Presentation on theme: "23 P A R T A The Digestive System."— Presentation transcript:
1 23 P A R T A The Digestive System
2 Digestive System: Overview
The alimentary canal or gastrointestinal (GI) tract digests and absorbs food Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas
3 Figure 23.1
4 The GI tract is a “disassembly” line
Digestive Process The GI tract is a “disassembly” line Nutrients become more available to the body in each step There are six essential activities: Ingestion, propulsion, and mechanical digestion Chemical digestion, absorption, and defecation
5 Figure 23.2
6 Intrinsic glands keep the mouth moist
Control of Salivation Intrinsic glands keep the mouth moist Extrinsic salivary glands secrete serous, enzyme-rich saliva in response to: Ingested food which stimulates chemoreceptors and pressoreceptors The thought of food Strong sympathetic stimulation inhibits salivation and results in dry mouth PLAY InterActive Physiology®: Secretion, pages 3-5
7 Gastrointestinal Tract Activities
Ingestion – taking food into the digestive tract Propulsion – swallowing and peristalsis Peristalsis – waves of contraction and relaxation of muscles in the organ walls Mechanical digestion – chewing, mixing, and churning food PLAY InterActive Physiology®: Motility, pages 3-5
8 Peristalsis and Segmentation
Figure 23.3
9 Gastrointestinal Tract Activities
Chemical digestion – catabolic breakdown of food Absorption – movement of nutrients from the GI tract to the blood or lymph Defecation – elimination of indigestible solid wastes
10 External environment for the digestive process
GI Tract External environment for the digestive process Regulation of digestion involves: Mechanical and chemical stimuli – stretch receptors, osmolarity, and presence of substrate in the lumen Extrinsic control by CNS centers Intrinsic control by local centers
11 Receptors of the GI Tract
Mechano- and chemoreceptors respond to: Stretch, osmolarity, and pH Presence of substrate, and end products of digestion They initiate reflexes that: Activate or inhibit digestive glands Mix lumen contents and move them along
12 Nervous Control of the GI Tract
Intrinsic controls Nerve plexuses near the GI tract initiate short reflexes Short reflexes are mediated by local enteric plexuses (gut brain) Extrinsic controls Long reflexes arising within or outside the GI tract CNS centers and extrinsic autonomic nerves PLAY InterActive Physiology®: Control of the Digestive System, pages 3-8
13 Nervous Control of the GI Tract
Figure 23.4
14 Histology of the Alimentary Canal
From esophagus to the anal canal the walls of the GI tract have the same four tunics From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa Each tunic has a predominant tissue type and a specific digestive function PLAY InterActive Physiology®: Anatomy Review, page 3
15 Histology of the Alimentary Canal
Figure 23.6
16 Moist epithelial layer that lines the lumen of the alimentary canal
Mucosa Moist epithelial layer that lines the lumen of the alimentary canal Three major functions: Secretion of mucus Absorption of end products of digestion Protection against infectious disease Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosae
17 Mucosa: Epithelial Lining
Simple columnar epithelium and mucus-secreting goblet cells Mucus secretions: Protect digestive organs from digesting themselves Ease food along the tract Stomach and small intestine mucosa contain: Enzyme-secreting cells Hormone-secreting cells (making them endocrine and digestive organs)
18 Mucosa: Lamina Propria and Muscularis Mucosae
Loose areolar and reticular connective tissue Nourishes the epithelium and absorbs nutrients Contains lymph nodes (part of MALT) important in defense against bacteria Muscularis mucosae – smooth muscle cells that produce local movements of mucosa
19 Mucosa: Other Sublayers
Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves Muscularis externa – responsible for segmentation and peristalsis Serosa – the protective visceral peritoneum Replaced by the fibrous adventitia in the esophagus Retroperitoneal organs have both an adventitia and serosa
20 Enteric Nervous System
Composed of two major intrinsic nerve plexuses: Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa Myenteric nerve plexus – Major nerve supply that controls GI tract mobility Segmentation and peristalsis are largely automatic involving local reflex arcs Linked to the CNS via long autonomic reflex arc PLAY InterActive Physiology®: Control of the Digestive System, page 5
21 Esophagus Figure 23.12
22 Figure 23.13 Bolus of food Tongue Uvula Pharynx Bolus Epiglottis
Trachea Esophagus Bolus (a) Upper esophageal sphincter contracted (b) Upper esophageal sphincter relaxed (c) Upper esophageal sphincter contracted Relaxed muscles Relaxed muscles Circular muscles contract, constricting passageway and pushing bolus down Bolus of food Gastroesophageal sphincter open Longitudinal muscles contract, shortening passageway ahead of bolus Stomach Gastroesophageal sphincter closed (d) (e) Figure 23.13
23 Stomach Chemical breakdown of proteins begins and food is converted to chyme Cardiac region – surrounds the cardiac orifice Fundus – dome-shaped region beneath the diaphragm Body – midportion of the stomach Pyloric region – made up of the antrum and canal which terminates at the pylorus The pylorus is continuous with the duodenum through the pyloric sphincter
24 Stomach Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous system Blood supply – celiac trunk, and corresponding veins (part of the hepatic portal system) PLAY InterActive Physiology®: Motility, page 6
25 Figure 23.14a
26 Microscopic Anatomy of the Stomach
Epithelial lining is composed of: Goblet cells that produce a coat of alkaline mucus The mucous surface layer traps a bicarbonate-rich fluid beneath it Gastric pits contain gastric glands that secrete gastric juice, mucus, and gastrin
27 Microscopic Anatomy of the Stomach
Muscularis – has an additional oblique layer that: Allows the stomach to churn, mix, and pummel food physically Breaks down food into smaller fragments
28 Microscopic Anatomy of the Stomach
Figure 23.15a
29 Microscopic Anatomy of the Stomach
Figure 23.15b
30 Microscopic Anatomy of the Stomach
Figure 23.15c
31 Digestion in the Stomach
Holds ingested food Degrades this food both physically and chemically Delivers chyme to the small intestine Enzymatically digests proteins with pepsin Secretes intrinsic factor required for absorption of vitamin B12
32 To keep from digesting itself, the stomach has a mucosal barrier with:
Stomach Lining The stomach is exposed to the harshest conditions in the digestive tract To keep from digesting itself, the stomach has a mucosal barrier with: A thick coat of bicarbonate-rich mucus on the stomach wall Epithelial cells that are joined by tight junctions Gastric glands that have cells impermeable to HCl Damaged epithelial cells are quickly replaced
33 Glands of the Stomach Fundus and Body
Gastric glands of the fundus and body have a variety of secretory cells Mucous neck cells – secrete acid mucus Parietal cells – secrete HCl and intrinsic factor
34 Glands of the Stomach Fundus and Body
Chief cells – produce pepsinogen Pepsinogen is activated to pepsin by: HCl in the stomach Pepsin itself via a positive feedback mechanism Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria PLAY InterActive Physiology®: Secretion, page 8 PLAY InterActive Physiology®: Motility, page 6
35 Regulation of Gastric Secretion
Neural and hormonal mechanisms regulate the release of gastric juice Stimulatory and inhibitory events occur in three phases Cephalic (reflex) phase: prior to food entry Gastric phase: once food enters the stomach Intestinal phase: as partially digested food enters the duodenum
36 Excitatory events include:
Cephalic Phase Excitatory events include: Sight or thought of food Stimulation of taste or smell receptors Inhibitory events include: Loss of appetite or depression Decrease in stimulation of the parasympathetic division
37 Excitatory events include:
Gastric Phase Excitatory events include: Stomach distension Activation of stretch receptors (neural activation) Activation of chemoreceptors by peptides, caffeine, and rising pH Release of gastrin to the blood
38 Inhibitory events include:
Gastric Phase Inhibitory events include: A pH lower than 2 Emotional upset that overrides the parasympathetic division
39 Intestinal Phase Excitatory phase – low pH; partially digested food enters the duodenum and encourages gastric gland activity Inhibitory phase – distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum Initiates inhibition of local reflexes and vagal nuclei Closes the pyloric sphincter Releases enterogastrones that inhibit gastric secretion
40 Release of Gastric Juice: Stimulatory Events
Figure
41 Release of Gastric Juice: Inhibitory Events
Figure
42 Regulation and Mechanism of HCl Secretion
HCl secretion is stimulated by ACh, histamine, and gastrin through second-messenger systems Release of hydrochloric acid: Is low if only one ligand binds to parietal cells Is high if all three ligands bind to parietal cells Antihistamines block H2 receptors and decrease HCl release
43 Regulation and Mechanism of HCl Secretion
Figure 23.17
44 Response of the Stomach to Filling
Stomach pressure remains constant until about 1L of food is ingested Relative unchanging pressure results from reflex-mediated relaxation and plasticity
45 Response of the Stomach to Filling
Reflex-mediated events include: Receptive relaxation – as food travels in the esophagus, stomach muscles relax Adaptive relaxation – the stomach dilates in response to gastric filling Plasticity – intrinsic ability of smooth muscle to exhibit the stress-relaxation response
46 Gastric Contractile Activity
Peristaltic waves move toward the pylorus at the rate of 3 per minute This basic electrical rhythm (BER) is initiated by pacemaker cells (cells of Cajal)
47 Gastric Contractile Activity
Most vigorous peristalsis and mixing occurs near the pylorus Chyme is either: Delivered in small amounts to the duodenum or Forced backward into the stomach for further mixing PLAY InterActive Physiology®: Motility, page 10
48 Gastric Contractile Activity
Figure 23.18
49 Regulation of Gastric Emptying
Gastric emptying is regulated by: The neural enterogastric reflex Hormonal (enterogastrone) mechanisms These mechanisms inhibit gastric secretion and duodenal filling
50 Regulation of Gastric Emptying
Carbohydrate-rich chyme quickly moves through the duodenum Fat-laden chyme is digested more slowly causing food to remain in the stomach longer
51 Regulation of Gastric Emptying
Figure 23.19
52 Small Intestine: Gross Anatomy
Runs from pyloric sphincter to the ileocecal valve Has three subdivisions: duodenum, jejunum, and ileum
53 Small Intestine: Gross Anatomy
The bile duct and main pancreatic duct: Join the duodenum at the hepatopancreatic ampulla Are controlled by the sphincter of Oddi The jejunum extends from the duodenum to the ileum The ileum joins the large intestine at the ileocecal valve
54 Small Intestine: Microscopic Anatomy
Structural modifications of the small intestine wall increase surface area Plicae circulares: deep circular folds of the mucosa and submucosa Villi – fingerlike extensions of the mucosa Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes PLAY InterActive Physiology®: Anatomy Review, page 5
55 Duodenum and Related Organs
Figure 23.20
56 Small Intestine: Microscopic Anatomy
Figure 23.21
57 Small Intestine: Histology of the Wall
The epithelium of the mucosa is made up of: Absorptive cells and goblet cells Enteroendocrine cells Interspersed T cells called intraepithelial lymphocytes (IELs) IELs immediately release cytokines upon encountering Ag
58 Small Intestine: Histology of the Wall
Cells of intestinal crypts secrete intestinal juice Peyer’s patches are found in the submucosa Brunner’s glands in the duodenum secrete alkaline mucus
59 Slightly alkaline and isotonic with blood plasma
Intestinal Juice Secreted by intestinal glands in response to distension or irritation of the mucosa Slightly alkaline and isotonic with blood plasma Largely water, enzyme-poor, but contains mucus PLAY InterActive Physiology®: Secretion, page 13
60 Liver: Microscopic Anatomy
Hepatocytes’ functions include: Production of bile Processing bloodborne nutrients Storage of fat-soluble vitamins Detoxification Secreted bile flows between hepatocytes toward the bile ducts in the portal triads
61 Liver: Microscopic Anatomy
Hexagonal-shaped liver lobules are the structural and functional units of the liver Composed of hepatocyte (liver cell) plates radiating outward from a central vein Portal triads are found at each of the six corners of each liver lobule Figure 23.24c
62 Gallbladder and Associated Ducts
Figure 23.20
63 The Gallbladder Thin-walled, green muscular sac on the ventral surface of the liver Stores and concentrates bile by absorbing its water and ions Releases bile via the cystic duct, which flows into the bile duct
64 Bile salts are cholesterol derivatives that:
Composition of Bile A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes Bile salts are cholesterol derivatives that: Emulsify fat Facilitate fat and cholesterol absorption Help solubilize cholesterol Enterohepatic circulation recycles bile salts The chief bile pigment is bilirubin, a waste product of heme
65 Regulation of Bile Release
Acidic, fatty chyme causes the duodenum to release: Cholecystokinin (CCK) and secretin into the bloodstream Bile salts and secretin transported in blood stimulate the liver to produce bile Vagal stimulation causes weak contractions of the gallbladder
66 Regulation of Bile Release
Cholecystokinin causes: The gallbladder to contract The hepatopancreatic sphincter to relax As a result, bile enters the duodenum
67 Pancreas Location Lies deep to the greater curvature of the stomach
The head is encircled by the duodenum and the tail abuts the spleen
68 Pancreas Exocrine function
Secretes pancreatic juice which breaks down all categories of foodstuff Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes The pancreas also has an endocrine function – release of insulin and glucagon
69 Acinus of the Pancreas Figure 23.26a
70 Regulation of Pancreatic Secretion
During cephalic and gastric phases, stimulation by vagal nerve fibers causes release of pancreatic juice and weak contractions of the gallbladder. 1 Acidic chyme entering duodenum causes the enteroendocrine cells of the duodenal wall to release secretin, whereas fatty, protein-rich chyme induces release of cholecystokinin. 2 Cholecystokinin and secretin enter bloodstream. 3 Upon reaching the pancreas, cholecystokinin induces the secretion of enzyme-rich pancreatic juice; secretin causes copious secretion of bicarbonate-rich pancreatic juice. Figure 23.28
71 Digestion in the Small Intestine
As chyme enters the duodenum: Carbohydrates and proteins are only partially digested No fat digestion has taken place
72 Digestion in the Small Intestine
Digestion continues in the small intestine Chyme is released slowly into the duodenum Because it is hypertonic and has low pH, mixing is required for proper digestion Required substances needed are supplied by the liver Virtually all nutrient absorption takes place in the small intestine
73 Motility in the Small Intestine
The most common motion of the small intestine is segmentation It is initiated by intrinsic pacemaker cells (Cajal cells) Moves contents steadily toward the ileocecal valve
74 Motility in the Small Intestine
After nutrients have been absorbed: Peristalsis begins with each wave starting distal to the previous Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine PLAY InterActive Physiology®: Motility, page 8
75 Local enteric neurons of the GI tract coordinate intestinal motility
Control of Motility Local enteric neurons of the GI tract coordinate intestinal motility Cholinergic neurons cause: Contraction and shortening of the circular muscle layer Shortening of longitudinal muscle Distension of the intestine
76 Other impulses relax the circular muscle
Control of Motility Other impulses relax the circular muscle The gastroileal reflex and gastrin: Relax the ileocecal sphincter Allow chyme to pass into the large intestine
77 Large Intestine Figure 23.29a
78 Functions of the Large Intestine
Other than digestion of enteric bacteria, no further digestion takes place Vitamins, water, and electrolytes are reclaimed Its major function is propulsion of fecal material toward the anus Though essential for comfort, the colon is not essential for life PLAY InterActive Physiology®: Secretion, page 15 PLAY InterActive Physiology®: Digestion and Absorption, page 10
79 The bacterial flora of the large intestine consist of:
Bacteria surviving the small intestine that enter the cecum and Those entering via the anus These bacteria: Colonize the colon Ferment indigestible carbohydrates Release irritating acids and gases (flatus) Synthesize B complex vitamins and vitamin K
80 Chemical Digestion: Carbohydrates
Absorption: via cotransport with Na+, and facilitated diffusion Enter the capillary bed in the villi Transported to the liver via the hepatic portal vein Enzymes used: salivary amylase, pancreatic amylase, and brush border enzymes PLAY InterActive Physiology®: Digestion and Absorption, pages 4 and 7
81 Carbohydrate digestion
Organ Substrate Enzyme End product(s) Oral cavity Starch Sal1vary amylase Maltose Stomach Amylase denatured Lumen of intestine Undigested polysaccharides Pancreatic amylase Brush border of small intestine Disaacharides: maltose Sucrose Lactose Maltase Sucrase Lactase Monosaccharides
82 Chemical Digestion: Proteins
Absorption: similar to carbohydrates Enzymes used: pepsin in the stomach Enzymes acting in the small intestine Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases PLAY InterActive Physiology®: Digestion and Absorption, pages 5 and 8
83 Protein digestion Organ Substrate Enzyme End product(s) Stomach
Organ Substrate Enzyme End product(s) Stomach Polypeptides Pepsinogen +HCl = pepsin Smaller peptides Lumen of intestine Trypsinogen, chymotrypsinogen (inactive enzymes released from the pancreas, transported to duodenum via pancreatic duct. These enzymes are activated by enterokinase from small intestine to trypsin and chymotrypsin Smaller polypeptides Aminopeptidase, carboxypeptidase Amino acids Brush border of small intestine Dipeptides Dipeptidase
84 Pancreatic Activation
Figure 23.27
85 Chemical Digestion: Fats
Glycerol and short chain fatty acids are: Absorbed into the capillary blood in villi Transported via the hepatic portal vein Enzymes/chemicals used: bile salts and pancreatic lipase
86 Chemical Digestion: Fats
Absorption: Diffusion into intestinal cells where they: Combine with proteins and extrude chylomicrons Enter lacteals and are transported to systemic circulation via lymph PLAY InterActive Physiology®: Digestion and Absorption, pages 6 and 9
87 Fat digestion Organ Substrate Enzyme End product(s) Oral cavity
Fat digestion Organ Substrate Enzyme End product(s) Oral cavity No enzyme to digest fat Stomach Lumen of intestine Fat globules Bile salt from gallbladder lipase Emulsified fat Glycerol, fatty acids Brush border of small intestine
88 Chemical Digestion: Fats
Figure 23.35
89 Fatty Acid Absorption Fatty acids and monoglycerides enter intestinal cells via diffusion They are combined with proteins within the cells Resulting chylomicrons are extruded They enter lacteals and are transported to the circulation via lymph
90 Fatty Acid Absorption Figure 23.36 Fatty acids and monoglycerides
associated with micelles in lumen of intestine Lumen of intestine 1 Fatty acids and monoglycerides resulting from fat digestion leave micelles and enter epithelial cell by diffusion. Absorptive epithelial cell cytoplasm 2 Fatty acids are used to synthesize triglycerides in smooth endo- plasmic reticulum. ER Golgi apparatus 3 Fatty globules are combined with proteins to form chylomicrons (within Golgi apparatus). 4 Vesicles containing chylomicrons migrate to the basal membrane, are extruded from the epithelial cell, and enter a lacteal (lymphatic capillary). 5 Lymph in the lacteal transports chylomicrons away from intestine. Chylomicron Lacteal Figure 23.36
91 Chemical Digestion: Nucleic Acids
Absorption: active transport via membrane carriers Absorbed in villi and transported to liver via hepatic portal vein Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines
92 Electrolyte Absorption
Most ions are actively absorbed along the length of small intestine Na+ is coupled with absorption of glucose and amino acids Ionic iron is transported into mucosal cells where it binds to ferritin Anions passively follow the electrical potential established by Na+
93 Electrolyte Absorption
K+ diffuses across the intestinal mucosa in response to osmotic gradients Ca2+ absorption: Is related to blood levels of ionic calcium Is regulated by vitamin D and parathyroid hormone (PTH)
94 Water Absorption 95% of water is absorbed in the small intestines by osmosis Water moves in both directions across intestinal mucosa Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients
95 Malabsorption of Nutrients
Results from anything that interferes with delivery of bile or pancreatic juice Factors that damage the intestinal mucosa (e.g., bacterial infection) Gluten enteropathy (adult celiac disease) – gluten damages the intestinal villi and reduces the length of microvilli Treated by eliminating gluten from the diet (all grains but rice and corn)
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