1AA3/2YY3 - Module 3e: Digestion - Small Intestines

The small intestine is crucial for digestion and absorption of nutrients.
Anatomy and histology will be examined along with digestive processes.
Topics covered: breakdown of carbohydrates, fats, and proteins, nutrient absorption, and properties of absorption.

Length and Structure:
- In a living individual, approximately 3 meters long and 2.5 centimeters in diameter.
- After death, can extend to 6.5 meters due to loss of muscle contraction.
Parts of the Small Intestine:
- Duodenum: about 25 cm long; connects to the stomach; retroperitoneal; has a C-shaped structure.
- Jejunum: approximately 1 meter (up to 2.5 meters post-mortem).
- Ileum: about 2 meters (up to 3.5 meters post-mortem); connects to the large intestine at the ileocecal sphincter.

Circular Folds (Plicae Circulares):
- Structure that increases surface area for absorption.
- Spiraled arrangement captures chyme, prolonging its contact time for absorption.
Layers:
- Serosa (outermost layer).
- Muscularis: longitudinal and circular layers.
- Submucosa: contains blood vessels, lymphatic tissue; facilitates absorption.
- Mucosa Layer: contains villi (finger-like projections) and microvilli (on absorptive cells) enhancing surface area.

Absorptive Cells: absorb nutrients; featured microvilli increase absorptive surface area.
Goblet Cells: secrete mucus for protection.
Paneth Cells: regulate microbial populations by secreting enzymes.
Enteroendocrine Cells: secrete hormones (ex. secretin, cholecystokinin) that signal other parts of the GI tract.

Intestinal Juice:
- Approximately 1-2 liters/day, pH 7.6.
- Contains water, mucus, bicarbonate ions (alkaline).
Brush Border Enzymes: Embedded in microvilli.
- Carbohydrate-digesting enzymes: alpha-dextrinase, maltase, sucrase, lactase.
- Protein-digesting enzymes: aminopeptidase, dipeptidase.
- Nucleotide-digesting enzymes: nucleosidases, phosphatases.

Segmentation: Local mixing, enhancing digestion of chyme with intestinal juices.
Migrating Motility Complex: Propels chyme from duodenum to ileum (90 minutes to 2 hours).

Carbohydrates: Pancreatic amylase breaks down polysaccharides to disaccharides and trisaccharides; brush border enzymes convert them to monosaccharides.
Proteins: Gastric digestion (HCl and pepsin) denatures proteins; pancreatic proteases further digest to amino acids and small peptides.
Lipids: Lingual lipase and gastric lipase begin cholesterol digestion, while bile emulsifies fats into smaller lipid globules for lipase action.
Nucleic Acids: Broken down by ribonuclease and deoxyribonuclease from the pancreas; further processed by brush border enzymes.

Carbohydrate Absorption:
- Monosaccharides (glucose, galactose via secondary active transport with sodium; fructose via facilitated diffusion).
- Taken up into capillary beds and directed to liver via the hepatic portal vein.
Amino Acid Absorption:
- Transported via specific transporters (some active transport, some secondary with sodium or H+ ions).
- Amino acids pass through basolateral surface via diffusion to capillaries for transport to liver.
Lipid Absorption:
- Short-chain fatty acids diffuse directly across cell membranes.
- Larger fatty acids undergo emulsification by bile salts, forming micelles for absorption through simple diffusion, repackaged as triglycerides into chylomicrons.

Around 9 liters of fluid enters the GI tract daily; approximately 90% is absorbed in the small intestine.
Mechanism: Osmosis driven by nutrient absorption creates an osmotic gradient, facilitating water movement into blood.

Overview of the small intestine's anatomy, histology, mechanical and chemical digestion, and nutrient absorption.
Preparation for subsequent lectures on the large intestines and the regulation of the digestive system.