Gastrointestinal System Physiology Fabian Omenankiti
Digestion • Digestion is the breakdown of large, complex organic molecules into smaller components that can be used by the body. • Molecules need to be small enough to diffuse across plasma membranes.
Steps of digestion • Ingestion – this is the consumption of or taking in of nutrients. • Digestion – the chemical breakdown of large organic molecules into smaller components by enzymes. • Absorption – the transport or delivery of digested nutrients to body tissues. • Assimilation-the conversion of absorbed food into the substance of the body. • Egestion – the elimination of food waste materials from the body.
Esophagus • Approximately 10” long • Functions include: • Secrete mucus • Moves food from the throat to the stomach using muscle movement called peristalsis • If acid from the stomach gets in here that’s heartburn.
STOMACH J-shaped muscular bag that stores the food you eat, breaks it down into tiny pieces. Mixes food with digestive juices that contain enzymes to break down proteins and lipids. Acid in the stomach kills bacteria. Food found in the stomach is called chyme.
SMALL INTESTINE Small intestines are roughly 7 meters long Lining of intestine walls has finger-like projections called villi, to increase surface area. The villi are covered in microvilliwhich further increases surface area for absorption.
SMALL INTESTINE The majority of chemical digestion occurs in the first of three sections of the small intestine known as the duodenum. This section also contains an opening from the bile duct and pancreatic duct through which bile and pancreatic enzymes enter the small intestine
Small Intestine • Food enters the small intestine as a semi-solid mixture known as chyme. The chyme is acidic due to the HCl in the stomach so it needs to be neutralized. • The presence of chyme in the small intestine triggers the conversion of prosecretin into secretin which is absorbed into the blood stream and carried to the pancreas
Large intestine About 5 feet long Accepts what small intestines don’t absorb Rectum (short term storage which holds feces before it is expelled).
Accessory Organs • Salivary gland • Liver • gall bladder • pancreas
Salivary glands parotid glands - produce a serous, watery secretion. submaxillary (mandibular) glands - produce a mixed serous and mucous secretion. sublingual glands - secrete a saliva that is predominantly mucous in character.
Liver • The liver is a large accessory organ of the digestive system that is constantly producing a fluid known as bile. • Bile is stored in the gall bladder until it is needed in the small intestine.
Liver and Gall Bladder • The presence of lipids in the small intestine trigger the release of the hormone cholecystokinin (CCK) which triggers the release of bile from the gall bladder. • Bile contains bile salts that emulsifies fats which means it breaks them into smaller droplets so they can be digested.
Pancreas • The pancreas is an accessory organ of the digestive system. It releases chemicals to aid in digestion. • Secretin will stimulate the pancreas to release a solution containing bicarbonate ion into the small intestine where it will neutralize the acidic chyme and raise the pH from 2.5 to 9.0. This inactivates the pepsin.
Pancreas and Carbohydrates • Amylase enzymes are released from the pancreas that break large carbohydrate chains into small chains called disaccharides. • Then the small intestine releases disaccharide enzymes which break those small chains into individual sugars.
Pancreas & Proteins • Trypsinogen, a protein-digesting enzyme is released into the small intestine where it is converted into trypsin and it breaks down large protein chains into smaller chains.
Digestion of Carbohydrates in the Small Intestine Pancreatic amylase • Polysaccharides + Water Disaccharides + Water Pancreatic amylase Monosaccharides
Digestion of Proteins in the Small Intestine Trypsin and Chymotrypsin • Polypeptides + Water Protein Fragments + Water Aminopeptidase Amino Acids
Carbohydrates • Broken down to monosaccharaides • Includes starch • Indigestible carbohydrates (cellulose) act as bulk and help move foodstuffs along the GI tract • Chemical digestion begins in the mouth with salivary amylase • Amylase works best in slightly acidic to neutral environment • Inactivated by stomach acid
Carbohydrates cont… • Digestion continues in small intestine with aid of pancreatic amylase • Amylases convert starch to oligosaccharides • Intestinal brush border enzymes convert oligosaccharides to monosaccharaides • Dextrinase,glucoamylase, maltase, sucrase, lactase • Digestion of carbohydrates ends with the small intestine (no digestion in large intestine)
Proteins • Digested to its amino acid monomers • Digestion begins in the stomach • Pepsinogen from chief cells is activated to pepsin which digests proteins • Optimal functioning in acidic environment (low pH) • Inactivated by high pH in duodenum • Activity restricted to stomach
Proteins cont… • Trypsin and chymotrypsin from the pancreas continue to cleave protein bonds into smaller peptide bonds in the intestine • Carboxypeptidase (brush border enzyme) cleaves amino acids one-by-one at the carboxyl end • Aminopeptidase does the same from the amino end
Lipids • Digestion only occurs in the small intestine • Pancreas provides lipase • Triglycerides are insoluble in water, therefore “pre-treated” with bile salts • Bile salts emulsify and increase surface area of the triglycerides to aid in fat digestion • Bile salts have both polar and nonpolar ends • Polar and faces aqueous environment. Nonpolar end faces fat molecules • Digestion occurs with LIPASE. BILE SALTS DO NOT DIGEST • Breakdown into fatty acids and monoglycerides
Nucleic Acids • DNA and RNA in the nuclei of cells of food hydrolyzed to their nucleotide monomers • Pancreatic nucleases responsible • Nucleotides later broken down by brush border enzymes nucleosidases and phosphatases
Carbohydrate Absorption • Glucose and galactose use secondary active transport (with Na+) to move into epithelial cells • Move out of epithelial cells and into blood via facilitated diffusion • Fructose moves entirely by facilitated diffusion
Protein Absorption • Several types of protein transporters move different amino acids • Most transporters are coupled to the active transport with Na+ • Short-chained amino acids are absorbed with H+cotransport into the epithelial cell • Broken down to single amino acids before moving into circulation
Lipid Absorption • Monoglycerides and fatty acids associate with bile salts and lecithin (phospholipid) to form micelles • Micelles collections of fatty elements clustered together with bile salts • Micelles easily diffuse between microvilli • Fat absorption complete in the ileum
Lipid Absorption • Once inside the epithelial cells, they are resynthesized into triglycerides by the smooth ER • Triglycerides coated with a “skin” of proteins to form chylomicrons (water-soluble lipoprotein droplet) • Chylomicrons leave via exocytosis and enter lacteals to join the lymph (not blood) • Later emptied into venous blood in the neck region • In the blood, chylomicrons liberate triglycerides • Triglycerides hydrolyzed to fatty acids and glycerol by lipoprotein lipase
Nucleic Acid Absorption • Pentose sugars, nitrogenous bases and phosphate ions from nucleotide breakdown are actively transported across epithelium by special carriers
Vitamin Absorption • Small intestine absorbs dietary vitamins and mostly water • Large intestine absorbs K and some B vitamins • Fat soluble vitamins (A, D, E and K) dissolve in dietary fats and are absorbed in micelles • Water soluble vitamins (B and C) absorbed via diffusion or specific active or passive transporters • Exception: Vitamin B12 is very large. Requires intrinsic factor (stomach) • Intrinsic factor binds to B12. Intrinsic factor binds to its receptor in the ileum allowing for endocytosis of B12