Digestive System Overview and Oral Cavity Structures

Digestive System Overview

• The digestive system is a continuous work in progress, with many organs performing specific functions for digestion.
• The digestive tract is a long, coiled tube extending from the mouth to the anus, spanning approximately nine meters in length.
• Digestion consists of two phases; Mechanical and Chemical.
  • Mechanical involves breaking food into smaller pieces, pushing it down the food tube, and mixing it with digestive juices.
  • Chemical involves further breaking down larger food molecules into smaller molecules through digestive enzymes.

Oral Cavity Structures

• Lips are muscular structures, predominantly formed by the Orbicularis Oris muscle.
• The oral cavity is responsible for the initial phase of digestion, which includes a small amount of absorption primarily of carbohydrates, specifically starch due to the presence of salivary amylase.

Tongue

• The tongue is a mass of striated muscle covered by mucosa.
• It's a major sensory organ for taste, and one of the essential organs for speech.
• The dorsal surface of the tongue is keratinized while the ventral surface is non-keratinized.

Tongue Papillae & Taste Receptors

• Filiform Papillae = Most numerous, cone-shaped, and the only papillae without taste buds.
• Fungiform Papillae = Mushroom or fungus shape, lightly keratinized, and located on the tip and sides of the tongue.
• Foliate Papillae = Found posterolaterally on the tongue, they have parallel ridges.
• Circumvallate Papillae = Largest, located in a V-shape at the terminal sulcus, and are the least numerous. They also have a serous minor salivary gland known as Von Ebner's gland.

Esophagus

• It's a muscular tube, approximately 25 centimeters long in adults.
• The mucosa has non-keratinized stratified squamous epithelium, and the submucosa contains small mucus-secreting glands.
• The esophageal glands are responsible for lubricating and protecting the mucosa.
• The esophagus has two sphincters: The upper and lower (cardiac) sphincters. These regulate the movement of materials into and out of the esophagus.

Stomach

• It's a J-shaped organ with a lining of simple columnar epithelium with microvilli.
• The stomach is responsible for the second phase of digestion and food storage.
• The stomach contains rugae, longitudinal folds that help with digestion.

Stomach Cells & Functions

• Surface mucous cells = "Foveolar cells", secrete alkaline mucous, and primarily line the gastric mucosa.
• Mucous neck cells = They secrete acidic mucous, which helps protect against stomach acid.
• Parietal or Oxyntic cells = Secrete gastric (hydrochloric) acid, gastroferrin (which binds to iron), and intrinsic factor (essential for vitamin B12 absorption). The parietal cells also help with digestion and activate pepsinogen.
• Chief cells = Secrete pepsinogen, an inactive form of pepsin that digests proteins.
• G-cells = "Enteroendocrine cells", secrete gastrin, which stimulates the parietal cells to produce gastric acid.

Liver

• The liver carries out several essential functions:
  • Albumin synthesis, crucial for maintaining fluid within blood vessels and preventing leakage.
  • Bile production, necessary for emulsification (breaking down fats) and waste removal.
  • Cholesterol metabolism.
  • Detoxification of drugs and alcohol.
  • Excretion.
  • Production of clotting factors.
  • Glycogen storage.
  • Hormone production: Betatrophin, Angiotensinogen, Thrombopoietin, Heparin.

Hepatocytes

• Hepatocytes are the most versatile cells in the liver, considered the functional unit of the liver.
• They perform diverse tasks: protein synthesis, detoxification of toxins, drug and alcohol metabolism, lipid synthesis, and bile production.

Liver Parts

• Lobes: The liver is composed of right and left major lobes, and the smaller caudate (tail) and quadrate lobes.
• Porta (Hilum): Contains the Hepatic Duct (right and left), the Hepatic Portal Vein, and the Hepatic Artery.
  • Kupffer Cells: These are macrophages within the liver.
  • Ito Cells: Also known as "HSC" or "Perisinusoidal cells", they're adipocytes located in the Space of Disse and store vitamin A.

Gallbladder

• The gallbladder is lined by simple columnar epithelium with microvilli.
• It stores and concentrates bile.
• The common bile duct is created by the joining of the cystic duct (from the gallbladder) and the hepatic duct (from the liver).
• The common bile duct empties into the duodenum.
• Rokitansky-Aschoff Sinuses: These are deep diverticula (outpouchings) found in the gallbladder mucosa.
• The gallbladder does not have a submucosa layer, unlike the stomach, small intestines, and large intestines.
• Glands of Luschka: These are developmental abnormalities, microscopic small bile ducts found in the subhepatic or subserosal connective tissue.

Pancreas

• The pancreas is a lobulated gland, divided into head, body, and tail segments.
• Ducts: The pancreas contains the Wirsung (main pancreatic duct) and the Santorini (accessory pancreatic duct).
• Exocrine Part: The exocrine part of the pancreas produces digestive enzymes, specifically proenzymes, which are inactive precursors to enzymes. The exocrine part includes pancreatic Acini and centroacinar cells.
• Sphincter of Oddi: This sphincter controls the release of bile and pancreatic juice into the duodenum.

Small Intestine

• The small intestine is the site where digestion is completed and nutrients are absorbed.
• It is made up of three segments: duodenum, jejunum, and ileum.
• The lining of the small intestine has permanent circular or semilunar folds called Plicae Circulares.
• The small intestine also contains villi.

Cells of the Small Intestine

• Enterocytes: Responsible for nutrient absorption.
• Goblet Cells: Secrete mucus to aid in lubrication and protection.
• Paneth Cells: Immune cells found at the base of intestinal crypts. They secrete lysozyme, which digests bacterial cell walls.
• Enteroendocrine Cells (APUD): Found in intestinal glands, they secrete hormones that regulate digestion.
• M (Microfold) Cells: They sample antigens from the gut lumen and carry them to immune cells.

Large Intestine

• The large intestine, also known as the "bowel," is responsible for water and electrolyte absorption.
• It forms indigestible material into feces.
• The large intestine is composed of the cecum, colon (ascending, transverse, descending), and rectum.
• Colonocytes: Columnar absorptive cells in the colon.
• Teniae Coli: Three longitudinal bands of muscle that help with peristalsis.
• Haustra: Individual segments of the colon that allow for independent contraction.

General Structure of the Digestive Tract

• The digestive tract is a hollow tube with variable lumen diameters and a wall made of four main layers:
  • Mucosa
  • Submucosa
  • Muscularis
  • Serosa

Mucosa

• The mucosa consists of:
  • An epithelial lining.
  • An underlying lamina propria composed of loose connective tissue.
  • A thin layer of smooth muscle known as the Muscularis Mucosae.

Submucosa

• The submucosa contains dense connective tissue, larger blood and lymph vessels, and the submucosal plexus (Meissner's Plexus) of the autonomic nervous system (ANS).
• The submucosa also includes Brunner's glands, which are only found in the duodenum.

Muscularis (Muscularis Externa)

• The muscularis is composed of smooth muscle cells arranged in two or more sublayers:
  • Internal Sublayer (Circular): Closer to the lumen, the fibers are generally circular.
  • External Sublayer (Longitudinal): The fibers run longitudinally.
• The muscularis contains the Myenteric Plexus (Auerbach's Plexus) of the ANS, which controls contractions for peristalsis.

Serosa

• The serosa is a thin layer of loose connective tissue rich in blood vessels, lymphatics, and adipose tissue. It is covered by mesothelium, a simple squamous epithelium.
• In the abdominal cavity, the serosa is continuous with mesenteries, which are covered by mesothelium on both sides and support the intestines.