Ch 23 - Digestive System

Accessory digestive organs

= secretes into the tube

1. teeth

2. tongue

3. gallbladder

4. salivary glands

5. liver

6. pancreas

Accessory digestive organs - functions

• produce a variety of secretions that help break down foodstuffs

GI tract (alimentary canal) organs

1. mouth

2. pharynx

3. esophagus

4. stomach

5. small intestine

6. large intestine

GI (Digestive) System - Functions

= prepares nutrients for absorption and use by body cells

1. Ingestion

2. Propulsion

   • swallowing (deflutition) - voluntarily

   • peristalsis - involuntarily

3. Mechanical Breakdown

4. Digestion

5. Absorption

   • active or passive

6. Defecation

Peritoneal cavity & peritoneum - Digestive System

• resides in the abdominopelvic cavity, protected by abdominal muscles (peritoneal cavity)

• wrapped in a double-layer membrane called the peritoneum

  • peritoneum that “overhangs” = omentum

  • peritoneum that attaches your intestines to your back abdominal wall = mesentery

layers from peritoneal cavity to food

1. peritoneal cavity

2. mesentery

3. serosa

4. muscularis

   • inner circular

   • outer longitudinal

5. submucosa

6. mucosa

7. food

peristalsis

• adjacent segments of the alimentary canal organs alternately contract (wavelike) and relax

• food is moved distally along the tract

• circular muscles block off backward movement while longitudinal muscles and coordinated circular contractions propel food in one direction

• primarily propulsive, some mixing may occur

segmentation

• nonadjacent segments of the alimentary canal organs contract and relax

• food is moved back and forth

• primarily mixes food and breaks it down mechanically, some propulsion may occur

mechanical digestion

• chewing, mixing food with saliva by the tongue, churning food in stomach, segmentation

• increases the surface area of ingested food

• prepare for chemical digestion

chemical digestion

• enzymes secrete into the lumen of the alimentary canal to break down complex food molecules to their chemical building blocks

  • pancreatic enzymes

  • bile

  • brush border enzymes

• catabolic process

mesentery - description

• double layer of peritoneum = sheet of 2 serous membranes used back to back

• extends to the “movable” digestive organs from the body wall

mesentery - function

1. provide routes for blood vessels, lymphatics, and nerves to reach the digestive viscera

2. hold organs in place

3. store fat

4. insulation and protection

mucus - function

• protects digestive organs from enzymes

• eases food passage

4 layers/tunics of GI tract

1. mucosa/mucous membrane

2. submucosa

3. muscularis externa

4. serosa

mucosa/mucous membrane - description

• innermost layer

• 3 layers

  1. epithelium

  2. lamina propria

  3. muscularis mucosae

epithelium - mucosa membrane

• stratified squamous - mouth, esophagus, anus

• simple columnar with mucus secreting cells everywhere else

• innermost layer that’s in contact with the food & has lymphatic nodules

lamina propria - mucosa membrane

• loose areolar CT

• nourishes epithelium, absorbs nutrients

• contains lymphoid follicles (part of MALT) mainly in pharynx and appendix

muscularis mucosae - mucosa membrane

• scant layer of smooth muscle cells

• Produces local movements to enhance absorption and secretion

mucosa/mucous membrane - function

1. Secrete mucus, digestive enzymes, and hormones

2. Absorb the end products of digestion into the blood

3. Protect against infectious disease

Special Functions of mucosa in Stomach and Small Intestine

• Enzyme-synthesizing and hormone-secreting cells

• Acts as a diffuse endocrine organ

submucosa - GI tract

• areolar CT

• rich supply of blood, lymphatic vessels, lymphoid follicles, nerve fibers

• abundant elastic fibers

muscularis externa - GI tract

• responsible for segmentation & peristalsis

• controlled by enteric nervous system

• inner circle layer

  • form sphincters that act as valves to control food passage from one organ to the next and prevent backflow

• outer longitudinal layer of smooth muscle cells

serosa - GI tract

• outermost layer

• also known as visceral peritoneum

• areolar CT

• make peritoneal fluid

• covered with mesothelium - squamous epithelial cells

serosa in esophagus

• serosa replaces by adventitia (dense CT)

membrane of retroperitoneal organs

• have both serosa and adventitia

splanchnic circulation

• arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation

hepatic portal circulation

• collects nutrients-rich venous blood flowing out from the digestive viscera(organs) and delivers it to the liver

oral cavity/mouth - epithelium

• walls of the mouth lined with stratified squamous epithelium - withstand friction

• gums, hard palate, dorsum of tongue = slightly keratinized for extra protection

oral cavity/mouth - digestive process

1. ingests

2. begins mechanical breakdown by chewing

3. initiates propulsion by swallowing

4. starts the digestion of polysaccharides

*absorption DOESN’T occur in the mouth

mastication/chewing

• voluntary and reflexive

• controlled mainly by stretch reflexes and in response to pressure inputs from receptors in mouth

soft palate

• mobile fold formed of skeletal muscles

• rise reflexively to close off the nasopharynx when we swallow

hard palate

• forms rigid surface against which the tongue forces during swallowing

salivary glands

• major or extrinsic salivary glands outside the oral cavity secrete saliva

• composed of 2 secretory cells

  1. serous cells

  2. mucous cells

serous cells

• produce a watery secretion containing enzymes, ions, mucin

  • lysozyme = enzyme to kill bacteria

mucous cells

• produce mucus (viscous solution)

saliva - function

1. cleanses the mouth

2. dissolves food chemical so they can be tasted

3. moistens food and helps compact it into a bolus

4. contains amylase (enzyme) that begins the digestion of starchy foods

5. protects against microorganisms

composition of saliva

• water - 97% to 99.5% = hypoosmotic

• slightly acidic (pH 6.75-7.00)

• include electrolytes, salivary amylase, lingual lipase, proteins mucin, lysozyme, IgA, defensins, metabolic wastes (urea, uric acid)

  • salivary amylase = start carbohydrate digestion

  • inactive lingual lipase = start fat digestion once it hits the acidic environment of the stomach

what can trigger the release of saliva

• sight

• smell

• though of food

• irritation of lower GI tract

minor salivary glands

• secrete saliva continuously

• keep the mouth moist

major salivary glands

• activated when food enters the mouth

• large amount of saliva are produced

control of salivation - parasympathetic

• chemoreceptors - activated by acidic substances

• mechanoreceptors - any mechanical stimulus in the mouth

• send signals to salivary nuclei in the brain stem (pons & medulla)

• facial (VII) & glossopharyngeal (IX)

control of salivation - sympathetic

• cause release of thick, mucin-rich saliva

• Strong activation constricts blood vessels serving salivary glands

• Almost completely inhibits saliva release, causing dry mouth

• Dehydration inhibits salivation due to low blood volume

esophagus

• collapsed when not involved in food propulsion

• pierces the diaphragm to enter abdomen

• contains upper and lower sphincters

• mucous cells on both sides of the sphincter help protect the esophagus from reflux of stomach acid

upper esophageal sphincter

• relaxes to let food enter the esophagus

• contracts after the food enters

• thick smooth muscles

lower esophageal sphincter

• gastroesophageal sphincter

• closed when food is not being swallowed

• open when bolus of food arrives

• after bolus of food enters, the sphincters close preventing regurgitation

• thick smooth muscles

mucosa - esophagus

• nonkeratinized stratified squamous epithelium

• changes to simple columnar epithelium at the esophagus-stomach junction

  • specialized for secretion

submucosa - esophagus

• contains mucus-secreting esophageal glands

  • compressed as bolus moves through

• greases esophageal wall and aids food passage

muscularis externa - esophagus

• superior third = skeletal muscle

• middle third = mixture of skeletal and smooth muscle

• inferior third = entirely smooth muscle

adventitia - esophagus

• instead of serosa have fibrous adventitia

• composed entirely of CT

• blends with surrounding structures along its route

buccal phase

• occurs in the mouth

• voluntary

• ends when a food bolus or a bit of saliva leaves the mouth and stimulates tactile receptors in the posterior pharynx

pharyngeal-esophageal phase

• involuntary

• controlled by the swallowing center in the brain stem (medulla and lower pons)

• vagus nerve transmit motor impulses from the swallowing center to the muscles of the pharynx and esophagus

• Once food enters the pharynx, respiration is momentarily inhibited and all routes except the desired one into the digestive tract are blocked off

stomach - function

1. propulsion

   • peristalsis

2. mechanical breakdown

   • peristaltic waves mix food with gastric juice and propel it into duodenum

3. Digestion (chemical breakdown)

   • pepsin begins the digestion of proteins

4. absorption

   • absorb fat-soluble substances (aspirin, alcohol, drugs)

5. storage

   • store food until it can be moved into duodenum

chyme

• partially digested food converted in the stomach

• hypertonic (higher concentration of solutes)

cardia - stomach

• first part of the stomach that is closest to esophagus

• food and liquids passes through

fundus - stomach

• stomach’s dome-shaped part, tucked beneath the diaphragm

body - stomach

• midportion of the stomach

• continuous inferiorly with the pyloric part

pylorus - stomach

• most distal part of stomach

• continuous with the duodenum with pyloric sphincter

pyloric antrum - stomach

• initial wider portion of the pyloric part

pyloric canal - stomach

• narrower, more distal portion of the pyloric part

• connects the pyloric antrum to the pylorus

pyloric sphincter - stomach

• valve of the distal end of the stomach that controls food entry int the duodenum

greater curvature - stomach

• larger, convex, left-side border of the stomach

• provides attachment point for the greater omentum

lesser curvature - stomach

• smaller, concave, right-side border of the stomach

• provides attachment point for the lesser omentum = stomach & duodenum to liver

rugae - stomach

• elevations or ridges in stomach mucosa

• prominent when the stomach is collapsed

muscularis externa - stomach

• incomplete innermost layer of smooth muscle fibrils that runs obliquely

• allow the stomach not only to mix, churn, and move food along the tract, but also to beat the food, physically breaking it down into smaller fragments

*only stomach has oblique layer

gastric glands - stomach

• produce gastric juice

stomach mucosa

• Simple columnar epithelium

• Composed entirely of mucous cells

• Produces a two-layer coat of alkaline mucus

• Viscous, insoluble mucus traps bicarbonate-rich fluid beneath it

mucous neck cells - stomach

• Located in the neck and deeper within glands

• Produce thin, soluble & acidic mucus

parietal cells - stomach

• Found mainly in the apical region of glands

• Secrete hydrochloric acid (HCl) and intrinsic factor

• Provide large surface area with microvilli

HCl - parietal cells of stomach

• Creates acidic environment (pH 1.5-3.5)

• Activates pepsin

• Denatures proteins, breaks down walls of plant foods

• Kills bacteria

Intrinsic factor - parietal cells of stomach

• is a glycoprotein required for vitamin B12 absorption in the small intestine

chief cells - stomach

• Located in the basal regions of glands

• Produce pepsinogen (inactive form of pepsin)

• Secrete lipases for fat digestion

pepsinogen - chief cells of stomach

• Inactive form of pepsin

• Activated by HCl in the apical region to become pepsin

• Positive feedback mechanism

  • pepsin activates more pepsinogen by catalyzing it, producing more pepsin

lipases - chief cells of stomach

• fat digesting enzyme

Enteroendocrine (G cells) - stomach

• Located deep in the gastric glands

• Release chemical messengers (histamine, serotonin, somatostatin, gastrin)

mucosal barrier

• thick coating of bicarbonate-rich mucus

• epithelial cells of the mucosa are joined together by tight junctions that prevent gastric juice from leaking

• damaged epithelial mucosal cells are shed and quickly replaced by division of undifferentiated stem cells that reside where the gastric pits join the gastric glands

what regulates gastric (juice) secretion

1. Neurol control

   • long nerve (vagus)

   • short nerve (local enteric)

2. hormonal control

   • gastrin stimulate release of HCl

• both release Ach, stimulating the output of gastric juice

what chemicals stimulates the release of HCl

1. Ach

2. gastrin

3. histamine

*when only 1 chemical binds to parietal cell receptor, HCl secretion is small

cephalic (reflex) phase

• occurs before food enters the stomach

• triggered by aroma, taste, sight, or thought of food

• vagus nerve stimulate gastric glands

gastric phase

• stomach distension activates stretch receptors and initiates both short and long reflex

  • long reflexes, impulses travel to the medulla and then back to stomach

• Partially digested proteins, caffeine, and rising pH directly activate gastrin-secreting G cells in the stomach antrum.

  • Gastrin stimulates parietal cells to secrete HCl by acting directly on their receptors and by stimulating enteroendocrine cells to release histamine

protein buffering - gastric phase

• Protein foods increase gastric pH, stimulating more gastrin and HCl release

• As proteins are digested, gastric contents become more acidic, inhibiting further gastrin secretion, maintaining optimal pH for enzyme activity

• negative feedback

Inhibition of gastric secretion - gastric phase

• Highly acidic gastric contents (pH below 2) inhibit gastrin secretion

• Sympathetic actions such as stress and fight-or-flight responses inhibit gastric secretion by overriding parasympathetic controls

stimulation - intestinal phase

• partially digested food entering the duodenum stimulates the release of intestinal gastrin, encouraging gastric glands to continue their secretory activity

4 main factors in the duodenum that inhibit gastric secretion - intestinal phase

1. distension

2. acidic chyme

3. fatty chyme

4. hypertonic chyme

How do the inhibitory factors protect the small intestine?

• prevent excessive acidity and regulate the influx of chyme to match the small intestine's processing abilities

What is the enterogastric reflex?

• inhibits stomach acid secretion through short reflexes via the enteric nervous system and long reflexes involving sympathetic and vagus nerves

• A nervous reflex whereby stretching of the wall of the duodenum results in inhibition of gastric motility and reduced rate of emptying of the stomach

What are enterogastrones, and what is their role?

• hormones released by the duodenum that inhibit gastric secretion

• two main enterogastrones are secretin and cholecystokinin (CCK)

mechanism of HCl secretion

1. H+ and HCO3- are generated from the dissociation of carbonic acid (H2CO3)

2. H+ K+ ATPase pumps H+ into the lumen and K+ into the cell

   • K+ returns to the lumen through membrane channels

3. HCO3- leaves the cell (alkaline tide) into the blood in exchange for interstitial fluid Cl-

4. Cl- diffuses through membrane channels into the lumen

gastric contractile activity

1. Propulsion

   • peristaltic waves move from the fundus toward the pylorus

2. Grinding

   • most vigorous peristalsis and mixing occur close to the pylorus

   • pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum

3. Retropulsion

   • peristaltic wave closes the pyloric valve, forcing most of the contents of the pylorus backwards into the stomach

pancreas

• encircled by the C-shaped duodenum

• retroperitoneal

• produces enzymes that breakdown/digest chyme and bicarbonate

pancreatic duct

• transports the secretions of the acinar cells (exocrine secretion)

• opens into the duodenum

accessory pancreatic duct

• empties directly into the duodenum

• proximal to the main pancreatic duct

• does NOT mix with bile beforehand

hepatopancreatic ampulla

• small reservoir where your common bile duct and pancreatic duct meet

hepatopancreatic sphincter

• controls the entry of bile and pancreatic juice

exocrine cells - pancreas

• produce pancreatic juice

• consists of Acini & pancreatic duct

endocrine cells - pancreas

• monitor blood levels and release insulin, glucagon, somatostatin

acinar cells

• produce enzyme rich component of pancreatic juice

  • proteases (for proteins)

  • amylase (for starch)

  • lipases (for fats)

  • nucleases (for nucleic acids)

• stimulated by CCK & vagus nerve (parasympathetic)

• full of rough ER

pancreatic duct cells

• secrete water that makes up the bulk of the pancreatic juice

• secrete bicarbonate that makes the juice alkaline

• stimulated by secretin & high pH change in duodenum

main pancreatic duct

• carry pancreatic juice from the pancreas

• unite at the wall of duodenum

major duodenal papilla

• where ampulla opens into the duodenum

liver - description

• blood rich

• largest abdominal gland/organ

• capable of regeneration

• located under diaphragm and lies almost entirely within the rib cage