Digestive system Is a disassemble line to break nutrients down into forms that can be used by the body for cells to perform metabolic reactions the cells need oxygen and organic molecules that can be broken down to release the energy in there bounds the digestive system provides these organic molecules digestive processes 1. ingestion 2. propulsion voluntary swallowing involuntary peristalsis alternate eaves of smooth muscle contraction and relaxation 3. mechanical digestion chewing and mixing food segmentation- rhythmic local contractions of intestines 4. chemical digestion food to monomers using digestive enzymes 5. secretion release of water, acids, enzymes, and salts 6. absorption end products from GI tract into blood or lymph 7. excretion or defecation elimination of indigestible substances digestive system is separated into two main groups 1. the alimentary canal or gastrointestinal tract includes mouth pharynx esophagus stomach small intestine large intestine 2. accessory digestive organs includes teeth tongue gallbladder salivary glands liver pancreas histology of alimentary canal 1
from the esophagus to the anal canal the walls of the GI tract are made of four layers or tunics mucosa submucosa muscularis externa serosa or adventitia 1. mucosa is the moist epithelial membrane that lines the lumen of the GI tract functions 1. secretion of mucus, digestive enzymes, and hormones 2. absorption of digestive produces 3. protection against infectious diseases three layers of the mucosa 1. epithelium oral cavity, pharynx, and esophagus stratified squamous most of remaining simple columnar epithelium lots of goblet cells produces mucous liberates food protects against enzymes some sites (stomach small intestine) contain glands that produces 1. digestive enzyme 2. hormone from secreting cells called enteroendocrine cell coordinate activates of digestive tract and accessory glands 2. lamina propria underlies the epithelium is loose areolar connective tissue contains 2
1. capillaries and lymphatic vessels for nutrients and absorption 2. lymph nodules for protection 3. sensory nerve endings 4. smooth muscle (very thin) 3. muscularis mucosae thin layer of smooth muscle adds strength and some movement 2. submucosa moderately dense connective tissue contains with much elastic fibers 1. blood and lymphatic vessels traveling to lamina propria 2. lymph nodules 3. glands 4. nerve fibers produces the submucosal nerve plexus regulates the activity of glands in the mucosa and submucosa layer 3. muscularis externa is an inner circular layer and outer longitudinal of smooth muscle also contains myenteric nerve plexus regulates the contraction of smooth muscle responsible for peristalsis to propel foodstuff in 4. serosa is the outermost protective layer of organs located the abdominopelvic cavity is the visceral peritoneum mesothelium prevents organs from sticking to each other not seen in oral cavity, pharynx, esophagus they have an adventitia is fibrous connective tissue no epithelium 3
oral cavity digestive processes occurring in the mouth 1. analysis of materials before swallowing 2. mastication (chewing of food) 3. lubrication mixing with mucus and salivary secretions 4. limited digestion by saliva saliva salivary amylase breakdown of polysaccharides into smaller fragments salivary lipase breakdowns triglycerides only works at low ph so works in the stomach 97% water moistens food lysozyme destroys bacteria IgA antibodies blocks infection mucin which forms thick mucus when water is added lubricates food growth factors release of saliva secretion of saliva are controlled by ANS salivatory nuclei of the medulla oblongata sends actions potentials down the facial and glossopharyngeal nerves to stimulate salivation both sympathetic and parasympathetic nervous system stimulates salivation thus always have a flow phases of saliva release: 1. oral phase chemoreceptors most strongly stimulated by acidic substances pressure receptors presence of food in mouth 4
Absorption of oral cavity very little if any absorption occurs in the mouth some lipid soluble substances results in the activation of the parasympathetic nervous system to produce abundant watery saliva rich in enzymes 2. cephalic phase thought, sight, smell of food also stimulates the parasympathetic nervous system to produce abundant watery saliva rich in enzymes sympathetic stimulation results in the production of a smaller viscous saliva with little enzyme content due to constriction of blood vessels feeding the glands stomach functions: 1. function is primarily as a food storage organ 50 ml empty 4 liters extremely full will extend nearly to the pelvis 2. also mechanically breaks up food particles 3. liquefies the food to produce chyme 4. some limited digestion of proteins and lipids 5. helps sterilize the food acidity of stomach actions of digestive enzyme pepsin histology of stomach stomach is lined by a simple columnar epithelium is a secretary sheet produces a carpet of mucus to protect the stomach for acid 5
the epithelium is organized into shallow depressions called gastric pits pits are lined with columnar epithelium called mucous epithelium release mucin cell at base of pit undergo mitosis to replace cells at the base of a gastric pit will be two or three gastric glands cells of gastric glands 1) mucous neck cells located at the top of the gland where it opens into the pit produces mucin 2) parietal cells produce intrinsic factors required for the absorption of B12 by the intestine B12 need to produce RBC Release drops as you age increasing chances of pernicious anemia produce HCL 1. CO2 diffuses into cell from blood 2. carbonic anhydrase produces H and HCO3 3. H/K ATPase pump out H in exchange for K 4. Cl shifts into cell as HCO3 goes to blood alkaline tide function of HCL kills micros denatures proteins breaks down plant cell wall activates pepsin converts Fe3 to Fe2 can be absorbed 3) chief cells produce pepsinogen is converted to pepsin by HCL 6
function of pepsin pepsin digests proteins to shorter peptide chains produce rennin and gastric lipase in infants milk digestion 4) enteroendocrine cells produces hormones gastrin produced by G cells stimulates secretion by parietal cells and chief cells somatostatin produced by D cells weekly inhibits gastrin release 5) regenerative cells located at the base of the gland undergoes mitosis to replace old cells Digestive process of the stomach begin with the production of a gastric secretion gastric secretion contains mucus, acid, enzymes, and hormones regulation of gastric secretion release is controlled by both nervous and hormonal mechanisms three overlapping phases cephalic, gastric, and intestinal 1. cephalic phase directed by CNS to prepare the stomach to receive food occurs before food enters the stomach triggered by sight, smell or thought of food PNS nerve fibers traveling down the vagus nerve stimulates the mucous cells, chief cells, parietal cells, and G cells 2. gastric phase 7
occurs once food has reached stomach results from 1)neural stretch reflexes 2)gastrin stimulation most important stimuli of the gastric phase are distention, peptides and low acidity neural response distention of stomach by the food activates local stretch reflexes stimulate the parietal cells to make HCl and the chief cells to release pepsinogen and stimulate the G cells to release gastrin hormonal response gastrin released do to low acidity and peptides and by stretch gastrin effects 1. gastrin stimulates the release of pepsinogen from chief cells 3. intestinal phase starts when chyme enters the SI 2. main effect of gastrin is the simulation of HCl from the parietal cells more acid = more gastrin which = more acid (positive feedback loop) gastrin release is inhibited when ph drops lower then 2 this stimulates release of somatostatin and provides a negative feed back control main function is to control the entrance of acidic chyme into the duodenum neural response 8
regulation of gastric emptying stretch on SI receptors by the chyme inhibits gastrin production by the stomach hormonal response four hormones are involved gastrin released from the small intestine if ph of chyme is 3 or above stimulates gastric secretion secretin released from the SI if ph of chyme is less than 3 inhibits gastric secretion cholecytokinin (CCK) gastric inhibitory polypeptide (GIP) released from SI due to the presence of fatty acid and lipids in the chyme both inhibit gastric secretions periodic gastric emptying must be closely regulated by neural reflexes and by hormones too fast and chyme is not formed too slow and stomach too acidic regulation is primarily accomplished by controlling the opening and closing of the pyloric sphincter stimulation of gastric emptying 1. stretch of the stomach mildly relaxes the pyloric sphincter 2. gastrin released form the stomach due to stretch acid and amino acids relaxes the pyloric sphincter inhibition of gastric emptying 1. stretch and chemical receptors (acid, amino acids and lipid) in the small intestine activate neural reflexes that slow gastric emptying by contracting the pyloric sphincter 9
2. hormones of small intestine acid, amino acids and lipid in the small intestine also stimulate the release of secretin, CCK and GIP from the small intestine inhibit emptying by contracting the pyloric sphincter small intestine site of most digestion and almost all absorption averages 10 feet in a living person (21 in a cadaver) and is 1 inch in diameter three regions 1. duodenum smallest region 10 inches is retroperitoneal receives: 1. stomach contents acid chyme 2. pancreatic juice digestive enzymes and bicarbonate solution 3. bile fat emulsifiers and waste products site where ducts from gall bladder and pancreas enter have a common entry point called the common bile ducts controlled by a valve called the sphincter of Oddi closed between meals 2. jejunum 3 feet long more digestion then absorption 3. ileum 6 feet long more absorption then digestion 10
to perform the roles of digestion and absorption there are specializations to increase surface area intestine 1. plica circulares deep folds of the inner surface (submucosa) of the intestine their shape causes the chyme to spiral not found in the distal half of the ileum 2. villi fingerlike projections of the mucosal surface of the 3. microvilli tiny projections of the plasma membrane of the epithelial cells (absorptive cells) also called the brush borders has brush border enzyme mostly for carbohydrates and proteins become progressively smaller in more distal regions of SI histology of small intestine cells of the mucosal epithelium cells of the villus simple columnar cells called absorptive cells contain brush borders mirovilli 1. produce digestive enzymes brush border enzymes 2. contain transporters for nutrient uptake goblet cells secretes mucus intestinal crypts also called intestinal glands start at the base between two villi secret intestinal juice cell types absorptive cells goblet cells enteroendocrine cells secretes hormones S cells = secretin, CCK cells= CCK G cell = gastrin paneth cells 11
secretes lysozyme and is phagocytic contains the epithilial cells that renew the old cells Peyer s patches mucosa-associated lymphoid tissue (MALT) structures of the submucosa duodenal glands or Brunner s glands secrete an abundant alkaline mucus neutralize stomach acid lymphatic nodules like Peyer patches more numerous closer to large intestine intestinal secretion 1. largely water and mucus 1.8 liters per day mainly secreted by duodenal glands and goblet cells water is necessary for acid hydrolysis most comes in to SI by osmosis 2. is enzyme poor most enzymes of SI are brush border enzymes 3. is slightly alkaline to buffer acid release of SI secretions cephalic phase ANS parasympathetic stimulation stimulates release of secretions before food arises mucus will protect the walls from enzymes sympathetic stimulation inhibits release of mucous so get duodenal ulcers intestinal phase local reflexes distention by the acid chyme 12
irritation of the intestinal wall by the acid chyme hormones gastrin, CCK and secretin released due to the presence of acid, amino acids and lipid small intestine is the site of most chemical digestion this requires 1. brush border enzymes 2. juices from the gall bladder 3. juices from the pancreas pancreas produces enzymes that break down all categories of foodstuff the enzymes are delivered to the duodenum by two pancreatic ducts. The larger one which fuses with the bile duct to form the common bile duct controlled by a valve called the sphincter of Oddi the smaller one (accessory duct) empties directly into the small intestine physiology of pancreas endocrine pancreas islets of Langerhans beta cells release insulin alpha cells release glucagon exocrine pancreas organized into small clusters of glandular epithelial cells called acini composition of pancreatic juice produce about 1.5 qt. Per day 1. mainly water 2. electrolytes most is sodium bicarbonate 3. is alkaline ph8 this is to neutralize the acid chyme from stomach provides optimal environment for enzymes 4. enzymes 13
most are release in inactive form proteases trypsinogen is activated to trypsin by enterokinase enterokinase is a brush border enzyme procarboxypeptidase = carboxypeptidase chymotrypsinogen = chymotrypsin proelastase= elastase starch enzymes pancreatic amylase lipid enzymes pancreatic lipase DNA and RNA enzymes nucleases regulation of pancreatic secretion 1. neural stimulation parasympathetic activity during cephalic phase of gastric secretion stimulates section of pancreases 2. hormone control due to food in the small intestine 1. CCK (released in response to fats in SI) stimulate the release of pancreatic enzymes 2. secretin (released in response to HCL in SI) stimulate pancreatic duct cells to release bicarb rich juice liver and gallbladder functions of the liver carbohydrate metabolism- maintaining normal blood glucose lipid metabolism- produce lipoproteins which traffic lipids amino acid metabolism- deaminate amino acids removal of waste products vitamin storage mineral storage drug inactivation phagocytosis and antigen presentation 14
Kupffer's cells plasma protein synthesis removal of circulation hormones removal of antibodies removal and storage of toxins- bilirubin for example synthesis and secretion of bile liver s role in digestion is the production of bile for export to the duodenum bile is a yellow-green alkaline solution its role in digestion is as a fat emulsifier (meaning it breaks up fat into tiny particles) gallbladder is storage organ for bile composition of bile 1. bile salts 1. cholesterol derivatives mostly cholic acid and chenodeoxycholic acid 2. emulsify fats breaks them up into many small droplets 3. help the absorption of fats and cholesterol bile salts are recycled by the enterohepatic circulation 1. bile salts are reabsorbed by a specific transporter by distal part of SI 2. return to the liver by the hepatic portal vein 3. transported to gall bladder for release 2. phospholipids important in fat emulsification and absorption 3. bile pigments most is bilirubin -- a waste product of heme bacteria breakdown to urobilinogen which gives feces a brown color 4. cholesterol 5. electrolytes mostly sodium bicarbonate 15
regulation of bile release into SI cephalic phase parasympathetic impulses via the vagus simulate release not real strong stimulator hormonal phase 1. CCK stimulates gallbladder contraction and relaxation of sphincter of Oddi of the common bile duct is released due to presence of fats in the chyme entering the SI 2. Secretin released by intestinal cells due to presence acid in the chyme entering the SI stimulates the bile duct cells to produce a watery bicarbonate-rich juice large intestine about 5 feet long and 2.5 inches in diameter four regions cecum the ileocecal sphincter allows material in from the ileum 2.4 inches long off to the side of the cecum is the appendix 3 inches long colon about four feet long ascending, transverse, descending, and sigmoid rectum about 8 inches long final 3cm is the anal cannel histology not as complicated as the small intestine no villi no plicea circulares few glands epithelium simple columnar with numerous goblet cells columnar cells are also called absorptive cells mainly absorb water muscularis 16
longitudinal muscles are thickened into three conspicuous longitudinal bands called taeniae coli contraction forms a series of pouches called haustra functions of the large intestine chemical digestion in the large intestine the finial stage of limited chemical digestion occurs here by bacteria 1. ferment some of the indigestible carbohydrates flatus hydrogen sulfide (H2S), H2, N2, CO2, methane (CH4) two amines indole and skatole together with H2S = odor of flatus and feces the 2. synthesize B complex vitamins and most vitamin K B vitamins necessary for some enzymes and DNA production K necessary for formation of clotting proteins by liver absorption water very important some electrolytes vitamins vitamin K released from bacteria required to synthesize four clotting factors biotin required for the mitochondria to convert pyruvate (from anaerobic glucose metabolism) to oxaloacetate during aerobic metabolism vitamin B5 (pantothenic acid) is a component of coenzyme A (CoA) required for fats, polysaccharides, and proteins to enter the citric acid cycle remaining bile salts for production of steroid hormones and some neurotransmitters 17
elimination of fecal material (defecation) Chemical digestion and absorption 1. catabolic process 2. breakdown foodstuff into their monomers which can be absorbed 3. accomplished by enzymes intrinsic enzymes found on surface (brush border) secreted enzymes from accessory glands 4. enzymatic breakdown is called hydrolysis part of a water molecule is added to each broken bond the diet mainly contains carbohydrates, proteins, and fats digestion of carbohydrates carbohydrates in the diet 1) starch, most of the digestible dietary carbohydrate are long glucose polymers 2) glycogen not much in diet digested like starch are glucose polymers 3) sugars disaccharides sucrose lactose small amount of maltose monosaccharides glucose galactose fructose these are ready to be absorbed (monomers) 4) cellulose can not be digested = dietary fiber all carbohydrates must be broken down into monomers mostly glucose fructose galactose 18
digestion of carbohydrates starches these are quickly absorbed 1. salivary amylase works in mouth and center of food while in stomach 2. pancreatic amylase breaks down starches into smaller pieces. Only takes 10 minutes to work in small intestine yields 1. maltose (two glucoses) 2. 3 to 9 glucose chain polymers (oligosaccharides) 3. dextrinase and glucoamylase found on brush border of SI works on small glucose polymers made of three or more sugars yeilds the sugars glucose and maltose 4. maltase found on the brush border of SI splits maltose into two glucose molecules sugars (disaccharides) major dietary sugars are sucrose (glucose + fructose) lactose (glucose + galactose) maltose (glucose + glucose) disaccharides are digested by brush border enzymes of SI 1. maltase breaks down maltose to two glucose molecules 2. sucrase breaks down sucrose to glucose and fructose 3. lactase breaks down lactose to galactose and glucose absorption glucose is absorbed 1) transcellular by sodium-glucose transport proteins secondary active transport coupled to sodium ions 19
2) paracellular by solvent drag galactose is absorbed 1) transcellular by galactose-sodium transport proteins secondary active transport coupled to sodium ions 2) paracellular by solvent drag fructose is absorbed 1) transcellular by facilitated diffusion that is not sodium coupled fructose is quickly converted to glucose inside the cell so there is always a strong gradient 2) paracellular by solvent drag protein digestion 1. stomach 1. pepsinogen secreted by chief cells cleaved to pepsin by acid 2. works in acidic conditions ph 1.5 to 3.5 stops in small intestine 3. attacks bonds involving tyrosine and phenylalanine so get smaller polypeptides and some free amino acids from terminal tyrosine and phenylalanine digests 10 to 15% of dietary protein into shorted polypeptides particularly effective in digesting collagen which is poorly digested by other enzymes prepares meat for other enzymes 2. small intestine 1. pancreatic enzymes trypsin chymotrypsin reduces size of polypeptides by attacking internal peptide bonds so have many smaller peptides carboxypeptidase removes single amino acids from the carboxyl end (-COOH) 20
2. brush border enzymes carboxypeptidase removes single amino acids from the carboxyl end aminopeptidase and dipeptidase removes single amino acids from the amine end (-NH2) these work best in neutral ph absorption there are specific amino acid-sodium transport proteins transporters for each class of amino acids classes are neutral charged amino acids positively charged amino acids negatively charged amino acids there are also specific sodium dependent transporters for some di and tripeptides once in the cell they are broken down infants can absorb proteins by pinocytosis allows IgA from breast milk to inter blood stream intact absorbed proteins also may triggers some food allergies lipid digestion almost all occurs in the small intestine very small amount by salivary lipase fats (triglycerides) are insoluble in water so enzymes can not get at them and form large droplets bile salts from gallbladder act as detergents bile salts have a polar end with combines with water and a nonpolar end which combines with the fats bile salts pull off small droplets from larger thus emulsifies fats this increases surface area of the droplets so enzymes can get at them 21
pancreatic lipases can now cleave two fatty acid chains leaving fatty acids and monoglycerides absorption lipase activity breaks down the triglycerides the fatty acids and monoglycerides combined with phospholipids from bile and with bile salts to form even smaller micelles micelles have a core of lipids shielded from water by a surface of bile salts also in the micelle is cholesterol and fat sol vitamins the micelles come in contact with the cell surface and melt into the plasma membrane releasing the contents into the cell once inside the cell the free fatty acids and monoglycerides are resynthesized into triglycerides the triglycerides combined with phospholipids and cholesterol and are surrounded with a coat of proteins to form a spherical particle called a lipoprotein the lipoprotein formed in the guy is called a chylomicrons this occurs in the endoplasmic reticulum with final assemble occurring in the Golgi and then exported from the cell most chylomicrons go to lacteals some to capillaries the chylomicrons enter the blood stream and lipoprotein lipase found on the surface of capillary endothelium break the triglycerides down to fatty acids and glycerol with can be used by the tissue for energy or stored as fat in adipose tissue the left over chylomicrons are taken up by the liver nucleic acid pancreatic nucleases hydrolize RNA and DNA to nucleotides Brush border nucleosidase and phosphatases hydrolize nucleotides to their free base, pentose sugars and phosphate ions 22
absorption specific carriers of purines and pyrmidines in the SI vitamin absorption 1. fat-soluble vitamins A, D, E, K incorporated into micelles 2. water soluble vitamins are small and easily absorbed by simple diffusion except B12 which is large binds with intrinsic factor from stomach then binds to a specific receptor in the ileum is endocytocytosised electrolyte absorption sodium most absorbed coupled to active absorption of glucose and amino acids potassium moves by simple diffusion as water is absorbed potassium is concentrated and moves into cells anions like Chloride passively follow the sodium and potassium iron 1. is actively transported into the GI cells where it is stored by binding to ferritin 2. when needed it is released into the blood and bound to transferritin calcium absorption when active form of Vitamin D is present parathyroid hormone converts vitamin D to active form and stimulates kidneys to reabsorb calcium water absorption 9 liters of water enter the GI tract daily most absorbed in SI due to uptake of nutrients i.e. follows the glucose and sodium 23
24