*** A&p Ch. 24 Digestive System

Alimentary canal: mouth to anus, digests food and absorbs fragments, mouth, pharynx, esophagus, stomach, small and large intestine.

Accessory digestive organs: teeth, tongue, gallbladder, digestive glands (salivary glands, liver, pancreas).

- processes food

- extracts nutrients from it

- eliminates the residue

- ingestion

- digestion

- absorption

- compaction

- defecation

Essentially our "disassembly line" to break down nutrients into a form that can be used by the body and to absorb then so they can be distributed to the tissues.

Study of the digestive tract and the diagnosis and treatment of its disorders.

Mechanical digestion: physical breakdown of food into smaller particles. Cutting and grinding action of teeth, churning action of stomach and small intestines, exposes more food surface to the action of digestive enzymes. Some nutrients are present in a usable form in ingested food, absorbed without being digested. (Vitamins, free amino acids, minerals, cholesterol, and water).

Chemical digestion: series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues). Carried out by digestive enzymes produced by salivary glands, stomach, pancreas and small intestine. Results: polysaccharides into monosaccharides, proteins into amino acids, fats into monoglycerides and fatty acids, nucleic acids into nucleotides.

Mucosa: epithelium that secretes digestive fluids.

Submucosa: contains blood, lymph and large glands.

Muscularis: circular and longitudinal layers, contracts by peristalsis.

Serosa: outer layer of connective and epithelial tissue- forms the mesentary.

PERITONEUM: serous membrane that lines the wall of the abdominal cavity.

- visceral peritoneum: covers organs in abdominal cavity.

- parietal peritoneum: lines wall of abdominal cavity.

- peritoneal cavity: space between that is filled with fluid, folds in membrane hold organs in place (mesentery, mesocolon, omentum, falicform ligament).

..... MESENTERY: of small intestines holds many blood vessels, extensions of the peritoneum, highly vascular.

..... LESSER OMENTUM: ventral mesentery that extends from the lesser curvature of the stomach to the liver.

..... GREATER OMENTUM: hangs from the greater curvature of the stomach. Covers small intestines like an apron, forming a deep pouch between its deep and superficial layers.

A nervous network in the esophagus, stomach, and intestines that regulates digestive tract motility, secretion and blood flow. Contains sensory neurons that monitor tension in gut wall and conditions in lumen.

* functions completely independently of the central nervous system. CNS exerts a significant influence on its action.

Functions include:

- ingestion (food intake)

- other sensory responses to food- chewing and chemical digestion

- swallowing, speech, and respiration

Masticate food into smaller pieces:

- makes food easier to swallow

- exposes more surface area for action of digestive enzymes speeding chemical digestion

- 32 adult teeth, - 20 deciduous (baby teeth)

Saliva:

- moisten mouth, aids in swallowing

- begin starch and fat digestion

- cleanse teeth

- inhibit bacterial growth

Pharyngeal constrictors (superior, middle and inferior): circular muscles that force food downward during swallowing. When not swallowing, the inferior constrictor remains contracted to exclude air from the esophagus.

A straight muscular tube 25-30cm long.

Peirces diaphram through esophageal hiatus into abdomen. Through peristatic contractions, passes food into stomach through lower esophageal sphincter.

A muscular sac in upper left abdominal cavity immediately inferior to the diaphram. Mechanically breaks up food particles, liquifies food, and begins chemical digestion of proteins and fat.

Internal volume of 50ml when empty, 1.0-1.5L after typical meal, up to 4L when extremely full and extend nearly as far as the pelvis.

Chyme: soupy or pasty mixture of semi-digested food in the stomach. Rugae: longitudinal wrinkles, allow for expansion of the stomach.

Gastric pits with glands: produce gastric juice.

Glands include 4 secretory cells:

- mucous neck: secrete acid mucus

- parietal cells: secrete HCl and intrinsic factor

- chief cells: produce pepsinogen and gastric lipase. Pepsinogen activated to Pepsin by HCl in the stomach. Pepsin itself via positive feedback mechanism.

- enteroendocrine cells: produce regulatory hormones. Gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK) and somatostatin.

Swallowing: a complex action involving over 22 muscles in the mouth, pharynx, and esophagus.

OCCURS IN 2 PHASES:

Buccal phase: is voluntary.

Pharyngoesophageal phase: is involuntary.

Dentin: hard, yellowish tissue that makes up most of the tooth.

Enamel: covers crown and neck.

Cementum: bone like material that covers root, holds tooth in place.

Enamel: non-living material secretion formed during development.

Pulp: contains nerves and blood vessels.

Parotid gland: located beneath the skin, anterior to the earlobe. Mumps is an inflammation and swelling of the parotid gland caused by a virus.

Submandibular gland: located halfway along the body of the mandible. Its duct empties at the side of the lingual frenulum, near the lower central incisors.

Sublingual gland: located in the floor of the mouth. Has multiple ducts that empty posterior to the papilla of the submandibular duct.

CO2 + H2O to H2CO3 to HCO3- + H+

HCO3- exchanged for CI- (chloride shift) from blood plasma.

Cl (chloride ion) pumped into the lumen of gastric gland to join H+ forming HCl.

- activates Pepsin and lingual lipase

- breaks up connective tissue and plant cells walls (helps liquefy food to form chyme)

- converts ingested ferric ions (FE3+) to ferrous ions (FE2+). FE2+ absorbed and used for hemoglobin synthesis

- contributes to nonspecific disease resistance by destroying most ingested pathogens

Cephalic phase: initiated by the sight, smell, taste or thought of food. Nerve impulses from the medulla stimulate hydrochloric acid, pepsinogen, gastrin, and histamine secretion.

Gastric phase: initiated by distention of the stomach, which stimulates gastrin secretion and activated CNS and local reflexes that promote secretion.

Gastrointestinal phase: acidic chyme, which enters the duodenum and stimulates neuronal reflexes and the secretion of hormones (secretin, cholecystokinin) that inhibit gastric secretions.

Mucous coat: thick, highly alkaline mucus resists action of acid and enzymes.

Tight junctions: between epithelial cells prevent gastric juice from seeping between them and digesting the connective tissue of the lamina propria and beyond.

Epithelial cell replacement: stomach epithelial cells live only 3-6 days. Sloughed off into the cyhme and digested with food. Replaced rapidly by cell division on the gastric pits.

* breakdown of these protective measures can result in inflammation and peptic ulcer.

- pyloric valve closed: propulsion: perstaltic waves move from the fundus toward the pylorus.

- pyloric valve closed: grinding: the most vigorous peristalsis and mixing action occur close to the pylorus.

- pyloric valve slightly opened: retropulsion: the pyloric end of the stomach acts as a pump that delivers small amount of chyme into the duodenum, simultaneously forcing most of its contained material backward into the stomach.

- as chyme enters duodenum, receptors response to stretch and chemical signals. Enterogastric reflex and enterogastrones inhibit gastric secretion and duodenal filling. Prescence of lipids and acid releases CCK and secretin, hormonal regulators of the process.

- carbohydrate-rich chyme noves quickly through duodenum.

- fatty chyme remains in duodenum 6 hours or more

This is the body's major digestive organ. Digestion is completed and absorption occurs. Runs from pyloric sphincter to the ileocecal valve.

Has 3 subdivisions:

Duodenum: ~10" long. The bile and pancreatic ducts empty here.

Jejunum: ~8" long

Ileum: ~12" long

Intestinal villi: intestinal crypts secrete 1-2L of intestinal juice per day. In response to acid and distension of the intestines. Ph of 7.4-7.8.

- Gradual movement of contents towards colon.

- Peristaltic wave begins in the duodenum, travels 10-70cm and dies out, followed by another wave starting further down the tract.

- Successive, overlapping waves of contraction.

- Move chyme toward colon over a period of 2 hours.

* purpose of segmentation is to mix and churn, NOT to move material along as in peristalsis.

Liver: main function: only digestive function is bile production. Bile: fat emulsifier. Synthesize cholesterol, lipoproteins, heparin, regulates blood glucose levels, manufacturers plasma proteins, stores vitamins, filters and breaks down harmful substances. Bodies largest gland, weighs about 3lbs. Located below diaphram. Divided into 4 lobes (right, left, quadrate, caudate).

* blood supply to the liver is mixture:

- venous blood with nutrients from intestine

- oxygenated arterial blood from celiac trunk

Gallbladder: chief function is bile storage

- after a meal, the hepatocytes absorb from the blood. Glucose, amino acids, iron, vitamins, another nutrients for metabolism or storage.

- removes and degrades. Hormones, toxins, bile pigments and drugs.

- secretes into the blood: albumin, lipoproteins, clotting factors, angiotensinogen, and other products.

- between meals, hepatocytes break down stored glycogen and releases glucose into the blood.

- yellow-green, alkaline solution containing:

1. Bile salts: cholesterol derivirates that function in fat emulsification and absorption.

2. Bilirubin: pigment formed from heme. Bacteria break down in intestine to stercobilin- brown color of feces.

3. Cholesterol, triglycerides, phospholipids, and electrolytes.

- acidic, fatty chyme causes the duodenum to release hormones.

- gallbladder contraction stimulated by cholecystokinin (CCK) from intestinal cells exposed to acidic, fatty chyme. Vagal stimulation (minor stimulation).

- CCK also causes secretion of pancreatic juice and hepatopancreatic sphincter to relax.

- pancreatic stimulation caused by secretion causes secretion of bicarbonate-rich pancreatic juice by duct cells

Spong retroperitineal gland posterior to the greater curvature of the stomach.

- made up of cells called acini that release pancreatic juice. Juice is clear, colorless liquid that contains enzymes.

Pancreatic juice- digests fats

Pancreatic amylase- digests starches

Trypsin- digests proteins

- pancreatic juices flow through pancreatic duct, which unites with common bile duct and empties into the duodenum of small intestine.

- chyme from stomach contains partially digested carbohydrates and proteins and undigested fats.

- 3-6 hours in small intestine. Most water absorbed and all nutrients absorbed.

- small intestine, like stomach, no role in ingestion or defaction.

- starch: is the most digestible carbohydrate.

- cellulose is indigestible

- starch is first digested to oligosaccharides up to 8 glucose residues long. Then into the dissacharide maltose. Finally to glucose which is absorbed by the small intestine.

- amino acids absorbed by the small intestine come from 3 sources:

- dietary proteins, digestive enzymes digested by each other.

- proteases (peptidases)- enzymes that digest proteins. Begin their work in the stomach in optimum pH of 1.5-3.5. Pepsin digests 10-15% of dietary protein into shorter peptides and some free amino acids.

- pancreatic enzymes take over protein digestion in small intestine by hydrolyzing polypeptides into shorter oligopeptides

- hydrophobic quality of lipids makes their digestion and absorption more complicated than carbohydrates and proteins.

- lipases- fat digesting enzymes

- lingual lipase secreted by the intrinsic salivary glands of the tongue.

- pancreatic lipase- in the small intestine digest most of the fats.

- globules broken up into smaller emulsification droplets by certain components of bile- Lecithin and bile acids.

Nucleic acid

- nucleases (deoxyribonuclease and ribonuclease) hydrolyze DNA and RNA to nucleotides.

- nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar, and nitrogenous bases.

Vitamins

- are absorbed unchanged.

- fat soluble vitamins: A, D, E and K absorbed with other lipids. If they are absorbed 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 B12 if bound to intrinsic factor from the stomach.

Once these macromolecules are broken down into smaller parts, they are absorbed through the epithelium into neighboring capillaries

Unique features:

- teniae coli: three bands of longitudinal smooth muscle in muscularis.

- haustra: pocketlike sacs caused by time of teniae coli.

- epiploic appendages: fat filled pouches of visceral peritoneum.

- mucosa: simple columnar epithelium through entire large intestine.

- no circular folds or villi to increase surface area in large intestine.

- have a greater density of mucous-secreting goblet cells.

Enter from small intestine or anus.

- colonize colon

- synthesize B complex vitamins and vitamin K

- metabolize some host-derived molecules (mucin, heparin, hyaluronic acid)

- ferment indigestible carbohydrates

- release irritating acids and gases (~500ml/day)

Digestive system is one of several systems involved in water balance.

Digestive tract received about 9L of water/day.

- 0.7L in food, 1.6L in drink, 6.7L in gastrointestinal secretions

- 8L is absorbed by small intestine and 0.8L by large intestine.

- 0.2L voided in daily fecal output.

- diarrhea occurs when large intestine absorbs too little water. Feces pass through too quickly if intestine is irritated. Feces contain high concentrations of a solute (lactose).

- constipation occurs when fecal movement is slow, too much water gets reabsorbed, and feces becomes hardened.

- residue remains in large intestine 12-24 hours

- no food breakdown except by enteric breakdown

- vitamins (made by bacterial flora), water and electrolytes (especially Na+ and Cl-) reclaimed

- major functions: propulsion of feces to anus; defecation.

- colon not essential for life

- feces consist of 75% water and 25% solids (bacteria, undigested fiber, fat, mucus and sloughed epithelial cells).

- haustal contractions occur every 30 minutes. This kind of colonic motility is a form of segmentation. Distention of a haustrum stimulates it to contract.

- mass movements occur 1-3 times a day

Rectum: 3 rectal valves stop feces from being passed with gas (flatus).

Anal canal: last segment of large intestine. Opens to body exterior to anus.

Sphincters:

- internal anal sphincter: smooth muscle.

- external anal sphincter: skeletal muscle

- stretching of the rectum stimulates defecation reflexes. Accounts for the urge to defecate that if often felt soon after a meal. Parasympathetic defecation reflex involves spinal cord.

- abdominal contractions (valsalva maneuver) increase abdominal pressure as levator ani lifts anal canal upwards.

Carbohydrates:

- ex. Starch

- enzyme: salivary amylase, pancreatic amylase

- what is absorbed: glucose?

Proteins:

- ex.

- enzyme: proteases (peptidases), pepsin?

- what is absorbed:

Lipids:

- ex.

- enzyme: lipases, lingual lipase, pancreatic lipase

- what is absorbed? Lectithin and bile acids

Nucleic acids:

- ex.

- enzyme: nucleases (deoxyribonuclease and ribonuclease)

- what is absorbed? Phosphate ions, ribose and deoxyribose sugar and nitrogenous bases.

Vitamins:

- are absorbed unchanged.

- fat-soluble vitamins: A, D, E and K absorbed with other lipids.

- water-soluble vitamins: B complex and C, absorbed by simple diffusion and B12 if bound to intrinsic factor from the stomach.

Carbohydrates:

- ex. Starch

- enzyme: salivary amylase, pancreatic amylase

- what is absorbed: glucose, fructose, galactose (all simple carbs)

Proteins:

- ex. Actine, myosin, collagen

- enzyme: proteases (peptidases), trypsin, (lesser tolerance than to carbs)

- what is absorbed:

Lipids:

- ex. Oil, butter, fatty acids

- enzyme: lipases, lingual lipase, pancreatic lipase

- what is absorbed? Lectithin, bile acids, fatty acids, glycerol (store as TGs)

Nucleic acids:

- ex. ACTG

- enzyme: nucleases (deoxyribonuclease and ribonuclease)

- what is absorbed? Phosphate ions, ribose and deoxyribose sugar and nitrogenous bases.

Vitamins:

- are absorbed unchanged.

- fat-soluble vitamins: A, D, E and K absorbed with other lipids.

- water-soluble vitamins: B complex and C, absorbed by simple diffusion and B12 if bound to intrinsic factor from the stomach.