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.

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.

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: 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.

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.

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).

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

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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.