Lecture Notes: Digestive System

Overview

• The digestive system comprises several organs and processes responsible for breaking down food and absorbing nutrients.
• Processes include: ingestion, digestion, absorption, compaction, and defecation.

Digestive Processes

1. Ingestion

   • Selective intake of food, starting in the mouth.

2. Digestion

   • Mechanical Digestion: Physical breakdown (e.g., chewing, stomach mixing).
   • Chemical Digestion: Enzymatic breakdown into monomers via hydrolysis.

3. Absorption

   • Nutrients enter the bloodstream or lymphatic fluid, primarily in the small intestine.

4. Compaction

   • Absorption of water and consolidation of indigestible residue in the large intestine.

5. Defecation

   • Elimination of feces through the rectum and anus.

Anatomy of the Digestive System

• Digestive Tract / Alimentary Canal: Mouth to anus.
• Accessory Organs: Teeth, tongue, liver, gallbladder, pancreas.

Layers of the Digestive Tract

• Mucosa: Epithelium, lamina propria, muscularis mucosa.
• Submucosa: Connective tissue with blood vessels, nerves.
• Muscularis Externa: Inner circular and outer longitudinal muscle layers.
• Serosa: Areolar tissue and mesothelium.

Nervous Control

• Enteric Nervous System: Regulates motility, secretion, blood flow independently but influenced by CNS.
• Networks: Submucosal plexus (controls secretion) and myenteric plexus (controls peristalsis).

Mesentery

• Connective tissue suspending the stomach and intestines, providing space for movement and passage for vessels and nerves.

Regulation of Digestive Tract

• Neural Control: Short myenteric reflexes and long vagovagal reflexes.
• Hormones: Gastrin and secretin.
• Paracrine Signaling: Nearby cell stimulation.

Oral Cavity

• Functions: Ingestion, taste, chewing, chemical digestion, swallowing, speech, respiration.
• Structures: Cheeks, lips, palate, tongue, teeth.

Teeth

• Adult teeth: 32 total (incisors, canines, premolars, molars).
• Tooth Structure: Crown, neck, root.

Saliva

• Functions: Moistens mouth, begins digestion, cleans teeth, binds food.
• Saliva contains water, enzymes (salivary amylase, lingual lipase), antibodies.

Pharynx and Esophagus

• Pharynx: Connects the oral cavity to the esophagus and nasal cavity to the larynx.
• Esophagus: Muscular tube connecting pharynx to stomach, has upper and lower sphincters.

Stomach

• Functions: Storage, mechanical and chemical digestion, converting bolus to chyme.
• Regions: Cardiac, fundic, body, pyloric.
• Curvatures: Greater and lesser.
• Muscle Layers: Additional oblique layer for mixing.
• Gastric Juice: Contains HCl, pepsin for protein digestion.

Protection from Self-Digestion

• Mucus coat, tight junctions, rapid epithelial cell replacement.

Liver and Gallbladder

• Liver: Secretes bile necessary for lipid digestion; has multiple lobes.
• Gallbladder: Stores and concentrates bile.
• Bile Function: Digestion and absorption of fats, removal of waste products.

Pancreas

• Functions: Endocrine (insulin) and exocrine (digestive enzymes).
• Pancreatic Juice: Neutralizes stomach acid, contains enzymes for nutrient digestion.

Small Intestine

• Regions: Duodenum, jejunum, ileum.
• Functions: Absorption of nutrients, complete digestion.
• Surface Area: Increased by circular folds, villi, microvilli.

Large Intestine

• Functions: Water and electrolyte absorption, feces formation.
• Sections: Cecum, colon, rectum, anus.
• Microbiome: Aids in digestion, synthesizes vitamins B and K.

Defecation

• Controlled by intrinsic and parasympathetic reflexes, external sphincter under voluntary control.

Digestive Health and Disorders

• Ulcers: Caused by H. pylori.
• Colorectal Cancer: Early detection important for treatment.
• Lactose Intolerance: Due to lack of lactase enzyme.

Summary

The digestive system involves a complex interaction of structures and processes aimed at breaking down food into absorbable nutrients, while also protecting the body from harmful substances and waste.