• "The process by which large ingested molecules are mechanically and chemically broken down to produce smaller molecules that can be absorbed across the wall of the GI tract."

• Mastication
• Saliva
• Enzymes to help breakdown simple sugars
• Mucus to lubricate the food for easier swallowing
• Lysozyme to kill bacteria
• Tongue
• Taste receptors
• Enzymes to help breakdown fatty acids

• Cardiac sphincter and pyloric sphincter
• Capacity of ~4 cups
• Secretion of acid and enzymes
• Holds food for 2-4 hours
• Result in the formation of chyme
• Mucus layer prevents autodigestion

• Prevents reflux of stomach content to cause heartburn and ulcers
• Controls the amount of stomach content into the small intestine
• Controls the amount of bile into the small intestine
• Prevents large intestine content (bacteria) to back up into the small intestine

• Duodenum
• ~10 inches in length
• Primary site of digestion
• Jejunum
• ~4 feet in length
• Some digestion
• Ileum
• ~5 feet in length
• Little digestion

• Pancreas secrets bicarbonate ions to neutralize the acidic chyme from the stomach
• Highly folded to increased surface area and maximize absorption
• Muscle contractions to mix the food with digestive fluids
• Food remains 3-10 hours in the small intestine
• ~95% of digestion takes place here

• Liver makes bile
• emulsification of fats

• Gall bladder stores bile

• Makes enzymes
• pancreatic amylase
• proteases
• lipases
• Enter small intestine at the duodenum
• Digestion of proteins, carbohydrates and fats

• ~3 1/2 feet in length
• Little digestion occurs
• Indigestible food stuff
• Absorption of water, some minerals, vitamins
• Formation of feces for elimination
• "Food" remains 24-72 hours

Physiology of the GI tract (Fig. 6-3)

• Peristalsis
• A ring of contraction propelling material along the GI tract
• Segmentation
• A back-and-forth action that breaks apart food
• Mass movement
• Peristaltic wave that contracts over a large area of the large intestine to help eliminate waste

• Enzymes speed up chemical reactions
• Enzymes lowers the amount of energy needed for the action to proceed
• Each enzyme acts on specific substance
• Enzyme release and activation is controlled by nerves and hormones
• Enzymes are only released when needed

• Usually synthesized from a specific site and must enter the bloodstream to reach target cell
• Gastrin
• Secretin
• Cholecystokinin
• Gastric Inhibitory Peptide

• Stimulated by food, thoughts of food
• Stimulates flow of stomach enzymes and HCl
• Stimulates contraction of cardiac sphincter
• Slows gastric emptying

• Stimulated by the presence of acidic chyme in the duodenum
• Stimulates the secretion of bicarbonate

• Stimulated by food, presence of fat and protein in the duodenum
• Stimulates contraction of gallbladder and flow of bile
• Stimulates the release of enzyme rich pancreatic fluids

• Stimulated by fats and protein
• Inhibits the secretion of stomach acid and enzymes
• Slows gastric emptying

• Proteins are denatured by the stomach acid
• Gastrin stimulates the release of pepsinogen from the chief cells in the stomach
• Pepsinogen => Pepsin
• Pepsin (an enzyme produced in the stomach) breaks down proteins into peptones

• Pancreas release the protein splitting enzymes: trypsin, chymotrypsin, and carboxypeptidase into the duodenum
• Proteins broken into smaller peptides and amino acids
• Peptides and amino acids are ready for absorption

• Gastric lipase works only in an acidic environment
• Gastric lipase acts on triglycerides containing short & medium chain fatty acid
• Longer fatty acid chain is not affected by the stomach

• Primary site of fat digestion
• Pancreas releases pancreatic lipase
• CCK stimulates the release of bile to help emulsify fat
• Fat is broken down to monoglycerides and fatty acids

• Saliva contains amylase
• Starch is broken down
• Enzymes inactivated at low pH

• The acidic environment stops action of salivary amylase
• No further starch digestion occurs

• Pancreatic amylase is released
• Intestinal cells release maltase, sucrase, and lactase

• Monosaccharides are absorbed

• Multiple hormones are released
• Multiple enzymes are released
• Content of food dictates the hormones and/or enzymes released

• "The process by which nutrient molecules are absorbed by the GI tract and enter the bloodstream"
• 95% of nutrients are absorbed by the absorptive cells in the small intestine

• The wall is folded
• Villi projections are located on the folds
• Absorptive cells (enterocytes) are located on the villi
• Microvilli are located on the villi
• Increases intestinal surface area 600 x

• Absorptive cells
• Produced in crypts
• Degradation of cells at the tips of the villi by digestive enzymes
• Newly formed cells constantly migrate to replace dying ones (every 2-5 days)
• High turnover causes the cells to deteriorate during nutrient deficiency

• Passive absorption
• Facilitated absorption
• Active absorption
• Endocytosis

• Simple diffusion of substances across the cell membrane
• No energy expended
• Going from high concentration gradient to low which drives the reaction
• Water, most fat, and some minerals are absorbed this manner

• Uses a carrier protein
• No energy expended
• Going from high concentration gradient to low which drives the reaction
• Fructose is absorbed in this manner

• Uses a carrier protein
• Energy is expended
• Going from low concentration gradient to high
• Glucose, galactose, amino acids are absorbed in this manner

• Phagocytosis ("cell eating") and pinocytosis ("cell drinking")
• Takes substances by forming an indentation in the cell membrane
• The surrounded particle is eventually incorporated into the absorptive cell
• Active transport
• Infant’s absorption of mother’s antibodies is absorbed in this manner

• Portal circulation
• Capillaries from the intestine and stomach drains into the portal vein leading to the liver
• Water soluble substances (proteins, carbohydrates, short- & med.-chain fatty acids, B vitamins and vitamin C)
• Lymphatic circulation
• Vessels that will carry particles that are fat soluble or too large in size

• Active absorption
• Whole proteins are broken down at the microvilli surface and within the absorptive cells
• Whole proteins are eventually broken down to amino acids
• Amino acids are sent to the bloodstream via portal circulation

• Passive absorption
• Short- & med. chain fatty acids travel through the portal vein
• Long chain (> 12 C) and monoglycerides travels through the lymphatic system
• Stomach is capable of absorbing limited amount of short-chain fatty acids

• Fructose is absorbed via facilitated absorption
• Sodium and glucose/galactose are actively absorbed into the cell with the use of a carrier
• The energy is used to pump sodium back out of the absorptive cell
• The monosaccharides can exit the absorptive cells and into the portal circulation by facilitated diffusion

Carbohydrate absorption (Fig. 6-15)

• Only ~5% of nutrients are unabsorbed by the small intestine
• Sodium, potassium, water, short-chain fatty acids, vitamin K and biotin are absorbed in the first 2/3 portion of the large intestine
• The remains are formed into a semisolid mass for elimination