The Human Digestive System

Overview

Mouth:
- Food broken by teeth, mixed with saliva
- Saliva contains amylase (enzyme for carbohydrate digestion)
- Bolus (food mix) formed
Esophagus:
- Bolus pushed by tongue, transported by flexible esophageal lumen
- Peristalsis moves bolus to stomach

Structure:
- Six areas, layers similar to esophagus
- Longitudinal, circular, and oblique muscle layers
- Rugae allow expansion
Function:
- Gastric glands produce mucus (protective) and gastric acid (digestive)
- Sphincter prevents acid reflux
- Gastric juice: hydrochloric acid, pepsin, intrinsic factor, lipase
- Hydrochloric acid destroys harmful bacteria, activates pepsin
- Pepsin breaks down proteins
- Intrinsic factor needed for vitamin B12 absorption
- Gastric movements mix food into chyme

Structure and Sections:
- Three sections: duodenum, jejunum, ileum
- Circular folds, villi, and microvilli increase surface area
- Villi contain blood capillaries and lacteal (lymphatic capillary)
Digestion:
- Bile and pancreatic secretions added in duodenum
- Pancreatic juice neutralizes stomach acid, contains digestive enzymes
- Bile emulsifies fats
- Nutrient absorption in jejunum and ileum
Absorption:
- Nutrients absorbed by villi into blood capillaries
- Some nutrients (e.g., glucose) transported freely, others need carriers (e.g., transferrin for iron)
- Fats transported by chylomicrons
- Ileum absorbs electrolytes, trace elements, vitamins (e.g., B12), and bile acids

Structure:
- Thicker than small intestine, about 1 meter long
Function:
- Connects to small intestine via Bauhin's valve
- No villi (main absorption in small intestine)
- Contains ~100 billion bacteria (vitamin production, fiber decomposition, immune function)
- Peristalsis transports chyme; water removal and mucus addition for waste excretion
- Ends in rectum, waste excreted through anus