Digestive System

BASIC DIGESTIVE PROCESSES Ingestion of food via the mouth Movement of food via the pharynx & esophagus Chemical & mechanical preparation of food in the stomach Completion of chemical digestion and absorption of food in the small intestine Elimination of wastes via the large intestine and rectum

Entire digestive tract lined with mucous membrane

GASTROINTESTINAL WALL-4 LAYERS Tunica mucosa-innermost-itself composed of 3 layers: a. epithelial layer b. lamina propria-areolar CT, blood vessels, lymphatic nodules, small glands c. muscularis mucosae-thin layer of smooth muscle Tunica submucosa-thick layer of CT  blood & lymph vessels, nerves, glands Tunica muscularis-double layer of smooth muscle tissue, with sphincters forming at several points along the tract Tunica serosa (adventitia in esophagous)-outermost Ct, covered by visceral peritoneum everywhere but esophagous & rectum

Plexuses innervate muscle & secretory cells of the gi tract

MOUTH=ORAL CAVITY Lined with stratified squamous epithelium Lips=outer surface Vestibule=space between teeth & lips or cheeks (all aid in mastication) Roof of the mouth: anterior hard palate & posterior soft palate Hard palate-bordered by upper teeth, formed by portions of palatine bones & maxillae. Provides hard surface against which tongue crushes & softens food

Soft palate-extends between oral & nasal portions of the pharynx. Functions to close off the nasopharynx during swallowing, preventing food from entering into the nasal cavity Tongue=floor of the mouth. Skeletal muscle covered with mucous membrane. Used in manipulating food, swallowing, speaking. Contains papillae in which taste buds are present

TEETH Protrude from alveoli (sockets) located along the mandible and maxillae. Surrounded by gums (=gingivae)  stratified squamous + dense fibrous CT 2 sets during human lifespan: Deciduous (milk)teeth-20 Permanent-32 Heterodont dentition-4 types of teeth: Incisors-chisel-shaped, for cutting Canines(cuspids)-tearing Premolars (bicuspids)-for crushing/grinding molars-- " " "

SALIVARY GLANDS Collectively secrete 1000-2000 ml saliva into the mouth daily. 3 distinct glands: Parotid-predominantly serous acini Submandibular-mixed acini Sublingual-predominantly mucous acini

OMENTA Coelom effectively subdivided into greater & lesser sacs by the stomach & 2 special mesenteries: greater & lesser omenta Greater omentum-extensive folded membrane, extending from the greater curvature of the stomach to the back wall & dowm to the pelvic cavity;  considerable fat ("pot belly" with excess)  hangs down like a fatty apron over abdominal organs to protect & insulate. Filled with plasma cells & other WBC. Can wraps around site of inflammation to wall off infectious organisms as with ruptured appendix, peptic ulcer, diverticulitis, etc

ESOPHAGUS Superior esophageal sphincter-closed by passive elastic tension in the wall of the esophagus when esophageal muscles relaxed Lower esophageal sphincter=cardiac sphincter  Last 4 cm of esophagus. Relaxes only long enough to allow food & liquids to pass into stomach-remains contracted otherwise to prevent food & HCl from being forced back into esophagus with  pressure in abdomen (breathing cycle, stomach contractions, late pregnancy) If lower esophageal sphincter doesn't close  heartburn

STOMACH J-shaped sac, 25 cm long, 1.5 l capacity Bolus enters through cardiac orifice, exits through pyloric orifice-both controlled by sphincters Empty stomach inner mucous membrane Branching wrinkles=rugae-flatten as the stomach fills Stomach lined with simple columnar epit indented by 3.5 x 10 6 gastric pits, 3-8 tubular gastric glands extend from each pit

4 regions of the stomach 1 3 3 4 1 2

CELL TYPES IN THE STOMACH Surface mucous cells-line the lumen & gastric pits  secrete alkaline mucus. Shed into the lumen Neck mucous cells-line the cardiac, pyloric, & fundic glands  secrete a more neutral mucus than at the surface. Replace lost surface cells Parietal (oxyntic) cells-eosinophilic, oval Secrete HCl & gastric intrinsic factor (Vit B 12 absorption) Chief cells-Lg, pyrimidal. Secrete pepsinogen Enteroendocrine cells-secrete hormones Undifferentiated cells-Replace others when they die (500,000 cells shed/d  renewed every 3 d

SMALL INTESTINE 6 m (20')long-completion of digestion & absorption of digestion products (EtOH absorbed in the stomach) Intestinal mucosa adapted for absorption: Plicae circulares-circular folds of the inner wall to  absorptive surface area Villi-fingerlike projections at the mucosal surface to  surface area 6000X Microvilli-projections from epit cells of the villi

INTESTINAL FEATURES See infolding of epit between bases of villi forming tubular intestinal glands=crypts of Lieberk ühn-extend into lamina propria Ea villus  blood capillaries & a lymph capillary=lacteal Brunner's glands=duodenal submucosal glands-secrete viscid, alkaline mucus to neutralize acidic chyme Peyer's patches=aggregated lmph nodules along the gi tract, esp in ileum

CELL TYPES IN THE SMALL INTESTINE Columnar absorptive cells-produce enzymes for terminal digestion of CHO & protein. Involved in the absorption of CHO, proteins, & fats from the intestinal lumen Undifferentiated cells-in depths of crypts  differentiate to replace other cell types as needed. Replaced cells disintegrate into intestinal lumen  discharge digestive enzymes

3. Mucous goblet cells-indepths ofcrypts & migrate upward  accumulate mucous until swelling into goblet shape  release mucus & die, esp abundant in duodenum (NOTE:mucous glands inhibited by sympathetic stimulation  superior portion of duodenum prone to peptic ulcers caused by nervous stress 4. Paneth cells-deep within intestinal crypts Secrete peptidases, lysozyme 5. Enteroendocrine cells-same as in stomach. Synth of > 20 gastrointestinal hormones

LARGE INTESTINE Aka colon=large bowel-Forms a rectangle that frames the tightly-packed sm intestine Cecum-cul-de-sac pouch, 6cm long Veriform appendix -opens into cecum, 2cm below ileocecal valve  bacteria & indigestible material easily trapped  inflammation (appendicitis) 2. Ascending colon-upward from cecum, bends @ R angle under liver @ R colic (hepatic) flexure

3. Transverse colon-extends across abdominal cavity R  L, makes R angle downward turn @ spleen=L colic (splenic) flexure 4. Descending colon-descends down to the rim of the pelvis 5. Sigmoid colon-S-shaped, travels transversely across the pelvis to the middle of the sacrum, continues to the rectum

DISTINCTIVE FEATURES OF THE LARGE INTESTINE An incomplete layer of longitudinal muscle forms 3 separate bands of muscle=taeniae coli along the full length of the colon Since taeniae coli don't cover all of the intestinal wall, the wall becomes puckered with bulges=haustra See epiploic appendages=fat-filled pouches which form at points where visceral peritoneum attached to taeniae in the serous layer

MICROSCOPIC ANATOMY OF THE LARGE INTESTINE Numerous goblet cells in the mucosal layer No villi or plicae circulares  smooth absorptive surface Lamina propria & submucosa  lymphoid nodules=gut-associated lymphoid tissue

RECTUM 15 cm, extends from the sigmoid colon to the anus. Retroperitoneal-no mesentery Mucosa & muscularis form shelves=plicae transversalis ,  vein network=hemorrhoidal plexus (may develop hemorrhoids) Anal canal-4cm, compressed by anal muscles. Upper part  5-10 anal (rectal) columns of mucous membranes united by folds=anal valves. Anus=slit opening to outside Anus & anal canal open only during defecation- otherwise closed by involuntary internal anal sphincter & voluntary external anal sphincter

ACCESSORY DIGESTIVE ORGANS Pancreas-12-15 cm, lies transversely across posterior abdominal wall. Retroperitoneal. 3 parts: Head, body, tail Exocrine cells arranged in groups=acini, ea with central lumen connecting to the main pancreatic duct

LIVER Lg, compound tubular gland. Ave 3 lb. Red Covered by network of CT=Glisson's capsule. Located under diaphragm, mostly on R side. Held in place by peritoneal attachments & by intra-abdominal pressure created by tonus of abdominal wall musculature Divided into 2 main lobes by falciform ligament, R lobe 6X larger, further subdivided into quadrate & caudate lobes

Bile ducts=bile canaliculi formed by bile capillaries that unite after collecting bile from liver cells  drain into R & L hepatic ducts  converge with cystic duct from the gallbladder to form the common bile duct Common bile duct joins with main pancreatic duct  enlarges into hepatopancreatic ampulla  joins duodenal papilla which opens into the 2 nd part of the duodenum

MICROSCOPIC ANATOMY OF THE LIVER Lobules=functional liver units, ea  branch of the hepatic vein. Hepatocytes within lobules  arranged in platelike layers, one cell thick Sinusoids-between radiating rows of hepatocytes  transport blood from the portal vein & hepatic artery. Walls lined with endothelial cells and Kupffer cells

GALLBLADDER Small, pear-shaped, saclike. In a depression under quadrate lobe of liver Layers of the gallbladder wall: outer, serous peritoneal coat middle muscular layer inner mucous membrane, continuous with the linings of the ducts-secretes mucin & actively transports H 2 O and salts out  bile more conc than in liver

PHYSIOLOGY OF DIGESTION Saliva: > 1 liter secreted/d 25-35% from parotid glands 60-70% from submandibular glands 3-5% from sublingual glands 99% H 2 O, 1% electrolytes & proteins, incl mucin & salivary amylase (breaks starch down into maltose & dextrin  works better on cooked starch because cooking disrupts cellulose of plant cell walls

Saliva also : -cleanses the mouth &  cellular & food debris on teeth -keeps soft parts of the mouth supple -buffers acidity in the oral cavity (high HCO 3 - , pH 6.35-6.85) -  IgA & lysozyme

Continuous secretion of saliva stimulated by parasympathetic nerve endings terminating in salivary glands Other factors stimulating saliva secretion: food in mouth & stomach, chewing, smell/taste/sight/thought of food Unpleasant stimuli related to food (eg rotten smells) inhibit parasympathetic system. Sympathetic stimulation (eg stress)  mouth & throat become dry

DIGESTIVE MOVEMENTS WITHIN THE STOMACH Food enters the cardiac orifice  slow, peristaltic mixing waves start in the smooth muscle pacemaker cells in the fundus and body= basic electrical rhythm As pyloric region fills, strong peristaltic waves churn chyme & propel it toward the pyloric orifice= pyloric pump As the stomach empties, peristaltic waves move farther up the body, ensuring that all chyme enters the pyloric region

REGULATION OF GASTRIC EMPTYING Fuller stomach  stronger force of pyloric pump. Distension of stomach  vagus nerve stimulated  strengthens peristaltic waves & causes gastrin secretion (promotes gastric secretion & motility). Pyloric sphincter relaxes  chyme enters duodenum. Secretagogues (EtOH, caffeine, partially digested protein) all stimulae gastric emptying

Gastric emptying inhibited by duodenum Neural response-enterogastric reflex-mediated by intrinsic nerve plexuses & autonomic nerves   stomach motility & gastric secretions Hormonal response-release of enterogastrones   peristaltic contractions & gastric motility

GASTRIC JUICE Colorless fluid, > 1.5 l secreted/d,  HCl, mucus, enzymes (gastric lipase, pepsin (for proteins  peptones), rennin (in children, caseinogen  casein)

DIGESTIVE MOVEMENTS IN THE SMALL INTESTINE Segmenting contractions-divide intestine into segments by sharp contractions of areas of circular smooth muscle in the intestinal wall. Duodenal segmentation caused by distension, ileal segmentation caused by gastrin Peristaltic contractions-propulsive  weak, repetitive waves which propel chyme through the sm intestine into colon. Regulated by motilin. Slow: ea wave takes 100-150 min from beginning to end of sm intestine

DIGESTIVE ENZYMES OF THE SMALL INTESTINE 1.5 l intestinal juice secreted/d  just dilute salt & mucus (enzymes from disintegration of brush border) Digestion in lumen via action of pancreatic enzymes & bile entering duodenum Bile: emulsifies fat & activates lipase Pancreatic enzymes: amylase-digests starch into disaccharides & monosaccharides proteases: trypsin, chymotrypsin, carboxypeptidases, aminopeptidases, etc

BILE-secreted by the liver. Alkaline,  H 2 O, HCO 3 - , bile salts & pigments, cholesterol, mucin, lecithin, bilirubin. 1 liter bile secreted/d. Most reabsorbed by a special active transport mechanism in the terminal ileum  returned to the liver

INTESTINAL MICROVILLI Have special actin-stiffened hairlike projections=brush border  3 categories of enzymes: Enterokinases: trypsinogen  trypsin Disaccharidases: Disaccs into monosaccs (eg lactase, sucrase, maltase) Aminopeptidases-aid enterokinase by breaking peptones into amino acids

ABSORPTION OF NUTRIENTS Monosaccharides like glucose & galactose taken up into intestinal epit cells by Na + cotransport (secondary active transport), fructose by passive transport. Most amino acids taken up by active transport mech similar to that for glucose (different carriers for basic, acidic, & neutral amino acids-basic come in by passive transport). Di- & tripeptides split into constituent amino acids once inside the intestinal epit cells

LIPID ABSORPTION & TRANSPORT Bile salts surround fatty acids & glycerol to form micelles (emulsification) Micelles attach to intestinal epit cell membrane  fatty acids & glycerol diffuse into the cell Once in, triglycerides resynthesized & are then coated with proteins forming chylomicrons Chylomicrons enter the lacteals & are transported through lymph

ABSORPTION OF WATER  9 l H 2 O enters gi tract/d. 92% absorbed in sm int, 6-7% absorbed in lg int. Water can move either way in sm int: when chyme is dilute  H 2 O absorbed across intestinal wall into the blood; when chyme is conc  H 2 O moves into the lumen of the sm int. Osmotic P in sm int  as nutrients absorbed & H 2 O moves from the intestine into the circulation

ABSORPTION OF VITAMINS H 2 O-soluble vits (most B vits, vit C) absorbed by facilitated transport Vit B 12 requires gastric intrinsic factor for absorption in terminal ileum Fat-soluble vits (A,D,E, &K) absorbed with fat

FUNCTIONS OF THE LARGE INTESTINE 100-500 ml chyme enters/d, only 1/3 excreted as feces  remainder (mostly H 2 O) absorbed in ascending or transverse colons  avoids dehydration Bacteria (living & dead) account for 25-50% dry weight of feces. Cells in crypts of Lieberk ühn secrete alkaline mucus  neutralizes H + produced by bacteria & lubricates lumen for the passage of feces

FORMATION OF FECES 150 g feces/d (100 g H 2 O + 50 g solids) Brown due to bilirubin; dark if  blood or from foods high in Fe, pale if high fat content Odor due to indole, skatole, H 2 S  result from decomposition of undigested food residue, unabsorbed amino acids, dead bacteria, cell debris

MOVEMENTS OF THE LARGE INTESTINE 1 ° motility  haustral contractions  depend on the slow rhythmicity of smooth muscle cells 3-4 X/d (following meals) motility  markedly=mass movements  drive feces into the descending colon-produced by the gastrocolic reflex When feces pushed into the rectum  the defecation reflex triggered

METABOLIC FUNCTIONS OF THE LIVER Deamination Formation of urea Synth of plasma proteins, fetal RBC, fibrinogen. Removal of bilirubin Amino acid synth Conversion of fructose & galactose into glucose Fatty acid oxidation Cholesterol & lipoprotein synth Detoxification rxns Vitamin A synth Production of metabolic heat

Digestive System

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Digestive System