Notes: Digestive System
- Mastication: chewing of food particles in the mouth; type of mechanical digestion.
- Deglutition: swallowing process of the food (bolus) in mouth region.
- Phases of
- Buccal phase: voluntary passing of food into pharyngeal region.
- Pharyngeal-esophageal phase: involuntary process controlled by medulla and pons areas; vagal stimulation of muscles of pharynx and esophagus.
- Peristalsis: propelling of food through the alimentary tract; alternate waves of contraction and relaxation
- Stomach digestion: proteins into polypeptides or long chains of amino acids.
- Pepsin: formed from pepsinogen under high HCl concentration
Rennin: coagulates milk protein in infants; not found in adults.
- Intrinsic factor: polypeptide produced in stomach; needed for absorption of vitamin B12, production of erythrocytes.
- Regulation of gastric secretion: nervous control= vagus nerve; hormonal: gastrin;
This stimulates secretion of enzymes and HCl and gastrin hormone antagonists.
- Cephalic reflex: happens prior to food entering stomach; produced by smell of food, sight of food, thought of food; primes stomach for digestion. Hypothalamus→ medulla→ Vagal stimulation (parasympathetic) of stomach glands.
Gastric reflex: presence of polypeptides in stomach and low pH leads to release of acetylcholine→ gastric juice; proteins tie up H+=pH↑ causing more gastrin and HCl; when proteins are digested=pH↓ slowing gastrin secretion (negative feedback)
- Role of gastrin: stimulates parietal cells to release HCl
- Release of HCl: stimulated by: acetylcholine, gastrin and histamine; when all 3 are present, secretion is high.
- Alkaline tide: accumulation of HCO3- into capillaries draining stomach
- Enterogastric reflex: inhibitory action (against vagal stimulation; activates sympathetic nerves causing closing of pyloric valve preventing food entering the small intestine; also activates series of other hormones to stop gastric secretion.
- Inhibition of gastric secretion: enterogastrone, secretin, cholecystokinin, gastric inhibitory protein.
- Receptive relaxation: relaxation of stomach muscles as food moves through esophagus and enters stomach
- Adaptive relaxation: stomach dilates when filled (nitric oxide release)
- Plasticity: stretching of stomach walls without increasing contractions.
- Electrical rhythm of stomach contractions: pacemaker cells in smooth muscle layers depolarize/repolarize 3 times per minute→cyclic slow waves of stomach; Gap junctions allow impulse transmission to rest of smooth muscle.
- Role of bile: contains bile salts, bile pigments, cholesterol, phospholipids; major job is emulsification of fats. Bile secretion by liver prompted by hormone secretin.
- Enterohepatic circulation of bile: bile salts are reabsorbed into the blood in the ileum of small intestine and returned to the liver through hepatic portal veins; They are then released in the next bile secretions.
- Bilirubin: waste product of heme (from hemoglobin) produced during old erythrocyte destruction; excreted into bile, it is the main bile pigment; it is metabolized in small intestine by bacteria and forms urobilinogen, the brown pigment found in feces.
- Bile flow regulation: intestinal hormone cholecystokinin (CCK); causes gall bladder contraction
- Release of pancreatic juice: secretin hormone; causes release of HCO3- ions; CCK hormone causes release of enzymes in pancreatic juice.
moving food down small intestine:
- Haustral contractions: local movements within each haustral segment
Mass movments: long, slow moving contractions forcing the contents toward the rectum.
- Defecation reflex: causes sigmoid colon and rectum to contract; feces move into anal canal stimulating external (voluntary) anal sphincter action.
- Salivary amylase: breaks down starch molecules in the mouth → maltose
- Pepsin: breaks the chemical bonds between amino acids tyrosine and phenylalamine→ polypeptides.
- Trypsin and chymotrypsin (from pancreas) digests polypeptides into small pieces within the small intestine
- Lipases: from pancreas: fat digesting enzymes; breaks down emulsified fats into 3 fatty acid molecules and one molecule of glycerol
- Pancreatic nucleases: hydrolyze DNA and RNA present in foods; form nucleotide monomers.
- Micelles: fatty acids and glycerol produced by lipase, combine with bile salts a phospholipid in bile (lecithin) to form micelles these allow for the fatty materials to diffuse into intestinal cells for absorption.
Chylomicrons: fatty acids and glycerol recombine to form triglycerides which combine with cholesterol and proteins→ chylomicrons; Golgi apparatus then prepares these for movement out of the cells.
- Transport of iron in the blood: the ionic form needed for hemoglobin is combined with ferritin, a protein where it is stored in intestinal mucosa. The iron moves through the blood bound to a plasma protein, transferring.
- Vitamin D: allows for the absorption of calcium.
- Ferritin: see #33 above.