Organs Associated with the Digestive Tract

ORGANS ASSOCIATED WITH THE DIGESTIVE TRACT

• Overview: The organs associated with the gastrointestinal tract (GIT) facilitate transport and digestion of food.

Salivary Glands

• Function:

  • Moisten and lubricate ingested food and oral mucosa.

  • Initiate digestion of carbohydrates and lipids.

  • Secrete immune components (e.g., lysozyme, lactoferrin).

Pancreas

• Function:

  • Produces digestive enzymes acting in the small intestine.

  • Secretes hormones important for metabolism of absorbed nutrients.

Liver

• Function:

  • Produces bile (stored in the gallbladder) essential for fat digestion and absorption.

  • Plays a role in carbohydrate and protein metabolism.

  • Inactivates toxins and drugs.

  • Synthesizes plasma proteins and blood coagulation factors.

SALIVARY GLANDS

• Major Glands:

  • Parotid Gland

  • Submandibular Gland

  • Sublingual Gland

Structure of Salivary Glands

• Characteristics:

  • Encapsulated exocrine glands in the mouth.

  • Composed of secretory units on a branching duct system, arranged in lobules separated by connective tissue septa.

• Types of Salivary Glands:

  • 3 Major Salivary Glands: Parotid, Submandibular, Sublingual.

  • Numerous intrinsic minor salivary glands throughout oral mucosa.

CELL TYPES IN SALIVARY GLANDS

1. Serous Cells

• Characteristics:

  • Polarized protein-secreting cells, usually pyramidal with round nuclei.

  • Secretes enzymes and proteins, forming the spherical unit "ACINUS".

2. Mucous Cells

• Characteristics:

  • Columnar shape with compressed basal nuclei.

  • Contains granules with mucins for lubrication.

3. Myoepithelial Cells

• Characteristics:

  • Found in basal lamina of acini and ducts; contractile processes help move secretory products.

DUCT SYSTEM

• Components:

  • Intralobular duct, secretory acini, and tubules empty into intercalated ducts.

  • These merge into striated ducts that reabsorb Na+ ions.

  • All striated ducts converge to form an excretory duct.

MAJOR SALIVARY GLANDS

1. Parotid Gland

• Location: Near the ear, in each cheek.

• Structure: Branched acinar glands with serous acini.

• Products: Secretes abundant α-amylase for carbohydrate digestion.

2. Submandibular Glands

• Structure: Branched tubuloacinar glands with serous acini and mucous tubular cells arranged in serous demilunes.

• Products: Secretes α-amylase and lysozyme.

3. Sublingual Glands

• Description: Smallest major glands, predominance of mucous cells.

• Products: Mucin, α-amylase, and lysozyme.

PANCREAS

• Type: Mixed exocrine-endocrine gland producing digestive enzymes and hormones.

• Anatomy: Elongated retroperitoneal organ with head near duodenum and a tail extending toward the left.

• Capsule: Thin capsule with septa separating parenchyma into lobules.

EXOCRINE PANCREAS

Structure

• Serous Acini:

  • Lacks myoepithelial cells and striated ducts; produces digestive enzymes.

  • Polarized cells with round basal nuclei and zymogen granules.

Function

• Fluid Secretion:

  • Drained by intercalated ducts secreting HCO3-; helps alkalinize and transport enzymes.

• Pancreatic Juice:

  • 1.5L of alkaline juice daily, neutralizes acidic chyme and optimizes pH for enzyme activity.

Digestive Enzymes

• Types:

  • Proteases: Trypsinogen, Chymotrypsinogen, α-amylase, Lipase, Nucleases.

• Protection Mechanisms:

  • Limiting protease activation to the duodenum; secretion of trypsin inhibitors and maintaining higher pH in acini.

HORMONES REGULATING EXOCRINE PANCREAS

1. Cholecystokinin:

   • Stimulates enzyme secretion by acinar cells from the small intestine.

2. Secretin:

   • Promotes water and HCO3- secretion by the duct cells.

ENDOCRINE PANCREAS

Pancreatic Islets

• Description: Spherical masses of endocrine cells within acinar exocrine tissue.

• Cell Populations:

  • α or A cells: Secrete glucagon, increase blood glucose.

  • β or B cells: Produce insulin, decrease blood glucose.

  • δ or D cells: Secrete somatostatin; inhibits hormones and gastric secretion.

LIVER

• Size: Largest internal organ located in the right upper quadrant of the abdomen.

• Function:

  • Production of bile, plasma proteins, detoxification, gluconeogenesis, nutrient storage.

Hepatocytes

• Major Functions:

  • Production of bile, major plasma proteins, detoxification, and storage of vitamins and minerals.

HEPATIC LOBULE

• Structure: Basic functional unit composed of hepatocytes, central vein, and portal triad (portal vein, hepatic artery, bile duct branch).

Vascular Sinusoids

• Located between hepatocytes, lined by fenestrated endothelial cells, serve blood flow toward the central vein.

Space of Disse

• Site of nutrient uptake and toxin filtration, separating hepatocytes and endothelial cells.

CELLS IN VASCULAR SINUSOIDS

1. Kupffer Cells

• Specialized macrophages recognizing and phagocytosing aged erythrocytes.

2. Ito Cells

• Store vitamin A, regulate liver regeneration and Kupffer cell activity.

HEPATIC ORGANIZATION

Classic Lobule

• Emphasizes blood flow toward the central vein representing hepatocyte endocrine function.

Portal Lobule

• Highlights bile flow towards the bile duct.

Hepatic Acinus

• Distinguishes oxygen and nutrient variation along the sinusoids; categorizes into zones based on oxygenation levels.

BILIARY TRACT

• Function: Bile produced by hepatocytes flows through bile canaliculi, bile ductules, and ducts toward the duodenum.

• Cholangiocytes: Lining of bile ducts made of simple columnar epithelium.

GALLBLADDER

• Structure: Pear-shaped organ attached to the liver, storing bile.

• Characteristics: Lacks submucosa, has a mucosa of columnar epithelium, muscularis, and adventitia or serosa.

GALLBLADDER FUNCTION

• Cholecystokinin: Stimulates contraction of gallbladder muscularis in response to fat in the small intestine.