Plant anatomy

• The digestive system has two main components: the Gastrointestinal (G.I.) Tract, and accessory structures and organs.

G.I. Tract

• Also the alimentary ('nourishment') canal, it extends from the oral cavity to the anus, where waste is expelled as fecal matter.

Accessory Structures and Organs

• Includes the salivary glands, mucous glands, tongue, teeth, liver, gallbladder, and pancreas, and all provide important functions in the digestive system.
• Assist the tract by secreting enzymes or substances to help transform, digest, absorb or transport food as it travels along the tract.

Primary Purpose of the Gastrointestinal Tract

• It is to break down large nutrients (polymers) into smaller units (monomers) for absorption and energy.

Basic Digestive Processes

• Ingestion: Taking food or drink into the mouth or oral cavity.
• Propulsion: Movement through the alimentary canal, involving the tongue, cheeks, and muscle contractions.
• Mechanical Digestion: Physical breakdown of food, including chewing and churning, to prepare for chemical digestion.
• Chemical Digestion: Enzymatic breakdown of food from complex to simple building blocks.
• Absorption: Transportation of digested products from the G.I. tract lumen across an epithelial lining into the blood and lymph vessels.
• Defecation: Elimination of indigestible material and waste products from the body (feces).
• Mechanical digestion mainly occurs in the mouth and stomach.
• Chemical (enzymatic) digestion begins in the stomach (protein digestion only) and becomes significant for all nutrients in the small intestine.
• The large intestine reabsorbs excess water and electrolytes.
• Undigested material and waste products are excreted via defecation.

Alimentary Canal

• A hollow, compartmentalized and specialized tube starting at the oral cavity and ending at the anus.

Oral Cavity

• The hollow space formed by the lips, cheeks, tongue, hard, soft palates, and throat allowing food, liquid, and air to enter the body.
• The mouth contains teeth, tongue, and the ducts of the extrinsic salivary glands, aiding in ingestion and digestion to produce speech.
• The lips contain collagen, elastin fibers, and adipose tissue covered by a thin layer of keratinized stratified squamous epithelium.
• The cheeks have layers of connective tissues, nerves, and muscles, including the buccinator, orbicularis oris, depressor anguli oris, and zygomaticus major which assist in the movement of the lips and cheeks.
• The tongue is an organ made of epithelium, skeletal muscles, nerves, and connective tissues.
• It contains papillae for gripping food and taste buds for detecting chemicals in food.
• It also helps to produce speech by altering or stopping the flow of air through the mouth and teeth to produce the sounds of many consonants.
• The hard palate is formed by the maxillae and palatine bones, while the soft palate is a flexible fleshy mass that ends in the uvula.
• These palates separate the mouth from the nasal cavity
• The teeth are hard organs specialized for biting (mastication) and grinding and converting food into tiny pieces.

Teeth Components

• Divided into the crown (above the gum line) and the root (below the gum line).
• Anchored into a bony socket (alveolus) by the cementum.
• Consists of three layers: pulp, dentin, and enamel.
• Adults typically have 32 permanent teeth arranged with 8 teeth in 4 quadrants: 2 incisors, 1 canine, 2 premolars, and 3 molars.
• Babies develop 20 deciduous teeth that are replaced by the permanent teeth.

Salivary Glands

• Saliva contains lingual amylase, lysozyme, water and mucous.
• Intrinsic salivary glands are small and located inside the oral cavity embedded in the lips and cheeks.
• Extrinsic salivary glands have ducts delivering secretion to the oral cavity; their secretory portions are rounded and called acini.
• The largest salivary gland is the parotid gland which produces 25% of saliva and is located anteriorly and inferiorly to the ear
• The serous secretions are rich in proteins like immunoglobins and a-amylase.
• The submandibular gland secretes 70% of the saliva and produces a more viscid secretion, rich in mucin.
• The smallest salivary gland is the sublingual gland, a mucus gland that produces only 5% of saliva and their secretions provide buffers and lubrication.

Pharynx

• The throat is a funnel-shaped tube posterior to the mouth connecting the nasal cavity and mouth to the esophagus and larynx in the neck.
• The oropharynx is the posterior wall of the mouth.
• Air inhaled through the mouth or nose also passes through the pharynx to the larynx.

G.I. Tract Layers

• The hollow G.I. tract has four distinct layers.

Tunica Mucosa

• The innermost layer of the digestive tract in direct contact with ingested nutrients.
• Composed of specialized epithelial cells, the lamina propria (areolar connective tissue), and the muscularis mucosae.
• Can be stratified (for protection) or simple (for absorption).

Tela Submucosa

• Deep to the muscularis mucosa and contains areolar connective tissue with blood vessels, nerves, adipose (fat), and fibrous connective tissue.
• Contains the submucosal (Meissner) plexus.

Tunica Muscularis Externa

• Contains inner circular and outer longitudinal smooth muscle layers separated by the Myenteric Plexus, which controls muscle contraction and peristalsis.

Tunica Serosa or Tunica Adventitia

• The outermost layer of the G.I. tract is a serous membrane (tunica serosa) or a fluffy fibrous connective tissue (tunica adventitia).

Oral Cavity

• Lined by a stratified squamous oral mucosa with keratin in areas subject to abrasion.
• Insalivation' refers to the mixing of oral cavity contents with salivary gland secretions
• The final function of the oral cavity is absorption of small molecules such as glucose and water, across the mucosa
• Food passes through the pharynx and esophagus via the action of swallowing.

Esophagus

• The esophagus is a muscular tube approximately 10 inches long and transports the bolus from the mouth to the stomach.
• Just proximal to the gastro-esophageal junction is the lower esophageal (cardiac) sphincter to prevents regurgitation of stomach contents into the esophagus.
• The muscular wall transitions from skeletal to smooth muscle along its length.

Stomach

• A J shaped expanded bag, located just left of the midline between the esophagus and small intestine.
• It is divided into four main regions and two borders called the greater and lesser curvatures.
• Regions include the cardia, fundus, body, and pylorus; gastric glands are mostly in the body.
• The inner surface has folds called rugae, which allow for expansion which allow the stomach to stretch and expand when food enters
• Functions include the short-term storage of ingested food, mechanical breakdown of food by churning and mixing motions and the chemical digestion of proteins by acids and enzymes.
• It has an acidic (pH 2) environment for storage of chyme.
• Unique three muscle layers (oblique, circular, longitudinal) assist churning.
• The enzymatic digestion of proteins starts here.
• Nutrient absorption does not occur in the stomach, only alcohol and aspirin are absorbed.
• Parietal cells make hydrochloric acid (HCl) and Chief cells make Pepsinogen and secrete these into gastric glands.

Small Intestine

• Extends for about 20 feet from the pyloric sphincter to the ileocecal valve.
• The duodenum receives digestive secretions from the pancreas and bile from the liver.
• Most digestion and absorption occur in the jejunum.
• The ileum is the final and longest segment, continuing nutrient absorption and empties into the cecum at the ileocecal junction.

Small Intestine Modifications

• The small intestine is made up of plicae circulares, villi, and microvilli (brush border) which increases the surface area for absorption.
• The mucosa contains specialized cells for absorption and secreting digestive enzymes/mucous.
• Mesentery Proper supports branches of blood vessels, lymphatics, and nerves.

Large Intestine

• Extends around the periphery of the small intestine and consists of the appendix, cecum, colon, and rectum.
• The cecum is the expanded pouch that receives material from the ileum.
• The colon has haustra held under tension by taenia coli.
• The rectum expands to hold fecal matter before elimination.
• The mucosa is flat with deep intestinal glands and goblet cells to lubricate fecal matter.
• Functions include reabsorption of water and compaction of feces, absorption of vitamins and electrolytes (including vitamin K), and storage of fecal matter.
• The cecum contains the Ileocecal valve, ascending, transverse, descending and the sigmoid colon.

Peritoneum

• Two layers, with serosa being the Visceral peritoneum (a.k.a. serosa) and the Parietal peritoneum lines inner surfaces of abdominal wall creating a cavity within the abdomen.
• Types include mesenteries, greater omentum and mesocolon.
• Mesenteries are fused double sheets of peritoneal membrane to suspend portions of digestive tract:
• Greater omentum covers drapes over all of the intestines and other abdominal the "apron of fat" providing insulation and protection.
• Lesser omentum tethers from the lesser curvature of the stomach connecting to the liver
• Mesentery proper supports the small intestine.
• Mesocolon supports the colon.

Liver

• The liver is a large, reddish-brown organ located in the right upper quadrant of the abdomen and is divided into four lobes.
• Functions as a mechanical filter, detoxifies several metabolites, and produces albumin, blood clotting factors, and bile.
• Metabolism of nutrients, all nutrients absorbed by the intestines must pass through the liver.
• Kupffer cells hang around in the sinusoids in the liver.

Gallbladder

• Stores and concentrates bile, releasing it when triggered by lipid ingestion.
• Does not make bile.
• Bile is made by hepatocytes of the liver and is composed of mostly of cholesterol salts for the emulsification of ingested fats in the intestines.

Pancreas

• A lobular, pinkish-grey organ behind the stomach with both exocrine and endocrine functions.
• Exocrine portion secretes pancreatic juices containing amylases, lipases, proteases, and nucleases into the duodenum.
• This fluid (pancreatic juices) is rich in carbohydrates and inactive enzymes.
• Endocrine function involves hormone production (insulin, glucagon) in the Islets of Langerhans.