Digestive System- Accesory Organs

The digestive system is a group of organs that break down food (physically and chemically) so that it can be used by the body cells.

The organs of the digestive system can be divided into two:

Gastrointestinal (GI) tract or alimentary canal 

Accessory digestive organs

These organs are called “accessory” because food does not pass through them, yet digestion would be impossible without them

The accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. 

Teeth: helps in physical breakdown of food

Tongue: helps in chewing and swallowing food

Other accessory digestive organs: produce or store secretions that help   in the chemical breakdown of food 

Teeth

Before any enzyme acts, mechanical digestion begins in the mouth with the teeth.

Structure and Types

Adult humans typically have 32 permanent teeth:

  • Incisors (8) – cutting
  • Canines (4) – tearing
  • Premolars (8) – crushing
  • Molars (12) – grinding (including 3rd molars / wisdom teeth, which may be absent in some individuals)

Each tooth has:

  • Crown – exposed part, covered with enamel (hardest substance in the human body, rich in hydroxyapatite).
  • Neck – junction between crown and root.
  • Root – embedded in the jawbone, covered by cementum and anchored by periodontal ligament.

Internally:

  • Dentin – forms the bulk of tooth, less hard than enamel.
  • Pulp cavity – contains nerves and blood vessels.

Functions

  • Break food into smaller particles → increases surface area for enzymatic action.
  • Important for proper mixing with saliva and formation of bolus.
  • Essential for clear speech and aesthetics, but in digestion, primarily a mechanical tool.

Fig: Sagittal Section of a Molar

Tongue 

The tongue is a muscular organ in the oral cavity that plays both mechanical and sensory roles.

Structure

  • Composed of skeletal muscle fibers covered by mucous membrane.
  • Upper surface (dorsum) has papillae:
    • Filiform papillae – most numerous, no taste buds, provide friction.
    • Fungiform papillae – scattered, contain taste buds.
    • Circumvallate (vallate) papillae – large, arranged in a V-shape near the back, contain many taste buds.
    • Foliate papillae – on lateral sides, more prominent in some species and in children.

Taste buds detect basic taste modalities: sweet, salty, sour, bitter, and umami.

Functions in Digestion

  • Manipulates food during chewing.
  • Helps form a bolus and initiates swallowing (deglutition) by pushing bolus toward the pharynx.
  • Contains lingual glands that secrete lingual lipase, which can begin triglyceride digestion (particularly active in the acidic environment of the stomach).
  • Taste helps trigger salivation and prepares the digestive system via cephalic phase responses.

Salivary Glands – Chemical Digestion Begins

“Saliva” is not just water—it is a carefully composed fluid crucial for digestion and oral health.

Fig: The salivary glands

Major Salivary Glands

There are three pairs of major salivary glands:

  1. Parotid glands

    • Largest; located anterior and inferior to the ear.
    • Secrete serous, watery saliva, rich in salivary amylase (ptyalin).
    • Duct: Stensen’s duct, opens near upper second molar.

  2. Submandibular glands

    • Located beneath the mandible.
    • Mixed glands – both serous and mucous secretions.
    • Duct: Wharton’s duct, opens at the floor of the mouth beside the lingual frenulum.

  3. Sublingual glands

    • Smallest; under the tongue.
    • Predominantly mucous secretion.
    • Multiple small ducts opening onto the floor of the mouth.

Composition of Saliva

Average daily secretion: about 1–1.5 liters.
Components include:

  • Water (~99%) – dissolves food, facilitates taste.
  • Electrolytes – Na⁺, K⁺, Cl⁻, HCO₃⁻.
  • Enzymes:
    • Salivary amylase – begins digestion of starch to maltose and dextrins (in neutral to slightly acidic pH; inactivated in low gastric pH).
    • Lingual lipase (from lingual glands but present in saliva) – starts fat digestion, especially in infants.
  • Mucus (mucins) – lubricates food, helps form bolus.
  • Lysozyme – antimicrobial.
  • IgA – immune protection.

Functions

  • Initiates carbohydrate digestion.
  • Lubricates food and oral mucosa.
  • Facilitates speech and swallowing.
  • Maintains oral hygiene (washes away food particles, has antimicrobial components).
  • Bicarbonate and phosphate provide buffering capacity, helping to maintain near-neutral pH in mouth.

Liver – The Metabolic and Secretory Giant

Fig: The pancreas, liver and gallbladder

The liver is the largest internal organ and an essential accessory digestive organ.

Location and Structure

  • Located in the right upper quadrant of the abdomen, just below the diaphragm.
  • Has four anatomical lobes: right, left, caudate, and quadrate.
  • Structural unit: hepatic lobule, consisting of plates of hepatocytes arranged around a central vein.
  • Receives dual blood supply:
    • Hepatic artery – oxygenated blood.
    • Portal vein – nutrient-rich blood from gastrointestinal tract, spleen, and pancreas.
Fig: Histology of the liver

Digestive Function: Bile Production

The liver produces bile, a crucial secretion for fat digestion and absorption.

  • Daily bile production: ~500–1000 mL.
  • Bile composition:
    • Bile salts (conjugated bile acids with Na⁺ or K⁺) – derived from cholesterol; amphipathic molecules that emulsify fats.
    • Bile pigments (mainly bilirubin, a breakdown product of hemoglobin).
    • Cholesterol, phospholipids (e.g., lecithin), electrolytes, and water.

Bile salts:

  • Break large fat globules into smaller micelles, increasing surface area for pancreatic lipase.
  • Facilitate absorption of fatty acids, monoglycerides, cholesterol, and fat-soluble vitamins (A, D, E, K) in the small intestine.
  • Are reabsorbed in the ileum and returned to liver via enterohepatic circulation.

Other Key Functions

  • Metabolism of carbohydrates, proteins, and lipids.
  • Biotransformation (drug metabolism) – Phase I and Phase II reactions.
  • Synthesis of plasma proteins – albumin, clotting factors.
  • Detoxification of endogenous and exogenous substances.

Students must connect liver function with drug action, clearance, and toxicity.

Fig: blood supply to the liver

Gallbladder – The Storage and Concentration Unit

The gallbladder is a pear-shaped sac located on the inferior surface of the liver.

Structure and Connection

  • Has fundus, body, and neck.
  • Connected to the cystic duct, which joins the common hepatic duct to form the common bile duct.
  • The common bile duct joins with the pancreatic duct and opens into the duodenum at the major duodenal papilla, controlled by the sphincter of Oddi.

Functions

  • Stores bile produced by the liver.
  • Concentrates bile by absorbing water and electrolytes (up to 5–10 times concentration).
  • Releases bile into the duodenum in response to cholecystokinin (CCK), a hormone secreted by duodenal mucosa when fatty food enters.

In exams, remember:

  • Liver makes bile; gallbladder stores and concentrates bile.
  • Gallbladder removal (cholecystectomy) does not stop bile production; it simply results in continuous flow of less concentrated bile into the intestine.

Pancreas – Exocrine and Endocrine Powerhouse

The pancreas is both a digestive (exocrine) and endocrine organ, located in the retroperitoneal space, extending from the duodenum to the spleen.

Structural Divisions

  • Head – nestled in the C-shaped curve of duodenum.
  • Body – lies behind the stomach.
  • Tail – near the spleen.

For digestion, focus on its exocrine component.

Exocrine Pancreas

  • Composed of acini that secrete pancreatic juice into a system of ducts → main pancreatic duct → joins common bile duct → opens into duodenum.

Pancreatic juice is:

  • Alkaline (rich in bicarbonate, pH ~8), which:
    • Neutralizes acidic chyme from the stomach.
    • Provides optimal pH for pancreatic enzymes.

Major Digestive Enzymes in Pancreatic Juice

  1. Carbohydrate-digesting enzyme

    • Pancreatic amylase – breaks down starch and glycogen into maltose and other oligosaccharides.

  2. Protein-digesting enzymes (secreted as inactive zymogens)

    • Trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase.
    • Activated in the intestinal lumen:
      • Enteropeptidase (enterokinase) from duodenal mucosa converts trypsinogen → trypsin.
      • Trypsin then activates other zymogens.
    • These enzymes break proteins and peptides into smaller peptides and amino acids.

  3. Fat-digesting enzyme

    • Pancreatic lipase – primary enzyme for triglyceride digestion, converting them to free fatty acids and monoglycerides, acting on emulsified fat droplets in presence of bile salts.

  4. Nucleic acid–digesting enzymes

    • Ribonuclease and deoxyribonuclease – digest RNA and DNA into nucleotides.

Regulation of Pancreatic Secretion

  • Secretin – released from duodenum in response to acidic chyme; stimulates bicarbonate-rich, watery secretion.
  • Cholecystokinin (CCK) – released in response to fats and proteins in duodenum; stimulates enzyme-rich pancreatic secretion and gallbladder contraction.

For exams, link:

  • Acid → Secretin → HCO₃⁻.
  • Fat/protein → CCK → enzymes + bile release.

How These Organs Work Together in Digestion

  1. Mouth

    • Teeth mechanically break food.
    • Tongue mixes food with saliva, forms bolus, aids swallowing.
    • Salivary glands begin starch digestion and lubricate food.

  2. Stomach → Duodenum

    • Chyme enters duodenum, which detects acid, fats, and proteins.
    • Hormones released:
      • Secretin → stimulates pancreatic bicarbonate.
      • CCK → stimulates pancreatic enzymes + gallbladder contraction.

  3. Liver & Gallbladder

    • Liver continuously produces bile.
    • Gallbladder stores and concentrates it; releases bile when stimulated by CCK.
    • Bile salts emulsify fats and aid absorption in small intestine.

  4. Pancreas

    • Releases enzymes to digest carbohydrates, proteins, fats, and nucleic acids.
    • Bicarbonate neutralizes gastric acid, providing optimal pH for enzyme activity.

All these accessory organs ensure that by the time chyme passes through the small intestine, complex macromolecules are broken down into absorbable units: monosaccharides, amino acids, fatty acids, monoglycerides, and nucleotides.

Quick Exam-Oriented Summary for UG Pharmacy Students

  • Teeth:

    • 32 permanent; types – incisors, canines, premolars, molars.
    • Function: mechanical digestion → increases surface area.

  • Tongue:

    • Skeletal muscle with papillae and taste buds.
    • Functions: taste, bolus formation, initiation of swallowing, lingual lipase secretion.

  • Salivary glands:

    • Parotid (serous), submandibular (mixed), sublingual (mainly mucous).
    • Saliva: water, electrolytes, amylase, mucins, lysozyme, IgA.
    • Functions: start starch digestion, lubrication, oral hygiene.

  • Liver:

    • Produces bile (bile salts, bile pigments, cholesterol, phospholipids).
    • Bile salts: emulsify fats, aid absorption of lipids and fat-soluble vitamins; undergo enterohepatic circulation.

  • Gallbladder:

    • Stores and concentrates bile; releases it in response to CCK.
    • Not essential for bile production, but important for regulated, concentrated bile delivery.

  • Pancreas (exocrine):

    • Secretes pancreatic juice (bicarbonate + enzymes).
    • Enzymes: pancreatic amylase, proteases (as zymogens), lipase, nucleases.
    • Secretin → bicarbonate; CCK → enzymes.





 


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