Digestive System Organization

Organization of the Digestive System

• The digestive system consists of the gastrointestinal tract (GIT) and accessory organs.
• The GIT includes the mouth, oropharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory organs include salivary glands, liver, gallbladder, and pancreas.

Functions of the Digestive Tract

• Ingestion: food and liquid enter the oral cavity.
• Mechanical digestion: crushing of food in the oral cavity and mixing in the stomach.
• Chemical digestion: breakdown of food into small organic molecules.
• Secretion: enzymes and digestive fluids secreted by the digestive tract and its accessory organs facilitate chemical digestion.
• Absorption: passage of nutrients from the digestive tract into blood or lymph for distribution to tissue cells.
• Elimination: undigested material is released through the rectum and anus by defecation.

Structure of the GIT

• Mucosa (inner lining) secretes mucus that lubricates and protects the GIT.
• Submucosa (layer of dense irregular connective tissue) contains blood vessels and lymphatic vessels.
• Muscularis propria (smooth muscle layer) has two layers: inner circular layer and outer longitudinal layer.
• Serosa (outermost layer) is a layer of loose connective tissue that secretes fluid to lubricate the outside of the GIT.

Musculature of the GIT

• Longitudinal muscle layer shortens the segment of the intestine and expands the lumen.
• Circular muscle layer reduces the diameter of the lumen and increases its length.
• Oblique muscle layer (found in the stomach only) helps to mix food with digestive enzymes.

Characteristics of GI Smooth Muscle

• Each muscle layer functions as a syncytium.
• Actin filaments are attached to dense bodies, which are bonded together by intracellular protein bridges called gap junctions.
• The slow waves of electrical activity occur at different frequencies in different parts of the GIT.

Regulation of GI Activities

• Autonomic nervous system (ANS) regulates GI tract activities:
  • Sympathetic nerves inhibit GI tract activities.
  • Parasympathetic nerves stimulate GI tract activities.
• Enteric nervous system (ENS) regulates GI tract activities:
  • Myenteric plexus controls movements and contraction.
  • Submucosal plexus controls secretions.
• Hormonal control:
  • Hormones from the GI tract and endocrine glands regulate GI tract activities.
  • Enteroendocrine cells release peptide hormones and biogenic amines.

Mouth

• Mechanical digestion through mastication.
• Chemical digestion through amylase enzyme in saliva that breaks down polysaccharides into disaccharides.
• Food particles are mixed with saliva during mastication, resulting in a moist lump called a bolus.

Salivary Glands

• Three pairs of salivary glands: sublingual, parotid, and submandibular glands.
• Saliva consists of 99.5% water and 0.5% dissolved substances, including HCO3-, electrolytes, and amylase enzyme.
• Functions of saliva:
  • Moistens food and the mouth during mastication.
  • Cleans the teeth and inhibits bacterial growth.
  • Digests a small amount of starch via amylase.

Esophagus

• Actively moves food from the throat to the stomach.
• Control of movement:
  • Upper esophageal sphincter prevents air from entering the esophagus.
  • Lower esophageal sphincter (cardiac sphincter) prevents backflow of stomach contents.

Stomach

• Muscular, expandable, J-shaped organ that can expand to contain 1-1.5 L of material (chyme).
• Regions:
  • Fundus
  • Cardia
  • Body
  • Antrum
  • Pylorus
• Functions:
  • Temporary storage of ingested food.
  • Mechanical digestion of ingested food.
  • Chemical digestion of food through the action of acid and enzymes.
  • Production of intrinsic factor.

Gastric Glands

• Glands in the fundus and body:
  • Secrete acid and enzymes.
  • Parietal cells secrete intrinsic factor and HCl.
  • Chief cells secrete pepsinogen.
• Glands in the pylorus:
  • Secrete mucus and hormones that coordinate and control digestive activity.

Gastric Gland Cells

• Parietal cells:
  • Secrete intrinsic factor.
  • Secrete HCl.
• Chief cells:
  • Secrete pepsinogen.
• G cells:
  • Produce gastrin.
• D cells:
  • Produce somatostatin.
• Enterochromaffin-like cells:
  • Produce histamine.

HCl Production

• Parietal cells do not create HCl in their cytoplasm.
• H+ and Cl- are transported and secreted separately.
• H+ is generated as carbonic anhydrase converts CO2 and H2O to carbonic acid.
• Bicarbonate ions are ejected into the interstitial fluid in exchange for a chloride ion.

Gastric Filling and Mixing

• Stomach accommodates a 20-fold increase in volume with little change in tension in its wall.
• Receptive relaxation of the stomach is mediated by the vagus nerve.
• Gastric mixing:
  • Each time a strong antral contraction occurs, it pushes food forward toward the pyloric sphincter, which is closed, so food returns back.
  • Such movements mix food with digestive enzymes, converting food into chyme.