OVERVIEW

Organs of digestion (two categories)

1. Alimentary canal (gastrointestinal tract) – mouth, esophagus, intestine

2. Accessory organs – salivary glands, liver, pancreas, teeth, tongue

Digestive processes

1. Ingestion – eating

2. Propulsion – moves food through the alimentary canal

-a. Swallowing – voluntary control

-b. Peristalsis – involuntary; alternate contraction of circ. And long. Smooth muscle

3. Mechanical digestion – chewing, mixing with saliva, churning in stomach

-a. Segmentation – similar to peristalsis

4. Chemical digestion – enzymatic bread down of food

5. Absorption – digested end products; move through mucosa into blood and lymph

6. Defecation – eliminating indigestible substances; feces

Control of digestion

1. Neural – intrinsic reflexes; extrinsic CNS and ANS

2. Hormonal – mostly intrinsic produced

Peritoneum

1. Visceral peritoneum – covers the surface of organs

2. Parietal peritoneum – lines the wall of the abdominopelvic cavity

3. Peritoneal cavity – small slit like cavity between the two; serous fluid

4. Mesentery – serous membrane connection; blood vessels, nerves, fat storage

5. Retroperitoneal – not suspended by mesentery; adhere to abdominopelvic wall

6. Peritonitis – wounds, ulcers, burst appendix; inflammation, infection; antibiotics

Splanchnic circulation

1. Arterial – branches of the celiac trunk; mesenteric arteries; 25% CO

2. Hepatic portal veins –collects nutrient rich venous blood

Alimentary canal histology

1. Mucosa – moist; mucus; lines alimentary canal

-a. Epithelium – simple columnar epithelium

-b. Lamina propria – areolar; capillaries; absorb digested nutrients; MALT

-c. muscularis mucosae – scant layer; folding of surface; villi

2. Submucosa – moderately dense connective tissue; blood supply

3. Muscularis externa – inner circularly; outer longitudinally arranged

4. Serosa – visceral peritoneum

FUNCTIONAL ANATOMY

Oral (buccal) cavity

1. Stratified squamous epithelium – lines oral cavity; much friction

2. Libia (lip) – from nose to chin

-a. Red margin – lipstick on; poorly keratinized; translucent; blood in capillaries

-b. Labial frenulum – medial fold joins lip and gum; inferior and superior

-c. Orbicularis oris - forms core of lips

3. Checks – lateral and inferior to lips

-a. Buccinator – forms core of check

4. Palate – forms roof of the oral cavity

-a. Hard palate – palatine and maxillary bone

-b. Soft palate – mostly skeletal muscle; shuts off nsopharynx during swallowing

-c. Uvula – finger like projection; hangs free of the soft palate

5. Tongue – helps in chewing; pushes food posteriorly; speech

-a. Skeletal muscle – what the tongue is composed of

-b. Papillae – peg like projections; rough appearance; taste buds

-c. Lingual frenulum – medial mucosal folds; secures tongue to floor of mouth

6. Intrinsic salivary gland – small scattered throughout oral cavity

7. Extrinsic salivary glands – three pairs of large; outside oral cavity

-a. Parotid – between masseter and skin; ducts between upper molars and check

-b. Submandibular – medial aspect mandible body; ducts at lingual frenulum

-c. Sublingual – under the tongue; 10-12 ducts floor of oral cavity

-d. Saliva – water; acidic; amylase (starch digestion); lysozyme; IgA; lingual lipase

Teeth

1. Location – alveolar margins of mandible and maxilla

2. Mastication – chewing; up and down; side to side

3. Teeth type – specialized

-a. Incisors – chisel shaped; cut off pieces (I)

-b. Canines – eyeteeth; conical; tearing and piercing (C)

-c. Premolar - bicuspid; 2 cusp; grinding (PM)

-d. Molar – 4 – 5 cusp; grinding (M)

4. Dental formula –number of different types of teeth

-a. Permanent – 2I,1C,2PM,3M/2I,1C,2PM,3M x 2 = 32

-b. Deciduous – 2I, 1C, 3M/2I, 1C, 3M x 2 = 20

5. Tooth structure

-a. Gingiva – gum

-b. Crown – above gingival

-c. Enamel – hardest substance; covers crown; acellular

-d. Root – embedded in jaw

-e. Cementum – calcified connective tissue; covers root

-f. Periodontal ligament – attaches cementum to alveolus of jaw

-g. Neck – constriction between crown and root

-i. Pulp cavity – blood vessels, nerves

-j. Root canal – pulp cavity in root; nerves and blood vessels leave

-k. Dentin – bone like material makes up most of tooth; radial striation of tubes

-l. Odontoblast – cells in the pulp cavity; produce dentin

-m. Trigeminal (V) – branches of this nerve to upper and lower teeth

6. Dental caries – cavities; plaque (sugar, bacteria) adheres; acid decalcifies

7. Calculus (tartar) – plaque calcifies; tooth and gum separate; gum disease

8. Gingivitis – infection; red, sore, bleeding gums

Pharynx

1. Oropharynx – immediately posterior to mouth

2. Laryngopharynx – posterior to oropharynx

3. Stratified squamous epithelium – area of high friction

4. Skeletal muscle – two layers

-a. Inner layer – longitudinal

-b. Pharyngeal constrictors – three outer; stacked; encircle

Esophagus

1. General – 25 cm tube; posterior to trachea

2. Esophageal hiatus – esophagus pierces diaphragm

3. Cardiac orifice – esophagus meets stomach

4. Cardiac (gastroesophageal) sphincter – slight thicken smooth muscle; diaphragm

5. Esophageal mucosa – squamous epithelium; to simple columnar in stomach

6. Longitudinal folds – mucosa and submucosa; collapsed when not filled

7. Mucus – secreted by glands when wall compressed with food

8. Muscularis externa – from skeletal; mixed; all smooth last third

9. Adventitia – no serosa; covered with layer of fibrous connective tissue

10. Heart burn – reflux of acidic gastric juices into esophagus

11. Hiatus hernia – abnormality; superior stomach above diaphragm

Mouth, pharynx, and esophagus: digestive processes

1. Mastication – teeth, tongue, saliva; mechanical; chemical amylase (later)

2. Deglutition – palate; tongue; muscle groups

-a. Buccal phase – voluntary; tongue against soft palate

-b. Paryngeal-esophaeal phase – swallowing center in medulla and pons

3. Bolus – food after mastication

Stomach: gross anatomy

1. Size – from 50 ml to 4 l; can extend to the pelvis

2. Rugae – longitudinal folds; when empty; mucosa and submucosa

3. Regions

-a. Cardia – region around cardiac orifice

-b. Fundus – domed shaped part; under diaphragm

-c. Body – midportion of stomach

-d. Pyloric region – inferior funnel shaped part

4. Pyloric sphincter – controls emptying of contents into duodenum

5. Greater curvature – convex lateral surface

6. Lesser curvature – concave medial surface

7. Greater omentum – from greater curvature; small intestine; transverse colon

8. Lesser omentum – from liver to lesser curvature

9. Autonomic nervous system

-a. Sympathetic fibers – splanchnic nerve; relayed through celiac plexus

-b. Parasympathetic fibers – from the vagus (X) nerve

10. Circulation

-a. Celiac trunk branches – gastric and splenic arteries

-b. Hepatic portal circulation – veins from the stomach drain into it

Stomach: microscopic anatomy

1. Muscularis externa – innermost oblique running smooth muscle layer

2. Mucosa – simple columnar; all goblet cells; protective alkaline mucus

3. Gastric pits – dot the mucosa; lead to gastric glands

4. Gastric glands – produce gastric juices, hormones and other substances

-a. Mucus neck cells – produce a more acidic mucus; function not known

-b. Parietal (oxynic) cells – HCl acid and intrinsic factor

-c. Chief (zymogenic) cells – pepsinogen; HCl to pepsin (rennin in infants)

-d. Enteroendocrine cells – hormones (gastrin) release into the lamina propria

5. Mucosal barrier – protects against harsh acidic environment

-a. Bicarbonate – rich mucus

-b. Tight junctions – between epithelial cells; prevent seepage to lower layers

-c. Undifferentiated stem cells – gastric pits; complete renewal 3 to 6 days

Gastric secretions

1. Pepsin – from pepsinogen; protein digestion

2. HCl – parietal cells stimulated by gastrin (others)

-a. H+ – CO₂, water, carbonic anhydrase, carbonic acid, HCO₃^-

-b. Alkaline tide – bicarbonate into capillaries; blood draining more alkaline

-c. Cl- – comes into cell as bicarbonate leaves; pumped into lumen with H⁺

3. Gastrin – into lamina propria; into capillaries to other cells

4. Intrinsic factor – for the absorption of vitamin B₁₂

5. Chyme – the food becomes this in the stomach

Gastric mobility

1. Filling – smooth muscles relax; coordinated with medullary swallowing centers

2. Contractile activity – about 3 per minute

-a. Peristalsis – because of the extra muscle layer; to and fro

-b. Pace maker cells – in the longitudinal layer; depolarize spontaneously

-c. Basic electrical rhythm – set by these cells

-d. Gap junctions – action potential spreads through our the stomach

-e. Subthreshold – depolarization brought to threshold by hormonal and neural

3. Gastric emptying – pyloric contractions; small amount of chyme at a time

Gastric regulation

1. Cephalic phase – sight smell of food; cortex; conditioned response

-a. Hypothalamus – stimulated by cortex; taste and smell receptors

-b. Medulla – also stimulated

-c. Vagus nerve – stimulated from above; stimulates release of gastric juices

2. Gastric phase – about 2/3^rd of the gastric juices released

-a. Distension – stretch receptors; reflex; stimulate gastric glands

-b. Food chemicals – such as peptides; stimulate G cells

-c. G cells – gastrin secreting enteroendocrine cells

-d. HCl – released from parietal cells; gastrin and other hormones cause this

3. Intestinal phase – stimulatory and inhibitory

-a. Intestinal mucosal cells – stimulated by low pH; food; release intestinal gastrin

-b. Intestinal (enteric) gastrin – stimulate gastric glands

-c. Inhibitory effect – lots of acidic chyme; enterogastrones; enterogastric reflex

-d. Enterogastric reflex – vagus & local reflexes inhibited; pyloric sphincter tighten

-e. Enterogastrones – secretin, cholecystokinin, and gastric inhibitory peptide

Stomach pathologies

1. Gastritis – inflammation; acid breaks mucosal barrier

2. Gastric ulcers – erosion; pain; perforation; hemorrhage; peritonitis

-a. Helicobacter pylori – acid resistant; cytotoxins; chemotaxis

3. Vomiting – emetic center (medulla); diaphragm, abdominal wall muscles contract

Small intestine: gross anatomy

1. Location – from pyloric sphincter to ileocecal valve

2. Size – 2.5 cm in diameter; 6 m in dead; 4 m in living

3. Function – finish digestion; almost all absorption

4. Divisions – 3 in order; duodenum; jejunum; ileum

5. Duodenum – mostly retroperitoneal; 0.25 m long

-a. Hepatopancreatic ampulla (Vader) – bile and main pancreatic ducts meet

-b. Major duodenal papilla – opening of above into duodenum

-c. Hepatopancreatic sphincter (Oddi) – muscular valve; bile and pancreatic juices

6. Jejunum – about 2.5 m

7. Ileum – about 3.6 m

8. Nerve supply – para from vagus (X); sympa from thoracic splanchnic

9. Vascularization – mostly superior mesenteric artery and vein; absorbed nutrients

Small intestine: microscopic anatomy

1. Plicae circulares – permanent circular folds; 1 cm high; slow and mix chyme

2. Villi – finger like projection of mucosa; 1 mm high

3. Simple columnar – absorptive cells; most of mucosa

4. Capillary bed – found in each villi; absorption

5. Lacteal – specialized lymph capillary; core of each villi; fat absorption

6. Microvilli (brush border) – tiny projection of plasma membranes; enzymes

7. Mucosa - simple columnar epithelial

-a. Simple columnar cells – the absorptive cells

-b. Goblet cells – secrete mucus

-c. Intestinal (Lieberkuhn) crypts – tubular glands; pit open between villi

-d. Intestinal juices – watery; carrier for absorption; secreted by crypts

-e. Stem cells – at base of crypts; all cells arise from these; rise up villi; shed at tip

-f. Changes – distally crypts decrease; goblet cells increases

8. Submucosa – typical areolar connective tissue

-a. Peyer’s patches – lymphoid follicles; increase in number distally

-b. Duodenal (Brunner’s) gland – set; bicarbonate (alkaline) mucus; neutralize acid

9. Muscularis externa – typical bilayered

10. Serosa – typical; except in retroperitoneal duodenum

11. Intestinal juices – watery; alkaline; enzyme poor

Liver: gross anatomy

1. Function – many functions; mostly metabolic

-a. Digestive function – production of bile for digestion of fats

-b. Gallbladder – storage organ for bile

2. Location – inferior to right diaphragm; mostly enclosed by rib cage

3. Size – largest organ in body; 1.4 kg (3 lbs)

4. Falciform ligament – mesentery; from diaphragm and anterior abdominal wall

5. Round ligament (ligamentum teres) – free end of above; umbilical vein

6. Lesser omentum – liver to lesser curvature of the stomach

7. Porta hepatis – hepatic artery, portal vein, common hepatic duct enter; from #6

9. Bare area – superior surface no serosa; fused to diaphragm

10. Gallbladder – in recess on inferior surface of right lobe

11. Sulcus for inferior vena cava – posterioinferior aspect

12. Hepatic ducts (right and left) – from fusion of many bile ducts

13. Common hepatic duct – fusion of left and right hepatic ducts

14. Cystic duct – duct leading from gallbladder

15. Common bile duct – fusion of common hepatic duct and cystic duct

Liver lobes

1. Traditional scheme - based on superficial structures

-a. Right lobe – largest

-b. Left lobe – separated from right lobe by falciform ligament

-c. Caudate lobe – most posterior; between fissure of left omentum and VC sulcus

-d. Quadrate lobe – between falciform ligament and gallbladder

2. New scheme –based on what is drained by right and left hepatic ducts

-a. Right lobe – same as above

-b. Left lobe – left, caudate, and quadrate lobes

Liver: microscopic anatomy

1. Liver lobule – small hexagonal functional unit of the liver

2. Hepatocytes – liver cells; form plates; most of lobule

3. Central vein – hepatocyte plates radiate from it; to hepatic veins; vena cava

4. Portal triads – found at the six corners of the lobule

-a. Portal arteriole – from hepatic artery; oxygen to hepatocytes

-b. Portal venule – from hepatic portal vein; nutrient rich venous blood

-c. Bile duct – bile from hepatocytes; transports out of liver

5. Liver sinusoids – large leaky capillaries; venous and arterial blood; central vein

6. Kupffer cells – macrophages in sinusoids; debris, bacteria, worn out blood cells

7. Hepatocytes – functions result in blood leaving having less waste and nutrients

-a. Nutrient processing – glucose to glycogen; amino acids to plasma proteins

-b. Fat soluble vitamin storage

-c. Detoxification – example; removing ammonia; convert to urea

-d. Bile production – chief digestive function

8. Bile canaliculi – small canals; transport bile; opposite direction from blood

9. Bile – yellow green alkaline solution; bile salts and bile pigments

-a. Bile salts – derived from cholesterol; digestion of fats and cholesterol

-b. Bilirubin – chief bile pigment; from heme; urobilinogen (brown)

Liver: Gallbladder

1. Gallbladder – green walled muscular sac; 10 cm; stores bile

-a. Cystic duct – bile secreted; from gallbladder; normally bile backs up

-b. Cholecystokinin – intestinal hormone; gallbladder contracts; HP sphincter opens

Liver disorders

1. Hepatitis – usually virus; A not too bad; B (blood/sex) C (enteric) cirrhosis

2. Cirrhosis – alcohol; connective scar tissue; fibrous liver; depressed activity

3. Gallstones (biliary calculi) – cholesterol crystallization; obstructive jaundice

Pancreas

1. Pancreas – head surrounded by duodenum; retroperitoneal; deep to stomach

2. Main pancreatic duct (Wirsung) – fuses with bile duct before hepatopancreatic ampula

-a. Major duodenal papilla – holds the hepatopancreatic (Oddi) sphincter

3. Accessory pancreatic duct (Santorini) – branches off main pancreatic duct

-a. Minor duodenal papilla – releases into duodenum

4. Acini (pancreatic) – clusters of secretory cells surrounding ducts

5. Pancreatic juices – water; enzymes; bicarbonate (alkaline)

6. Proteases – released in inactive form to prevent autodigestion

-a. Trypsinogen – converted to trypsin by brush border enzyme

-b. Other proteases – activated by trypsin; such as chymotripsinogen

7. Other enzymes – active when released; bile may increase activity

8. Hormonal control – of pancreatic activity

-a. Secretin – acid stimulus; into blood; stimulates bicarbonate release

-b. Cholecystokinin – protein and fat stimuli; acini release enzymes

9. Parasympathetic control – vagus; cephalic and gastric phases of gastric secretion

Small intestine: digestive processes

1. Chyme – delivered slowly; acidic; hypertonic; loss of fluid to intestinal lumen

2. Segmentation – alteration of contraction of nonadjacent segments; mixing mostly

3. Peristalsis – alteration of contraction of adjacent segments; moves along

4. Migrating mobility complex – wave up to 70 cm; dies; next one more distal

5. Ileocecal valve – back pressure keeps close; two mechanisms open it

-a. Gastroileal reflex – enhanced activity of stomach increases force of segmentation

-b. Gastrin – released by the stomach; increased mobility; relaxes ileocecal valve

Large intestine: gross anatomy

1. Teniae coli – longitudinal layer muscularis externa; 3 bands of smooth muscle

2. Haustra – pocket like sacs; from tone of taniae coli

3. Cecum – 1^st part; sac like; inferior to ileocecal valve

4. Vermiform appendix – posteriomedial surface of cecum; MALT; appendicitis

5. Colon – 2nd part of the large intestine; several distinct areas\

-a. Ascending colon – goes up the right side

-b. Right (hepatic) flexure – right turn of the colon

-c. Transverse colon – crosses abdominal cavity

-d. Left (splenic) colon – left turn of the colon

-e. Descending colon – down the left side of abdominal cavity

-f. Sigmoid colon – in pelvis; S-shaped

-g. Retroperitoneal – most of the colon

-h. Transverse mesocolon – transverse colon to posterior abdominal wall

-i. Sigmoid mesocolon – sigmoid colon to posterior abdominal wall

6. Rectum – runs posterioinferiorly in front of sacrum

-a. Rectal valves – three transverse folds; separates flatus from feces

-b. Muscularis externa – well developed; must contract to expel feces

7. Anal canal – last 3 cm to outside

-a. Internal anal sphincter – smooth muscle (muscularis externa)

-b. External anal sphincter – skeletal muscle

Large intestine: microscopic anatomy

1. Mucosa – except for anal canal; simple columnar

-a. Crypts – numerous and deep

-b. Goblet cells – many; much mucus produced

-c. Special features – no plicae circulares or villi

1. Anal mucosa – stratified squamous epithelium; much friction

-a. Anal columns – long folds of stratified squamous

-b. Anal sinuses – recesses between these sinuses; secrete mucus

-c. Hemorrhoidal veins – two set of superficial veins; can become hemorrhoids

Bacterial flora

1. Fermentation – of indigestible material, like cellulose

-a. Flatus – mostly swallowed air; bacterial hydrogen sulfide and methane

2. Vitamin synthesis – two important ones produced

-a. B complex vitamins – needed in many metabolic activities

-b. Vitamin K – needed by liver for the coagulation of clotting proteins

Large intestine: digestive processes

1. Absorption – of water, electrolytes, and some vitamins

2. Motility – contractions are sluggish and short lived

-a. Haustral contractions – slow segmental movements; from haustra to haustra

-b. Mass movements – long, slow moving peristalsis; large areas of colon

3. Defecation – elimination of feces; most important function

-a. Fecal content – undigested food, bacteria, epithelial cells, mucus

-b. Defecation reflex – rectum filled; parasympathetic reflex

-c. Rectal contraction – and sigmoid colon

-d. Internal anal sphincter – relaxes

-e. External anal sphincter – voluntary; no go; reflex ends until next mass movement

-f. Valsalva’s maneuver – diaphragm; abdominals contract; abdominal pressure

Large intestine: disorders

1. Appendicitis – inflammation; bacteria; burst; peritonitis; appendectomy

2. Hemorrhoids – itchy varicosities; inflammation of hemorrhoidal veins

3. Salmonella – eggs; food workers; if not contained, bacteremia; antibiotics

CHEMICAL DIGESTION AND ABSORPTION

Chemical Digestion

General

1. Chemical digestion – catabolic; large molecules to monomers

2. Hydolysis – adding a water molecule to break it down

Carbohydrates

1. Salivary amylase – starch to oligosaccharides

2. Pancreatic amylase – starch to oligosaccharides

3. Oligosaccharide – made up of two to eight simple sugars

4. Brush border enzymes – on microvilli small intestine

-a. Oligosaccharide substrate – dextrinase and glucoamylase; disaccharides

-b. Disaccharide substrate – maltase, sucrase, and lactase; monosaccharides

Proteins

1. Pepsin – stomach; proteins into smaller polypeptide; some amino acids

2. Pancreatic enzymes – activated in duodenum

-a. Trypsin – proteins into smaller peptides

-b. Chymotrypsin – same as trypsin

-c. Carboxypeptidase – one amino acid at a time; the end with the carboxyl group

3. Brush border enzymes – microvilli of small intestine

-a. Carbosypeptidase – same as pancreatic carboxypeptidase

-b. Aminopeptidase – one amino acid at a time; from the amino end

-c. Dipeptidase – another one

4. End products – mostly amino acids; some di- and tripeptides

Lipids

1. Bile salts – synthesized from cholesterol; hydrophobic and hydrophilic sides

2. Lecithin – phospholipids; same as above

3. Emulsification – of the fat droplet by the above; increased surface area

4. Lipase (pancreatic) – triglycerides to 2 fatty acids and monoglyceride

Nucleic acids

1. Nucleases (pancreatic) – nucleic acids into nucleotide monomers

2. Brush border enzymes – nucleosidases; to nitrogenous bases; sugars; phoshate

Absorption

Carbohydrates

1. Secondary active transport – glucose and galactose cotransported with sodium

2. Facilitated transport – fructose in; all move to capillary bed this way

Proteins

1. Secondary active transport – of amino acids; di- and tripeptides

2. Endothelial cells – di and tripeptides are broken down to amino acids

Lipids

1. Micells – bile salts and lecithin; lipids at core

-a. Core – fatty acids; monoglycerides; cholesterol; fat soluble vitamins

2. Simple diffusion – of micelle core; bile salts and lecithin do not

3. Intracellular processing – inside the epithelial cell

-a. Triglycerides – resynthesized in smooth endoplasmic reticulum

-b. Chylomicrons – triglycerides; phospholipids; cholesterol; protein coat

-c. Golgi apparatus – responsible for extrusion of chylomicrons

4. Lacteals – chylomicrons too large for capillaries; into lymph; to blood

5. Lipoprotein lipase – in blood; chylomicrons broken down; fatty acids; glycerol

Nucleic acids

1. Active transport – special carriers for pentose; bases; phosphate

Vitamins

1. Large intestine – absorb vitamins B and K made by bacteria

2. Fat soluble vitamins – A, D, K, and E; with dietary fat

3. Water soluble vitamins – most B vitamins and vitamin C; simple diffusion

4. Vitamin B₁₂ – intrinsic factor complexes with; ileum mucosa; endocytosis

Electrolytes

1. Sodium – active transport of glucose and amino acids

2. Chloride – terminus of small intestine; actively exchanged for bicarbonate

3. Potassium – simple diffusion

4. Iron – actively transported according to need

-a. Ferritin – protein which binds to iron

-b. Shed epithelial cells – loss of excess iron

-c. Inadequate iron – increased release into blood

-d. Transferrin – iron transfer protein in blood

5. Calcium – absorbed according to need

-a. Vitamin D – activated in kidney by PTH; low calcium levels

Chemical digestion and absorption: disorders

1. Lactose intolerance – lactase deficiency; bacteria in large intestine feast

2. Infant food allergies – endocytosis of protein; immune system sees as foreign

3. Gluten enteropathy – grain protein; damage intestinal villi; mirovilli size reduced