The Digestive System The organ system that processes food, extracts nutrients from it, and eliminates the residue.
Digestive System The digestive system allows your body to obtain substances required to sustain life that your body cannot make on its own including: monosaccharides, amino acids, nucleic acids, fats, vitamins, electrolytes (ions) and water The alimentary canal or gastrointestinal (GI) tract is a long muscular tube lined with epithelial tissue passing through the body which is closed off at each end by a sphincter of skeletal muscle Opens to the outside world therefore the lumen and its contents are part of the external environment Its primary function is to move water, nutrients and electrolytes from the external environment into the body s internal environment
Digestive system: 2 subdivisions 1. Digestive tract (alimentary canal) 30 ft long muscular tube extending from mouth to anus Mouth, pharynx, esophagus, stomach, small intestine, and large intestine, rectum, anus Gastrointestinal (GI) tract is the stomach and intestines General Anatomy Oral cavity Tongue Teeth Sublingual gland Submandibular gland Diaphragm Liver Gallbladder Bile duct Ascending colon Small intestine Cecum Appendix Parotid gland Pharynx Esophagus Stomach Pancreas Transverse colon Descending colon Sigmoid colon Rectum Anal canal Anus
2. Accessory organs and exocrine glands (teeth, salivary glands, pancreas and the liver) aid in the process of digestion in the alimentary canal by physical manipulation of food or through the secretion of substances into the GI tract
Digestive System Begins with the oral cavity (mouth and pharynx) where chewing and the secretion of saliva starts digestion Digestive secretions are added to the food by GI epithelium, liver and pancreas, turning it into a soupy mixture called chyme The products of digestion pass out of the lumen into the ECF where they pass into blood or lymph for distribution throughout the body Any material remaining in the lumen at the end of the GI tract is defecated through the anus
Peritoneum 2 connective tissue membranes in the abdominal cavity: protect the organs in the abdominal cavity from damage by friction/abrasion Visceral peritoneum covers external surface of digestive organs Parietal peritoneum lines the internal wall of the abdominal cavity Between the 2 layers of the peritoneum is a peritoneal cavity which is filled with the peritoneal fluid secreted by the cells of each layer the fluid functions to lubricate digestive organs, allowing them to slide across one another without creating friction which would lead to inflammation
Serous Membranes Liver Gallbladder Stomach Lesser omentum Ascending colon Greater omentum Small intestine (a) Lesser omentum attaches stomach to liver Greater omentum covers small intestines like an apron
Wall of the Alimentary Canal 4 principle layers of the GI tract Mucosa (superficial) inner layer of epithelial, connective and muscular tissues that faces lumen Submucosa loose connective tissue with blood and lymph vessels and submucosal plexus of the Enteric Nervous System Muscularis 2 layers of smooth muscle (superficial circular and deeper longitudinal) responsible for motility which is innervated by the myenteric plexus of the Enteric Nervous System Serosa (deep) strong connective tissue membrane that maintains the structural integrity of the alimentary canal (visceral layer of the peritoneum)
Layers of the Alimentary Canal Wall
Tissue Layers of the Digestive Tract Diaphragm Esophageal hiatus Enteric nervous system: Mucosa: Stratified squamous epithelium Lamina propria Muscularis mucosae Submucosa: Esophageal gland Muscularis externa: Inner circular layer Outer longitudinal layer Serosa Myenteric plexus Submucosal plexus Parasympathetic ganglion of myenteric plexus Lumen Blood vessels
Enteric Nervous System A specialized division of the nervous system associated only with the alimentary canal Connected to the CNS via the Parasympathetic NS (stimulates digestion) and Sympathetic NS (inhibits digestion) Composed of two major nerve plexuses (groups) which send both sensory and motor information throughout the alimentary canal to control digestion Submucosal nerve plexus (submucosa layer) associated with mechano- and chemoreceptors in the mucosa controls the endo- and exocrine secretion of the mucosa Myenteric nerve plexus (muscularis layer) controls the contraction of smooth muscle
4 Basic Processes of the Digestive System 1. Digestion mechanical and chemical breakdown of food 2. Motility/Propulsion movement of material along the GI tract 3. Secretion release of substances (hydrolytic enzymes, mucus, acid, bicarbonate, water, ions) from salivary glands, GI epithelial cells, hepatocytes or pancreatic acinar cells into the GI tract lumen or ECF 4. Absorption active or passive transfer of substances from the lumen of the GI tract to ECF Processes are regulated by the nervous and endocrine systems as well as paracrine signals
4 Basic Processes of the Digestive System
Simple epithelium Mucosal Epithelium absorptive cells use integral transporting proteins in the apical and basal membranes to absorb ions, water and nutrients out of the lumen into the body facilitated diffusion, primary and secondary active transport processes secretory cells (endocrine and exocrine) exocytose digestive enzymes, and mucus exocytose hormones and/or paracrine molecules into the ECF for digestive regulation sensory cells act as mechano- and chemoreceptors which detect the presence of food by the distension of the GI wall and by the presence of specific chemicals (proteins, salts, acids, fats )
Chemical Digestion Chemical digestion occurs by Enzyme action Occurs in the mouth, stomach and small intestine
Contraction of the muscularis: 2 types: Motility 1. Peristalsis is characterized by progressive waves of contraction that move from one section to the next moves food between 2 and 25 cm / sec occurs over long distances in esophagus to move food from the pharynx to the stomach and within the stomach where it contributes to the mixing of food may occur over short distances in the small intestine 2. Segmentation is characterized by short segments of the small and large intestines alternately contracting and relaxing which mixes contents and keeps them in contact with absorptive epithelium
Secretion and Absorption Exocrine cells of the salivary glands, pancreas, liver and the GI mucosal cells secrete ~7 liters of enzymes, mucus, electrolytes and water into the lumen daily Occurs in all segments from the mouth to the rectum The 7 liters of fluid secreted daily into the lumen of the GI tract must be absorbed to prevent dehydration excessive vomiting or diarrhea can be dangerous By the time the food and secretions reach the rectum ~98.9% will be absorbed and moved into the body leaving 100 ml of fluid to be defecated Occurs in the small and large intestines
3 Stages of Digestion The processes of digestion, secretion, motility and absorption take place throughout the entire length of the GI tract in 3 overlapping stages named by the location of food 1. Cephalic (head) phase thinking about, smelling, or seeing food which has not entered the alimentary canal food is in the mouth 2. Gastric phase food is in the stomach 3. Intestinal phase food is in the small intestine
Cephalic phase Attributed to activation of Parasympathetic Nervous System (PNS) controlled reflexes coordinated by the medulla oblongata Stimulated by anticipatory and ingestion activities Prepares the digestive system BEFORE ingestion of food Decreases time required for digestion and absorption Action potentials from the medulla To the salivary glands and pancreas To the stomach, small and large intestines Secretion of saliva and pancreatic juice Secretion of mucosal cells Peristalsis in the esophagus and stomach Contraction of the muscularis layer
The Oral Cavity Palatine tonsil Tongue Salivary duct orifices: Sublingual Submandibular Hard palate and palatine rugae Soft palate Uvula 32 adult teeth 16 in mandible 16 in maxilla From midline to the rear of each jaw 2 incisors 1 canine 2 premolars 3 molars
Salivary Glands Odor, sight, thought of food stimulates salivation Masseter muscle Parotid gland Parotid duct Tongue Sublingual ducts Bolus mass swallowed as a result of saliva binding food particles into a soft, slippery, easily swallowed mass Submandibular duct Submandibular gland Sublingual gland Mandible Lingual frenulum Opening of submandibular duct
Composition and Functions of Saliva Water Ions Hydrolytic enzymes salivary amylase begins the chemical digestion of carbohydrates lingual lipase begins the chemical digestion of lipids Mucin protein that aids in the lubrication of a bolus ball of chewed food and saliva that is swallowed Lysozyme antibacterial enzyme that reduces the risk of infections
Swallowing (Deglutition) The tongue forces the bolus into the pharynx causing the epiglottis to fold over the opening to the airways (glottis) and diverts the bolus into the esophagus The tonically contracted upper esophageal sphincter which keeps the proximal esophagus closed, relaxes and opens momentarily while breathing is inhibited Once the bolus enters the esophagus, a strong peristaltic wave pushes the bolus to the stomach Peristalsis ensures you can swallow regardless of body position Liquid reaches the stomach in 1 to 2 seconds Food bolus in 4 to 8 seconds
At distal end of the esophagus, the tonically contracted lower esophageal sphincter (cardiac sphincter) momentarily relaxes and opens momentarily to allow the bolus to enter the stomach if the cardiac sphincter does not close properly, gastric juice containing acid can enter the esophagus causing painful irritatation (heart burn)
Swallowing Upper esophagus Peristaltic contraction Bolus of ingested matter passing down esophagus
Gastric Phase As food enters the stomach, the gastric phase of digestion begins The cephalic phase continues until the food has been completely ingested and swallowed The presence of food in the stomach detected by mechanoreceptors and chemoreceptors: stimulates the secretion of gastric juice from the gastric mucosa into the lumen of the stomach to further hydrolyze ingested food stimulates the contraction of the muscularis which mixes the food with gastric juice making a soupy mixture called chyme
The Stomach The stomach is divided into the 3 regions: the fundus, the body and the antrum and is able to hold up to 2 liters of food and fluid when completely filled When the stomach is empty, the mucosa folds into rugae when filled, the expanded wall of the stomach causes these folds to disappear (flatten)
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Stomach Diaphragm Lesser omentum Lesser curvature Pyloric region: Antrum Pyloric canal Pylorus Pyloric sphincter Fundic region Cardiac region Body Longitudinal muscle Circular muscle Oblique muscle Gastric rugae Greater curvature Duodenum Greater omentum
The Stomach Wall Tubular invaginations (depressions) of the surface epithelium called gastric glands extend down into the supporting connective tissue
Chief cells and parietal cells of gastric glands secrete substances into the lumen of the stomach which combine to make gastric juice
Gastric Juice Chief cells secrete the inactive enzyme (zymogen) pepsinogen Begins the chemical hydrolysis of proteins when pepsinogen is converted to pepsin (active) secrete gastric lipase continues the chemical hydrolysis of lipids Parietal cells secrete HCl (hydrochloric acid) to lower ph to 2.0 (optimal for chemical digestion in the stomach) denatures swallowed proteins, kills bacteria and activates pepsin secrete intrinsic factor which forms complexes with vitamin B 12 and is essential for its absorption in the intestine lack of intrinsic factor results in pernicious anemia which is a reduction of red blood cell synthesis due to a vitamin B 12 deficiency
Protection of the Stomach Mucosa Mucous cells secrete a combination of mucus (superficial physical barrier) and bicarbonate (chemical buffer barrier under mucus) to protect the mucosa from autodigestion by HCl
Peptic Ulcer (a) Normal CNR/SPL/Photo Researchers, Inc. (b) Peptic ulcer Gastritis, inflammation of the stomach, can lead to a peptic ulcer as pepsin and hydrochloric acid erode the stomach wall. Most ulcers are caused by acid-resistant bacteria Helicobacter pylori, that can be treated with antibiotics and Pepto-Bismol.
ph Hormonal Control of Gastric Function Gastric Phase: Gastrin secreted by g cells Intestinal phase: Secretin and Cholecystokinin Inhibition of peristalsis Maintainance of intentinal ph Enhancement of intestinal digestion Food Enters the Stomach Gastric cavity becomes less acidic G cells release gastrin onto parietal cells Parietal cells release HCl ph
Gastric Motility As food particles are reduced to a more uniform texture, each peristaltic wave forces a contracted pyloric sphincter open which allows only a small amount of chyme to move into the small intestine
Intestinal Phase Food is in the Small Intestine The 3 segments of the small intestine include: the duodenum (proximal 25 cm) the jejunum (middle 250 cm) the ileum (distal 360 cm) Completion of digestion aided by the secretions of the liver and pancreas enter the lumen of the small intestine at the duodenum through a duct which is guarded by the contracted sphincter of Oddi a combination of segmental and peristaltic contractions which mixes the chyme with pancreatic juice and bile and moves it toward the large intestine
Small Intestine
Structural modifications increase the amount of surface area for digestion and absorption When the small intestine is empty, the mucosa is folded into structures called plicae or circular folds when filled, the expanded wall of the small intestine causes these folds to disappear (flatten) Villi: fingerlike projections Crypts: tubular invaginations of the surface epithelium Small Intestinal Mucosa
Individual epithelial cells of the small intestinal mucosa have a highly folded apical cell membrane each fold is called a microvilli and increase the number of integral membrane proteins for digestion (enzymes) and absorption (transporters) that can be exposed to the lumen aka the brush border for its bristle-like appearance
Pancreas A triangular gland located behind the stomach which has both exocrine and endocrine functions Acinar (epithelial) cells secrete pancreatic juice into a duct that empties through the sphincter of Oddi at the duodenum Pancreatic islets (islets of Langerhans) secrete the hormones insulin and glucagon to control blood glucose levels
Pancreatic Juice Acinar cells exocytose pancreatic juice into the pancreatic duct which flows into the duodenum Pancreatic juice contains: pancreatic amylase hydrolyzes carbohydrates pancreatic proteases hydrolyze proteins secreted as zymogens pancreatic lipase hydrolyzes lipids pancreatic nucleases hydrolyze nucleic acids bicarbonate (HCO 3- ) a buffer secreted by duct cells that neutralizes the gastric acid, raising the ph to 8.0 (optimal for both pancreatic and intestinal enzymes)
Pancreatic Zymogens The pancreatic zymogens include: trypsinogen chymotrypsinogen Procarboxypeptidase The zymogens are converted to their active form by a series of biochemical reactions initially catalyzed by the duodenal brush-border enzyme enterokinase enzymatically hydrolyzes trypsinogen to trypsin trypsin then activates the other 2 zymogens chymotrypsinogen to chymotrypsin procarboxypeptidase to carboxypeptidase
Activation of Pancreatic Zymogens
Liver and Gallbladder Hepatocytes of the liver secrete bile into the hepatic ducts leading to the gallbladder composed of bile acids and phospholipids a detergent which causes fat emulsification increases the surface area of fat globules increases of lipid hydrolysis by lipase Gallbladder a muscular sac that stores bile secreted from the liver when the sphincter of Oddi is closed
Gallbladder Disorders Cholelithiasis Formation of gallstones in bile ducts, gallbladder, or cystic duct May consist of: Cholesterol or bile pigment Mixed content with calcium salts 48
Hormonal Control of Intestinal Phase The presence of acidic chyme in the small intestine causes the secretion of the intestinal hormone secretin stimulates the secretion of pancreatic bicarbonate The presence of fatty acids and peptides in the chyme in the small intestine causes the secretion of the intestinal hormone CCK stimulates the secretion of pancreatic enzymes stimulates the contraction of the gallbladder to squeeze the bile into the bile duct relaxes (opens) the sphincter of Oddi, allowing the entry of pancreatic juice and bile into duodenum Both secretin and CCK inhibit HCl secretion from parietal cells and inhibit the muscularis of the stomach thus limiting the rate of acidic chyme movement into the small intestine
ph < 8 fats and proteins small Intestine distension CCK secretin inhibits peristalsis and HCl secretion Stomach stimulates muscularis causes relaxation Sphincter of Oddi stimulates contraction Gallbladder stimulates enzyme secretion Pancreas stimulates HCO 3 - secretion slow gastric emptying Small Intestine bile and pancreatic juice flows into duodenum expel bile chemically digest chyme increase ph in the small intestine segmentation and peristalsis
Summary of Chemical Digestion in the GI Tract Carbohydrates salivary amylase (minor) in the mouth pancreatic amylase (major) in the small intestine Proteins pepsin (minor) in the stomach pancreatic proteases (major) in the small intestine Lipids lingual lipase (minor) in the mouth gastric lipase (minor) in the stomach pancreatic lipase (major) in the small intestine bile significantly increases the rate of hydrolysis Nucleic acids pancreatic nucleases (deoxyribonucleases and ribonucleases) in the small intestine Brush-border enzymes of the small intestine complete the final hydrolysis of carbohydrates, proteins and nucleotides prior to their absorption
Absorption of Polar Substances Molecules cross the apical cell membrane of an absorptive cell of a villus and diffuse to the basal side of the cell Absorption is completed as the molecules cross the basal cell membrane of an absorptive cell and is moved into a blood vessel (polar molecules) in the submucosa of a villus Most polar molecules cross the apical cell membrane by secondary active transport, using the Na + gradient generated by the Na +, K + - ATPase across the cell
Absorption of Nonpolar Substances 1. Fatty acids and monoglycerides assemble with cholesterol, lipid soluble vitamins and bile into a micelle 2. The nonpolar micelle diffuses across the apical cell membrane of the absorptive cell into the Golgi apparatus 3. Once inside Golgi apparatus, the lipids are packaged with proteins forming a chylomicron 4. The chylomicrons are packed into vesicles that are exocytosed out of the basal membrane 5. Chylomicrons, which are too large to enter a blood vessel, enter lymph lacteals
Vitamins Absorbed unchanged Fat-soluble vitamins: A, D, E, and K absorbed with other lipids If they are ingested without fat-containing food, they are not absorbed at all, but are passed in the feces and wasted Water-soluble vitamins, B complex and C, absorbed by simple diffusion and B 12 if bound to intrinsic factor from the stomach Malabsorption Syndrome
Distribution of Absorbed Substances The polar molecules are absorbed into the blood vessels flow directly to the liver via the hepatic portal vein connects the blood supply of the gastrointestinal system and liver The liver uptake (transport) some of the nutrients into the hepatocytes (liver cells) to store a portion of the absorbed nutrients, the remainder is distributed to all other cells of the body Chylomicrons move through lymphatic vessels until these vessels merge with the heart where they enter the circulatory system to be distributed to all cells of the body
Large Intestine Any food in the small intestine that could not be chemically digested is moved into the large intestine (~500 ml/day) where most of the remaining water and ions are absorbed and the remaining material (~150 ml/day) removed by defecation Subdivided into 3 anatomical segments 1. the colon ascending colon transverse colon descending colon sigmoid colon 2. rectum 3. anal canal
The Large Intestine Rectum Rectal valve Anal canal Levator ani muscle Hemorrhoidal veins Internal anal sphincter External anal sphincter Anus Anal sinuses Anal columns
Absorption and Motility Large intestine takes about 12 to 24 hours to reduce the residue of a meal to feces Does not chemically change the residue Reabsorbs water and electrolytes Neural control of defecation: 1. Filling of the rectum, detected by stretch receptors 2. Reflex contraction of rectum and relaxation of internal anal sphincter (involuntary) 3. Voluntary relaxation of external sphincter Feces consist of 75% water and 25% solids, of which 30% is bacteria, 30% undigested fiber, 10% to 20% fat, small amount of mucus, and sloughed epithelial cells
Bacterial Flora and Intestinal Gas Bacterial flora populate large intestine About 800 species of bacteria Digest cellulose and other undigested carbohydrates Body absorbs resulting sugars Help in synthesis of vitamins B and K Flatus intestinal gas Average person produces 500 ml per day (flatus) from 7 to 10 L of gas present but reabsorbed Most is swallowed air
Absorptive and Postabsorptive States The absorptive state is the period of time when the alimentary canal is absorbing nutrients into the body increasing their levels (most importantly glucose) The postabsorptive state is the period of time between absorptive states (when the alimentary canal is NOT absorbing nutrients into the body) nutrient levels decrease during this time as they are constantly used 2 antagonistic hormones from 2 different cell types of the pancreatic islets of Langerhans are secreted during the absorptive or the postabsorptive state in order to control the levels of circulating glucose (blood sugar) Alpha ( ) cells secrete glucagon Beta ( ) cells secrete insulin
Pancreatic Islet
Insulin A peptide hormone that is synthesized first in beta cells as proinsulin (inactive) proinsulin is then converted to active insulin in the Golgi apparatus Secreted into circulation during the absorptive state in response to an increase in the blood glucose level Insulin causes a decrease in the blood glucose level returning it to the set point stimulates the uptake of glucose out of the blood into skeletal muscle cells stimulates the uptake of glucose out of the blood into adipose cells and its subsequent conversion to glycerol which is stored in adipose as triglycerides stimulates the enzymatic conversion of glucose into glycogen (glycogenesis)
Glucagon A peptide hormone that is secreted into circulation during the postabsorptive state in response to a decrease in the blood glucose level Glucagon causes an increase in the blood glucose level returning it to the set point stimulates lipolysis of triglycerides in adipose cells and the release of fatty acids into circulation stimulates the enzymatic hydrolysis of glycogen (glycogenolysis) in the liver stimulates gluconeogenesis in the liver The glucose that is synthesized by the liver is transported out of the liver into the blood increasing the blood glucose level
Insulin vs. Glucagon Insulin decreases blood glucose levels hypersecretion causes hypoglycemia low blood sugar hyposecretion causes hyperglycemia high blood sugar can lead to diabetes mellitus Glucagon increases blood glucose levels hypersecretion causes hyperglycemia hyposecretion causes hypoglycemia
Diabetes Mellitus A metabolic condition caused by inability of insulin to function properly The 5 symptoms of diabetes mellitus are: Hyperglycemia Glycosuria glucose found in urine Polyuria large urine output Polydipsia large thirst due to dehydration caused by polyuria Polyphagia large hunger and food consumption
Diabetes Mellitus Type I insulin dependent diabetes mellitus (IDDM) or juvenile onset diabetes caused by the loss of insulin synthesis by beta cells and require insulin injections to control blood sugar Type II non-insulin dependent diabetes mellitus (NIDDM) or adult onset diabetes mainly caused by a defect/reduction in the insulin receptor on target cells these individuals produce insulin, therefore insulin injections are ineffective exhibit insulin insensitivity The types can be differentially diagnosed in part by a glucose tolerance test