Biology 322 Human Anatomy Digestive System Digestive System Overview Digestion is the process of consuming and processing the food we eat in order to sustain life Digestion can be broken down into 4 basic processes: 1.Ingestion the selective consumption of food products 2.Digestion mechanical and chemical breakdown of food into molecules that can be (1) absorbed and (2) used by the body for energy 3.Absorption uptake of nutrients into the bloodstream 4.Defecation elimination of the un-useable leftovers Digestive system carries out two main processes related to digestion: 1.Mechanical digestion physically breaking down food into smaller pieces Teeth, stomach, sm. Intestine 2.Chemical digestion breaking down nutrient polymers into monomers Proteins amino acids Polysaccharides monosaccharides Lipids fatty acids, monoglycerides Nucleic acids nucleotides Digestive System Anatomy Two distinct divisions (1) digestive tract and (2) accessory organs Digestive tract (alimentary canal) Tube that extends from the mouth to the anus Open at both ends so ANATOMICALLY, the tract is OUTSIDE the body Propulsive movements move food/material along the digestive tract Mechanical and chemical digestion occurs here (with help of secretions from acc. organs) Accessory organs Structures which assist in mechanical and chemical digestion Teeth, tongue, salivary glands, liver (and gall bladder), pancreas Carry out mechanical digestion (teeth, tongue) or provide secretions which assist in chemical digestion (liver, gall bladder, pancreas) - Secretions are deposited INTO the digestive tract!! Digestive Tract Anatomy Mouth Esophagus Stomach Small intestine Large intestine Rectum Anus FOOD FECES Food is broken down into smaller pieces Macromolecules are broken down into monomers Nutrients are absorbed Organization of the digestive tract Digestive tract is essentially one long, continuous tube with slight changes in shape The wall of the dig. tract consists of 4 main layers: 1.Mucosa 2.Submucosa 3.Muscularis externa 4.Serosa/adventitia Internal External Mucosa Innermost layer of digestive tract Faces lumen (exposed to food/fluids) Consists of : a)Epithelium type varies as you move down the tract Strat. Squamous in oral cavity, esophagus, anal canal Simple columnar everywhere else (stomach, intestines) b)Lamina propria loose connective tissue layer c)Muscularis mucosae thin layer of smooth muscle Contraction creates grooves and folds that increase surface area for digestion/absorption Submucosa Thicker layer of loose connective tissue (areolar C.T.) Contains a large concentration of Blood vessels (for nutrient absorption) Nerves (control contraction of smooth muscle) Regulates movement of material through tract as well as secretions of some glands Mucus glands Lubricate and protect the mucosa Lots of acids/digestive enzymes in GI tract! Lymphatic vessels Series of vessels that (1) reabsorb excess fluid from tissues, (2) transport pathogens from tissues important for immune system, (3) transport MOST lipids from dig. tract Muscularis Externa TWO layers of smooth muscle that surround the dig. tract Inner layer is circular, outer layer runs longitudinally (lengthwise) Coordinated contractions squeeze and push food/material along the tract (DIGESTIVE MOTILITY) Serosa/Adventitia Outermost layer of the digestive tract SEROSA thin layer of loose areolar tissue surrounding stomach and intestines ADVENTITIA thin layer of FIBROUS C.T. mainly surrounding esophagus and rectum Both play a role in attachment of tract to other structures in thoracic and abdominal/pelvic cavities Innervation of the Digestive Tract Motility, gland secretion, blood flow to tract is controlled by two nerve networks called PLEXUSES These plexuses make up the ENTERIC NERVOUS SYSTEM Basically thought to be under control of ANS Meissners plexus (submucosal) Bundles of nerves found in the submucosal layer Control contraction of muscularis mucosae Control secretion of glands (mucus) Auerbachs plexus (myenteric) Bundle of nerves found IN BETWEEN LAYERS of muscularis externa Controls contraction of muscularis externa controls digestive tract motility Organization of Digestive Tract within the Abdominal/Pelvic cavities Organs undergo a lot of movement during digestion Loosely held in place by sheets of connective tissue called MESENTARY that extend from posterior body wall Where this C.T. surrounds the organs, its referred to as VISCERAL PERITONEUM C.T. that lines the abdominal or pelvic cavity is known as PARIETAL PERITONEUM Function of peritoneum and mesentary is to allow movement, lubricate surfaces between organs, maintain organs in proper orientation In some places, large extensions of mesentary are given unique names Greater Omentum is a wide sheet of mesentary that hangs off of the stomach and large intestine covers the small intestine like an apron Lesser Omentum is a sheet of mesentary that extends from the liver out to the stomach Organs that are enclosed by mesentary are called INTRAPERITONEAL (stomach, most of intestine) Organs posterior to the mesentary are RETROPERITONEAL (kidneys, pancreas, part of small intestine) Movement of Food Through the Digestive Tract Entire process of digestion begins when food enters the oral cavity Lips, cheeks, tongue, palate, and teeth work together to: Keep food within cavity (lips, cheeks, palate) Move food within cavity (tongue, cheeks) Grind food into smaller pieces for ingestion (teeth) also a very important part of MECHANICAL DIGESTION Mechanical digestion is critical since it increases surface area of food to CHEMICAL DIGESTION (breakdown by enzymes, acids) Mechanical breakdown of food in the mouth is known as MASTICATION Movement of Food Through the Digestive Tract In the mouth, food is mixed with saliva from three main pairs of salivary glands Parotid, Submandibular, Sublingual Total saliva production averages L/day! Salivary glands are MIXED GLANDS Contain cells that produce SEROUS (thin,watery) or MUCUS (thick, stringy) secretions Also secrete : 1.Amylase : enzyme that starts starch digestion 2.Lipase : breaks down fats (not active until reaches stomach) 3.Lysozyme : enzyme that inhibits bacterial growth Regulation of salivary gland secretion Secretion is regulated by SALIVATORY nuclei in brainstem Smell of food, eating food triggers secretion Signals are delivered to gland by BOTH division of ANS Parasympathetic division : triggers release of serous secretions (watery) Sympathetic division : triggers release of mucus secretions Movement of Food Through the Digestive Tract Food mixed with saliva forms a small, moist ball called a BOLUS Bolus is pushed to back of oropharynx for entry into ESOPHAGUS Esophagus is straight muscular tube (~1ft long) Lined by stratified squamous epithelium ESOPHAGEAL SPHINCTER regulates entry of bolus into stomach Prevents reflux of stomach acid back up into espohagus (ACID REFLUX heartburn) Esophagus also contains a large number of mucus glands Swallowing (DEGLUTITION) Coordinated movement of many muscles regulated by SWALLOWING CENTER in brainstem 2 phases: BUCCAL phase tongue and cheeks push bolus to back of oropharynx Activates stretch receptors which trigger second phase Pharyngeal/Esophageal phase coordinated constriction of pharynx pushes bolus into esophagus and downwards via PERISTALSIS Proximal 1/3 is skeletal muscle, distal 2/3 is smooth muscle Movement of Food Through the Digestive Tract Stomach 2 main functions : Store food (very short term; < 1hr) Continue mechanical digestion AND begins chemical digestion Mechanical digestion is carried out by grinding/churning actions of the stomach Secretions of the stomach also help begin chemical digestion Stomach wall consists of same three layers as other parts of digestive tract, with a few unique modifications.. Mucosa is lined by columnar epithelial cells and contains many pockets called GASTRIC PITS Cells at the bottom of the pits are actively dividing and migrate up to replace lost cells Several types of glands also empty into the gastric pits 1.Cardiac glands in cardiac region 2.Pyloric glands in pyloric region 3.Gastric glands all other parts of stomach Movement of Food Through the Digestive Tract and cardiac glands Cardiac and pyloric glands: Primarily secrete mucus (MUCUS CELLS) and small amounts of HCl, intrinsic factor (PARIETAL CELLS) Intrinsic factor promotes Vit. B12 absorption Gastric glands: Much more complex than cardiac and pyloric glands Widespread throughout stomach Secrete mucus and HCl like other glands Also secrete enzymes and hormones: CHIEF CELLS produce PEPSINOGEN, a protease that is converted to PEPSIN when exposed to low pH (acidic) environment of stomach G-CELLS secrete hormone GASTRIN (stimulates HCl release and promotes gastric motility) Movement of Food Through the Digestive Tract Gastric secretion Gastric glands produce large amounts of gastric juice each day Mostly H2O, HCl, and pepsin Very low pH (~1) Mucus helps protect lining Mucus also contains a large quantity of bicarbonate ions (HCO3-) that buffer acid Acidic gastric juices have a number of functions Activates pro-enzymes called ZYMOGENS Degrades proteins, connective tissues of food we eat Kills most bacteria/viruses found in food we eat Gastric secretion is stimulated by GASTRIN (from G cells), ACh (from parasympathetic neurons), histamine (produced by gastric glands) Food mixes with gastric juice and is turned into a liquid called CHYME that is released into the duodenum Gastric secretion and motility is regulated by both hormones as well as by neural reflexes Generally, sympathetic stimulation = DECREASED secretion/motility and parasympathetic stimulation = INCREASED secretion/motility Hormones released from stomach (gastrin, histamine) or duodenum (gastrin, GIP, others) generally act as paracrine hormones Movement of Food Through the Digestive Tract Small Intestine Long, coiled tube (~20 ft long!) A little shorter in living human since smooth muscle tone causes it to contract Main site of chemical digestion and absorption Divided into 3 segments (all continuous) Duodenum first segment, receives chyme from stomach Jejunum middle segment, MOST digestion and absorption occurs in this segment Ileum distal segment Duodenum is retroperitoneal, jejunum and ileum are intraperitoneal Movement of Food Through the Digestive Tract Duodenum Chyme from the stomach is released into duodenum in a slow, controlled manner In addition, the secretions of the liver/gall bladder and pancreas enter here through an opening in the wall of the duodenum (duodenal papillae) Alkaline secretions from pancreas help neutralize some acid of the chyme Additional lipases, proteases, nucleases from pancreas carry out rest of chemical Brunners glands found in the submucosa secretes a mucus that has a lot of HCO3 (bicorbonate ions) Bile from liver helps emulsify fats from diet Turns BIG droplets into millions of tiny ones Basically, the duodenum serves to: 1.Mix chyme with bile and pancreatic secretions 2.Neutralize acids from stomach Movement of Food Through the Digestive Tract Jejunum and Ileum Jejunum Second segment of S.I. Very muscular Contractions are important for mixing of chyme with digestive enzymes Richly supplied with blood vessels Most chemical digestion and nutrient/water absorption occur here Ileum Third segment of S.I. Less muscular, vascular than jejunum Nutrient absorption is finished up here Vit. B12 is absorbed here Ileoccal valve regulates movement of food residue into the cecum (first part of Large Intestine) Nearly all nutrients and 90% of water have been absorbed at this point What moves into L.I. is basically feces Movement of Food Through the Digestive Tract Small Intestine Histology Mucosa and submucosa contract and cause wall of SI to form CIRCULAR FOLDS (plicae circularis) Folds cause contents of SI to slowly spiral through Finger-like projections called VILLI extend into the lumen of the SI Lined by simple columnar epithelium (absorption) Also contain GOBLET cells (mucus secreting) Villi are very tall in duodenum and get shorter as you move to jejunum and ileum Each villus contains blood vessels and LACTEALS to absorb nutrients Lacteal is a small lymphatic vessel needed to absorb lipids Movement of Food Through the Digestive Tract Small Intestine Histology Columnar cells also possess MICROVILLI on the apical surface (BRUSH BORDER) Increases absorptive surface area Also produce BRUSH BORDER enzymes which activate some pancreatic enzymes and also carry out chemical digestion on their own Enzymes stay on surface of cells.so chyme must come in contact with surface of cells for final digestion and absorption Therefore, a lot of mixing and churning is required for proper digestion and absorption INTESTINAL CRYPTS are pits in between villi Bases of villi possess actively dividing cells that replace damaged or scraped off cells at the top of the villus Movement of Food Through the Digestive Tract Large intestine (colon) ~5 ft long in most humans Larger diameter than SI Main function is to finish up H2O absorption as well as absorb electrolytes (NaCl) Absorption of nutrients DOES NOT OCCUR HERE Also functions to store feces prior to elimination Movement of material from SI into colon is regulated by ILEOCECAL VALVE (a smooth muscle sphincter) CECUM is a large sac-like portion of the colon that receives material from SI APPENDIX hangs off here (vestigial organ) 4 segments of colon Ascending, transverse, descending, sigmoid colon (S- shaped segment that leads into the rectum Thin band of muscle called TENIAE COLI runs the length of the colon Causes colon to bunch up into pouches called HAUSTRA Movement of Food Through the Digestive Tract Large Intestine Motility Both segmentation AND peristaltic movements Ensure proper mixing of contents and reabsorption of H2O/electrolytes Peristaltic movements are pretty rare (only forceful when we want to eliminate feces) During most of the day, mild peristaltic movements actually push material TOWARDS ileum prevents constant filling of rectum MASS MOVEMENTS occur a couple times per day in preparation for defecation Strong movement pushes material toward rectum Stimuli that promote gastric and small intestine motility generally also increase large intestine motility May experience urge to defecate after a large meal Accessory organs of the digestive system Liver Mixed function organ Endocrine function includes production of erythropoietin, angiotensinogen Main digestive function is through production of BILE Bile is released and stored in the GALL BLADDER Bile is also concentrated (H2O removed) Bile contains water, minerals, phospholipids, and bile salts Main digestive function of bile is to emulsify lipids Phospholipids and bile salts coat lipid droplet and break it up into smaller drops Accessory organs of the digestive system Pancreas Like, liver the pancreas is a mixed function gland Endocrine function = insulin, glucagon secretion Digestive function is to produce alkaline PANCREATIC JUICE Pancreatic juice contains many enzymes required for chemical digestion Amylase (starch digestion) Lipase (lipid digestion) Nucleases (RNA/DNA digestion) Proteases (protein digestion) Most proteases are synthesized in the pancreas as inactive ZYMOGENS Chymotrypsinogen chymotrypsin Trypsinogen trypsin Procarboxypeptidase carboxypeptidase Trypsinogen is converted to trypsin by brush border enzymes of the intestine Trypsin then activates chymotrypsin and carboxypeptidase Accessory organs of the digestive system Regulation of Pancreatic and Liver Secretion ANS regulates secretion same way as other digestive functions Sympathetic stimulation = less secretion Parasymp. stimulation = more secretion Also regulated by hormone produced by duodenum called CHOLECYSTOKININ (CCK) CCK promotes contraction of gall bladder AND secretion of pancreatic enzymes Also DECREASES gastric secretion and motility Soshifts focus from gastric digestion to intestinal digestion Pancreatic and liver secretions enter the duodenum through the same opening called the MAJOR DUODENAL PAPILLA Bile duct and pancreatic duct basically join just before entering duodenum