The alimentary canal, also known as the digestive tract, is a continuous tube from the oral cavity to the anus.
Organs include the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
The gastrointestinal tract refers specifically to the stomach and intestines.
Accessory Organs
Accessory organs aid in digestion.
Functions of the Digestive System
Ingestion: Taking food and liquids into the oral cavity.
Digestion: Breaking down food into smaller pieces.
Mechanical Digestion:
Physically breaking down food to increase surface area for chemical digestion.
Examples:
Mastication (chewing).
Churning of the stomach.
Segmentation (contractions of the inner circular layer of the muscularis externa in the small intestine to mix contents).
Chemical Digestion:
Breaking chemical bonds in macromolecules using enzymes.
Macromolecules (e.g., proteins, starch, glycogen) are broken down into monomers (e.g., amino acids, glucose).
Propulsion: Moving contents through the alimentary canal.
Deglutition (swallowing): Moving food from the oral cavity to the pharynx.
Peristalsis: Alternating contractions of the inner circular and outer longitudinal layers of the muscularis externa to push food down the digestive tract.
Secretion: Cells producing and releasing substances.
Enzymes, saliva, hydrochloric acid.
Absorption: Bringing substances from outside cells into cells, blood, and the lymphatic system.
Water, vitamins, and nutrients are absorbed into the blood.
Lipids are absorbed into the lymph.
Elimination: Removing undigested and unabsorbed material through defecation.
Layers of the Digestive Tract
Typical layers include the mucosa, submucosa, muscularis externa, and serosa/adventitia.
Mucosa Layer (Mucous Membrane)
Innermost layer closest to the lumen.
Sublayers:
Epithelium: Varies depending on the organ (e.g., nonkeratinized stratified squamous in the esophagus, simple columnar in the stomach and small intestine).
Lamina Propria: Areolar connective tissue.
Muscularis Mucosae: Smooth muscle that can create ridges and grooves to increase surface area.
Mucosa-associated lymphatic tissue (MALT): Lymphocytes to fight pathogens.
Submucosa Layer
Connective tissue with blood vessels, lymphatic vessels, nerve plexuses, and glands (depending on the organ).
Also contains MALT.
Muscularis Externa
Typically two layers of smooth muscle (but can vary):
Inner Circular Layer: Can form sphincters to prevent backflow or aid in peristalsis and segmentation.
Outer Longitudinal Layer: Pushes food down the alimentary tract.
Serosa or Adventitia
Serosa: Serous membrane made of simple squamous epithelium and areolar connective tissue; also known as the visceral peritoneum in the abdominal pelvic cavity.
Visceral Pericardium - outer surface of the heart.
Visceral Pleura - outer surface of the lungs.
Adventitia: Connective tissue that connects one organ to another; found in the pharynx, most of the esophagus, and the rectum.
Smooth Muscle
Smooth muscle cells contain dense bodies similar to Z discs for myofilament attachment.
Types of smooth muscle:
Single-Unit (Visceral): Contains gap junctions for communication; autorhythmic.
Multi-Unit: Requires individual neuron innervation; found in the iris, ciliary body, and erector pili muscles.
Calcium source: Sarcoplasmic reticulum and extracellular fluid.
Enteric Nervous System
Intrinsic nervous system regulating movement and secretion.
Influenced by the parasympathetic nervous system.
Submucosal Plexus: located in the submucosal layer; controls glands in the submucosa and muscularis mucosa.
Myenteric Plexus: located between the inner circular and outer longitudinal layers of the muscularis externa; controls the muscularis externa.
Peritoneum
Serous membrane lining the viscera and walls of the abdominal cavity.
Parietal Peritoneum: Lines the abdominal walls.
Peritoneal Cavity: Space containing peritoneal fluid to reduce friction.
Visceral Peritoneum: Serosa; outer layer of organs.
Retroperitoneal Organs: Behind the peritoneum (e.g., duodenum, pancreas, parts of the colon, rectum).
Mesenteries
Structures made of peritoneum.
Falciform Ligament: Connects the liver to the abdominal wall and diaphragm.
Greater Omentum: Apron-like structure hanging from the greater curvature of the stomach; contains blood vessels and fat.
Lesser Omentum: Connects the lesser curvature of the stomach to the liver.
Mesentery Proper (Mesentery): Connects the small intestine and contains blood vessels.
Mesocolon: Holds the colon to the posterior abdominal wall.
Transverse Mesocolon: Holds the transverse colon.
Sigmoid Mesocolon: Holds the sigmoid colon.
Oral Cavity
Where food enters the mouth; site of ingestion.
Made of nonkeratinized stratified squamous epithelium.
Structures: Labia (lips), buccae (cheeks), palate (roof of the mouth), tongue (floor of the mouth).
Functions:
Ingestion.
Gustation (taste) and touch.
Mastication (chewing): Mechanical digestion to increase surface area.
Chemical Digestion:
Salivary amylase: Breaks down starch. amylase→starch
Lingual lipase: Breaks down triglycerides (fats) at low pH. lipase→triglycerides
Propulsion: Deglutition (swallowing).
Respiration.
Organs within the Oral Cavity
Labia:
Made of thin keratinized stratified squamous epithelium, giving a reddish appearance.
Buccae: Hold food in.
Teeth: Support and are used for chewing.
Tongue:
For gustation (taste), making sounds (speech), mastication (chewing).
Palate: Roof of the mouth.
Hard Palate: Anterior portion made of the maxilla and palatine bones.
Soft Palate: Posterior portion that closes off the nasopharynx.
Uvula: Prevents food from entering the nasal pharynx and produces certain sounds.
Fauces: Passageway from the oral cavity to the oropharynx.
Salivary Glands
Accessory organs that produce saliva.
Saliva Composition:
Water, enzymes, electrolytes, etc.
Bolus: Food mixed with saliva.
Serous Acini: Cells that produce water, digestive enzymes (salivary amylase, lingual lipase), electrolytes, and defensive secretions.
Mucous Acini: Cells that produce mucus for lubrication.
Types of Salivary Glands:
Parotid Gland: Lateral side of the face; mostly serous acini.
Submandibular Gland: Below the mandible; both serous and mucous acini; produces the most saliva.
Sublingual Gland: Under the tongue; mostly mucous acini; produces the least amount of saliva.
Teeth
Accessory digestive organs used for mastication (chewing) to increase surface area for chemical digestion.
Regions of the Tooth:
Crown: Visible part.
Neck: Within the gingiva (gums).
Root: In the alveolar parts of the maxilla and mandible.
Structures of the Tooth:
Enamel: Hardest substance in the body, made of calcium phosphate crystals.
Cementum: Outer surface of the neck and root.
Dentin: Contains tubules that connect to the pulp.
Pulp Cavity: Space containing nerves, blood vessels, and lymph vessels; connects to the root canal.
Periodontal Ligament: Dense regular connective tissue that forms a gomphosis joint between the tooth and the alveolus.
Types of Teeth:
Primary (Deciduous) Teeth: 20 teeth.
Incisors (Central and Lateral): Flat teeth for cutting.
Canines (Cuspids): Sharp teeth for puncturing.
Molars: Cusps for grinding.
Secondary (Permanent) Teeth: More than 20 teeth.
Incisors (Central and Lateral): for cutting.
Canines (Cuspids): for puncturing.
Premolars (Bicuspids): Cusps for grinding.
Molars: Cusps for grinding, including the third molar (wisdom tooth).
Pharynx
Food passes from the oral cavity to the pharynx.
Regions:
Oropharynx: Food and liquids pass through first.
Laryngopharynx: Next region food passes through.
Esophagus
Peristalsis moves the bolus down to the stomach.
Made of nonkeratinized stratified squamous epithelium to protect against abrasion.
Contains both skeletal and smooth muscle.
Lower Esophageal Sphincter (Cardiac Sphincter): Prevents backflow of stomach contents into the esophagus.
Gastroesophageal Reflux Disease (GERD): Acidic gastric juices erode the esophagus.
Submucosal area: esophageal glands for lubrication
Muscularis Externa: Upper third is skeletal muscle for deglutition, middle third transitions from skeletal to smooth muscle, and lower third is all smooth muscle for peristalsis.
Most of the esophagus has adventitia, but the portion in the abdominal cavity has serosa.
Stomach
Food mixed with gastric juices becomes chyme.
Mechanical Digestion: Churning due to the inner oblique layer of the muscularis externa.
Chemical Digestion:
Gastric lipase: Digests lipids (triglycerides).
Pepsinogen (becomes pepsin): Digests proteins.
Lingual lipase: Continues chemical digestion.
Hydrochloric acid: Breaks down hydrogen bonds and Van der Waals forces within proteins, denaturing them.
Storage of Food: Approximately four hours.
Some Absorption: Aspirin and lipid-soluble drugs (including alcohol).
Regions:
Cardiac region.
Fundus/Fundic region.
Body.
Pylorus/Pyloric region: Includes the pyloric antrum and pyloric sphincter (which controls food entry into the small intestine).
Greater Curvature, Greater Omentum hangs from here.
Lesser Curvature - site of the left gastric artery.
Rugae: Folds that allow the stomach to expand.
Layers:
Mucosa: Simple columnar epithelium with mucous cells to protect the stomach from enzymes and hydrochloric acid.
Gastric Pits and Glands: Important for identifying the stomach.
G Cells (Enteroendocrine Cells): Secrete gastrin to promote digestion.
Chemical Digestion of brush border enzymes and pancreatic enzyme secretions.
Most absorption of nutrients and water takes place in small intestine.
Regions:
Duodenum: Short segment; contains the greater and lesser duodenal papilla; Brunner's glands/duodenal glands in the submucosa produce alkaline mucus to neutralize acidity.
Jejunum: Most chemical digestion and absorption occur here; no distinguishing features in the submucosa layer.
Ileum: Contains Peyer's patches in the lamina propria to fight bacteria; ileocecal valve connects to the cecum.