Transcript Ch. 25 D
Mixes chyme with enzymes and bile Churns chyme to increase contact with mucosa for absorption and digestion Moves residue towards large intestine 25-1 Segmentation random ringlike constrictions mix and churn contents 12 times per minute in duodenum Peristaltic waves begin in duodenum but each one moves further down push chyme along for 2 hours suppressed by refilling of stomach Food in stomach causes gastroileal reflex (relaxing of valve and filling of cecum) 25-2 Purpose of segmentation is to mix and churn not to move material along as in peristalsis 25-3 Gradual movement of contents towards colon Begins after absorption occurs Migrating motor complex controls waves of contraction second wave begins distal to where first wave began 25-4 Salivary amylase stops working in stomach (pH < 4.5) 50% of dietary starch digested before it reaches small intestine Pancreatic amylase completes first step in 10 minutes Brush border enzymes act upon oligosaccharides, maltose, sucrose, lactose and fructose lactose indigestible after age 4 in most humans (lactase declines) 25-5 Glucose may also be transported by “solvent drag”. Water leaving the stomach or intestine ‘drags’ solutes with it. Sodium-glucose transport proteins (SGLT) in membrane help absorb glucose and galactose Fructose absorbed by facilitated diffusion then converted to glucose inside the cell 25-6 Pepsin has optimal pH of 1.5 to 3.5 -- inactivated when passes into duodenum and mixes with alkaline pancreatic juice (pH 8) 25-7 Pancreatic enzymes take over protein digestion by hydrolyzing polypeptides into shorter oligopeptides 25-8 Brush border enzymes finish task, producing amino acids that are absorbed into intestinal epithelial cells amino acid cotransporters move into epithelial cells and facilitated diffusion moves amino acids out into blood stream Infants absorb proteins by pinocytosis (maternal IgA) 25-9 Bile acids are the same as bile salts. 25-10 25-11 Chylomicrons are produced in the epithelial cells of the small intestine as lipids are absorbed. The triglycerides (lipids) are taken up by the lacteals NOT the capillaries. 25-12 Nucleases hydrolyze DNA and RNA to nucleotides Vitamins are absorbed unchanged nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar and nitrogenous bases A, D, E and K with other lipids -- B complex and C by simple diffusion and B12 if bound to intrinsic factor Minerals are absorbed all along small intestine Na+ cotransported with sugars and amino acids Cl- exchanged for bicarbonate by stomach Iron and calcium absorbed as needed 25-13 Digestive tract receives about 9 L of water/day .7 L in food, 1.6 L in drink, 6.7 L in secretions 8 L is absorbed by small intestine and 0.8 L by large intestine Water is absorbed by osmosis following the absorption of salts and organic nutrients Diarrhea occurs when too little water is absorbed 25-14 25-15 5 feet long and 2.5 inches in diameter in cadaver Begins as cecum and appendix in lower right corner Ascending, transverse and descending colon frame the small intestine Sigmoid colon is S-shaped portion leading down into pelvis Rectum - straight portion ending at anal canal 25-16 Mucosa - simple columnar epithelium No circular folds or villi to increase surface area Intestinal crypts (glands sunken into lamina propria) produce mucus only Muscularis externa anal canal has stratified squamous epithelium muscle tone in longitudinal muscle fibers (concentrated in taeniae coli) form pouches (haustra) Transverse and sigmoid have a serosa, rest retroperitoneal epiploic appendages are suspended fatty sacs 25-17 Bacterial flora populate large intestine Make up about 30% of the volume of feces ferment cellulose and other undigested carbohydrates; we absorb resulting sugars synthesize vitamins B and K Flatus (gas) average person produces 500 mL per day most is swallowed air but hydrogen sulfide, indole and skatole produce odor 25-18 Transit time is 12 to 24 hours reabsorbs water and electrolytes Feces consist of water and solids (bacteria, mucus, undigested fiber, fat and sloughed epithelial cells) Haustral contractions occur every 30 minutes Mass movements occur 1 to 3 times a day triggered by gastrocolic and duodenocolic reflexes 25-19 Anal canal is 3 cm total length Anal columns are longitudinal ridges separated by mucus secreting anal sinuses Hemorrhoids are permanently distended veins 25-20 1. Filling of the rectum stimulates stretch receptors 2. A spinal reflex stimulates contraction of rectum 3. The reflex also relaxes the internal anal sphincter 3. The brain controls voluntary relaxation of external sphincter 25-21 25-22