PNB 2775 Exam 3

Alimentary Canal

The gastrointestinal tract. Mouth, esophagus, stomach, small intestine, large intestine.

Accessory Organs (digestive)

Are rich in secretions. Salivary glands, gallbladder, liver, pancreas.

Peritoneum

Serous membranes within the abdominal cavity. Parietal or visceral.

Serous

Able to secrete.

Parietal Peritoneum

The inside surface of the body’s wall. Encases everything.

Visceral Peritoneum

The surface of the internal organs. Encases each separately. Supports blood vessels, nerves, and lymphatic vessels.

Peritoneal Fluid

Produced by the serous membrane lining. Provides lubrication to allow sliding without friction or irritation.

Intraperitoneal Organs

Completely encased by the visceral peritoneum. Stomach, liver, spleen, and all of the intestines other than the duodenum.

Retroperitoneal Organs

Organs behind the peritoneum. Kidneys, pancreas, duodenum, ascending and descending colon.

Mesentery

Continuous organ formed by the peritoneum. Holds intestines to the body. Supports and stabilizes intestines. Conduit for blood vessels, nerves, lymphatic vessels.

Crohn’s Disease (cause)

Mesentery attacked by the immune system.

GI Histologic Layers

Transporting/epithelial layer, defense/immune and blood vessel layer, muscle layers, protection/connective tissue layer (everywhere).

Transport layer (orientation and function)

Face lumen, secrete, exchange nutrients.

Mucosa

Crucial for secretion, digestion, and absorption. Consists of the mucosal epithelium, lamina propria, and muscularis mucosae. Secretes more in a day than water drank.

Mucosal Epithelium (Mucosa)

The most variable feature of the GI system. GI stem cells, cell-to-cell junctions, endocrine and exocrine function, enterocytes transport nutrients.

Lamina Propria (Mucosa)

Connective tissue contains nerve fibers, vessels, lymph nodes. Contains wandering immune cells.

Muscularis Mucosae (Mucosa)

Thin layer of smooth muscle. Shapes intestinal folds, moves the villi.

Submucosa

Loose connective tissue containing larger blood and lymph vessels. Transports absorbed nutrients. Contains the submucosal plexus.

Submucosal Plexus

Nerve supply to muscularis mucosae.

Muscularis Externa

Two layers of smooth muscle (circular and longitudinal) that facilitate GI contraction.

Circular Muscularis Externa

Contract and relax the radius of the lumen.

Longitudinal Muscularis Externa

Contract and relax the length of the lumen.

Myenteric Plexus

The nerve bed that innervates the two layers of the muscularis externa. Slower pacemaker cells that change activation, not really speed of contraction.

Rugae

Folds in the stomach.

Plicae

Folds in the small intestine.

Gastric glands

Invaginations in the stomach.

Crypts

Invaginations in the small intestine.

Peristalsis

Muscle contraction that moves food down the tract.

Segmentation

Muscle contraction that mixes and breaks food down.

Basic Electrical Rhythm (BER)

Sets contraction frequency. Usually 3-20 seconds. set by pacemaker cells (interstitial cells of Cajal) between muscle and nerve plexus.

The Short Reflex

Enteric nervous system acts independently to stimulate motility and secretion in the intestines. Intrinsic neurons and glial cells respond to neurotransmitters, GI peptides, ACh.

Saliva

Softens and lubricates food, begins chemical digestion with salivary amylase, lipase. Contains lysozymes, immunoglobulins like IgA, fluoride, HCO3-. About 1-1.5 gallons secreted per day.

Three salivary gland types

Parotid, sublingual, submandibular. All pairs.

Parotid Glands

Secrete watery solution.

Sublingual Glands

Secrete mucus-rich solution.

Submandibular Glands

Secrete mixed solution (watery and mucus)

The Cephalic Phase

Salivary secretion and function, swallowing reflex. Reflexes initiated in the brain based on food-related stimulus.

Acinar Cells

Epithelial cells at the endpieces (innermost areas) of salivary glands. Secrete isotonic fluid resembling extracellular fluid and plasma ion concentrations.

Ductal Cells

Epithelial cells along the shaft of salivary glands that are impermeable to water. Absorb Na+ and Cl- and secrete K+ and HCO3- via striated and excretory ducts, leading to net removal of solute.

Ductal Modification

The process of acinar secretions passing by ductal cells and exchanging ions with them. Reduces when salivation rate increases.

Secretagogues

Keep HCO3- levels high during higher salivary stimulation.

Swallowing Reflex

The second part of the cephalic phase. Primary and secondary peristalsis.

Primary Peristalsis

The initial swallow. Esophageal sphincter relaxes, epiglottis closes.

Secondary (Repetitive) Peristalsis

If bolus (food) does not go all the way down right away, esophageal muscles continue to contract.

Gastroesophageal Junction

The lower esophageal sphincter, the junction of the esophagus and stomach. Resting pressure about 30mmHg, much lower than upper esophagus to encourage downward momentum.

Esophageal Stricture

High pressure in the esophageal junction caused by diseases, medications.

Achalasia

Degeneration of the esophageal myenteric plexus causes solid dysphagia, fullness in chest while eating, chest pain, weight loss. Esophageal peristalsis is weak and uncoordinated.

The Gastric Phase

A series of short reflexes initiated when food enters the stomach.

Functional Regions of the Stomach

Anterior/proximal, posterior/distal.

Anterior/Proximal Stomach

The cardia, fundus, and body of the stomach. Serves as a reservoir.

Posterior/Distal Stomach

The antrum and pyloric sphincter. Grinds and mixes, then pumps out to intestines.

Rugae

Folds inside the stomach that flatten out in order to aid expansion.

Oblique Layer

The third muscle layer in the stomach (in addition to the circular and longitudinal muscles). Overlying the mucosa, provides strength and integrity.

Lower Esophageal Sphincter and Cardia secretion/motility

Secrete mucus, HCO3-. Prevents reflux, allows entry of food, regulates belching.

Fundus and Body secretion/motility

Secretes H+, intrinsic factor, mucus, HCO3-, pepsinogens, lipase. Is a reservoir and provides tonic force during emptying.

Antrum and Pylorus secretion/motility

Secretes mucus and HCO3-, which protects from HCl. Mixes, grinds, sieves, and regulates emptying into the duodenum.

Gastric Secretion types (4)

Acid, gastrin, paracrine, and enzyme secretions.

Acid Secretion

Type of gastric secretion. Secreted by parietal cells in gastric glands. Blood pH increases after eating. Parietal cells excrete H+ and Cl- on the lumen side, Cl- and Na+ on the basolateral side. Main limiter is proton pump activity. If HCl rises past necessary, proton pumps activate to prevent H+ and Cl- binding.

Acid Secretion Stimulators

Gastrin, ACh, histamine.

Acid Secretion Inhibitors

Prostaglandin E2, somatostatin (SST)

Gq-Coupled Pathways

Pathways beginning with ACh and Gastrin. Increase gastric acid secretion.

Gs-Coupled Pathway

Pathway beginning with Histamine. Increases cAMP production, which increases secretion rates.

Gi-Coupled Pathways

Pathways beginning with Somatostatin and Prostaglandins/PGE2. Decrease cAMP production, which decreases acid secretion.

Gastrin Secretion

Type of gastric secretion. Secreted by G cells in the pits of the gastric mucosa and acts on parietal and ECL cells to stimulate acid release and mucosa growth in stomach and intestines. Triggered only by GRP and amino acids. Inhibited by SST.

Paracrine Secretion

Type of gastric secretion. Histamine, intrinsic factor, somatostatin.

Histamine

Strong paracrine stimulus for acid secretion. Secreted by ECL cells. Stimulated by gastrin, ACh.

Intrinsic Factor

Most important paracrine stomach secretion to health. Secreted by parietal cells. Absorbs vitamin B12 and helps RBC production. Deficiency causes pernicious anemia.

Somatostatin (SST)

Primary feedback signal secreted by D cells as a result of high H+. Inhibits insulin and glucagon.

Enzyme Secretion

Type of gastric secretion. Pepsin, gastric lipase.

Pepsin

Secreted by chief cells as pepsinogen, then activated by acid and preexisting pepsin once in the stomach (out of the mucosa).

Gastrin

Co-secreted with pepsin(ogen) by stomach chief cells.

Gastric Phase Motility Control

Mechanical (stretch) and chemical (protein digestion products) stimulation. Response varies from proximal (fundus and body) to distal (antrum) stomach portions.

Fundus and Body Motor Functions (stomach)

Receptive Relaxation, Adaptive Relaxation.

Receptive Relaxation

Relaxation of the stomach muscles as food moves down esophagus and enters stomach. Makes room for approaching mass.

Adaptive Relaxation

Relaxation of the stomach muscles when full to make room for what’s in there.

Antrum Motor Functions (stomach)

Mixing, peristalsis.

Stomach Protective Characteristics

Thick, alkaline mucus. Tight epithelial cell junctions. Rapid cell replacement by GI stem cells.

Gastric Ulcers

Necrosis of the gastric mucosa lining caused by over-secretion of gastric acid. Can be treated using anti-H. pylori antibiotics.

Intestinal Phase

Third phase of digestion. A series of responses after chyme enters the small intestine.

Duodenum

First part of small intestine. Contains common bile duct and pancreatic duct.

Jejunum

Second part of small intestine. Contains pronounced villi and microvilli and performs the most absorption.

Ileum

Third part of small intestine. Contains Peyer’s Patches (lymph nodules) and the ileocecal valve.

Ileocecal Valve

The sphincter between the small and large intestines.

Plicae

Circular folds in the small intestine that increase surface area. Don’t stretch.

Gastric Emptying

Strong peristaltic waves in gastric pump, increased muscle tone in gastric reservoir. Pyloric sphincter opens through GIP and NO action. Duodenal segmental contractions are inhibited to let chyme in.

Promoters of Gastric Emptying

Gastric volume increase, neural input, gastrin.

Inhibitors of Gastric Emptying

GIP (lipids/carbs), CCK (lipids/carbs), secretin (high acidity), enterogastric reflex.

Enterogastric Reflex

Duodenum senses distention and acid via mechano- and chemoreceptors and slows gastric emptying.

Dumping Syndrome

Loss of gastric emptying control due to gastrectomy (for cancer, weight loss). Sugary foods leave stomach quickly, causing excess fluid influx and swelling/cramping/pain in intestine, hyper- and hypo-glycemia. May lead to malabsorption.

Small Intestine Motility

Segmentation contractions for mixing and slow propulsion due to frequency gradient. Weak peristalsis. Migrating motor complex.

Migrating Motor Complex (MMC)

Clears out small intestine during fasting (~2hrs after eating). Stimulated by motilin, inhibited by eating.

Cholecystokinin (CCK)

Secreted by I cells in the mucosa of the upper small intestine. Stimulates pancreatic enzyme secretion, contracts gallbladder, relaxes sphincter of Oddi, inhibits gastric emptying and secretion. Triggered by amino acids, peptides, fatty acids. Inhibited when digestion products move to lower small intestine.

Sphincter of Oddi

Sphincter between middle duodenum and liver/gallbladder/pancreatic secretions. Regulates secretion flow into intestine.

Secretin

Secreted by S cells in the mucosa of the upper small intestine. Stimulates pancreatic bicarbonate secretion, inhibits gastric emptying and gastric secretion (augments CCK functions). Triggered by acids, products of protein digestion. Inhibited when digestion products move to lower small intestine.

Absorption of Carbohydrates

SGLT1 and GLUT5 on apical sides and GLUT2 and the Na+/K+ ATP pump on basolateral/capillary sides of enterocytes work to bring glucose from the small intestine lumen to the capillaries.

SGLT1

On small intestine enterocyte apical sides. Secondary active transporter of Na+ and glucose (sometimes galactose) from lumen into cells. Powered by the concentration gradients of those substances and regulated by glucose concentration in the lumen.

GLUT5

On small intestine enterocyte apical sides. Facilitated diffusion of fructose from lumen into cells.

Na+/K+ Proton Pump (enterocytes)

On small intestine enterocyte basolateral sides. Actively (using ATP) transports one potassium in and three sodium out of the small intestine enterocytes. Removal of sodium balances inflow from SGLT1.

GLUT2

Transports all monosaccharides (glucose, galactose, fructose) from enterocytes into the bloodstream. Facilitated diffusion.

Endopeptidases (proteases)

Attack internal peptide bonds (not on the ends), breaking peptides into two smaller peptides. Secreted by the stomach, intestine, and pancreas as inactive proenzymes. Pepsin in the stomach, trypsin in the small intestine.