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Chapter 18LectureOutline
Functions of GI Tract
18-5
Is movement of food through GI tract by means of: Ingestion--taking food into mouth Mastication--chewing food and mixing it with saliva Deglutition--swallowing food Peristalsis--rhythmic wave-like contractions that
move food through GI tract
Motility
18-6
Includes release of exocrine and endocrine products into GI tract
Exocrine secretions include: HCl, H2O, HCO3-, bile,
lipase, pepsin, amylase, trypsin, elastase, and histamine
Endocrine includes hormones secreted into stomach and small intestine to help regulate GI system e.g. gastrin, secretin, CCK, GIP, GLP-1, guanylin,
VIP, and somatostatin
Secretion
18-7
Is passage of digested end products into blood or lymph
Absorption
18-8
Digestion
Refers to breakdown of food molecules into smaller subunits
Includes temporary storage and subsequent elimination of indigestible components of food
Storage and Elimination
18-9
Immune Barrier
Includes physical barrier formed by tight junctions between cells of small intestine
And cells of the immune system that reside in connective tissue just below epithelium
Structure of Digestive System
18-10
Digestive System continued
Organs include oral cavity, pharynx, esophagus, stomach, and small and large intestine
Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas
18-12
Layers of GI Tract
18-13
Is the absorptive and secretory layer lining lumen of GI tract In places is highly folded with villi to increase absorptive area Contains lymph nodules, mucus-secreting goblet cells, and thin
layer of muscle
Mucosa
18-14
Is a thick, highly vascular layer of connective tissue where absorbed molecules enter blood and lymphatic vessels
Contains glands and nerve plexuses (submucosal plexus) that carry ANS activity to muscularis mucosae of small and large int.
Submucosa
18-15
Is responsible for segmental contractions and peristaltic movement through GI tract
Has an inner circular and outer longitudinal layer of smooth muscle Activity of these layers moves food through tract
while pulverizing and mixing it Myenteric plexus between these layers is major
nerve supply to GI tract Includes fibers and ganglia from both Symp and
Parasymp systems
Muscularis
18-16
Serosa
Is outermost layer; serves to bind and protect Consists of areolar connective tissue covered with layer of
simple squamous epithelium
18-17
From Mouth to Stomach
18-19
Mastication (chewing) mixes food with saliva which contains salivary amylase An enzyme that catalyzes partial digestion of starch
From Mouth to Stomach
18-20
Deglutition (swallowing) begins as voluntary activity Oral phase is voluntary and forms a food bolus Pharyngeal and esophageal phases are involuntary
and cannot be stopped To swallow, larynx is raised so that epiglottis covers
entrance to respiratory tract A swallowing center in medulla orchestrates
complex pattern of contractions required for swallowing
From Mouth to Stomach continued
18-21
Peristalsis propels food thru GI tract = wave-like
muscular contractions
After food passes into stomach, the gastroesophageal sphincter constricts, preventing reflux
From Mouth to Stomach continued
18-23
Stomach
18-24
Is most distensible part of GI tract Empties into the duodenum Functions in: storage of food; initial digestion of
proteins; killing bacteria with high acidity; moving soupy food mixture (chyme) into intestine
Stomach
18-25
Stomach continued
Is enclosed by gastroesophageal sphincter on top and pyloric sphincter on bottom
Is divided into 3 regions: Fundus Body Antrum
18-26
Inner surface of stomach is highly folded into rugae
Contractions of stomach churn chyme, mixing it with gastric secretions Eventually these
will propel food into small intestine
Stomach continued
18-27
Stomach continued
Gastric mucosa has gastric pits in its folds
Cells that line folds deeper in the mucosa, are exocrine gastric glands
18-28
Gastric glands contain cells that secrete different products that form gastric juice Goblet cells secrete
mucus Parietal cells secrete
HCl and intrinsic factor (necessary for B12 absorption in intestine)
Chief cells secrete pepsinogen (precursor for pepsin)
Stomach continued
18-29
HCl in Stomach
Is produced by parietal cells which pump H+ into lumen via an H+/ K+ pump (pH ~1)
Cl- is secreted by facilitated diffusion
H+ comes from dissociation of H2CO3
Cl- comes from blood side of cell in exchange for HCO3
-
18-31
Is secreted in response to the hormone gastrin; and ACh from vagus These are indirect effects since both stimulate
release of histamine which causes parietal cells to secrete HCl
HCl in Stomach continued
18-32
Makes gastric juice very acidic which denatures proteins to make them more digestible
Converts pepsinogen into pepsin Pepsin is more
active at low pHs
HCl in Stomach continued
18-33
Both HCL and pepsin can damage lining and produce a peptic ulcer
1st line of defense is the adherent layer of mucus = a stable gel of mucus coating the gastric
epithelium Contains bicarbonate for neutralizing HCL Is a barrier to actions of pepsin
Gastric epithelial cells contain tight junctions to prevent HCL and pepsin from penetrating the surface
Gastric epithelial cells are replaced every 3 days
Protection of Stomach Against HCL and Pepsin
18-34
Digestion and Absorption in Stomach
Proteins are partially digested by pepsin Carbohydrate digestion by salivary amylase is soon
inactivated by acidity Alcohol and aspirin are the only commonly ingested
substances that are absorbed
18-35
Gastric and Peptic Ulcers
Peptic ulcers are erosions of mucous membranes of stomach or duodenum caused by action of HCl
In Zollinger-Ellison syndrome, duodenal ulcers result from excessive gastric acid in response to high levels of gastrin
Helicobacter pylori infection is associated with ulcers Antibiotics are useful in treating ulcers And also proton pump inhibitors such as Prilosec
Acute gastritis is an inflammation that results in acid damage due to histamine released by inflammation Is why histamine receptor blockers such as Tagamet
and Zantac can treat gastritis
18-36
Small Intestine
18-37
Is longest part of GI tract; approximately 3m long Duodenum is 1st 25cm after pyloric sphincter Jejunum is next 2/5 of length Ileum is last 3/5 of length; empties into large intestine
Small Intestine (SI)
18-38
Absorption of digested food occurs in SI Facilitated by long length and tremendous surface
area
Small Intestine (SI) continued
18-39
Surface area increased by foldings and projections
Large folds are plicae circulares
Microscopic finger-like projections are villi
Apical hair-like projections are microvilli
Small Intestine (SI) continued
18-40
Small Intestine (SI) continued
Each villus is covered with columnar epithelial cells interspersed with goblet cells
Epithelial cells at tips of villi are exfoliated and replaced by mitosis in crypts of Lieberkuhn
Inside each villus are lymphocytes, capillaries, and central lacteal
18-41
Small Intestine (SI) continued
A carpet of hair-like microvilli project from apical surface of each epithelial cell Create a brush border
18-42
Intestinal Contractions and Motility
2 major types of contractions occur in SI: Peristalsis is weak and
slow and occurs mostly because pressure at pyloric end is greater than at distal end
Segmentation is major contractile activity of SI Is contraction of circular
smooth muscle to mix chyme (shown in diagram)
18-44
Large Intestine
18-48
Has no digestive function but absorbs H2O, electrolytes, B and K vitamins, and folic acid
Internal surface has no villi or crypts and is not very elaborate
Contains large population of microflora = 1013 to 1014 commensal bacteria of 400 species
which produce folic acid and vitamin K and ferment indigestible food to produce fatty acids
And reduce ability of pathogenic bacteria to infect LI antibiotics can negatively affect commensals
Large Intestine (LI) or Colon
18-49
Extends from ileocecal valve at end of SI to anus
Outer surface bulges to form pouches (haustra)
Large Intestine (LI) or Colon continued
18-50
SI absorbs most water but LI absorbs 90% of water it receives Begins with osmotic gradient set up by Na+/K+
pumpsWater follows by osmosis
Salt and water reabsorption stimulated by aldosterone LI can also secrete H2O via active transport of NaCl
into intestinal lumen
Fluid and Electrolyte Absorption in LI
18-51
Liver
18-53
Structure of Liver Liver is the largest internal organ Hepatocytes form hepatic plates that are 1–2 cells thick
Plates are separated by sinusoids which are fenestrated and permeable even to proteins Contain phagocytic Kupffer cells
18-54
Structure of Liver continued
A damaged liver can regenerate itself from mitosis of surviving hepatocytes In some cases, such as alcohol abuse or viral
hepatitis, regeneration does not occurCan lead to liver fibrosis and ultimately cirrhosis
18-55
Food absorbed in SI is delivered 1st to liver Capillaries in digestive tract drain into the hepatic
portal vein which carries blood to liver Hepatic vein drains liver Liver also receives blood from the hepatic artery
Hepatic Portal System
18-56
Liver Lobules
Are functional units formed by hepatic plates In middle of each is central vein At edge of each lobule are branches of hepatic portal vein and
artery which open into sinusoids
18-57
Liver Lobules continued
Bile is secreted by hepatocytes in bile canaliculi Empty into bile ducts which flow into hepatic ducts that carry
bile away from liver
18-58
Enterohepatic Circulation
Is recirculation of compounds between liver and intestine
Many compounds are released in bile, reabsorbed in SI, and returned to liver to be recycled
Liver excretes drug metabolites into bile to pass out in feces
18-59
Detoxication of Blood
Liver can remove hormones, drugs, and other biologically active molecules from blood by: Excretion into bile Phagocytosis by Kupffer cells Chemical alteration of molecules
e.g. ammonia is produced by deamination of amino acids in liver Liver converts it to urea which is excreted in
urine
18-64
Gallbladder and Pancreas
18-68
Is a sac-like organ attached to inferior surface of liver
Stores and concentrates bile continuously produced by liver When SI is empty, sphincter
of Oddi in common bile duct closes and bile is forced into gallbladder Expands as it fills with bile
When food is in SI, sphincter of Oddi opens, gall bladder contracts, and bile is ejected thru cystic duct into common bile duct then to duodenum
Gallbladder
18-69
Pancreas
Is located behind stomach Has both endocrine and exocrine functions
Endocrine function performed by islets of LangerhansSecretes insulin and glucagon
Exocrine secretions include bicarbonate solution and digestive enzymesThese pass in pancreatic duct to SIExocrine secretory units are acini
18-70
18-71
Pancreatic juice and bile are secreted into the duodenum
18-72
The Pancreas is Both an Exocrine and Endocrine Gland
(a) A photomicrograph of the endocrine and exocrine portions of the pancreas
(b) An illustration depicting the exocrine acini, where acinar cells prod. Inactive enzymes stored as zymogen granules which are secreted via ducts into duodenum
Contains water, bicarbonate, and digestive enzymes Digestive enzymes include amylase for starch, trypsin
for proteins, and lipase for fats Brush border enzymes are also required for
complete digestion
Pancreatic Juice
18-73
The Activation of Pancreatic Juice Enzymes
Most pancreatic enzymes are produced in inactive form (zymogens) Trypsin is activated
by brush border enzyme, enterokinase
Trypsin in turn activates other zymogens
18-74