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• The digestive system supplies the body with fuel and building blocks.
• The digestive tract, also known as the gastrointestinal (GI) tract or alimentary canal, starts in the oral cavity.

Digestive System Components and Functions

• Oral cavity is involved in ingestion and mechanical digestion, supported by accessory organs like teeth and tongue, and saliva for moistening and mixing.
• Pharynx facilitates muscular propulsion of materials into the esophagus.
• Esophagus transports materials to the stomach.
• Stomach performs chemical digestion using acids and enzymes, alongside mechanical digestion through muscular contractions.
• Small intestine is the primary site for enzymatic digestion and absorption of water, organic substrates, vitamins, and ions.
• Large intestine handles dehydration and compaction of indigestible materials for elimination.

Accessory Digestive Organs

• Teeth are responsible for mechanical digestion through mastication (chewing).
• Tongue assists mechanical digestion with teeth and provides sensory analysis of food.
• Salivary glands secrete lubricating fluid containing enzymes for carbohydrate breakdown.
• Liver secretes bile, stores nutrients, and performs many other vital functions.
• Gallbladder stores and concentrates bile.
• Pancreas has exocrine cells that secrete buffers and digestive enzymes, and endocrine cells that secrete hormones.

Peritoneal Cavity Details

• The peritoneal cavity is lined by the peritoneum and consists of two parts: the visceral peritoneum and the parietal peritoneum.
• Peritoneal fluid is present within the cavity, and some organs are supported by mesenteries.
• Mesenteries stabilize the position of organs and provide a pathway for blood vessels, nerves, and lymphatic vessels
• Mesenteries include the falciform ligament (separates the liver into right and left lobes), greater omentum (provides padding, protection, insulation, and energy reserves), and lesser omentum (extends from the liver to the stomach and duodenum)
• The mesentery proper attaches the small intestine (jejunum and ileum) to the posterior abdominal wall.
• The mesocolon is associated with the large intestine, and the duodenum and pancreas are retroperitoneal (behind the peritoneum).
• Intraperitoneal organs are located within the peritoneum, while retroperitoneal organs are located behind the peritoneum.

Generalized Wall of the Digestive Tract

• The general wall of the digestive tract has 4 layers: the mucosa, submucosa, muscularis, and serosa.
• The mucosa is the innermost layer facing the lumen, consisting of an epithelium (simple or stratified) and a lamina propria.
• Lamina propria contains blood vessels, arteries, veins, nerve endings, and scattered lymphatic tissue.
• Muscularis mucosae is a sheet of smooth muscle and elastic muscle.
• Submucosa is made of dense irregular connective tissue, binding the mucosa to the muscular wall.
• The submucosa contains blood vessels, lymphatics, and exocrine glands that secrete buffers and enzymes into the lumen; it is innervated by the submucosal neural plexus..
• Muscularis consists of two layers: an inner circular wall and an outer longitudinal layer.
• The myenteric plexus is a network of nerves that control the muscles of the GI tract, forming the intramural plexus with the submucosal plexus
• Serosa is a serous membrane that covers the digestive tract in the peritoneal cavity.

Esophagus Adaptations

• Esophagus: Has stratified squamous epithelium to withstand abrasion.
• Has two muscularis layers: circular (striated) and longitudinal (smooth).
• Features an outer fibrous layer called adventitia around the thoracic part.

Stomach Adaptations

• Stomach: Has longitudinal folds (rugae) for distension and gastric pits with glands.
• Has three muscularis layers: circular, longitudinal, and oblique.
• Contains two sphincters, the lower esophageal (LES) and pyloric, formed by circular fibers.
• Has an outer visceral peritoneum layer, the serosa.

Small Intestine Adaptations

• Small Intestine: Mucosa has permanent circular folds.
• Muscularis has circular and longitudinal fibers.
• Serosa is continuous with the mesentery.

Large Intestine Adaptations

• Large Intestine: Mucosa features lymph nodes, intestinal glands and anal columns.
• Muscularis has outer longitudinal and circular layers.
• Serosa is continuous with the mesocolon.

Movement in the Digestive Tract

• Motility involves peristalsis (forward movement) and segmentation (mixing).
• Segmentation involves ring-like contractions that mix contents.

Oral Cavity Details

• The buccal cavity is involved in sensory analysis, mechanical digestion, lubrication, and limited chemical digestion.
• The cavity boasts stratified squamous epithelium, keratinized in areas of abrasion.
• Includes hard and soft palates.
• It's equipped with a tongue, buccal fat pads, and buccinator muscle.
• Lips have a philtrum (runs from lip to nose).
• Vestibule is the space between the lips/cheeks.
• Features gingivae (gums)
• Fauces is a narrow opening between the mouth and throat.
• There are 2 pharyngeal arches to each side of the uvula which guards opening to pharynx
• Includes the palatoglossal arch and palatopharyngeal arch (palatine tonsils in between)
• The palatoglossal arch is soft tissue ridge in the throat from the soft palate to the base of the tongue

Tongue Specifications

• The tongue has 4 main functions: mechanical digestion, food manipulation for swallowing, sensory analysis, production and secretion of mucins and lingual lipase.
• The body of the tongue is the oral portion, while the root is the pharyngeal portion.
• There are papillae on the dorsum and a frenulum on the inferior midline.
• Plica fimbriata are fimbriated folds, along with lingual vein.
• There are extrinsic and intrinsic tongue muscles, both controlled by CN XII (hypoglossal).

Teeth Characteristics

• Teeth contain a crown, neck, and root.
• The gingival sulcus margins the gums.
• A periodontal membrane and ligaments provide its support.
• Teeth have a pulp cavity with vessels and nerves involved in mastication.
• There are 4 dental tissues, the innermost is the pulp is a layer with nerves, blood vessels, and connective tissues
• Dentin, underlies the enamel, composing most of the tooth.
• Cementum holds the root in place.
• Enamel on occlusal surfaces is the hard, protective outer layer.
• Types of teeth consist of a dentition of either 20 primary/deciduous teeth or 32/28 permanent teeth, lacking primary premolars and 3rd molars.

Salivary Gland Facts

• There are 3 pairs of major salivary glands secreting into the oral cavity: parotid (25%), sublingual (5%), and submandibular (70%).
• Minor salivary glands secrete mucous.
• Saliva : Contains water, electrolytes, buffers, glycoproteins, antibodies, enzymes, and wastes.
• Regulates antibacterial protection of the mouth.
• Contains salivary amylase and lingual lipase.

Pharynx Composition

• Commonly called the throat, used as a common passageway.
• Stratified squamous epithelium in it's composition
• Has 3 regions: nasopharynx, oropharynx, and laryngopharynx.

Esophagus Properties

• Begins at C6 level, connects to the cricoid cartilage of the larynx, and posterior to the trachea.
• The esophagus is split into cervical, thoracic and abdominal regions.
• passes through esophageal hiatus to get to the stomach
• Mucosa and Submucosa are folded
• Muscularis features both skeletal and smooth muscles.
• Lacks serosa (retroperitoneal).
• It has an sphincter at both of it's ends
• Upper sphincter (UES) prevents air from entering the esophagus.
• Lower esophageal sphincter (LES) prevents food from exiting the esophagus.

Swallowing/Deglutition Process

• Consists of three phases: buccal, pharyngeal, and esophageal.
• First phase involves compression of bolus and retraction of the tongue.
• Second phase includes contact with palatal arches and blocking of nasopharynx passage.
• Third phase is bolus is pushed into the stomach
• This process starts as voluntary, then proceeds automatically in pharynx and esophagus.

Stomach Composition

• The stomach is tubular, in the upper left quadrant. It stores, digests (mechanical/chemical), and forms chyme(Food bolus, partially digested food, gastric juices)
• Lesser curvature runs from cardia to the pylorus, whereas greater curvature is on the lateral surface.
• There are 4 regions: cardia, fundus, body, and pyloric.
• Other features include the pyloric sphincter and rugae.

Gastric Histology

• Stomach histology includes simple columnar epithelium, gastric pits, and gastric glands.
• Parietal cells produce HCl and carbonic acid and transport bicarbonate and hydrogen, with chloride shift.
• Chief cells secrete digestive enzymes.

Stomach Digestion and Absorption

• Enables continued carbohydrate digestion, preliminary protein digestion, and absorption of drugs/alcohol.
• Absorption levels are generally low in the stomach

Small Intestine Details

• Location of completed chemical digestion and product absorption
• This organ consists of 3 segments: duodenum (25 cm/10 in), jejunum (2.5 m/8.2ft), and ileum (3.5 m/11.5ft).

Small Intestine Wall

• Transitions into the large intestine via ileocecal valve.
• Wall features circular folds (plicae circulares) to increase surface area; intestinal villi that expand simple columnar epithelia with microvilli (brush border enzymes) and lacteals.
• Mucosa in regional specialization and features Intestinal and Duodenal glands for control

Intestinal Control Factors

• Neural, hormonal, duodenal submucosal glands
• Involves local, parasympathetic, myenteric and enterogastric reflexes.
• Hormones may enchance or suppress reflexes when related to digestion.

Pancreas Profile

• Positioned posterior to the stomach, with the head in the duodenal bend and a 15 cm body/tail.
• It joins the bile duct and enters the duodenum.
• Acinar cells of the pancreas, the islets only account for approximately 1%.
• It produces digestive enzymes controlled by hormones
• Pancreatic enzymes do most of the digestive work aided by Secretin and Cholecystokinin (CCK)

Liver Parameters

• The liver is the largest visceral organ and has four lobes.
• Ligaments include, falciform hilum(porta hepatitis)
• Each lobe gets dived into 100,000 lobules which are hexagonally shaped
• The parenchyma forms plates arranged with sinusoids (draining into central vein) and Kupffer cells.
• Bile is produced in bile canaliculi which combine with bile ductules and combine with right and left hepatic ducts that flow either direclty or indirectlty via the cystic duct to the duodenal via major duodenal papilla
• It holds 3 categories, metabolic and hematological regulation and bile production

Gallbladder Highlights

• Hollow and pear-shaped, attached to the posterior right liver lobe.
• Responsible for the secretion, and concentraion of bile.

Large Intestine Aspects

• The organ stores and compacts waste, and has an expanded pouch (cecum), colon, and rectum region.
• Absorption of water/vitamins from bacteria
• Large intestine is in segments with small connectives (mesentery), that separate larger segments (colon) divided into a certain direction: Ascending → Transverse → Descending → Sigmod.
• Has anal canal, consisting of hemorrhoid plexus. Internal with smooth muscle for involuntary process. While Exteryal has a skeletal muscle, involoving consciou control

Elimination Rundown

• Feces exit from the body that results in stimulation of receptors in rectal mucosa caused by Distention of rectum.
• Reflexive relaxation of internal sphincter, including Contraction of colonic smooth muscle
• The residues of the process consist mostly cellulose, with some undigested connective tissues, fats, and bacteria.

Large Intestine Histology & Physiology

• Lamina propria contains large lymphoid nodules, reduced amounts Teniae coli, few microvilli, abundant goblet cells and distinct intestinal glands.
• Absorption and reabsorption of water, vitamins, salts, and ~10% nutrient recovery happens in the large intestine.

Absorption Types

• Includes absorption by transcellular means (across cells) and paracelluar means (passive)
• Mostly occurs in the small intestine but not entirely

Carbohydrate Digestion

• Starts in the mouth with salivary amylase, finishes in the brush border of the small intestine with pancreatic alpha-amylase aided by brush border enzymes to break carbs in monosaccharides

Lipids Digestion

• Breakdown begins in the mouth then in the small intestine. The process includes the involvement of water soluble enzymes interacting with insoluble lipids. Bile salts emulsify the lipids forming micelles able to diffuse for easy synthesis.

Protein Digestion

• Mechanical (oral), chemical processing (stomach) which helps expose peptides to enzymes
• Acidic pH is required for optimal protein breakdown
• The peptides must traverse the duodenum with the help of carboxylpepitdases.
• Epithelial must chain must be shortened to amino acids for optimal results which goes through the hepatic portal vein.

Nuclei Acid Conversion

• Brush border enzymes involved to convert constituents through active transport

Water, lons, and Vitamin Absorption

• Water is absorbed by osmosis gradient , especially in small intestine, but even more in the large intestine
• ions- goes through multiple transports
• Vitamins goes throuh water solublity or fat solubility