Digestive System: Small Intestine

Overview

• The small intestine is a critical part of the digestive system.
• Food passes from the stomach into the small intestine, where it undergoes further digestion and nutrient absorption.
• The presence of hydrochloric acid in the stomach reduces gastric activity and prepares the food for the small intestine.

Sections of the Small Intestine

• Duodenum: First part of the small intestine (25 cm long), receives chyme from the stomach.
• Jejunum: Middle part (2.5 meters long), significant for nutrient absorption.
• Ileum: Longest part (about 3.6 meters long), connects to the large intestine.

Digestive Process

1. Chyme enters the duodenum in small amounts (approx. 3 ml at a time).
2. Gastric contractions are powerful near the pylorus, pushing the chyme forward.
3. Mechanical receptors and hormones like secretin are stimulated to enhance digestion.

Hormones and Secretions

• Anti-progesterone and digestive hormones: Released by the small intestine to regulate digestive activity.
• Secretin: Stimulates bicarbonate release to neutralize stomach acid.
• Gastric inhibitory peptide (GIP): Reduces gastric motility and secretion to allow digestive activities.

Structure and Function

• Mucosa: Inner lining with permanent folds called plicae circulares that slow down chyme movement and increase surface area.
• Villi: Finger-like projections on the mucosa, aid in nutrient absorption.
  • Core contains capillaries and a lymphatic vessel called lacteal.
  • Microvilli (brush border): Cytoplasmic extensions of absorptive cells, enhance absorptive capacity.
• Submucosa, muscularis externa, and serosa: Other layers providing structural support.

Blood Supply and Nerve Innervation

• Blood supply: Mesenteric arteries ensure blood flow to the small intestine.
• Nerve supply: Sympathetic and parasympathetic fibers regulate digestive activities.

Role of Liver and Gallbladder

• Liver: Largest digestive organ, produces bile for fat emulsification, stores nutrients.
• Gallbladder: Stores and concentrates bile, releases it into the duodenum when needed.
  • Bile helps in the breakdown of fats.

Key Takeaways

• The small intestine is essential for nutrient absorption and digestion continuation from the stomach.
• Structural components like villi and microvilli are crucial for maximizing nutrient absorption.
• The liver and gallbladder play significant roles in supporting digestive activities in the small intestine.