Digestion and Absorption in the Small Intestine

Chyme and Small Intestine

• Chyme, a liquid mixture from the stomach, contains partially digested carbohydrates and proteins.
• Enters the duodenum, the upper 5-10 inches of the small intestine.
• Stomach empties slowly due to chyme acidity and the need for proper digestion and absorption in the duodenum.

Regions of the Small Intestine

• Duodenum: Upper region where most digestion and absorption occur; receives bile and pancreatic juice.
• Jejunum: Middle region of the small intestine.
• Ileum: Lowermost region that connects to the large intestine.

Length and Structure of Intestines

• Human intestinal tract is approximately 20-25 feet long (shorter in a living body due to muscle tone).
• Length reflects the ability to digest various foods.
• Intestines are anchored to the abdominal cavity wall by mesentery.
• Loops of intestine are attached to each other via mesentery extensions.

Mesentery and Greater Omentum

• Mesentery: Extensions of parietal peritoneum that cover the GI tract and attach organs to the wall or each other.
• Greater Omentum: A large mesentery that lies over the intestinal bundle, containing adipose tissue (omental fat).
• Adipose tissue can also accumulate within the organs themselves.

Hepatic Portal Vein

• Veins draining the intestines merge to form the hepatic portal vein.
• The hepatic portal vein carries nutrient-rich blood from the digestive tract to the liver.

Folding in the Small Intestine

• The small intestine is heavily folded to maximize surface area.
• Three levels of folding:
  • Plicae circulares: Visible to the naked eye; permanent folds that triple the surface area.
  • Villi: Microscopic folds in the mucosa (epithelial lining), increasing surface area tenfold.
  • Microvilli: Folds in the apical membrane of columnar cells.
• Total surface area in the small intestine is approximately 2,700 square feet (the size of a tennis court).

Intestinal Glands

• Downfolds between villi form intestinal glands.
• Surface cells of villi function in absorption.
• Goblet cells secrete mucus.
• Glandular regions have cell diversity.
  • Columnar and goblet cells produce intestinal juice (about 1 liter/day).
  • Intestinal juice has a basic pH of 8.1, containing mucus and bicarbonate.
  • Paneth cells secrete antimicrobial lysozyme and are phagocytic.
  • Enteroendocrine cells produce hormones like cholecystokinin (CCK) and secretin.

Submucosa and Lymphatic Tissue

• Submucosa contains mucous glands that secrete alkaline mucus.
• Mucosa-associated lymphatic tissue (MALT) filters absorbed substances and traps pathogens.
• Lymphocytes act on filtered material.

Smooth Muscle Layers and Peristalsis

• The small intestine has two layers of smooth muscle.
• Peristalsis propels food through the small intestine.

Pancreatic Juice

• Digestion in the small intestine requires pancreatic juice (no enzymes are produced by intestinal cells).
• Exocrine cells of the pancreas form acini, spherical clusters with microscopic ducts.
• Acinar cells produce about 1 liter/day of pancreatic juice, which is basic and contains enzymes:
  • Proteolytic enzymes: trypsin, chymotrypsin, carboxypeptidase (break down proteins)
  • Pancreatic amylase: Breaks down carbohydrates (glycosidic bonds)
  • Pancreatic lipase: Breaks down triglycerides
• Ducts from acini merge into the main pancreatic duct, joined by the bile duct from the liver.

Bile

• Produced in the liver and stored in the gallbladder.
• Basic in nature and produced at a constant rate.
• Contains:
  • Bile salts: Amphipathic emulsifiers derived from cholesterol
  • Lecithin: another amphipathic molecule
  • Bile pigments: Breakdown products of heme (e.g., bilirubin)
  • Avenues to get rid of cholesterol and ions

Mechanical Digestion and Segmentation

• Involves asynchronous contractions of circular and longitudinal muscle layers (segmentation).
• Segmentation mixes chyme with intestinal juice, pancreatic juice, and bile.

Chemical Digestion

• Involves pancreatic amylase acting on short-chain carbohydrates.
• Pancreatic amylase breaks down carbohydrates to disaccharides but cannot break the final glycosidic bond.
• Disaccharides: Membrane enzymes (brush border enzymes) that break the last glycosidic bond.
• Brush border enzymes break down disaccharides into monosaccharides.

Protein Digestion

• Proteins are denatured and partially digested in the stomach by pepsin.
• Pancreatic juice enzymes further break them down to dipeptides.
• Dipeptidases (brush border enzymes) break the last peptide bond in dipeptides, resulting in free amino acids.

Lipid Digestion

• Lipids form large globules due to hydrophobic interactions.
• Pancreatic lipase cannot access bonds within these globules.
• Bile salts and lecithin emulsify large fat globules into smaller ones, exposing bonds.
• Pancreatic lipase breaks down triglycerides into free fatty acids, glycerol, and monoglycerides.
• Bile salts form micelles, transport vesicles that carry the digestion products to absorptive cells.
• Micelles move contents across the cell membrane.

Triglyceride Synthesis and Chylomicrons

• Within absorptive cells, triglycerides are resynthesized.
• Resynthesized triglycerides are surrounded by amphipathic proteins to form chylomicrons.
• Chylomicrons exit the basal surface and enter a lymphatic capillary (lacteal) in the villus because they are too large to enter a blood capillary.

Villi Core

• The core of each villus contains a blood capillary and a lacteal (lymphatic capillary).
• Absorbed nutrients (except lipids) enter the blood capillaries.
• Lipids get packaged into chylomicrons and are absorbed into the lymphatic system before entering the bloodstream.

Absorption of Monosaccharides

• The final step of digestion occurs on the brush border via disaccharidases.
• Monosaccharides (e.g., glucose) are absorbed via secondary active transport.
• Na^+/K^+ ATPase pumps maintain low intracellular Na^+ levels.
• As Na^+ follows its gradient, monosaccharides are pulled in.
• Some monosaccharides follow their gradient via facilitated diffusion.
• Facilitated diffusion transports monosaccharides across the basal surface into the bloodstream.

Absorption of Amino Acids

• Similar to monosaccharide absorption, amino acids are absorbed via secondary active transport.
• Sodium-potassium pump creates low sodium levels within the cells.
• As sodium follows its gradient, amino acids are pulled along.
• Amino acids pass through a facilitating protein as they travel into the bloodstream.

Lipid Absorption

• Triglycerides are broken down in the intestinal lumen and resynthesized in the absorptive cells.
• The triglycerides get packaged into a chylomicron then get absorbed into the lacteal.
• This is where we looked at the manufacture of very low density lipoproteins and LDLs and HDLs.