Digestive System PDF

**DIGESTIVE SYSTEM** Disassemble line that breaks nutrients down into a form that can be absorbed by the digestive tract and used by the cells for metabolic reactions. The cells need oxygen and organic molecules that can be broken down to release the energy in their bounds - The digestive syste...

**DIGESTIVE SYSTEM** Disassemble line that breaks nutrients down into a form that can be absorbed by the digestive tract and used by the cells for metabolic reactions. The cells need oxygen and organic molecules that can be broken down to release the energy in their bounds - The digestive system provides these organic molecules Digestive processes 1. Ingestion 2. Propulsion: voluntary swallowing, involuntary peristalsis, alternate waves of smooth muscle contraction and relaxation 3. Mechanical digestion: chewing and mixing food, segmentation- rhythmic local contractions of intestines 4. Chemical digestion: breakdown of food to monomers using digestive enzymes 5. Secretion: release of water, acids, enzymes, and salt 6. Absorption: end products from GI tract into blood or lymph 7. Excretion or defecation: elimination of indigestible substances 2 main groups: 1. The alimentary canal or gastrointestinal tract: - Mouth - Pharynx - Esophagus - Stomach - Small intestine - Large intestine 1. Analysis of materials before swallowing 2. Mastication 3. Lubrication: mixing with mucus and salivary secretions 4. Limited digestion by saliva Saliva contains: - Salivary amylase: breakdown of polysaccharides into smaller fragments - Salivar lipase: breakdowns triglycerides and only works at low pH so works in the stomach - 97% water: moistens food - Lysozyme: destroys bacteria - IgA antibodies: blocks infections - Mucin: which forms thick mucus when water is added; lubricates food - Growth factors Release of saliva: secretion of saliva is controlled by ANS - Salivatory nuclei of medulla oblongata sends actions potentials down the facial and glossopharyngeal nerves to stimulate salivation -both sympathetic and para sympathetic nervous system stimulate salivation (so they always have flow) Phases: 1. Cephalic phase - Thought, sight, smell of food - Also stimulates the parasympathetic nervous system to produce abundant watery saliva rich in enzymes 2. Oral phase - Chemoreceptors: most strongly stimulated by acidic substances - Pressure receptors: presence of food in mouth - Results in the activation of the parasympathetic nervous system to produce abundant watery saliva rich in enzymes 3. Sympathetic stimulation - results in the production of a smaller viscous saliva with little enzyme content  - due to constriction of blood vessels feeding the glands **Absorption of oral cavity:** Very little if any absorption occurs in the mouth - some lipid soluble substances   **Histology of stomach:** Stomach is lined by a simple columnar epithelium - Is a secretary sheet  - Produces a carpet of mucus to protect the stomach form acid - The epithelium is organized into shallow depressions called gastric pits - Pits are lined with columnar epithelium called mucous epithelium (which will release mucin) \*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 **Function of HCL** - Kills micros - Denatures proteins - Breaks down plant cell wall and connective tissue - Activates pepsin and lingual lipase - Converts Fe3 (ferric) to Fe2 (ferrous)  - Fe2 now binds to a protein produced by the - stomach called gastroferritin and is absorbed  by - the SI **Chief cells** (which are in gastric gland) 1.Produce pepsinogen=Is converted to pepsin by HCL 2.Function of pepsin 3.Pepsin digests proteins to shorter peptide chains 4.Produce rennin and gastric lipase in infants=milk digestion Enteroendocrine cells - Eight or more types - Produces hormones: Gastrin = Produced by G cells and Stimulates secretion by parietal cells and chief cells Somatostatin: Produced by D cells and weekly inhibits gastrin release Regenerative cells - located at the base of the gland - undergoes mitosis to replace old cells  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: Occurs once food has reached stomach - Results from  1\) Neural response 2\) Hormonal response Most important stimuli of the gastric phase are distention, peptides and low pH (high acidity levels)  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] Gastric phase (cont.) Hormonal response - Gastrin is released due to low pH (high acidity) and peptides and by stretch  Gastrin effects 1\. Gastrin stimulates the release of pepsinogen from chief cells 2\. Main effect of gastrin is the simulation of HCl from the parietal cells 3. More acid = more gastrin which = more acid (positive feedback loop) 4. Gastrin release is inhibited when pH drops lower then 2 This stimulates release of somatostatin and provides a negative feed back control Intestinal phase: Starts when acid chyme enters the SI Main function is to control the entrance of acidic chyme into the duodenum - Results from - Neural response - stretch on SI receptors by the chyme inhibits gastrin production by the stomach - Intestinal phase - Hormonal response - Four hormones are involved: Gastrin is released from the small intestine if pH of chyme is 3 or above  Stimulates gastric secretion Secretin is released from the SI if pH of chyme is less than 3 Inhibits gastric secretion - Cholecystokinin (CCK)  - Gastric inhibitory polypeptide (GIP)  Both released from SI due to the presence of fatty acid and lipids in the chyme **Regulation of gastric emptying:** - periodic gastric emptying must be closely regulated by neural reflexes and by hormones  - too fast and chyme is not formed - too slow and stomach becomes 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 from the stomach due to stretch acid and amino acids relaxes the pyloric sphincter **Inhibition of gastric emptying ** 1\. Stretch of the small intestine activate neural reflexes that slow gastric emptying by contracting the pyloric sphincter 2\. Chemical receptors for acid, amino acids and lipid in the small intestine activate neural reflexes that slow gastric emptying by contracting the pyloric sphincter  3\. 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 2\. jejunum 3\. ileum **Duodenum** Smallest region - 10 inches (25.4 cm) - is retroperitoneal  Receives: 1\. stomach contents 2\. pancreatic juice 3\. bile 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 or hepatopancreatic sphincter - closed between meals   **Jejunum**: 3 feet long (.91 meters) - more digestion then absorption **Ileum**: 6 feet long (1.82 meters) - more absorption then digestion  **Small intestine histology:** To perform the roles of digestion and absorption there are specializations to increase surface area 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 intestine 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 Cells of the mucosal epithelium including cells of the villus - Simple columnar cells called absorptive cells - contain brush borders or mirovilli - produce digestive enzymes - brush border enzymes - contain transporters for nutrient uptake - Goblet cells - secretes mucus **Histology cont.** Cells of the 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 - contains the epithelial stem cells that renew the old cells - paneth cells secretes lysozyme and is phagocytic - Cells of the submucosal layer - Duodenal glands or Brunner's glands - Peyer's patches  - Mucosa-associated lymphoid tissue (MALT): more numerous closer to large intestine Intestinal secretion: - Largely water and mucus - 1.8 liters per day - mainly secreted by duodenal glands and goblet cells - water is necessary for acid hydrolysis  - slightly alkaline to buffer acid - Is enzyme poor - most enzymes of SI are brush border enzyme Release of Small Intestine Secretions: - Cephalic phase - 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 neural reflexes: distention by the acid chyme  - 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 Chemical digestion: Small intestine is the site of most chemical digestion This requires Pancreas: Produces enzymes that break down all categories of foodstuff The enzymes are delivered to the duodenum by two pancreatic ducts. 1\) The larger one which fuses with the bile duct to form the common bile duct - Controlled by a valve called the sphincter of Oddi 2\) 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: produces about 1.5 qt. (1.41 liters) per day 1\. mainly water 2\. electrolytes: most is sodium bicarbonate 3\. is alkaline pH8  4. enzymes: most are release in inactive form Pancreatic Enzymes: 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: - neural control - parasympathetic activity during cephalic phase of gastric secretion stimulates section of pancreases - 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 - 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: 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 - cholesterol derivatives - mostly cholic acid and chenodeoxycholic acid - emulsify fats (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) Regulation of bile release: Cephalic phase - Parasympathetic impulses via the vagus simulate release - Not a 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 (1.52 M) long and 2.5 inches (6.35cm) in diameter - Four regions - Cecum - the ileocecal sphincter allows material in from the ileum - 2.4 inches (6.1cm) long - Appendix off to the side  of the cecum is the - inches (7.62cm) long - Colon - about four feet (1.22 M) long - ascending, transverse, descending, and sigmoid - Rectum - about 8 inches (20.32cm) long - final 3cm is the anal cannel **Chemical digestion in large intestine:** Occurs here by the actions of **bacteria** 1\. Fermentation of some the indigestible carbohydrates - flatus - hydrogen sulfide (H2S), H2, N2, CO2, methane (CH4) two amines, indole and skatole  - indole and skatole together with H2S = odor of flatus and feces 2\. Cellulose digested to glucose with is absorbed here Bactria also synthesize B complex vitamins and most vitamin K - B vitamins necessary for some enzymes and DNA production - K is necessary for blood clotting Functions of Large Intestine: Absorption  - Water: very important - Some electrolytes - 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 - for production of steroid hormones and some neurotransmitters - Remaining bile salts **Functions of large intestine cont.** - Elimination of fecal material (defecation) **Chemical digestion and absorption:** 1\. Catabolic process 2\. Breakdown foodstuff into their monomers which can be absorbed - 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 For chemical digestion: 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 Starches in the diet: 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 - maltose (two glucoses) - 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  **Absorption of carbohydrates**: Glucose  1\) transcellular by sodium-glucose transport proteins  - secondary active transport coupled to sodium ions 2\) paracellular by solvent drag  Galactose  1\) transcellular by galactose-sodium transport proteins - secondary active transport coupled to sodium ions 2\) paracellular by solvent drag  Fructose 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 Pepsinogen secreted by chief cells cleaved to pepsin by acid Works in acidic conditions pH 1.5 to 3.5 - stops in small intestine Attacks bonds involving tyrosine and phenylalanine - so get smaller polypeptides and some free amino acids from terminal tyrosine and phenylalanine Stops in small intestine Particularly effective in digesting collagen which is poorly digested by other enzymes Prepares meat for other enzymes **Protein digestion**: 2\. Small intestine 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) 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 **Amino acid absorption**: There are specific amino acid-sodium transport proteins transporters for each class of amino acids Classes  - 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:** Most occurs in the small intestine Small amount by salivary lipase (stomach) Fats (triglycerides) are insoluble in water  So enzymes can not get at them and they form large droplets with small surface area 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 droplets - Thus emulsifies fats - This increases surface area of the droplets so enzymes can get at them - Pancreatic lipases can now cleave two fatty acid chains leaving fatty acids and monoglycerides **Lipid absorption:** - Lipase activity breaks down the triglycerides in the droplets - The fatty acids and monoglycerides combined with phospholipids from bile and with bile salts to form smaller droplets called 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 cell 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 - 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 - Converted to very low-density lipoproteins - Travel to the adipose tissue - Converted to low density lipoproteins - Mostly hold cholesterol - Absorbed by the cells - High density lipoproteins made by the liver pickup cholesterol and phospholipids form the tissues to return to the liver