Digestive system Csaba Bödör, bodor@korb1.sote.hu Chapter 45, (page 875- )

Food processing and nutrition ingestion: taking of food into the digestive cavity digestion: process of breaking down food mechanically and chemically absorption: nutrients pass through the lining of the digestive tract and into the blood egestion (or elimination): food that is not digested and absorbed is discharged from the body

Digestive system in most invertebrates and in all vertebrates digestive tract is a complete tube with openings to each ends digestion takes place as food passes through the tube

Digestive system some parts of the tract are specialized to perform specific functions Food passes through: mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus salivary glands, pancreas, liver

Digestive system In the wall of the digestive tract: 4 layers (Fig. 45-5) mucosa: a layer of epithelial tissue and underlying connective tissue lines the lumen of the digestive tract highly folded in stomach and intestine > incr. surface submucosa: connective tissue layer rich in blood vessels, lymph vessels and nerves muscle layer: 2 sublayers of smooth muscle fibers inner circular fibers, outer longitudinal fibers peritoneum: under the diaphragm, the outer connective tissue coat of the dig. tract > visceral peritoneum it is connected to the parietal peritoneum that lines abdominal and pelvic cavities > peritoneal cavity

Digestive system some parts of the tract are specialized to perform specific functions Food passes through: mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus salivary glands, pancreas, liver

Digestive system Food processing begins in the mouth Mechanical and enzymatic digestion of carbohydrates begins in the mouth teeth and salivary glands (3 pairs)

Digestive system Types of teeth Type Shape Function Incisors chisel biting Canines long, pointed tearing Premolars flattened surface crushing grinding Molars (2-1-2-3) x 4 = 32

Digestive system Structure of teeth the hardest substance in the body !!! Resembles the hardness of bones

Digestive system Food processing begins in the mouth Mechanical and enzymatic digestion of carbohydrates begins in the mouth teeth and salivary glands (3 pairs) They produce saliva (1 liter each day) enzyme: salivary amilase (carbohydrates (starch) > sugars)

Digestive system After chewing bolus (lump) is formed … A bite of food is swallowed and moved through the pharynx and esophagus to the stomach Role of the epiglottis (see respiratory system!) Food is moved by the propulsive activity if the digestive tract It is peristalsis; waves of muscular contractions > that push the food in one direction The mixing and propulsive movements of the digestive tract are known as motility

Digestive system Stomach: mechanical and enzymatic digestion Large, muscular organ The entrance is normally closed by the sphincter Folds of the wall: rugae, wrinkled appearance lined with columnar epith. large amounts of mucus gastric glands > deep in the stomach wall parietal cells: HCl, chief cells: pepsinogen

Digestive system Stomach: mechanical and enzymatic digestion food is mechanically digested by vigorous churning gastric glands; parietal cells > HCl, intrinsic factor chief cells > pepsinogen pepsinogen: inactive enzyme precursor comes in contact with gastric juice converted into pepsin the active enzyme pepsin: the main digestive enzyme of the stomach proteins > short polypeptides the wall of the stomach is protected from acidic gastric juice > alkaline mucus, epithelial cells fit tightly together

Digestive system After several hours in the stomach… partially digested food: chyme („thick soap”) salivary amilase inactivated by acidic pH chyme is released through the pylorus (exit) into the small intestine Digestion is completed in the small intestine Small intestine (5-6 meters long): 3 regions duodenum: most chemical digestion takes place here bile (emulsifies fats) from liver and enzymes from pancreas are released here, they act on the chyme jejunum and ileum Surface area is increased by folds in the wall, intestinal villi and intestinal microvilli

Digestive system

To increase the surface Digestive system Villi and microvilli To increase the surface Digestion, absorption

Digestive system Liver, the largest internal organ secretes bile: important in mechanic digestion fats adding or removing nutrients to and from the blood (homeost.) converts glucose to glycogen and stores it converts excess amino acids into fatty acids stores iron and vitamins detoxifies alcohol and other drugs and poisons a single liver cell: more than 500 metabolic activities bile (water, bile salts, bile pigments, cholesterol), released by the gall bladder, no digestive enzymes !!!

Digestive system Pancreas secretes digestive enzymes The pancreas releases enzymes that digest protein, lipid, and carbohydrates, as well as RNA and DNA trypsin, chymotripsin: polypeptides > dipeptides pancreatic lipase: digests fats pancreatic amilase: almost all types of carbohydrates > disacch. (except cellulose) ribonucleases, deoxyribonucleases: RNA and DNA > nucleotides they act on the chyme in the duodenum! nutrients are digested as they move through the digestive tract

a cells: glucagon b cells: insulin

Digestive system Summary of the digestion

Digestive system Polysaccarides are digested to the disaccharide maltose by salivary and pancreatic amylases Maltase splits maltose into glucose Proteins are split by pepsin, trypsin, chymotripsin into small peptides, dipeptides Small peptides, dipeptides > proteolytic enzymes (dipeptidases) into the final product of amino acids Lipids are emulsified by bile salts and then hydrolyzed by pancreatic lipase into fatty acids and monoacidglycerols

Digestive system Regulation by nervous system and hormones Extensive network of neurons: enteric nervous system Polypeptides secreted by endocrine cells of the mucosa of the digestive tract.

Digestive system Absorption Takes place mainly through the villi of the small intestine Water, simple sugars, alcohol, salts … are small enough for absorption 2 single epithelial linings Absorption occurs by: Diffusion Facilitated diffusion Active transport Amino acids, glucose: directly transported to the liver (by hepatic portal vein) Free fatty acids and monoacylglycerols > reassembled in epithelial cell > triacylglycerols > +cholesterol > chylomicrons > lymph vessel > circulation (90% of fats)

Digestive system Large intestine eliminates waste Undigested material, unabsorbed chyme passed into large intestine Shorter but larger in diameter, water and sodium absorbed Incubates bacteria that produce Vitamin K and certain B vitamins the small intestine joins it few cm-s from its end: cecum (blind pouch) vermiform appendix: projects from the end of the cecum regions of the large intestine: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus Elimination of wastes (defecation)

Digestive system

Digestive system Minerals Vitamins Organic compounds required in small amounts for many biochemical processes Many serve as components of coenzymes Fat soluble vitamins: D, E, K, A Water soluble vitamins: C and B complex Minerals Minerals are inorganic nutrients ingested as salts dissolved in food and water Trace elements are minerals required in amounts less than 100 mg per day

Vitamins

Some important minerals and their functions

How the body uses protein How the body uses fat

Most carbohydrates are ingested in the form of complex carbohydrates Fiber is a mixture of cellulose and other indigestible carbohydrates Carbohydrates are used mainly as an energy source Lipids are used as an energy source, Components of cell membranes To synthesize steroid hormones and other lipid substances Most lipids are ingested in the form of triacyglycerols Proteins serve as enzymes, Are essential structural components of cells The best distribution of essential amino acids is found in the complex proteins of animal foods

Under-nutrition > protein deficiency > kwashiorkor > edema