Digestive System Overview

Digestive System Part 2

• Oral Vestibule: Space anterior to teeth and posterior to cheeks and lips. When you puff out your cheeks, this space is filled.

• Oral Cavity Proper: Teeth and tongue are found here. Teeth and tongue participate in mechanical breakdown of food by chewing (mastication).

• Taste Buds: Found on the surface of the tongue, they help detect harmful substances. Taste receptors are found around projections called papillae on the tongue.

• Papillae: Projections on the tongue where taste buds are found. There are four types of papillae:

  • Filiform papillae: Sharp edges, like waves at a beach.
  • Foliate papillae: Boxy in shape, with three folds of lamina propria.
  • Fungiform papillae: Mushroom-shaped, projecting upward and expanding at the top.
  • Circumvallate papillae: Bubbly, extending below the surface of the tongue.

Teeth

• Primary Teeth (Deciduous): First set of teeth that come in, also known as milk teeth.
• Permanent Teeth: Second set of teeth, formed between the mandible and maxilla, and are not lost.
• Teeth Structure: Crown (superior), Neck (middle), and Root (inferior) make up the tooth structure.
• Layers of Teeth: Enamel (crown), Dentin (bony region, found in crown, neck, and root), Pulp Cavity (contains blood vessels and nerves, found in crown), and Cementum (attaches tooth to alveolus).

Pharynx

• Pharynx: Region behind the nose, shared by the respiratory and digestive systems. Breath air through the pharynx and swallow food through the pharynx.
• Three Parts of Pharynx: Nasopharynx (upper), Oropharynx (middle), and Laryngopharynx (lower).
• Epiglottis: A cartilage that protects the larynx from food, preventing it from entering the respiratory tract.
• Bolus: A small mass of chewed food that is swallowed.

Esophagus

• Esophagus: Part of the GIT in the thoracic cavity, responsible for pushing food from the pharynx to the stomach through peristalsis.
• Layers of Esophagus: Mucosa (non-keratinized stratified squamous epithelium), Submucosa, Muscularis Externa (same inner circular and longitudinal layers), and Adventitia (outermost layer, special to the esophagus).

Stomach

• Stomach: Receives the bolus from the esophagus and is responsible for chemically and mechanically breaking down food.
• Four Areas of Stomach: Cardia (receives bolus), Fundus (pouches upward), Body, and Pylorus (funnel-shaped).
• Two Sphincters: Cardiac Sphincter (prevents stomach contents from flowing back into the esophagus) and Pyloric Sphincter (contracts to prevent stomach contents from leaving before being fully digested).
• Stomach Layers: Mucosa (simple columnar epithelium with microvilli), Submucosa, Muscularis Externa (three layers: outer longitudinal, middle circular, and inner oblique), and Serosa (visceral peritoneum).

Gastric Glands

• Chief Cells: Secrete pepsinogen, an inactive form of the enzyme pepsin.
• Parietal Cells: Secrete HCl, which mixes with pepsinogen to form pepsin, an active enzyme that breaks down protein.

Small Intestine

• Duodenum: First part of the small intestine, receives very acidic chyme from the stomach.
• Jejunum: Middle part of the small intestine.
• Ileum: Last part of the small intestine, has lymph nodes under the mucosa and submucosa.
• Adaptations for Absorption: Length, folds, and villi increase the surface area for absorption.
• Villi: Folds of mucosa and submucosa, composed of epithelium and lamina propria, increase the surface area for absorption.
• Microvilli: Projections of plasma membrane and cytoplasm on top of columnar cells, further increasing the surface area for absorption.
• Lacteal or Lymphatic Capillaries: Found in each villus, act as screeners to ensure only nutrients are absorbed.

Peritoneum

• Peritoneum: Lines the abdominal cavity, made up of simple squamous epithelium and loose areolar connective tissue.
• Parietal Peritoneum: Lines the abdominal cavity, separates the abdominal cavity from the pelvic cavity.
• Visceral Peritoneum: Lines multiple organs, double-layered folds.
• Lesser Omentum: Connects the liver to the lesser curvature of the stomach.
• Greater Omentum: Connects the greater curvature of the stomach to the transverse colon.
• Mesentery: Suspends the small intestine to the posterior abdominal wall.

Accessory Glands

• Salivary Glands: Produce saliva that breaks down carbohydrates.
• Parotid Gland: Produces enzymes that break down carbohydrates.
• Sublingual Gland: Produces mucus that lubricates the bolus.
• Submandibular Gland: Produces a combination of mucous and serous secretions.

Pancreas

• Pancreas: Found in the abdomen, retroperitoneal, and has three parts: head, neck, and tail.
• Endocrine Function: Produces hormones like glucagon and insulin to regulate blood glucose levels.
• Exocrine Function: Produces digestive enzymes that empty into the duodenum.
• Pancreatic Ducts: Two ducts: the main pancreatic duct (duct of Wirsung) and the accessory pancreatic duct (duct of Santorini).

Gallbladder

• Gallbladder: Stores bile, which is produced by the liver.
• Histology: Similar to the GIT, with a mucosa, muscular layer, and serosa.
• Layers: Mucosa (simple columnar epithelium with microvilli), Muscular layer (smooth muscle), and Serosa (in the abdominal cavity and peritoneal cavity).

Biliary Tract

• Biliary Tract: Starts at the liver, where two ducts (right and left hepatic ducts) combine to form the common hepatic duct.
• Common Bile Duct: Formed by the combination of the common hepatic duct and the cystic duct.
• Ampulla of Vater: The common bile duct and pancreatic duct combine here, regulated by the sphincter of Oddi.

Liver

• Liver Functions: Produces bile, removes aged RBC, WBC, and bacteria from the blood, and regulates blood glucose levels.
• Lobes: Right, left, quadrate, and caudate.
• Hepatic Lobule: Hexagonal, with rows of hepatocytes (liver cells) arranged radially around a central vein.
• Hepatocytes: Filter blood coming through, removing toxins and waste products.
• Sinusoids: Blood passes through these rows of hepatocytes, getting filtered into the central vein.
• Kupffer Cells: Phagocytic cells that line the sinusoids, removing pathogens and debris from the blood.
• Portal Triad: Found at the corners of the hepatic lobule, contains branches of three vessels: the portal venule, hepatic arteriole, and bile duct.### Overview of the Digestive System
• The digestive system consists of organs that make up the gastrointestinal tract (GIT) and additional organs that function in the digestive system but are not part of the GIT.

Mouth Structures

• The mouth contains salivary glands, tongue, and teeth.
• Salivary glands produce salivary enzymes that break down food chemically.

Gastrointestinal Tract

• The GIT consists of the pharynx, esophagus, stomach, small intestine, large intestine, and anus.
• Food passes through the GIT, undergoing mechanical and chemical breakdown.

Functions of the Digestive System

• Ingestion: the function of the mouth, tongue, and teeth to bring food into the body.
• Digestion: the breakdown of food into smaller molecules.
• Mechanical breakdown: occurs through chewing food with teeth, smushing food with the tongue, and muscle contractions.
• Chemical breakdown: occurs through salivary enzymes produced by salivary glands.
• Absorption: passes through the small intestine (main organ of absorption) and large intestine.
• Storage of feces and release of feces (defecation) occur in the large intestine.
• Peristalsis: muscle contractions that push food through the GIT.

Types of Muscular Contractions

• Peristalsis: sequential contractions along the path of the organ, from esophagus to small intestine.
• Segmentation: contractions occur in segments along the organ at the same time, pushing food in both directions and mixing it.

Peritoneum

• The peritoneum lines the abdominal cavity and is made of simple squamous epithelium and loose areolar connective tissue.
• It wraps around organs and has two areas: parietal peritoneum and visceral peritoneum.
• Parietal peritoneum: lines the abdominal cavity, sits above pelvic organs, and separates the abdominal cavity from the pelvic cavity.
• Visceral peritoneum: lines multiple organs and has double-layered folds.

Retroperitoneal Structures

• Structures found behind the peritoneum, not in the peritoneal cavity.
• Examples: lesser omentum, greater omentum, and mesentery.

General Histology of the Gastrointestinal Tract

• The wall of the GIT has four layers from the esophagus to the anal canal.
• The four layers are: mucosa, submucosa, muscularis externa, and serosa (not mentioned in the text).
• Mucosa: consists of epithelium, lamina propria, and muscularis mucosa.
• Submucosa: consists of connective tissue, glands, lymphatics, nerves, arteries, and veins.
• Muscularis externa: consists of inner circular layer and outer longitudinal layer of smooth muscle.