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The Digestive System Part 2. What has happened to the food so far? Food has been ingested Mechanical digestion has occurred in the mouth increasing the.

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Presentation on theme: "The Digestive System Part 2. What has happened to the food so far? Food has been ingested Mechanical digestion has occurred in the mouth increasing the."— Presentation transcript:

1 The Digestive System Part 2

2 What has happened to the food so far? Food has been ingested Mechanical digestion has occurred in the mouth increasing the surface area and forming a food bolus Salivary amylase has been secreted in the saliva and carbohydrates have been broken down into smaller chains Lingual lipase has been added to the food bolus Peristaltic motion of the esophagus has moved the bolus to the cardiac sphincter of the stomach

3 The Stomach Mechanically breaks up food particles Liquefies the food Begins the chemical digestion of proteins and a small amount of fat Produces CHYME—pasty mixture of semidigested food that is passed into the small intestine

4 Four Regions of the Stomach Cardiac Region—anterior section (nearest the heart); receives food from esophagus Fundic Region—Dome shape portion superior to the esophageal attachment Body Region—Greatest part of the stomach Pyloric Region—posterior section; where food moves into the small intestine

5 The Wall of the Stomach Has a layer of mucosa on the inside of the stomach The gastric mucosa contains depressions called gastric pits Cells in bottom of pits replace epithelial cells that are sloughed off into chyme Gastric pits also contain glands that are made of various cell types

6 The Role of Hydrochloric Acid in the Stomach Gastric juice has high concentration of HCl and can have a pH as low as 0.8 Functions of acidic environment: 1.Activates enzymes pepsin and lingual lipase 2.Breaks up connective tissues & plant cells walls, helps to liquefy food & form chyme 3.Converts ingested ferric ions (Fe 3+ ) to ferrous ions (Fe 2+ ) to be used in hemoglobin production

7 Types of Cells found in Gastric Pits Mucous Cells—predominate cardiac & pyloric glands; secrete mucous Regenerative (stem) Cells—produce a continual supply of new cells to replaced sloughed off cells Parietal Cells—secrete HCl & intrinsic factor; found mostly in gastric glands Chief Cells—Most numerous cell type, secrete chymosin & lipase in infancy and pepsinogen throughout life Enteroendocrine Cells—Secrete hormones & other messengers that regulate digestion

8 Digestive Enzymes in the Stomach Pepsinogen is converted to PEPSIN, an enzyme that breaks down proteins to small amino acid chains In infancy, gastric lipase digests the butterfat of milk In infancy, chymosin (renin) curdles milk Intrinsic Factor—Essential for absorption of vitamin B 12 in the small intestine

9 Gastric Motility The medulla swallowing center signals the stomach to relax when we swallow; this prepares the stomach to receive food Arriving food stretches the stomach The stomach begins peristaltic contractions governed by pacemaker cells in the stomach wall Contractions occur every 20 seconds and get stronger as they move throughout the stomach After 30 minutes the contractions are strong About 3 mL of chyme at a time is squirted into the small intestine Typical meal is emptied from the stomach in 4 hours

10 Why does the stomach not digest itself? It has a mucous coat that is highly alkaline (basic) Epithelial cells live only 3 to 6 days and then are sloughed off into the chyme and replaced Epithelial cells are joined by tight junctions that prevent gastric juice from seeping between them

11 Accessory Organs Liver—Secretes bile into small intestine; home to the gall bladder; after a meal the liver removes glucose, amino acids, vitamins, and other nutrients for its (liver’s) metabolism & storage Gall Bladder—Sac on the underside of the liver that stores & concentrates bile Bile—Yellow-green fluid containing minerals, cholesterol, fats, phospholipids, bile pigments & bile acids; principal pigment is bilirubin derived from the breakdown of hemoglobin; bile acids aid in fat digestion & absorption

12 Accessory Organs Pancreas—both an endocrine (insulin & glucagon) and exocrine organ; secretes about 1,200 to 1,500 mL of pancreatic juice per day; pancreatic juice is alkaline (basic) and buffers chyme from stomach in the small intestine; pancreatic enzymes include trypsinogen, chymotrypsinogen, and procarboxypeptidase; pancreatic amylase, pancreatic lipase, ribonuclease, and deoxyribonuclease

13 What has happened to the food so far? The bolus from the esophagus has moved into the cardiac sphincter of the stomach The stomach mechanically digests the food The churning mixes the food with pepsin that breaks down protein into smaller amino acid chains And, lingual lipase is activated beginning to break down lipid (fat) molecules The food material becomes a pasty substance called CHYME The chyme is released into the small intestine through the pyloric sphincter about 3 mL at a time.

14 The Small Intestine The site of nearly all of the chemical digestion The site of nearly all nutrient absorption into the bloodstream Has a large surface area exposed to chyme because of the presence of villi and microvilli— folds of the mucous membranes of the small intestine 6 to 7 m (18 – 22 ft) in cadaver; 2 m long (6.5 ft) in living person because of muscle tone Named “small” intestine because diameter is smaller than the “large” intestine

15 Three Regions of the Small Intestine Duodenum—First 25 cm beginning at the pyloric valve; receives chyme from stomach and the pancreatic juice & bile; stomach acid is neutralized here; fats are broken up by bile acids, pepsin is inactivated by high pH, and pancreatic enzymes take over chemical digestion Jejunum—Next 2.5 m Ileium—Last 3.6 m

16 Specialized Structures of the Small Intestine Villi—Fingerlike projections of the mucosa that increase SA Covered with absorptive cells (to move nutrients to blood stream) Core contains an arteriole, a capillary network (for nutrient absorption), a venule, and a lymphatic capillary called a lacteal (absorbs fat)

17 Specialized Structures of the Small Intestine Microvilli—Tiny projections from each villus; called a brush border; further increases SA; contains brush border enzymes; chyme must come in contact with these enzymes for digestion to occur Intestinal Crypts—Similar to gastric pits; dividing stem cells replacing mucosal cells every 3 to 6 days Paneth Cells—Secrete lysozyme, & other chemicals protecting against bacterial infection Duodenal Glands—Secrete bicarbonate-rich mucus to neutralize stomach acid

18 Intestinal Secretion Intestinal crypts secrete 1 to 2 L of intestinal juice per day Fluid has a pH of 7.4 to 7.8 Contains water & mucus but little enzyme

19 Intestinal Motility Contractions of small intestine serve 3 functions: 1.Mix chyme with intestinal juice, bile & pancreatic juice for acid neutralization and chyme digestion 2.Churn chyme and bring it into contact with mucosa for contact digestion & nutrient absorption 3.Move residue toward the large intestine

20 Intestinal Motility Segmentation, ringlike constrictions, appear at several places of the small intestine simultaneously Internal pacemaker cells control contractions Contractions occur 12 times per minute in duodenum & 8 – 9 times per minute in ileum When most nutrients have been absorbed, segmentation is replaced with peristalsis which moves remaining chyme toward large intestine

21 Chemical Digestion & Absorption CARBOHYDRATES Starch is digested into oligosaccharides (chains of up to 8 glucose molecules), then to disaccharide maltose (2 glucose molecules) and finally to glucose molecules which are absorbed by the small intestine Salivary amylase breaks starch into oligosaccharides Pancreatic amylase breaks oligosaccharides into maltose in the small intestine by contact digestion Maltase hydrolyzes maltose into individual glucose molecules which are absorbed into the bloodstream

22 Chemical Digestion & Absorption PROTEINS Proteases (peptidases)—enzymes that digest proteins Pepsin begins protein digestion in the stomach; 10% to 15% of protein is digested here; pepsin works at pH 1.5 to 3.5 so only works in the stomach In the small intestine trypsin & chymotrypsin hydrolyze proteins into smaller amino acid chains Other enzymes remove one amino acid at a time by contact digestion Individual amino acids are absorbed into the bloodstream.

23 Chemical Digestion & Absorption LIPIDS—Hydrophobic quality makes their digestion & absorption more complicated Lipases—enzymes that digest fats Lingual lipase secreted in saliva is activated by stomach pH to start fat digestion; digests 10% of fat Pancreatic lipase digests most of fat in the small intestine Bile emulsifies (breaks down into droplets) fat to make digestion easier (exposing more SA to digestive enzymes) Average daily intake of fat can be digested in 1 to 2 minutes The free fatty acids and glycerol and then absorbed into the bloodstream or the lacteal duct

24 Chemical Digestion & Absorption Nucleic Acids (DNA & RNA) Present in much smaller quantities than other macromolecules Nucleases break DNA and RNA into their individual nucleotides Contact digestion then break nucleotides into sugar, phosphate and nitrogen base These products are then absorbed into the bloodstream

25 Chemical Digestion & Absorption WATER Digestive tract receives about 9 L of water per day, 0.7 L in food, 1.6 L in drink, and 6.7 L in gastrointestinal secretions 8 L of this is absorbed in the small intestine 0.8 L is absorbed by the large intestine 0.2 L voided in feces Too little water absorption in large intestine is diarrhea Too much water absorption in large intestine is constipation

26 What has happened to the food so far? After the chyme reaches the small intestine, it is mixed with pancreatic juice and bile Segmentation and peristalsis move the chyme through the small intestine Carbohydrate, protein, lipid, and nucleic acid digestion is completed Small nutrient molecules are absorbed into the bloodstream Undigested material is moved into the large intestine

27 The Large Intestine 1.5 m (5 ft) long aka colon Begins with cecum—blind pouch which has the appendix attached to it Contains ascending, transverse, descending, and sigmoid regions

28 The Large Intestine There are no villi in the large intestine There are intestinal crypts that have mucus as their only secretion Large intestine is heavily populated with normal bacterial flora that ferment cellulose, & other undigested nutrients and synthesize B vitamins & vitamin K Average person expels 500 mL of flatus (gas) per day “The hydrogen gas in flatus is combustible & has been known to explode in surgery that used electrical cauterization.”

29 Absorption & Motility It takes the large intestine about 12 to 24 hours to reduce a meal’s residue to feces It removes water & electrolytes from the residue and reabsorbs it Contractions occur every 30 minutes that move residue short distances MASS MOVEMENTS occur 1 to 3 times per day, lasting about 15 minutes that move residue several cm; they often occur after the stomach fills & within an hour after breakfast

30 Defecation Removal of waste residue from the body Internal & external anal sphincter is composed of skeletal muscle and is under voluntary control


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