Gastrointestinal System Chapter 23. GI: Overview: Organ systems Gastrointestinal (GI) tract [Alimentary canal] a continuous muscular digestive tube

Gastrointestinal SystemChapter 23

GI: Overview: Organ systems

Gastrointestinal (GI) tract [Alimentary canal] a continuous muscular digestive tube Digests:

breaks food into smaller fragments

Absorbs: digested material is moved through mucosa into the blood

Eliminates: unabsorbed & secreted wastes.

Organ systems

Includes: Mouth, pharynx &

esophagus Stomach Small intestine Large intestine

Accessory digestive organs: teeth, tongue, gall bladder, salivary glands, liver & pancreas

Figure 23.1

Processes

Ingestion Propulsion Mechanical digestion Chemical digestion Absorption Defecation

Processes

Ingestion: obtaining food Propulsion: moves food along the GI tract by

peristalsis (wave-like muscular contraction) Mechanical digestion :

chewing & mixing with saliva mixing in stomach segmentation (local constriction in intestine to mix food

& digestive juices)

Processes

Chemical digestion: breaks down food to molecular fragments (monomers) (Hydrolysis). Begins in the mouth with saliva & continues into the

small intestine. Absorption: movement of nutrients across the

mucosal membrane into blood/lymph Defecation: eliminates unused/indigestible &

secreted substances from the body

Functional Considerations :

Substances in the GI tract lumen are outside of the body.

Multiple sensors & receptors line the GI tract to monitor contents & respond to conditions.

Controls: intrinsic (local control) & extrinsic (CNS)

Peritoneum : serous membrane

Visceral peritoneum: covers the external surfaces of most digestive organs

Parietal Peritoneum: lines the body wall Peritoneal Space: potential space containing fluid

that separates the visceral & parietal peritoneum

Figure 23.5a

Peritoneum

Mesentery: double layer of peritoneum fused together that extends to the organs from the posterior body wall. Provides support for the organs Provides support for vessels & nerves supplying the organs

Figure 23.5a

Peritoneum

Retroperitoneal organs Organs that adhere to the posterior abdominal wall &

lose their peritoneum by resorption Parts of the large & small intestine & most of the

pancreas; (also kidneys)

Figure 23.5b

GI blood supply

Blood supply: about 25% of cardiac output Arterial: Abdominal aorta celiac trunk Celiac trunk Hepatic, splenic & gastric branches

which serve the liver, spleen & stomach Celiac trunk superior & inferior mesenteric

branches serve small & large intestine

Histology

GI tract wall has 4 layers: Mucosa Submucosa Muscularis Externa Serosa or Adventitia

Histology of the Alimentary Canal

Figure 23.6

Histology

Mucosa: The epithelial membrane that lines the GI tract from mouth anus. Secretes mucous, digestive enzymes & hormones Absorbs nutrients Protects from disease & from the GI contents

Histology

Mucosa; 3 layers: Epidermis Lamina propria (loose ct : contain capillaries & some

elements of MALT) Muscularis mucosa

Histology

Submucosa: moderately dense CT with blood, nerve, lymph vessels & lymphoid follicles; rich in elastic fibers

Muscularis externa: smooth muscle Responsible for peristalsis &

segmentation Circular layer Longitudinal layer Sphincters: in some areas

the circular layer thickens; act as valves

Histology

Serosa of intraperitoneal organs = visceral peritoneum

Esophagus has an outer covering of fibrous connective tissue = adventitia

Retroperitoneal organs: visceral serosa on the surface facing the peritoneal cavity & adventitia on the surface facing the body wall.

Nerves

Intrinsic: (Local): Short reflex Submucosal nerve plexus:

regulates glands & mucosal muscle Myenteric plexus: controls GI wall & GI motility

Extrinsic: (CNS): Long reflex Parasympathetic NS: enhances gut motility &

secretion SNS: inhibits gut motility & secretion

Nerves

Intrinsic: (Local): Short reflex Submucosal nerve plexus: regulates glands & mucosal muscle Myenteric plexus: controls GI wall & GI motility

Extrinsic: (CNS): Long reflex Parasympathetic NS: enhances gut motility & secretion SNS: inhibits gut motility & secretion

Figure 23.4

Functional Anatomy: Mouth

Mouth: lips, palate, & tongue

Mouth cavity = Buccal cavity


Lips: extend from inferior margin of the nose to the superior margin of the chin. Red area = red margin, is poorly keratinized & lacks sweat or sebaceous glands.

Palate: Hard palate: rigid surface against which food is

forced in chewing Soft palate: muscular structure that rises &

blocks off the nasopharynx during swallowing


Tongue: muscular tentacle composed of interlaced muscle fibers that grips & repositions food, mixes food with saliva & compresses food to form a food bolus, prior to swallowing.


Filiform papillae: rough surface Fungiform papillae: house taste buds Circumvallate papillae: house taste buds, Foliate papillae: posterolateral; taste buds

Functional Anatomy: Mouth Salivary Glands: intrinsic & extrinsic

Intrinsic glands: scattered throughout the buccal cavity mucosa

Extrinsic glands: supply most of the saliva; outside buccal cavity & supply secretions via ducts:

Parotid Submandibular Sublingual

Functional Anatomy: Mouth Composition of saliva:

97-99.5% H2O Electrolytes: pH 6.75-7.0 Amylase: (digestive enzyme) Proteins: mucin, lysozyme, & IgA

Protection from microbes by saliva: IgA: immunglobulins in secretions Lysozyme: bacteriostatic (inhibits bacterial growth) Cyanide Defensins: local antibiotic activity & when activated

promote chemotaxis by WBCs Normal flora: convert salivary components to nitrates

then to NO. NO is toxic & bacteriocidal


Control of Salivation: Continuous baseline secretory activity

With food ingestion, salivation increases dramatically Parasympathetic NS: chemoreceptors & pressoreceptors

stimulate salivatory nuclei to increase salivation

Functional Anatomy: Mouth, Pharynx

Teeth: Primary: 2I 1C 2M x 2 = 20

2I 1C 2M Permanent: 2I 1C 2PM 3M x 2 = 32

2I 1C 2PM 3M Structures

Crown: exposed above gingiva (gum) Root: anchored by periodontal ligament to the bone by a

fibrous joint (gomphosis)

Figure 23.07

Figure 23.11

Functional Anatomy: Throat & Esophagus

Pharynx: oropharynx & laryngopharynx; muscular wall propels food to the esophagus

Esophagus: Muscular 25cm tube from laryngopharynx to

stomach Passes through the diaphragm at the

esophageal hiatus Gastroesophageal (cardiac) sphincter: A

physiologic sphincter that helps keep esophagus closed when empty

Functional Anatomy: Esophagus

Esophagus (continued) Wall has all 4 GI tract tunics:

Epithelial layer changes at the junction with the stomach from stratified squamous epithelium to simple columnar epithelium

Esophageal mucous glands lubricate food bolus Muscularis externa

Superior 1/3 of muscularis externa is skeletal muscle Middle 1/3 is mixed skeletal & smooth Lower 1/3 is smooth muscle

Adventitia: external covering

Digestive Processes: Mouth, Pharynx & Esophagus Ingestion Mechanical digestion: chewing Chemical digestion: mixing food with saliva Propulsion: swallowing & initiating peristalsis

Functional Anatomy: Stomach

Cardiac region: narrow, receives food bolus Fundus: bulge that extends supero-laterally to the cardia,

reaches the diaphragm Body: mid-portion Pyloric antrum : funnel shaped portion narrows to form the; Pyloric canal Pylorous Pyloric sphincter small intestine Rugae

longitudinal mucosalfolds

volume about 4L

Figure 23.14a

Rugae

Microscopic Anatomy : Stomach

Stomach: has the 4 tunics of the GI tract. Epithelium: Simple columnar epithelium

(goblet cells-mucous); Muscularis externa has an additional oblique

layer of muscle (allows another dimension of contraction).

Figure 23.15

Gastric glands secrete gastric juices

MicroscopicAnatomy : Stomach


Mucous neck cells: in the duct portion

Figure 23.15


Gastric glands secrete gastric juices Mucous neck cells: in

the duct portion Parietal cells: mid

portion secrete HCl & intrinsic factor for B12 absorption

Figure 23.15


Gastric glands secrete gastric juices Mucous neck cells: in

the duct portion Parietal cells: mid

portion of glands secrete HCl & intrinsic factor

Chief cells: base of gland; secretes pepsinogen a precursor molecule to pepsin (an enzyme that digests protein)

Figure 23.15


Enteroendocrine cells: secrete multiple hormonal products; Gastrin, histamine,

endorphins, serotonin, cholecystokinin, & somatostatin, which influence several digestive system organs

Figure 23.15


Mucosal barrier: protects the stomach from its own secretions Viscous mucous overlies a thick coating of HCO3

- rich mucous

Tight junctions between epithelial cellPM of glandular cells are impermeable to HCl

Epithelium is replaced every 3-6 days

Digestive Processes (Stomach)

Acts as a holding vessel for ingested food

Participates in mechanical & chemical digestion Propulsion: Delivers its product (chyme) to the small

intestine

Digestive Processes (Stomach)

Protein digestion: HCl denatures protein HCl activates pepsinogen to pepsin Pepsin breaks peptide bonds of proteins Rennin: an enzyme that breaks down casein

(milk protein) secreted in infants Intrinsic factor: required for Vit. B12 absorption

(needed to mature RBC); Absence of B12 results in pernicious anemia

Regulation of gastric secretion (3 phases)

Cephalic Phase Gastric Phase Intestinal Phase

Cephalic phase: Stimulation

Cephalic phase: CNS response to presentation of food; enhances gastric gland secretion

Loss of appetite; satiety / depression

Cephalic phase: Inhibition

Gastric phase: Stimulation

Gastric phase: food entering stomach; Stretch Change in pH (increase) Peptides All cause increased gastric gland secretion


Stretch: reflex arc causes increased Acetylcholine release which then causes increased gastric gland secretions

Increased pH / polypeptides / caffeine All enhance Gastrin secretion by enteroendocrine G cells


3 chemicals: cause enhanced HCl secretion through 2nd messenger systems Gastrin Acetylcholine Histamine

Gastric phase: Inhibition

pH <2.0 inhibits Gastrin secretion

SNS inhibits Gastrin (G cell) activity

Intestinal phase: Stimulation

Excitatory: As chyme enters the duodenum the mucosa secretes enteric gastrin which stimulates secretion by gastric glands

Intestinal phase: Inhibition

Inhibitory: As more chyme fills the small intestine, the enterogastric reflex is triggered Inhibits CNS stimulation Inhibits local reflexes Controls gastric emptying Activates sympathetic fibers that tighten the

pyloric sphincter

Regulation of Gastric Activity

Figure 23.16

Small Intestine: Gross Anatomy

6-7m long: from pyloric sphincter to the ileocecal valve

3 subdivisions: Duodenum Jejunum Ileum


Duodenum : Curves around the pancreatic head (~25cm long) Contains the hepatopancreatic ampulla : formed

by the merger of the bile duct & the pancreatic duct.

Hepatopancreatic sphincter controls admission of bile & pancreatic enzymes to the duodenum

Duodenum is retroperitoneal

Fig 23.20

Small Intestine; Regulatory Function

Duodenenal regulation of gastric emptying: Feedback mechanisms monitor the contents being delivered from the stomach High fat content Low pH (high acidity) Hypertonicity (high osmolality) All result in decreased stomach emptying.

Regulation of Gastric Emptying

Figure 23.19


Jejunum: extends from duodenum to ileum (~2.5m long)

Ileum: from jejunum to ileocecal valve (~3.6m long)

Figure 23.1


Jejunum & Ileum function in absorption; Intraperitoneal Suspended from mesentery whose veins &

lymph vessels carry nutrients away from small intestine

Small Intestine: Microanatomy

Figure 23.21

Plicae circularis: mucosal folds cause chyme to spiral slowly through the lumen

Villi

Villi: velvety mucosa of absorptive epithelium containing capillaries & lymph lacteals

Microvilli: PM projections forming “brush border”; Hold enzymes that complete protein & carbohydrate digestion

Small intestine Wall

Cell types: Mostly absorptive cells Goblet (mucous) cells increase in number as

the small intestine progresses Enteroendocrine cells T-Lymphocytes


Intestinal crypts with cells that secrete intestinal juice & contain Paneth cells that secrete protective lysozyme (antibacterial)

Peyer’s Patches: lymphoid follicle in submucosa


Brunners glands (duodenum) secrete HCO3- rich

mucous to increase the pH of chyme Villus epithelium is replaced every 3-6 days Intestinal Juice : isotonic with blood plasma,

slightly alkaline, low enzyme content

Liver & Gall Bladder

Liver produces bile (fat emulsifier) that is stored in & concentrated by the gall bladder. (metabolic role of the liver; Ch. 24)

Liver: Gross Anatomy

Largest gland 4 Lobes Falciform ligament

mesentery supports liver from diaphragm & anterior body wall

separates R & L lobes

Round ligament fibrous remnant of umbilical vein

Fig 23.23

Blood supply hepatic artery & hepatic

portal vein Bile

drains from biliary ducts to common hepatic duct which fuses with cystic duct from gallbladder to form the bile duct

Fig 23.23

Liver: Gross Anatomy

Liver: Microscopic Anatomy

Liver lobule: hexagonal (Fig 23.24) Central vein: drains the lobule Hepatocytes form plates that radiate from the

central vein

Microanatomy of the Liver

Figure 23.24c, d

Liver: Anatomy

Portal triad at each corner of the hexagonal system. Consists of branches of: Hepatic Artery delivers O2 Hepatic portal vein delivers

nutrients from small intestine Bile duct receives bile from

the bile canaliculi that lie between layers of hepatocytes

Figure 23.24c, d

Liver: Anatomy

Liver (cont) Liver sinusoids Large

leaky capillaries conduct blood from the artery & portal vein to the central vein

Hepatic macrophages Kupffer cells lie in sinusoid walls

Central veins flow into hepatic veins then to the inferior vena cava

Figure 23.24c, d


Composition of bile: Alkaline solution: Bile salts, bile pigments, cholesterol, fats & phospholipids Bile salts & phospholipids participate in fat

absorption


Bile salts are conserved by enterohepatic circulation Reabsorbed in the ileum Return to Liver in hepatic portal blood Re-secreted by the Liver

Bile pigments & bilirubin break down to urobilin then stercobilin

Gall Bladder

Gall Bladder = a muscular pouch that stores bile & expels bile when needed via the cystic duct & the bile duct.

Figure 23.20

Regulation of Bile Release

Cholecystokinin (CCK) & secretin released by the small intestine in response to increased fats in chyme

CCK: Stimulates both Gall bladder &

pancreatic secretion Relaxes hepatopancreatic sphincter

Secretin: stimulates bile secretion

Figure 23.25

Pancreas: Gross Anatomy

Head encircled by duodenum Tail abuts the spleen Mostly retroperitoneal

Pancreatic Acini

Large numbers of Acinar cells in clusters around ducts; exocrine (Acini) Acinar cells: Secrete

pancreatic digestive enzymes Endocrine cell clusters form the

pancreatic islets that produce insulin & glucagon

Pancreatic Juice

Pancreatic Juice: Alkaline, watery, contains enzymes & electrolytes

Pancreatic Juice

Proteolytic enzymes are released as inactive forms then are activated in the duodenum Trypsinogen Trypsin by the brush border

enzyme enterokinase Trypsin activates precursors to form

carboxypeptidase & chymotrypsin Amylase, lipase, & nuclease are released in their

active forms

Regulation of pancreatic secretion

Neural via PSNS Hormonal: Intestinal Hormones

CCK: released in response to fats & protein. Stimulates pancreatic secretion of enzymes

Secretin: released in response to HCl. Stimulates pancreatic duct cells to release HCO3

-

Figure 23.28

Digestive Processes : Small Intestine

Optimal digestion requires adequate motility & control of chemical composition


pH: acidic chyme must be buffered to allow proper enzyme activity

Osmolality: chyme is hypertonic & would pull H2O out of circulation; thus chyme is released in small amounts

Liver & pancreatic function are required for appropriate delivery of bile salts & enzymes to the small intestine


Segmentation: moves intestinal contents back & forth to mix

Duodenal rhythm for segmentation is greater than the rhythm in the ileum. Contents move toward the ileum.


After most digestion has occurred, Peristalsis begins sweeping from duodenum distally

Occurs in series with each peristaltic wave, originating more distally (migrating mobility complex)

Ileocecal valve (sphincter) is relaxed by neural (gastroileal reflex) impulses from the stomach & hormonal (gastrin) secretion by the stomach

Large Intestine: Gross Anatomy

Ileocecal valve to anus (~1.5m) Teniae coli: 3 ribbons of

longitudinal smooth muscle Haustra: pocket-like segments

of large intestine Epiploic appendages: fat filled

pouches of visceral peritoneum

Figure 23.29a

Large Intestine: Gross Anatomy

Cecum: blind pouch (below ileocecal valve) Appendix: attached to cecum (lymphoid) Colon: ascending, transverse, descending, sigmoid

Transverse colon & sigmoid colon are intraperitoneal; anchored by mesentery. The rest of the colon is retroperitoneal

Rectum Anal canal

Figure 23.29a

Large Intestine: Microscopic Anatomy

All 4 layers present Mucosa: simple columnar epithelium until anal

canal (stratified squamous) No folds or villi. No significant enzyme secretions Crypts: invaginations of mucosa contain large numbers

of goblet cells that secrete mucous for protection & lubrication

Large Intestine: Microscopic Anatomy

Bacterial flora Ferment indigestible CHO: produces about 500ml of gas

per day Synthesize B complex vitamins & most vitamin K

Large Intestine: Digestive Processes

Propulsion: Haustra contractions: stretch stimulate haustra to

contract moving (& mixing) contents to next haustra Mass peristalsis: long, slow contractile waves moving

contents toward rectum (3-4 per day) Gastrocolic reflex: food intake causes mass peristalsis

Large Intestine: Digestive Processes

H2O reabsorption & vitamin absorption Defecation

Empty rectum receives waste, causing stretch Stretch initiates reflex contraction of the rectum &

relaxation of both anal sphincters Voluntary control of the external anal sphincter can

postpone defecation If suppressed the contraction stops & is reinitiated later With defecation rectal muscle contractions are aided by

increased abdominal pressure (valsalva maneuver)

Structure of the Anal Canal

Figure 23.29b Figure 23.32

Chemical Digestion of Specific Food Groups

Chemical Digestion: Mechanism Catabolic process: break down of food molecules to

monomeric form that can be absorbed Enzymes break molecules by hydrolysis (Adding H2O into

a molecular bond)

Chemical Digestion: Carbohydrates

Carbohydrates (CHO): complex sugars are broken down to simple sugars (monomers) Simple sugars (monosaccharides): glucose,

fructose, & galactose can be absorbed Disaccharides (not absorbed): sucrose,

maltose, & lactose are hydrolyzed by sucrase, maltase, & lactase (brush border enzymes) into monosaccharides


Polysaccharides: broken down to smaller & smaller oligosaccharides Salivary amylase: breaks starch into 2-8 linked

monosaccharide Salivary amylase is denatured by HCl in the stomach


Polysaccharides (cont) Pancreatic amylase: continues breakdown (mostly to

maltose) Brush border enzymes act on oligosaccharides of more than

3 simple sugars (brush border) Sucrose, maltose & lactose are broken down by specific

brush border enzymes (sucrase, maltase, & lactase) Monosaccharides are absorbed.

Chemical Digestion: Proteins

Proteins: broken down to amino acid monomers. Begins in the stomach. Pepsinogen is activated to pepsin by HCL Pepsin: cleaves peptide bonds associated with tyrosine &

phenylalanine forming polypeptides (+ a few amino acids). Pepsin is inactivated by increased pH in the duodenum

Trypsin & chymotrypsin: pancreatic enzymes that further breakdown polypeptides.

Chemical Digestion: Proteins

Proteins (cont): Carboxypeptidase (pancreatic & brush border

enzyme): Splits off single amino acid from the end of

polypeptide chain that contains the carboxyl group

Aminopeptidase & dipeptidase (brush border enzymes) release final amino acid monomers

Figure 23.34

Figure 23.34

Chemical Digestion: Lipids

Lipids: require emulsification to disperse fats into microdroplets which enhances enzyme activity Bile salts: surround fat droplets & keep them

in suspension in the aqueous digestive juices Pancreatic lipase: cleaves off two fatty acid

chains from triglyceride molecules Yields a monoglyceride & 2 free fatty acids

Figure 23.35

Chemical Digestion: Nucleic acids

Nucleic acids: small amounts occur in the diet Pancreatic nucleases: Hydrolyze nucleic acids to

nucleotide monomers Nucleosidases & phosphatases (brush border) break

down nucleotides

Absorption: Small Intestine

Most nutrients are absorbed from chyme prior to the ileum (ileum primarily reabsorbs bile salts)

Most nutrients cross the mucosa by active transport. Some products of lipid digestion are absorbed by diffusion. CHO, amino acids & nucleic acid remnants are actively

transported into mucosal cells & then enter capillary blood

Absorption: Small Intestine (Fats)

Monoglycerides & free fatty acids are associated with bile salts & lecithin (a phospholipid) to form micelles Micelles diffuse between microvilli The lipid substances leave the micelles & diffuse across the

PM of microvilli Inside the intestinal epithelial cell, the components are

reassembled into triglycerides

Absorption: Small Intestine (Fats)

Monoglycerides (cont) Triglycerides are then packaged with cholesterol &

phospholipids into chylomicrons Chylomicrons are exocytosed & diffuse to lymphatic lacteals

to eventually be placed into blood by the lymphatic system Chylomicrons in the blood are hydrolyzed by lipoprotein lipase

to free fatty acids & glycerol prior to tissue absorption Fig. 23.36

Fatty Acid Absorption

Figure 23.36


Vitamins Some Vit. K & B complex vitamins are absorbed in the

large intestine Dietary vitamins are absorbed in the small intestine Fat soluble vitamins (A, D, E, K) dissolve in dietary fats &

are absorbed after being incorporated into micelles


Electrolytes: most ions are actively absorbed throughout small intestine

Na+ is coupled to absorption of glucose & amino acids (cotransport)

Cl- actively transported into cells; exchange for HCO3-


K diffuses into the cells Iron is transported into cells & bound to ferritin Ca2+ absorption regulated by blood Ca2+ levels &

Vit D ( Vit D is a required cofactor for Ca2+ absorption)


Water moves freely in both directions across the mucosa Active transport of solutes create an osmotic gradient &

H2O follows H2O movement changes solute concentrations & effects

absorption of substances that move by diffusion


Malabsorption: can result from Disruption of secretion Mucosal damage Reduction of absorptive surface area

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Gastrointestinal System Chapter 23. GI: Overview: Organ systems Gastrointestinal (GI) tract [Alimentary canal] a continuous muscular digestive tube