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1 Chapter 18 The Digestive System

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2 I. Introduction to the Digestive System

3 Digestion From food, humans must get basic organic molecules to make ATP, build tissues, and serve as cofactors and coenzymes. Digestion breaks polymers (carbohydrates, fats, and proteins) into monomer building blocks. Via hydrolysis reactions Absorption takes these monomers into the bloodstream to be allocated.

4 Hydrolysis of Polymers

5 Digestive Tract Functions

Motility Ingestion: taking food into the mouth Mastication: chewing Deglutination: swallowing Peristalsis: one-way movement through tract Segmentation: churning/mixing

6 Digestive Tract Functions

2. Secretion Exocrine: digestive enzymes, acid, mucus Endocrine: hormones to regulate digestion 3. Digestion Breaking food down into smaller units 4. Absorption Movement of food molecules into blood or lymph

7 Digestive Tract Functions

5. Storage and elimination Temporary storage and elimination of undigested food Immune barrier Simple columnar epithelium with tight junctions prevents swallowed pathogens from entering body. Peyer’s patch (lymphoid tissues)

8 Digestive System Divisions

Gastrointestinal tract: 30 feet long Mouth  Stomach  Pharynx  Esophagus  Small intestines  Large intestines  Anus Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, pancreas

9 Digestive System Divisions

10 Digestive Tract Open at both ends and continuous with the environment

Considered to be “outside” the body Materials that cannot be digested (cellulose) never actually “enter” the body.

11 GI Tract Layers There are four tunics:

Mucosa: inner secretory and absorptive layer; may be folded to increase surface area Submucosa: underlying connective tissue; very vascular, to pick up nutrients; also has some glands Muscularis externa: smooth muscle; responsible for peristalsis and segmentation Serosa: outer connective tissue layer

12 GI Tract Layers

13 Regulation of the GI Tract

EXTRINSIC: ANS and hormones Parasympathetic division: Vagus nerve stimulates esophagus, stomach, small intestine, pancreas, gallbladder, and large intestine up to the splenic flexure Pelvic splanchnic nerves in sacral region stimulate descending colon, sigmoid colon, rectum, and anus. Preganglionic neurons synapse on submucosal plexus and myenteric plexus.

14 Regulation of the GI Tract

Sympathetic division: Inhibits peristalsis and secretion Stimulates contraction of sphincters Hormones: From brain and digestive organs

15 Regulation of the GI Tract

INTRINSIC: Intrinsic sensory neurons in gut wall help in intrinsic regulation via separate enteric nervous system Myenteric plexus (Auerbach’s plexus) Submucosal plexus (Meissner’s plexus) Paracrine signals

16 II. From Mouth to Stomach

17 Mouth Mastication: Chewing breaks food down into smaller pieces for deglutition and mixes it with saliva. Muscles of mastication – masseter, temporalis, buccinator, medial pterygoid, lateral pterygoid Saliva: contains mucus, an antimicrobial agent, and salivary amylase to start digestion of starch.

18 Deglutition Three parts:

Oral: voluntary; muscles of mouth and tongue mix food with saliva to form a bolus. Pharyngeal: initiated by receptors in the posterior oral cavity and oropharynx Uvula ascends to cover nasopharynx, and epiglottis folds down to cover the larynx; diverts food into the esophagus Upper esophageal sphincter relaxes.

19 Deglutition Esophageal: controlled by swallowing center of brain stem (medulla) Upper 1/3 contains skeletal muscle (voluntary) Lower 2/3 contains smooth muscle (involuntary) Bolus is moved down esophagus to stomach via peristalsis

20 Esophagus Passes through the diaphragm via the esophageal hiatus

Lined with nonkeratinized stratified squamous epithelium Upper portion has skeletal muscle; lower portion smooth muscle Lower esophageal sphincter opens to allow food to pass into stomach. It stays closed to prevent regurgitation.

21 Stomach Functions: Stores food

Churns food to mix with gastric secretions Begins protein digestion Acidic environment kills bacteria in the food Moves food into small intestine in the form of chyme

22 Stomach – Gross Structures

Connected to esophagus = cardia Upper region = fundus Lower region = body Distal region = pyloris Muscular gate that separates stomach from duodenum = pyloric sphincter Lining has folds called rugae.

23 Stomach – Gross Structures

24 Stomach – Microscopic Structures

Gastric pits at base of folds lead to gastric glands with secretory cells: Mucus neck cells secrete mucus to help protect stomach lining from acid. Parietal cells secrete HCl acid and intrinsic factor (helps small intestine absorb vitamin B12). Chief cells secrete pepsinogen.

25 Stomach – Microscopic Structures

Enterochromaffin-like (ECL) cells secrete histamine and serotonin (paracrine signals). G cells secrete gastrin (hormone). D cells secrete somatostatin (hormone). Stomach cells also secretes ghrelin hormone, which is a hunger signal.

26 Stomach – Microscopic Structures

27 HCl Secretion Primary active transport of H+ via H+/K+ ATPase pumps (“proton pump”) Secondary active transport of Cl- into cell w/ HCO3- out Facilitated diffusion of Cl- into the lumen of stomach

28 Stimulation of HCl Secretion

G cells secrete Gastrin – carried to parietal cells in blood  stimulates secretion of HCl ECL cells, stimulated by gastrin, secrete histamine Histamine stimulates parietal cells via H2 histamine receptors to secrete HCl Vagus nerve (parasympathetic) stimulate parietal and ECL cells

29 Function of HCl Decrease pH < 2, which serves 3 functions:

Proteins are denatured (allows enzymes access). Pepsinogen is converted to active pepsin (digests proteins). Serves as the optimal pH for pepsin activity

30 Function of HCl

31 Stomach Defenses Acid and pepsin could errode the stomach lining.

Three processes help prevent errosion: Adherent layer of mucus with bicarbonate Tight junctions between epithelial cells Rapid epithelial mitosis that replaces epithelium every three days

32 Digestion and Absorption in the Stomach

Proteins begin digestion in the stomach. Starches begin digestion in the mouth, but salivary amylase is not active at pH 2, so this activity stops in the stomach. Alcohol and NSAIDs (aspirin) are the only common substances absorbed in the stomach (due to high lipid solubility).

33 Peptic Ulcers Peptic ulcers: erosions of the mucosa of the stomach or duodenum Helicobacter pylori: bacterium that reduces mucosal barriers to acid Treatment for ulcers combines K+/H+ pump inhibitors (Prilosec) and antibiotics.

34 Gastritis Inflammation of the submucosa caused by acid irritation

Histamine released as part of the inflammatory response can stimulate more acid release. Prostaglandins are needed to stimulate protective alkaline mucus production. NSAIDs inhibit prostaglandin activity and can lead to gastritis. H2 blockers (Tagamet and Zantac) inhibit H2 receptors.

35 III. Small Intestine

36 Small Intestine Structures

Three sections: Duodenum Jejunum Ileum Mucosa and submucosa folded into plicae circulares; mucosa further folded into villi; and epithelial plasma membranes folded into microvilli

37 Small Intestine Structure

38 Small Intestine Functions

Complete digestion of carbohydrates, proteins, and fats Absorption of nutrients Sugars, lipids, amino acids, calcium, and iron absorbed in duodenum and jejunum Bile salts, vitamin B12, water, and electrolytes absorbed in ileum Very rapid due to villi and microvilli

39 Villi and Microvilli Columnar epithelium with goblet cells (mucus)

Capillaries absorb sugars and amino acids, and lacteals absorb fatty acids. Intestinal crypts with Paneth cells (secrete antibacterial molecules) and mitotic stem cells

40 Intestinal Enzymes Called brush border enzymes

Not released into lumen, but stay attached to plasma membrane with active site exposed to chyme

41 Intestinal Enzymes

42 Intestinal Contractions/Motility

Peristalsis is weak. Movement of food is much slower due to pressure at pyloric end. Segmentation is stronger and serves to mix the chyme.

43 Intestinal Contractions/Motility

44 Intestinal Contractions/Motility

Smooth muscle contractions occur automatically. Graded depolarizations called slow waves produced by pacemaker cells called interstitial cells of Cajal produce action potentials in muscle cells.

45 Intestinal Contractions/Motility

Depolarization from the interstitial cells of Cajal opens voltage-gated Ca2+ channels in muscle cells  action potential and contraction. Stimulation travels short distances through gap junctions but must be regenerated in neighboring pacemaker regions. Produces contractions needed for segmentation

46 Regulation of Contraction

Autonomic nerves influence enteric nervous system to stimulate or inhibit cells of Cajal. Acetylcholine from parasympathetic system interacts with muscarinic ACh receptors to increase amplitude and duration of slow waves.

47 Regulation of Contraction

48 IV. Large Intestine

49 Large Intestine Structure

Chyme from ileum passes through ileocecal valve into: Cecum Ascending colon  Transverse colon  Descending colon  Sigmoid colon  Rectum  Anal canal  Anus

50 Large Intestine Functions:

Absorption of water, electrolytes, vitamin K, and some B vitamins Production of vitamin K and B vitamins via microbial organisms Storage of feces

51 Large Intestine

52 Microbial Biota Several hundred different species of bacteria live in the large intestine. Some are commensal. The bacteria benefit, and we aren’t harmed. Others are mutualistic. Both (human and bacteria) benefit from each other.

53 Benefits from Microbes

Microbes make vitamin K and some B vitamins. They also make fatty acids from cellulose. Some of these are used for energy by large intestine epithelial cells. They help absorb electrolytes such as sodium, calcium, bicarbonate, magnesium, and iron. They outcompete harmful species of bacteria. Disruption of normal microflora can lead to irritable bowel disease.

54 Absorption of Fluids Most absorption occurs in small intestine, but some is left for large intestine. Not all water is absorbed; about 200 mL is left per day to be excreted with feces. Water is absorbed passively following an osmotic gradient set up by active Na+/K+pumps. Aldosterone stimulates greater salt and water absorption here.

55 Defecation As feces passes to the rectum, pressure there increases, the internal anal sphincter relaxes, and the need to defecate rises. The external anal sphincter controls defecation voluntarily. During defecation, longitudinal rectal muscles contract to increase pressure as the anal sphincters relax.

56 V. Liver, Gallbladder, and Pancreas

57 Liver Largest abdominal organ

Has amazing regenerative abilities due to mitosis of hepatocytes Composed of hepatocytes that form hepatic plates separated by capillaries called sinusoids Very permeable, allowing passage of blood proteins, fat, and cholesterol

58 *Major Liver Functions

Carbohydrate metabolism Glucose ↔ glycogen ↔ fat Glycogen, AA, lactic acid → glucose Secretion of glucose into the blood Lipid metabolism Synthesis of triglyceride and cholesterol Excretion of CHO in bile Fatty acids → ketone bodies

59 *Major Liver Functions

Protein synthesis Albumins, clotting factors, transport proteins Detoxification of blood Phagocytosis by Kupffer cells Chemical alterations of hormones and drugs Production of urea, uric acid, and other less toxic compounds Secretion of bile Synthesis of bile salts Conjugation and excretion of bile pigment

60 Hepatic Portal System Products of digestion absorbed in intestines are delivered to the liver via the hepatic portal vein Recall that hepatic portal vein receives venous blood from splenic, superior mesenteric, and inferior mesenteric veins Inside the liver, a branch of hepatic portal vein, branch of hepatic artery, and bile ductule make up the portal triad

61 Hepatic Portal System

62 Liver Lobules Hepatic plates are arranged in hexagonal patterns with portal triads at each end and a central vein in the center Hepatic artery nourish hepatocytes Hepatic portal vein delivers nutrients and toxins from gut to hepatocytes Bile secreted by the hepatocytes is released into bile canaliculi, which drain into bile ducts. Central vein drains venous blood from hepatocytes and dump into IVC

63 Liver Lobules

64 Hepatic Portal System

65 Liver Lobules

66 Secretion of Drugs into Bile

Aside from bile, the liver secretes other substances into the bile ducts to clear them from the blood. These are then excreted in feces.

67 Secretion of Drugs into Bile

68 Enterohepatic Circulation

Some of the molecules released into the bile are absorbed again in the small intestine and returned to the liver. These molecules are part of enterohepatic circulation.

69 Bile Production The liver makes 250–1,500 ml of bile per day.

Function: Act as detergent to emulsify lipids in food Bile is composed of: Bile pigments (bilirubin) Bile salts Phospholipids (lecithin) Cholesterol Inorganic ions

70 Bilirubin Produced in spleen, liver, and bone marrow

Also derived from RBC breakdown Macrophages converts heme  biliverdin  unconjugated bilirubin Unconjugated bilirubin (not water-soluble) Transported by albumin in the blood to liver Liver binds glucuronic acid to bilirubin (becomes conjugated bilirubin) to make it water-soluble in order for it to be secreted as bile

71 Bilirubin Conjugated bilirubin is secreted into the bile, where it is taken to the small intestine. Bacteria there turn it into urobilinogen, which makes feces brown. 30−50% is absorbed by the intestines and taken back to the liver. Some is used to make bile, and some remains in blood to be filtered by the kidneys.

72 Bilirubin Production

73 Conjugated Bilirubin

74 Bile Salts Made from bile acids conjugated with glycine or taurine

Bile acids (derived from cholesterol) Four polar groups on each molecule Cholic acid and chenodeoxycholic acid Most is recycled in enterohepatic circulation. ½ gram of cholesterol is broken down and lost in the feces through this pathway.

75 Bile Salts Form micelles with polar groups toward water

Fats enter the micelle and are emulsified.

76 Detoxification of Blood

The liver can remove hormones, drugs, and other substances in three ways: Secreted into bile Phagocytized by Kupffer cells lining sinusoids Chemically altered by hepatocytes Ammonia is converted into urea. Urea is returned to the blood to be filtered by the kidneys. Steroids are altered and then secreted into bile.

77 Gallbladder Stores and concentrates bile from the liver:

Liver  Bile ducts  Hepatic duct  Cystic duct  Gallbladder Gallbladder  Cystic duct  Common bile duct  joins Main pancreatic duct at Sphincter of Oddi  duodenum

78 Gallbladder

79 Pancreas Has endocrine and exocrine functions

Endocrine: Islets of Langerhans cells (alpha and beta) make insulin and glucagon. Exocrine: Acini cells make pancreatic juice, which is delivered to the duodenum via the pancreatic duct.

80 Pancreatic Enzymes

81 Bicarbonate Made by cells lining ductules Made from CO2 from the blood

First, carbonic acid is made. This dissociates to form H+ and bicarbonate. The bicarbonate is secreted into pancreatic juice, and H+ goes back into the blood. Bicarbonate is counter-transported with Cl−. People with cystic fibrosis have trouble secreting bicarbonate, which can lead to destruction of the pancreas.

82 Bicarbonate

83 Pancreatic Enzymes Most are inactive until they reach the small intestine. Enterokinase (brush border enzyme) activates trypsinogen  trypsin (to digest protein). Trypsin activates other enzymes.

84 VI. Neural and Endocrine Regulation of the Digestive System

85 Gastric Regulation Cephalic phase: control by brain via vagus nerves

Divided into three phases: Cephalic phase: control by brain via vagus nerves Sight/smell/thought of food can stimulate salivation and gastric secretions Stimulation goes from brain to organ via vagus nerve Stimulates G cells, ECL cells, chief cells, and parietal cells Lasts for the first 30 minutes of a meal

86 Gastric Regulation Divided into three phases: Gastric phase: triggered by arrival of food into stomach Gastric distension (due to amount of food that enters) and amino acids in food stimulate vagus nerve to signal acid secretion (mainly via ECL cells). Positive feedback occurs; as more proteins are broken down, more secretions are released to break them down.

87 Gastric Regulation Gastric phase (cont)

There is also a negative-feedback system. As pH drops < 2.5 (due to more HCl), somatostatin is released. This inhibits gastrin secretion. Lots of proteins buffer pH, so secretion matches protein concentration.

88 Gastric Regulation Divided into three phases: 3. Intestinal phase: inhibition of gastric activity when chyme enters the small intestine Stretch of the duodenum stimulates a neural reflex that inhibits gastric stimulation via the vagus nerve. The presence of fats stimulates the duodenum to make enterogastrone. Enterogastrone inhibits gastric secretions. Several specific hormones have been identified with enterogastrone activity (CCK, GIP, GLP-1).

89 Gastric Regulation Intestinal phase (cont) - when chyme enters the duodenum, two hormones are produced: Secretin is produced by duodenum in response to a the low pH in chyme Stimulate production and secretion of bicarbonate and H2O in pancreatic juice Cholecystokinin (CCK) is produced in response to the presence of partially digested proteins and fats in chyme. Stimulate secretion of pancreatic enzymes to help digest these molecules Stimulate gallbladder to secrete bile

90 Summary of Gastric Regulation

91 Summary of Gastric Acid Secretion

92 Regulation of Intestinal Function

Enteric nervous system: neurons and glial cells that innervate the intestines Includes myenteric plexus (Auerbach) and submucosal plexus (Meissner) Acts independently from CNS but with some feedback to CNS via vagus nerve Innervates interstitial cells of Cajal

93 Enteric Nervous System Coordinates Peristalsis

94 Regulation of Intestinal Function

Paracrine regulation: Enterochromaffin-like (ECL) cells in intestinal mucosa secrete serotonin and motilin in response to pressure (filling) and chemicals in the food. This stimulates muscle contractions. Guanylin: made in ileum and colon; stimulates the secretion of water and Cl−and inhibits absorption of Na+. More water and salt are lost in feces.

95 Regulation of Intestinal Function

Intestinal reflexes: Gastroileal reflex: increased gastric activity = increased ileum activity and movement of food through ileocecal valve Ileogastric reflex: distension of ileum = decrease in gastric motility Intestino-intestinal reflex: over-distension of one portion of the intestine causes relaxation of other portions.

96 Summary of Gastrointestinal Hormones

97 VII. Digestion and Absorption of Carbohydrates, Lipids, and Proteins

98 Digestion of Carbohydrates

Starch digestion begins in mouth with salivary amylase and continues in intestines with pancreatic amylase. Brush border enzymes (maltase, sucrase, lactase) finish breaking down resulting products and other disaccharides (maltose, sucrose, lactose).

99 Absorption of Carbohydrates

Monosaccharides are absorbed across the epithelium via: Secondary active transport with sodium Facilitated diffusion of glucose into blood

100 Digestion of Proteins Begins in stomach with pepsin to produce short-chain polypeptides Finishes in duodenum and jejunum with pancreatic trypsin, chymotrypsin, elastase, and carboxypeptidase, and the brush border enzyme aminopeptidase.

101 Absorption of Proteins

Free amino acids cotransported with Na+ Dipeptides and tripeptides cross via secondary active transport using a H+gradient.

102 Digestion of Fats Fat digestion begins in duodenum when bile emulsifies the fat and the pancreatic enzyme lipase breaks it down into fatty acids. Phospholipase A (from pancreas) digests phospholipids into fatty acids.

103 Digestion of Fats

104 Fat Emulsification and Digestion

105 Absorption of Fats Fatty acids and monoglycerides move into bile micelles and are transported to brush border. Inside the epithelial cells, they are regenerated into triglycerides, cholesterol, and phospholipids and combined with proteins to form chylomicrons. These enter the lacteals.

106 Fat Absorption

107 Transport of Lipids in Blood

The lymphatic system drops chylomicrons into the bloodstream at the thoracic duct. They pick up an apolipoprotein, which allows them to bind to receptors on the capillary endothelium within muscles and adipose tissue. Here they are digested by lipoprotein lipase, which releases free fatty acids for use by muscle cells or for storage by fat cells.

108 Transport of Lipids in Blood

Cholesterol and triglycerides made in the liver are combined with other apolipoproteins to form very-low-density lipoproteins (VLDLs) to deliver triglycerides to organs. Once triglycerides are removed, they become low-density lipoproteins (LDLs), which transport cholesterol to organs. Excess cholesterol is returned to the liver on high-density lipids (HDLs).

109 Lipid Carrier Proteins

110 *Summary of Major Digestive Enzymes