Alimentary Canal Organs

Alimentary Canal Organs

• Oral cavity contains cheeks, lips, hard/soft palates, and the tongue
• Stratified squamous epithelium exists within
• Mechanical and chemical digestion occurs, with the tongue tasting and forming food boluses
• Pharynx contains the nasopharynx, oropharynx, and laryngopharynx
• Stratified squamous epithelium exists within
• Muscular tube connects the mouth to the esophagus
• Functions as a passageway for food and air while assisting with swallowing
• Esophagus connects the pharynx to the stomach
• Stratified squamous epithelium exists within
• Muscular tube conducts food from mouth to stomach
• Stomach is a J-shaped organ comprised of cardia, fundus, body, and pylorus regions
• Simple columnar epithelium exists within
• It stores and breaks down food, digests proteins, produces acid, and acts as a pathogen barrier
• Small intestine is comprised of the duodenum, jejunum, and ileum
• Simple columnar epithelium with microvilli exists within
• Large intestine consists of the cecum, colon, rectum, and anal canal
• Simple columnar epithelium with microvilli exists, though less than in the small intestine
• Functions: water/electrolyte absorption, feces formation/storage, and housing bacteria
• Anus is the digestive tract opening, comprised of the anal canal and external anal sphincter
• Stratified squamous epithelium exists within

Accessory Organs

• Tongue is a muscular organ in the mouth with taste buds
• Functions: mixes food, tastes, initiates swallowing, and forms boluses
• Teeth's enamel, dentin, and pulp provide mechanical chewing and food breakdown
• Tonsils are lymphoid tissue in the pharynx composed of palatine, lingual, and pharyngeal tonsils
• Part of the lymphatic system, immune defense by trapping pathogens occurs
• Salivary glands contain parotid, submandibular, and sublingual glands that secrete saliva
• Saliva contains enzymes to break down carbs, lubricates food for swallowing, and has antimicrobial properties
• Liver is located on the right side of the abdomen
• Produces bile, processes nutrients, detoxifies substances, and stores glycogen
• Gallbladder stores and concentrates bile, releasing it into the duodenum when fat is present in the stomach
• Pancreas is located behind the stomach
• Exocrine portion produces digestive enzymes
• Endocrine portion secretes hormones that regulate blood sugar levels

Digestive Processes

• Ingestion is eating and occurs in the mouth
• Mastication is chewing and occurs in the mouth
• Propulsion is movement of food from one end to the other that occurs in the:
  • Mouth
  • Pharynx/esophagus
  • Stomach
  • Small and large intestine
  • Anus
• Swallowing is the movement of liquids or soft masses of food called a bolus
• Peristalsis propels materials through most of the digestive tract
• Mass movements are contractions moving material in distal large intestines to anus
• Mixing doesn't propel food, instead it uses segmentation
  • Occurs in the mouth, stomach, and small/large intestines
• Secretion of mucous and enzymes occurs in the mouth, pharynx/esophagus, stomach, and small/large intestines
• Digestion is mechanical and chemical to break down large organic molecules into individual components
  • Occurs in the mouth, stomach, and small/large intestines
• Absorption is the movement of molecules out of the digestive tract and into the blood or lymph system
  • Occurs a little in the stomach and mainly in the small/large intestines
• Elimination is the removal of waste from the body and occurs in the anus

Alimentary Canal Wall Layers

• Mucosa contains epithelium and secretes mucus/hormones, and can contain digestive enzymes that absorb nutrients
• Submucosa has glands and a submucosal nerve plexus, which is part of the enteric nervous system (ENS), which controls when glands release secretions
• Muscularis has longitudinal muscle that's parallel to the flow of food
  • Contains circular muscle that circles around the canal and the myenteric nerve plexus is part of the ENS, and controls when longitudinal/circular muscles contract
• Serosa is called Adventitia in the esophagus and parts of the large intestine and is the inner serous layer (visceral peritoneum) in some organs

Enteric Nervous System (ENS)

• Mostly local control of organs through the submucosal and myenteric nerve plexus
• Autonomic nervous system influences the enteric nervous system
• Sympathetic nervous system inhibits digestion and parasympathetic nervous system stimulates digestion

Nervous System Regulation

• Norepinephrine generally inhibits digestion
• Acetylcholine generally stimulates digestion
• Serotonin has 95% located in the digestive system, meaning antidepressants and cancer chemotherapies often have digestive side effects

Serous Membranes

• The peritoneum is a serous membrane of the abdominal cavity and is a double-layered sac
  • The other two in the body are the pleura and pericardium
  • Allows movement of food and adjustment of organs without friction
• The visceral layer lines the inner surface of the abdominal wall
• The parietal layer covers the surface of abdominal organs
• Peritoneal cavity exists between the parietal/visceral layers of the peritoneum
  • Contains serous fluid to lubricate and allow organs to move smoothly
• Greater omentum is an apron-like fold of visceral peritoneum, draping over the intestines that consists of 4 layers of peritoneum
  • Contains fat/blood vessel deposits and lymphatic vessels that functions include fat storage for energy reserves, immune response to combat diseases, and infection isolation
• Mesentery proper is a double-layered fold of peritoneum that attaches to the small intestine
  • It holds the small intestine in place and contains blood vessels, nerves, and lymphatics

Salivary Glands

• Parotid gland secretes salivary amylase that breaks down carbs
• Submandibular gland secretes salivary amylase that breaks down carbs
• Sublingual gland secretes salivary amylase that breaks down carbs

Pharynx

• Nasopharynx is respiratory
• Oropharynx is digestive
• Stratified squamous epithelium exists within
• Laryngopharynx is digestive
• Stratified squamous epithelium exists within
• Oropharynx and laryngopharynx are involved in swallowing

Esophagus

• Stratified squamous epithelium exists that functions as the passageway to the stomach

Deglutition Steps (Swallowing)

• Voluntary phase: The tongue presses upward to move food to the back of the mouth
• Pharyngeal phase: Involuntary and is comprised of smooth muscle
  • Uvula rises to block the nasal cavity
  • Larynx rises and the epiglottis folds over to block the respiratory tract
  • Tongue rises to block the mouth
  • Food is forced down the esophagus
• Esophageal phase: Peristalsis (smooth muscle helps move food particles down) down the esophagus

Stomach Microanatomy

• Mucous neck cells line the entryway and secrete mucous
• Parietal cells are scattered through the wall and release HCl which is acidic and denatures bacteria, activating pepsin
  • Intrinsic factor (IF) assists the small intestine in absorbing vitamin B12
• Chief cells are mainly at the bottom of the pit and secrete pepsin which breaks down proteins
  • Pepsin can only activate when the pH level is 3, which happens in the stomach
• Enteroendocrine cells line along the bottom with chief cells when secreting hormones into the blood
  • Hormones travel far and paracrines target near

Regulation of Gastric Secretions

• Cephalic phase is stimulated by thinking and smelling and is inhibited by depression
• Gastric phase is stimulated by stretch and sensing food chemicals and is inhibited by emotional upset
• Intestinal phase has a brief stimulation effect ,and is inhibited by stretch and pH drops
• Stimulation occurs for the cephalic and gastric phase
• Inhibition occurs for the intestinal phase

Gastric Motility Processes

• Propulsion occurs 3 times a minute and relies on baseline electrical rhythm
• Grinding mixes in the pyloric region
• Retropulsion causes a small amount of chyme to squirt out while the rest goes backwards
• Emptying allows the small intestine to control the stomach
• Small intestine sensitivity factors are:
  • Stretch
  • pH
  • Food content where fatty foods means slower emptying and carbohydrates mean the stomach empties faster

Small Intestine Sections

• Duodenum (first 12 inches)
• Jejunum
• Ileum (closer to the large intestine)

Increased Surface Area of Small Intestine

• Circular folds (plicae circulares) are permanent at 1cm deep
• Villi are fingerlike at 1mm
• Microvilli contain bound enzymes

Small Intestine Microanatomy

• Secretory cells secrete intestinal juice to move food along
• Intraepithelial lymphocytes secrete cytokines
• Granular/paneth cells are antimicrobial agents
• Stem cells differentiate into many cells and give rise to new cells in the small intestine
• Enteroendocrine cells secrete substances into the bloodstream
• These are all located in the intestinal crypt

Small Intestine Enzymes

• Bruch border is the microvilli
• Enzymes are bound to the microvilli
  • Disaccharidase breaks up disaccharides for molecule absorption
  • Peptidase breaks up peptide bonds for absorption

Liver's Function

• Creates bile for digestive function
• Processes nutrients/toxins via the hepatic portal system
• Food goes (hepatic portal) to the liver (hepatic vein) and then to the heart
• Blood from the heart goes to the intestine then the liver via the hepatic portal

Gallbladder Functions

• Stores and concentrates bile, then contracts and releases it

Bile Composition

• Green and contains bile salts
  • Has a hydrophilic head and a hydrophobic tail
  • Hydrophobic ends surround the fat
  • Emulsifies fat to make it easier to get water in between fat

Enterohepatic Circulation

• Requires the intestines and hepatic portal system
• Bile salts from the liver go into the small intestines (sometimes going to the gallbladder first)
• Then they go to the ileum, are reabsorbed, go back into the blood (hepatic portal vein) and are recycled back to the liver
• This happens about 5 times per meal

Pancreas Secretions

• The endocrine function secretes hormones into the blood to target all over the body and secretes insulin/glucagon
• The exocrine function secretes something locally, but not into the blood
  • It secretes pancreatic juice (alkaline/basic to balance acidic chyme from the stomach)
  • Enzymes are secreted from pancreatic juice to breakdown all 4 kinds of macromolecules

Large Intestines

• Absorbs water as its main function
• Bacteria flora digests the indigestible carbs and synthesizes B and K vitamins
• Motility moves food around
  • Haustral contractions rely on small pockets that contract
  • Gastrocolic reflex relies on stomach tells the colon it is time to go
  • Mass movement lasts 10-30 minutes and occurs 3-4 times a day triggering a urge to defecate
• Defecation (elimination) needs 2 sphincters in order
  • Internal anal sphincters are smooth muscles that are involuntary with no control of relaxation/contraction
  • External anal sphincters are skeletal muscles and are voluntary and able to fully contract and relax

Macromolecules

• Carbohydrates
  • Monomer: Monosaccharide
  • Subcomponents: C, H, O
  • Function: Fuel for ATP
  • Example: Glucose, fructose, sucrose and starch
• Proteins
  • Monomer: Amino acids
  • Subcomponents: C, H, O, N
  • Function: Catalyze reactions, structural components
  • Examples: Enzymes, hormones, keratin and collagen
• Lipids
  • Monomer: Triglyceride (1 glycerol, 3 fatty acids)
  • Subcomponents: C, H, O
  • Function: Energy storage
  • Examples: Triglycerides, estrogen and cholesterol
• Nucleic acids
  • Monomer: Nucleotides (A, C, T, G)
  • Subcomponents: Phosphate group, 5-carbon sugar, nitrogenous base
  • Function: genetic storage
  • Examples: DNA, RNA

Digestive Enzymes

• Carbohydrates
  • Enzymes: Salivary amylase, pancreatic amylase, brush border enzymes
  • Produced: Salivary glands, pancreas, microvilli
  • Site: Mouth and small intestines
• Proteins
  • Enzymes: Pepsin, pancreatic proteases, brush border enzymes
  • Produced: Stomach, pancreas, microvilli
  • Site: Stomach and small intestines
• Lipids
  • Enzymes: Lipase and bile salts
  • Produced: Pancreas and liver
  • Site: Small intestine
• Nucleic acids
  • Enzymes: Deoxyribonuclease and ribonuclease
  • Produced: Pancreas
  • Site: Small intestine