An elderly, male patient with stomach cancer had a feeding tube inserted into his alimentary canal to bypass esophagus, stomach and duodenum. He also had gallbladder removed years ago.
Anatomy of Digestive Tract
Alimentary Canal: continuous with the muscular tube that runs from mouth to anus. Digests food through breaking it down and absorbs fragments through lining in blood.
Accessory Digestive organs: include teeth tongue and gallbladder. Include digestive glands that produce secretions that help break down food.
Mouth: bounded by lips anteriorly, cheeks laterally, palate superiorly and tongue inferiorly
Palate: forms roof of mouth and has two parts
Salivary Glands: cleanses the mouth, dissolves food chemicals for taste, moistens food for compaction and begins breakdown of starch for amylase.
Hard palate: formed by palatine bones and palatine process. Mucosa is slightly corrugated to help create friction
Soft Palate: formed by mostly skeletal muscle and closes off nasopharynx during swallowing
Major Salivary Glands: located outside oral cavity
Minor Salivary Glands: scattered throughout oral cavity
Parotoid: anterior to ear and external to master muscle
Submandibular: medial to body of mandible
Sublingual: anterior to submandibular gland under tongue
2 types of secretory cells:
Serous Cells: produce watery secretions, enzymes, ions and bits of mucin
Mucous Cells: produce mucus which lubricates oral cavity and hydrates foodstuff
Pharynx: food passes from mouth into oropharynx and then into laryngopharynx.
Allows passage of food, fluids and air.
Lined with stratified squamous epithelium with mucus producing glands.
External muscle layers consist of two skeletal muscle layers.
Inner layer of muscle runs longitudinally while outer pharyngeal constrictors encircle wall of pharynx
Esophagus: flat muscular tube that runs from laryngopharynx to stomach. It is collapsed when not involved in food propulsion.
Gastroesophageal sphincter: surrounds cardiac orifice. Keeps orifice closed when food is not being swallowed.
Esophageal mucosa contains stratified squamous epithelium which changes to columnar at stomach.
Submucosa has mucus secreting esophageal glands that aid in food passage. Also contains adventitia instead of serosa.
Stomach
Gross Anatomy
Microscopic Anatomy
Cardial region: surrounds cardiac orifice
Fundus: dome shaped region beneath diaphragm
Body: mid portion
pyloric region: terminates in pylorus which is continuous with duodenum through pyloric valve
Greater Curvature: convex lateral surface of stomach
Lesser Curvature: concave medial surface of stomach
Lesser omentum: runs from lesser curvature to liver
Greater omentum: drapes inferiorly from greater curvature over intestines, spleen and transverse colon. Blends with mesocolon: anchors to large intestines to posterior abdominal wall. Contains fatty deposits and lymph.
Stomach wall contains all 4 tunics but muscular and mucosa are modified.
Muscular has extra third layer which is called oblique.
Mucosa consists of simple columnar epithelium entirely composed of mucous cells.
Secrete coat of alkaline mucus that traps bicarbonate rich fluid layer beneath it.
Dotted with gastric pits which lead into gastric glands that produce gastric juice.
Smooth muscle allows stomach to not only churn, mix and move chyme but also to physically break it down into smaller fragments.
Types of gastric gland cells
Parietal: secrete hydrochloric acid, denatures proteins, activates pepsin, breaks down plant cell walls and kills many bacteria.
Required for absorption of b12 in small intestines
Chief cells: secrete pepsinogen, inactive enzyme that is activated by pepsin by hydrochloric acid and pepsin itself.
Enteroendocrine: secrete chemical messengers. Some are paracrine (histamine) while others are hormones (gastrin)
Mucosal barrier: protects stomach from harsh conditions and is created by three factors
Thick layer of bicarbonate rich mucus.
Tight junctions between epithelial cells prevent juice from seeping underneath.
Damaged epithelial cells are quickly replaced by division of stem cells
Small Intestines: major organ of digestion and absorption
Duodenum: mostly retroperitoneal, curves around head of pancreas
Ileum: attached posteriorly by mesentery; joins large intestine at ileocecal valve
Jejunum: attached posteriorly by mesentery
Structural modifications for absorption:
Length and surface area is increased for nutrient absorption.
Circular folds: permanent folds that force chyme to slowly spiral through lumen, allowing more time for absorption
Villi: fingerlike projections of mucosa with core containing dense capillary bed and lymphatic capillary called lacteal for absorption
Microville: contain membrane bound enzymes called brush border enzymes which are used for final digestion of proteins, carbohydrates and nucleic acids.
Small intestine wall histology
Intestinal crypts: tubular glands scattered between villi
Enterocyctes: make up bulk of epithelium
In ville: absorb nutrients and electrolytes, brush border enzymes bound to plasma membrane
In crypts: produce intestinal juice, watery mixture of mucus that acts as carrier fluid for absorbing nutrients from chyme
Goblet cells: mucus secreting cells
Enteroendorcien cells: source of enetrogastrones and secretin
Paneth cells: secrete antimicrobial agents(defensins and lysozyme)
Mucosa associated lymphoid tissue (MALT) protects against microorganisms
Peyer's patches located in the mucosa of the ileum and extend into the submucosa
Duodenal glands: in submucosa of the duodenum and secrete alkaline mucus to neutralize acidic chyme
Teeth: lie in sockets in gum covered margins of mandible and maxilla
Mastication: process of chewing that tears and grinds food into smaller fragments
Incisors: chisel shape for cutting
Canines: fanlike that tear or pierce
Premolars: broad crowns with rounded cusps used to grind or crush
Molars: broad crowns and rounded cusps. These are the best grinders
Each tooth has 2 major regions:
Roots: portion embedded in jawbone
Crown: exposed part above gums. Covered by enamel which is heavily mineralized with calcium salts and hydroxyapatite crystals
Tongue: composed of interlacing skeletal muscle. Functions include gripping, reposition and mixing of food during chewing.
Filiform Papillae: gives tongue roughness to provide friction. Only one that doesn't contain taste buds
Liver: digestive function is production of bile
Consists of four primary lobes which are right, left caudate and quadrate
Falcifrom ligament: separates larger right and smaller left lobes. suspends liver from diaphragm and anterior abdominal wall
Round Ligament: remnant of fetal umbilical vein along with free edge of falciform ligament
Blood enters through hepatic artery and hepatic portal vein which enters at porta hepatis
Ducts transporting bile include common hepatic duct leaving the liver which unites with cystic duct from gallbladder to form the bile duct
Microscopic anatomy of liver
Liver lobules: hexagonal structural and functional units
Composed of plates of hepatocytes that filter and process nutrient rich blood
Portal triad: in each corner of lubule
Branch of hepatic portal vein that brings nutrient rich blood from intestines
Bile duct: receives bile from bile canaliculi
Branch of hepatic artery that supplies oxygen
Liver Sinusoids: leaky capillaries located between hepatic plates
Blood from both hepatic portal vein and hepatic artery proper percolates from triad regions through sinusoid and empties into central vein.
Pancreas: deep to greater curvature of stomach with head encircled by c-shaped duodenum and tail next to spleen.
Exocrine function: secrete pancreatic juice via duct
Acini: clusters of secretory cells that produce zymogen granules contains proenzymes
Composition of pancreatic juice
Endocrine: secrete insulin and glucagon by pancreatic islet cells
Alkaline solution to neutralize acidic chyme coming from stomach
Protease (for proteins) which is secreted in inactive form
amylase (carbs)
Lipase (lipids)
Nuclease (for nucleic acids)
Large Intestines
Teniae Coli: three bands of longitudinal smooth muscle in the muscularis
Haustra: rocketlike sacs caused by tone of teniae coli
Subdivisions
Cecum: first part of large intestines
Appendix: contains masses of lymphoid tissue; bacterial storehouse
Colon: regions are retroperitoneal
Ascending Transverse and Descending colon: travels up right side, across and down left side of abdominal cavity
Sigmoid colon: s-shaped portion that travels through pelvis
Rectum: three rectal valves stop feces from being passed with gas
Anal canal: last segment that opens to body exterior at anus
Has internal (smooth) and external anal sphincter (skeletal)
Microscopic Anatomy
Contains thicker mucosa made up of simple columnar epithelium
Except in anal canal where it becomes stratified squamous epithelium
Contains abundant deep crypts with many mucus producing goblet cells
Physiology of Digestive organs
Pharynx and esophagus are conduits to pass food from mouth to stomach
The major function of both includes propulsion that starts with deglutition
Two phases of deglutition
Buccal phase: voluntary contraction of tongue. pushes the food bolus into the oropharynx
Pharyngeal-esophageal phase: involuntary phase controlled by the swallowing center in the brain
Once food enters pharynx, respiration is momentarily prohibited
Talking or inhaling while swallowing can short circuit these protective mechanisms
Stomach: serves as holding area for food
Protein digestion begins in stomach
Exhibits propulsion to deliver chyme to small intestines
Carries out mechanical breakdown of food
Lipid soluble alcohol, aspirin, and other drugs are absorbed into blood.
HCl denatures proteins by unfolding amino acid chain so it is more accessible to enzyme
Pepsin carries out enzymatic digestion of proteins
Only stomach function essential to life is secretion of intrinsic factor
Required absorption go B12
B12 needed to mature red blood cells
Lack of intrinsic factor causes pernicious anemia
Treated with B12 injections
Liver
Bile: yellow, green alkaline solution containing:
Bile salts: cholesterol derivatives; functioning fat emulsification and absorption
Bilirubin: pigment formed from heme; broken down by bacteria in intestines to stercobilin
Enterohepatic Circulation: recycling mechanism that conserves bile salts
Returned to liver via hepatic portal blood where it is used in new bile
95% of secreted bile salts are recycled each time
Small Intestines
Chyme from stomach contains partially digested carbohydrates, proteins and undigested fats
Chyme entering duodenum is usually hypertonic; chyme delivery has to be slow to prevent osmotic loss of water from blood.
Low PH of chyme os adjusted upward
Chyme is mixed with bile and pancreatic juice to continue digestion.
Enterogastric reflex and entergastrones control movement of food into duodenum to prevent it from being overwhelmed
Motility of small intestines
After meal segmentation is most common motion. Mixes chyme with pancreatic and brush border enzymes and bile
Between meals peristaltic waves are initiated every 90-120 minutes
Meal remnants, bacteria and debris are moved toward large intestines
Ileocecal valve opens and admits chyme into large intestines when force of segmentation and motility in ileum increase
ileocecal valve closes when chyme exerts backward pressure
Large Intestines
Absorbs vitamins made by bacterial flora, water and electrolytes.
Major function of large intestines is propulsion of feces to anus and defecation.
Digestion: some remaining food residues are digested by gut bacteria
Mass movements: slow, powerful peristaltic waves
Gastrocolic reflex: initiated by presence of food in stomach and amount of fiber increase strength of contraction
Defecation reflex: initiated by mass movements forcing feces toward rectum
Parasympathetic signals stimulate contraction of sigmoid colon and rectum relaxation of internal anal sphincter.
Conscious control allow relaxation of external anal sphincter band muscle of rectum contract to expel feces
Assisted by valsalva's maneuver: closing of glottis, contraction of diaphragm and abdominal wall muscles cause an increase in intra- abdominal pressure
Mouth and accessory organs
Ingestion: food is voluntary placed in oral cavity
Propulsion: voluntary buccal phase of deglutition ignited by tongue, propels food into pharynx
Mechanical Breakdown: mastication by teeth and mixing movement by tongue
Digestion: salivary amylase in saliva, produced bu salivary glands begins digestion of starch
Basic Nutrients
Digestion of Carbohydrates
Only Monosaccharides can be absorbed.
Starch and disaccharides are broken down into oligosaccharides and disaccharides; begins in mouth with salivary amylase.
Then broken down into lactose, maltose and sucrose.
Final break down into monosaccharides (glucose, fructose and galactose)
Digestion of Proteins
Source of protein not only dietary but also includes digestive enzymes and proteins from breakdown of mucosal cells.
Broken into:
Small peptides
Amino acid monomers
Large/ small polypeptides
Some dipeptides and tripeptides
Digestion begins when pepsinogen is converted to pepsin
Converted at a PH of 1.5-2.5 because it becomes inactive in high PH of duodenum
Digestion of Lipids
Steps of lipid digestion in intestine
Emulsification: triglycerides and their break down products are insoluble in water
Digestion: pancreatic lipase breaks down fat into three fatty acids and monoglycerides
Micelle formation: products from digestion become coated with bile salts
Diffusion: lipid products leave micelles and cross epithelial membrane via diffusion
Lipid products are converted back to triglycerides and enter lymphatic lacteal
Eventually emptied into venous blood at thoracic duct
Short chain fatty acid can diffuse directly into blood stream
Digestion of Nucleic acids
Nuclei of ingested cells in food contain DNA and RNA.
Pancreatic nuclease hydrolyze nucleic acid to nucleotide monomer.
Brush border enzymes break nucleotides down into three nitrogenous bases, pentose sugar and phosphate ions.
Enzymes Responsible for digestion
Pancreas
Acini: cluster of cells that produce Zymogen granules containing pro- enzyme
Direct: some symptoms include weight loss, difficulty swallowing and blood in the stool
Indirect: elderly man who had stomach cancer
Without gallbladder you can't eat large amounts of fatty, greasy or high fiber food.
Bile is released from the gallbladder to digest any fats that you eat so when you no longer have a gallbladder there is no storage unit to call upon.
Bile from liver drips out. This means that not only is there less bile to break down fats but also bile can't make its way into large intestine where it contributes to abdominal pain.
Feeding tubes cause discomfort and nutritional formula instead of solid food is sent through the tubes.
Feeding tubes may supply more fluid and nutrition than what is needed.
This leads to edema, which is watery fluid building up in tissues or body cavities including the lungs.
Pancreatic nucleases: for nucleic acids; hydrolyze nucleic acids to nucleotides monomers
Pancreatic lipases: for lipids; break down fat into three fatty acids and monoglycerides
Pancreatic Proteases: for proteins; cleave proteins into smaller peptides
Pancreatic amylase: break down starch or glycogen that escape salivary amylase into oligosaccharides and disaccharides
Salivary Glands
Salivary amylase: begins breakdown of starch and carbohydrates
Small Intestines
Brush border enzymes; for final digestion of proteins, carbohydrates and nucleic acids
Proteins: breaks oligopeptides and dipeptides into amino acids
Carbohydrates: further breaks them into lactose, maltose and sucrose then finally into monosaccharides