THE DIGESTIVE SYSTEM
DIGESTIVE FUNCTIONS
INGESTION intake of food
DIGESTION breakdown of molecules
ABSORPTION uptake nutrients into blood/lymph
DEFECATION elimination of undigested material
STAGES OF DIGESTION
MECHANICAL DIGESTION IS PHYSICAL BREAKDOWN OF FOOD INTO SMALLER PARTICLES ( teeth & churning action of stomach and intestines )
CHEMICAL DIGESTION IS SERIES OF HYDROLYSIS REACTIONS THAT BREAK MACROMOLECULES INTO THEIR MONOMERS ( enzymes from saliva, stomach, pancreas & intestines)
and the results are
Polysaccharides into monosaccharides
Proteins into amino acids
Fats into glycerol and fatty acids
DIGESTIVE PROCESSES
MOTILITY muscular contractions that break up food, mix it w/enzymes & move it along.
SECRETION digestive enzymes & hormones
MEMBRANE TRANSPORT absorption of nutrients.
SUBDIVISIONS OF THE
DIGESTIVE SYSTEM
DIGESTIVE TRACT 30 foot long tube extending from mouth to anus.( ALIMENTARY CANAL )
ACCESORY ORGANS teeth, tongue, salivary glands, liver, gallbladder and pancreas
TISSUE LAYERS OF G.I. TRACT
MUCOSA
SUBMUCOSA
MUSCULARIS EXTERNA
ADVENTITIA OR SEROSA
MUCOSA
DIGESTIVE EPITHELIUM
LAMINA PROPIA ( connective tissue )
MUSCULARIS MUCOSAE ( thin muscle layer )
SUBMUCOSA
Loose connective tissue layer containing large blood vessels, lymphatics, and nerves.
THE SUBMUCOSA PLEXUS ( between submucosa and muscularis externa ) CONTROL THE TONE OF THE MUSCULARIS MUCOSAE.
MUSCULARIS EXTERNA
Several layers of smooth muscle tissue
INNER CIRCULAR LAYER
OUTER LONGITUDINAL LAYER
Produces waves of contraction called PERISTALSIS.
ADVENTITIA OR SEROSA
Areolar tissue or mesothelium
Covering that attaches and holds the G.I. tract in position.
RELATIONSHIP TO THE PERITONEUM
Only duodenum, pancreas and parts of large intestine are retroperitoneal
Dorsal mesentery suspends GI tract and forms Serosa ( VISCERAL PERITONEUM ) of stomach and intestines.
Ventral mesentery forms lesser and greater omentum ( layer of CT contains lymph nodes, lymphatic vessels and blood vessels )
LESSER & GREATER OMENTUM
Lesser attaches stomach to liver
Greater covers small intestines like an apron
MESENTERY OF SMALL
INTESTINES HOLDS MANY BLOOD VESSELS
MESOCOLON ANCHORS THE COLON TO THE BACK BODY WALL
REGULATION OF DIGESTIVE TRACT
NEURAL CONTROL
Short myenteric reflexes ( swallowing )
Long vagovagal reflexes ( parasympathetic stimulation of digestive motility and secretion )
HORMONES messengers diffuse into bloodstream, distant target cells.
PARACRINE SECRETIONS messengers diffuse to nearby target cells
THE MOUTH ( ORAL CAVITY )
FUNCTIONS
INGESTION ( food intake )
ANALYSIS OF POTENTIAL FOOD (taste)
MASTICATION ( chewing )
CHEMICAL DIGESTION
DEGLUTITION
SPEECH
RESPIRATION
ACCESORY STRUCTURES include the SALIVARY GLANDS and the TEETH.
THE LINING OF THE ORAL CAVITY CONSISTS OF NONKERATINIZED STRATIFIED SQUAMOUS EPITHELIUM
TOOTH STRUCTURE
DENTIN
CROWN
ENAMEL
ROOT
CEMENTUM
PERIODONTAL LIGAMENT
ALVEOLAR SOCKET
MASTICATION
Breaks food into smaller pieces to be swallowed ( increase surface area exposed to digestive enzymes )
Contact of food with sensory receptors triggers chewing reflex
Tongue, buccinator and orbicularis oris manipulate food
Masseter and temporalis elevate teeth to crush food
Medial and lateral pterygoids swing teeth in side-to-side grinding action of molars.
SALIVARY GLANDS
SALIVA
FUNCTIONS
Begin starch and fat digestion
Cleanse teeth
Inhibit bacterial growth
Bind food together into bolus
COMPOSITION OF SALIVA
SALIVARY AMILASE begins starch digestion
LINGUAL LIPASE digest fat after reaches stomach
MUCUS aids in swallowing
LYSOZYME enzyme that kills bacteria
Ig A antibody that inhibits bacterial growth
ELECTROLYTES Na, K, Cl, HCO3, Phosphate
HISTOLOGY OF SALIVARY GLANDS
Compound tubuloacinar glands
Mucous cells secrete mucus
Serous cells secrete thin fluid rich in amylase
Mixed acinus is possible
SALIVARY GLANDS
Small intrinsic glands found under mucous membrane of mouth, lips, cheeks and tongue
Three pairs extrinsic glands connected to oral cavity by ducts ( PAROTID GLAND, SUBMANDIBULAR GLAND, AND SUBLINGUAL GLAND )
PAROTID GLAND
LARGEST SALIVARY GLAND
Located in front of the ear, between the skin and masseter muscle
Secretion is a clearly watery fluid ( contains salivary amilase )
SUBMANDIBULAR GLAND
Located under the mandible
Secretions are thicker because the presence of MUCIN
The duct opens near the lingual frenulum
SUBLINGUAL GLAND
Located under the tongue
Saliva is secreted into numerous lesser sublingual ducts along the base of the tongue.
It is primarily a MUCOUS GLAND
THERE ARE FOUR PRINCIPAL
GROUPS OF SMALLER SALIVARY GLANDS
( INTRINSIC GLANDS )
IN THE MOUTH
PALATINE GLANDS
LINGUAL GLANDS
BUCCAL GLANDS
LABIAL GLANDS
PHARYNX
Is a common passageway for nutrients and air.
Is divided in NASOPHARYNX, OROPHARYNX and LARYNGOPHARYNX.
Propulsion of the bolus results from the contractions of the pharyngeal constrictors and palatal muscles
ESOPHAGUS
STRAIGH MUSCULAR TUBE 25-30 cm long
Nonkeratinized stratified squamous epithelium
Esophageal glands in submucosa
Skeletal muscle in upper part & smooth in bottom.
Extends from pharynx to cardiac stomach passing through esophageal hiatus in diaphragm
SWALLOWING ( DEGLUTITION )
SERIES OF MUSCULAR CONTRACTIONS COORDINATED BY CENTER IN MEDULLA AND PONS ( Signals from C.Nerves V, VII, IX and XII )
OCCURS IN STAGES
BUCCAL PHASE
PHARYNGEAL-ESOPHAGEAL PHASE
BUCCAL PHASE tongue collects food and pushes it back into oropharynx
PHARYNGEAL-ESOPHAGEAL PHASE - soft palate rises and blocks nasopharynx, infrahyoid muscles lift larynx and epiglottis is folded back, pharyngeal constrictors push bolus down esophagus ( lower esophageal sphincter relaxes )
STOMACH
MECHANICALLY BREAKS UP FOOD PARTICLES, LIQUIFIES THE FOOD, AND BEGINS CHEMICAL DIGESTION OF PROTEIN AND FAT ( resulting mixture called chyme )
STOMACH DOES NOT ABSORB ANY SIGNIFICANT AMOUNT OF NUTRIENTS.
GROSS ANATOMY OF THE STOMACH
J SHAPED ORGAN WITH LESSER & GREATER CURVATURE
REGIONAL DIFFERENCES
Cardiac region just inside cardiac orifice
Fundus is domed portion superior to esophageal opening
Body is the main portion of the organ
Pyloric region is narrowed inferior end (antrum & pyloric canal )
PYLORUS is the opening to duodenum
INNERVATION AND CIRCULATION
Innervation by parasympathetic fibers from vagus and sympathetic fibers from the celiac plexus
All blood drained from stomach is filtered through the liver before returning to heart.
MUCOSA
simple columnar glandular epithelium, lamina propia has tubular glands (
GASTRIC PITS )
MUSCULARIS EXTERNA three layers of smooth muscle
CELLS OF THE GASTRIC GLANDS
MUCOUS CELLS secrete mucus
REGENERATIVE CELLS divide rapidly to produce new cells that migrate upwards
PARIETAL CELLS secrete HCL acid and INTRINSIC FACTOR
CHIEF CELLS secrete chymosin & lipase in kids, and pepsinogen throughout life
ENTEROENDOCRINE CELLS secrete hormones and paracrine messengers
GASTRIC SECRETIONS
FUNCTIONS OF HYDROCHLORYD ACID
Activates enzymes pepsin & lingual lipase
Liquifying food to form chyme
Converts ingested Ferric ions to Ferrous ions that can be absorbed and utilized for hemoglobin synthesis
Destroys ingested bacteria and pathogens
INTRINSIC FACTOR
Produced by parietal cells
Essential for absorption of B12 by small intestine
Necessary for RBC production (pernicious anemia)
PEPSIN
Chief cells
Protein digestion
Secreted as pepsinogen ( inactive zymogen )
HCl acid converts it to pensin ( active form )
GASTRIC LIPASE & CHYMOSIN
Chief cells
Lypase digests butterfat of milk in infant
Chymosin curdles milk by coagulating its proteins
CHEMICAL MESSENGERS
Many produced by enteroendocrine cells
Hormones enter blood and reach distant target cells
Paracrine secretions reach neighboring cells
Gut-brain peptides signaling molecules produced in digestive tract and Central Nervous system. ( SUBSTANCE P, VASOACTIVE INTESTINAL PEPTIDE, CHOLECYSTOKININ )
GASTRIC MOTILITY
SWALLOWING CENTER SIGNALS STOMACH TO RELAX.
ARRIVING FOOD STRETCHES STOMACH ACTIVATING A RECEPTIVE-RELAXATION RESPONSE.
RHYTHM OF PERISTALSIS CONTROLLED BY PACEMAKER CELLS IN LONGITUDINAL MUSCLE LAYER
REGULATION OF GASTRIC SECRETION
CEPHALIC PHASE
Vagus nerve stimulates gastric secretion & motility just with sight, smell, taste or thought of food.
GASTRIC PHASE
Activated by presence of food or semidigested protein ( by stretch or increase in pH )
Secretion is stimulated by:
ACh parasympathetic fibers
Histamine - enteroendocrine cells
Gastrin from pyloric G cells
INTESTINAL PHASE
Duodenum regulates gastric activity through hormones & nervous reflexes
At first gastric activity increases
Enterogastric reflex= duodenum inhibiting stomach
Chyme stimulates duodenal cells to release secretin, cholecystokinin and gastric inhibitory peptide
THE LIVER
3 lb. organ located inferior to diaphragm.
Four lobes right, left, quadrate and caudate
Falciform ligaments separates left and right
Round ligament is remnant of umblical vein
Gallbladder adheres to ventral surface between right and quadrate lobes.
Perform metabolic and hematological regulation, and synthesis and excretion of BILE.
GALLBLADDER AND BILE
Located inferior to right lobe of the liver
10 cm long
STORES & CONCENTRATES BILE
BILE is a yellow-green fluid containing minerals, bile acids, cholesterol, bile pigments and phospholipids
Bile acid emulsify fats & aid in their digestion
Bilirubin pigment is from hemoglobin breakdown
PANCREAS
Retroperitoneal gland posterior to the stomach
Has a head, body, and tail
Exocrine and endocrine gland
Secretes INSULIN & GLUCAGON into the blood
Secretes 1500 ml of Pancreatic juice into duodenum ( water, enzymes, zymogenes, and sodium bicarbonate )
OTHER ENZYMES
AMYLASE
LIPASE
RIBONUCLEASE
DEOXYRIBONUCLEASE
ACTIVATION OF ZYMOGENS
TRYPSINOGEN converted to TRYPSIN by intestinal epithelium
TRYPSIN converts other two as well as digests dietary protein
HORMONAL CONTROLOF SECRETION
CHOLECYSTOKININ released from duodenum in response to arrival of acid and fat ( causes contraction of gallbladder, secretion of pancreatic enzymes, relaxation of hepatopancreatic sphincter )
SECRETIN released from duodenum in response to acidic chyme ( stimulates all ducts to secrete more bicarbonate )
GASTRIN from stomach & duodenum weakly stimulates gallbladder contraction & pancreatic enzyme secretion
DUCTS OF GALLBLADDER, LIVER AND PANCREAS
Bile passes from BILE CANALICULI between cells to BILE DUCTULES to RIGHT & LEFT HEPATIC DUCTS
Right and Left Hepatic ducts join outside the liver to form COMMON HEPATIC DUCT
CYSTIC DUCT from Gallbladder joins common hepatic duct to form the BILE DUCT
DUCT OF PANCREAS and bile duct combine to form HEPATOPANCREATIC AMPULLA emptying into duodenum at the major duodenal papilla
SPHINCTER OF ODDI ( Hepatopancreatic sphincter ) regulates release of bile and pancreatic juice.
SMALL INTESTINE
NEARLY ALL CHEMICAL DIGESTION AND NUTRIENT ABSORPTION OCCURS IN THE SMALL INTESTINE
GROSS ANATOMY OF SMALL INTESTINE
DUODENUM curves around head of pancreas ( 10 in )
Retroperitoneal along with pancreas
Receives stomach contents, pancreatic juice, and bile
Neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH increase, pancreatic enzymes
JEJUNUM is next 8 ft. ( Covered with serosa and suspended by mesentery )
ILEUM is last 12 ft ( covered with serosa and suspended by mesentery, ends at ILEOCECAL JUNCTION with large intestine
SURFACE AREA OF SMALL INTESTINE
CIRCULAR FOLDS up to 10 mm tall ( plica circularis ) involve only mucosa and submucosa
VILLI are fingerlike projections ( contain blood vessels and lymphatics- lacteal for nutrient absorption )
MICROVILLI have brush border on cells, and brush border enzymes for final stages of digestion
INTESTINAL CRIPTS
Pores opening between villi lead to intestinal cripts
BRUNNERS GLANDS in submucosa secrete bicarbonate mucus
PEYER PATCHES are populations of lymphocytes to fight pathogens
Secrete 1-2 L of intestinal juice/day
INTESTINAL MOTILITY
Mixes chyme with intestinal juice, bile, and pancreatic juice
Churns chyme to increase contact with mucosa for absorption and digestion
Moves residue towards large intestine ( by Segmentation Random ringlike constrictions mix & churn contents; and by Peristaltic waves that begin in duodenum but each one moves further down )
FOOD IN STOMACH CAUSES GASTROILEAL REFLEX ( relaxing of ileocecal valve and filling of cecum )
SEGMENTATION
( for mix & churn, not to move )
PERISTALSIS
GRADUAL MOVEMENT OF CONTENTS TOWARDS THE COLON
BEGINS AFTER ABSORPTION OCCURS
MIGRATING MOTOR COMPLEX CONTROLS WAVES OF CONTRACTION ( SECOND WAVE BEGINS DISTAL TO WHERE FIRST WAVE BEGAN )
CARBOHYDRATE DIGESTION AND ABSORPTION
NUCLEIC ACIDS, VITAMINS, AND MINERALS
NUCLEASES hydrolyze DNA & RNA to nucleotides
VITAMINS are absorbed unchanged ( A,D,E, & K with other lipids; B complex & C by simple diffusion, and B12 if bound to Intrinsic factor )
MINERALS are absorbed all along small intestine ( Na cotransported with sugars & amino acids; Cl exchanged by bicarbonate reversing stomach, Iron & calcium absorbed as needed )
WATER BALANCE
Digestive tract receives about 9 L of water/day ( 0.7 L in food, 1.6 L in drink, 6.7 L in secretions )
8 L is absorbed by small intestine & 0.8 L by the large intestine
WATER is absorbed by OSMOSIS following the absorption of salts & organic nutrients
LARGE INTESTINE
5 feet long and 2.5 inches in diameter ( in cadaver )
Begins as cecum & appendix in lower right corner
ASCENDING, TRANSVERSE, and DESCENDING COLON frame the small intestine
SIGMOID COLON is S-shaped pofrtion leading down into pelvis
RECTUM is straight portion ending at anal canal
MICROSCOPIC ANATOMY
MUCOSA is SIMPLE COLUMNAR EPITHELIUM ( anal canal is Stratified squamous epithelium )
No circular folds or villi to increase suraface area
INTESTINAL CRYPTS produce mucus only
MUSCULARIS EXTERNA ( longitudinal muscle fibers form taenia coli producing haustra pouches - )
Transverse & Sigmoid have a serosa; the rest is retroperitoneal.
ABSORPTION & MOTILITY
Transit time is 12 to 24 hours ( reabsorbs water and electrolytes )
Feces consist of water & solids
Haustral contractions occur every 30 minutes ( distension of a haustrum stimulates it to contract )
Mass movements occur 1 to 3 times a day ( triggered by gastrocolic and duodenocolic reflexes )
DEFECATION
STRETCHING OF THE RECTUM STIMULATES DEFECATION
Intrinsic defecation reflex via myenteric plexus
Causes muscularis to contract & internal sphincter to relax
Defecation occurs only if external anal sphincter is voluntary relaxed
Parasympathetic defecation reflex involves spinal cord
Stretching of rectum sends sensory signals to spinal cord
Splachnic nerves return signals intensifying peristalsis
ABDOMINAL CONTRACTIONS INCREASE ABDOMINAL PRESSURE as levator ani lifts anal canal upwards