Digestion and Absorption in the Small Intestine

Digestion of Nucleic Acids

• Nuclei of ingested cells in food contain DNA and RNA
• Pancreatic nucleases break down nucleic acids into nucleotide monomers
• Further broken down into free nitrogenous bases, pentose sugars, and phosphate ions
• Breakdown products are actively transported by special carriers in epithelium of villi

Absorption of Vitamins, Electrolytes, and Water

• Fat-soluble vitamins (A, D, E, and K) are absorbed by diffusion
• Water-soluble vitamins (C and B) are absorbed by passive or active transport
• Vitamin B12 binds with intrinsic factor and is absorbed
• In large intestine, vitamin K and B vitamins from bacterial metabolism are absorbed

Absorption of Electrolytes and Water

• Iron and calcium are absorbed in duodenum
• Iron and calcium absorption is related to need
• Ionic iron is stored in mucosal cells with ferritin
• When needed, iron is transported in blood by transferrin
• Ca2+ absorption is regulated by vitamin D and parathyroid hormone (PTH)
• 9 L of water from GI tract secretions enters small intestine
• 95% of water is absorbed in small intestine by osmosis
• Most of the rest is absorbed in large intestine

Mechanical Breakdown and Digestive Processes

• Mechanical breakdown: processes that physically mix or break down food into smaller fragments
• Segmentation: local constriction of intestine that mixes food with digestive juices
• Digestion: catabolic sets that involves enzymes that break down food molecules
• Absorption: passage of digested end products from intestinal mucosa into blood or lymph
• Defecation: elimination of undigested substances via anus in form of feces

Organization of Digestive System

• Peritoneum: serous membranes of abdominal cavity
• Visceral peritoneum: membrane on external surface of most digestive organs
• Parietal peritoneum: membrane that lines body wall
• Peritoneal cavity: fluid-filled space between two peritoneums
• Serous fluid lubricates mobile organs

The Pancreas

• Mostly retroperitoneal; deep to greater curvature of stomach; between spleen and small intestine
• Exocrine function: produce pancreatic juice
• Acini: clusters of secretory cells to produce pancreatic juices
• Ducts: secrete to duodenum via main pancreatic duct; smaller duct cells produce water and bicarbonate
• Endocrine function: secretion of insulin and glucagon by pancreatic islet cells
• Composition of pancreatic juice: watery, alkaline solution (pH 8) with electrolytes, digestive enzymes, and nucleases

Regulation of Bile and Pancreatic Secretions

• Bile and pancreatic juice secretions are stimulated by neural and hormonal controls
• Hormonal controls include: cholecystokinin (CCK) and secretin

Lipids

• Dietary requirements: unsaturated fats, saturated fats, and cholesterol
• Goal is to keep total cholesterol < 5.2 mmol/L
• 20-35% of calorie intake should be from fat

Proteins

• Dietary sources: animal products, soybeans, legumes, nuts, and cereals
• Complete proteins contain all essential amino acids
• Incomplete proteins lack some essential amino acids
• Uses in body: structural materials, functional molecules
• Three factors determine whether amino acids are used to synthesize proteins or burned as energy:
  • All-or-none rule
  • Adequacy of caloric intake
  • Hormonal controls

Carbohydrate Metabolism

• Carbohydrate metabolism is essentially glucose metabolism
• All food carbohydrates are transformed to glucose
• Glucose is converted to CO2 and water in various processes
• When cellular energy is high, glucose breakdown is inhibited
• Glucose is converted to glycogen (glycogenesis) or fat (lipogenesis)
• When blood glucose levels fall, glycogenolysis occurs

Lipid Metabolism

• Lipids provide a greater energy yield than glucose or protein catabolism
• Most products of fat digestion are transported in lymph as chylomicrons
• Broken down into fatty acids and glycerol

Lipogenesis

• Triglyceride synthesis occurs when cellular ATP and glucose levels are high
• Dietary glycerol and fatty acids not needed for energy are stored as triglycerides
• 50% is stored in adipose tissue; other 50% is deposited in other areas
• Glucose is easily converted to fat