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