hepatitis – inflammation of the liver as a result of alcohol consumption, viral infection (hepatitis A, B, and C are most common), or drugs. d. Cirrhosis – chronic inflammation of the liver leading to scarring of the liver. e. Gallstones – concentrated collections of cholesterol and bile salts. f. Jaundice – accumulation of bilirubin (yellow) in the skin as a result of a blockage or liver disease. 15