Digestive System

A&P Exam 4

Describe anatomy of GI tract

Continuous tube from mouth to anus, has 7 sphincters of significance

Define accessory organs, give examples

Vital organs for digestion; salivary glands, liver, gallbladder, pancreas

List and describe the 7 sphincters of the GI tract

1. Upper esophageal sphincter (keeps air out, prevents reflux)

2. Lower esophageal sphincter (prevents reflux)

3. Pylorus sphincter (controls emptying of stomach into duodenum)

4. Sphincter of Oddi (allows entry of bile and enzymes)

5. Ileocecal valve (one way gate between intestines)

6. Internal anal sphincter (involuntary controlled)

7. External anal sphincter (voluntarily controlled, skeletal)

List the 4 layers of the GI wall

Mucosa, submucosa, muscularis externa, serosa

Explain components of mucosa layer

Villi - finger like projections that increase SA of epithelial layer

Crypts - contain exocrine cells that produce secretions into lumen

Lamina propria - connective tissue

Muscularis mucosa - thin layer of smooth muscle that makes flux and agitation

Explain components of submucosa layer

Blood and lymphatic vessels - allows for lots of reabsorption

Submucosal plexus - ENS innervates and influences secretion of exocrine cells

Explain components of muscularis externa layer

Circular muscle - changes diameter of GI tract wall

Myenteric plexus - communicates with submucosal plexus, engages in ENS control of circular and longitudinal smooth muscle and muscularis mucosa

Longitudinal layer - shortens and lengthens

Explain the serosa layer

Connective tissue, reduces friction between organs

How does the ENS impact the GI wall

Locally controls the myenteric plexus to control GI motility and the submucosal plexus to control secretions

How does the ANS impact the GI wall

Is an external overrider of the ENS and modulates its activity

Parasympathetic stimulates ENS activity, sympathetic inhibits ENS activity

Explain functions of digestive system

Ingesting foods, liquids, nutrients, ions

Digest large molecules into absorbable molecular forms and transport it

Absorption of nutrients from gut lumen to internal environemnt

Metabolic transformation of fuel molecules and detoxification

Excretion of metabolic end products secreted by liver (bilirubin)

Immune functions (production of antibodies)

List the 4 major processes of the DS

Motility, digestion, absorption, secretion

What enzymes break down carbohydrates and what are the products

Amylase breaks down polysaccharides (starch, glycogen) to maltose → maltase breaks down maltose → glucose

Sucrase and lactase break down disaccharides (sucrose, lactose) → fructose, galactose

What enzymes break down proteins and what are the products

Pepsin, trypsin, chymotrypsin break down polypeptides → carboxypeptidase, aminopeptidase breaks down peptide fragments → amino acids

What enzymes break down triglycerides and what are the products

Lipase breaks down glycerol (3 pieces) → monoglyceride + two free fatty acids

Why is the amount of fluid so much greater than amount consumed in GI tract

Because the DS adds large volumes of fluid secretions

Explain feed-forward regulation of GI tract

Sight, smell, taste of food → CNS → efferent autonomic neurons —> nerve plexus → GI glands and smooth muscle —> response

Explain short reflex regulation of GI tract

Stimulus in GI tract → sensory receptors (chemo, osmo, mechano) → nerve plexus ENS → smooth muscle/gland → response

Occurs all in gut wall

Explain long reflex regulation of GI tract

Stimulus → sensory neuron → CNS → efferent autonomic neurons → nerve plexus ENS → smooth gland/muscle → response

Goes to the brain and spinal cord first before going to the digestive tract

Name the GI hormones secreted by the tract that regulates the smooth muscles/glands

Gastrin, CCK, secretin, GIP (all peptides)

Explain properties of gastrin

Produced in stomach

Stimulated by amino acids, peptides in stomach, parasympathetic nerves

Inhibited by acid in stomach, somatostatin

Stimulates acid secretion (parietal cells), motility, ileum, and mass movement

Explain properties of CCK

Produced in small intestine

Stimulated by amino acids, fatty acids in small intestine

Stimulates insulin secretion, gallbladder

Inhibits acid secrection, motility

Relaxes sphincter of Oddi

Explain properties of secretin

Produced in small intestine

Stimulated by acid in small intestine

Stimulates enzyme secretion, liver

Inhibits acid secretion and motility

Explain properties of GIP

Produced in small intestine

Stimulated by glucose, fat in small intestine

Goes to pancreas → stimulates insulin into plasma

Name phases of GI control and its stimulus origins

Cephalic - head

Gastric - stomach

Intestinal - small intestine

Explain the cephalic phase of GI control

Stimulated by sight, smell, taste, chewing, emotional state

Increases secretory and contractile activity mainly by parasympathetic system
Lots of feed-forwarding

Explain the gastric phase of GI control

Stimulated by stretch, acidity, amino acids, peptides

Mediated by short and long reflexes, hormone gastrin (increases secretion and motility)

Explain the intestinal phase of GI control

Stimulated by stretch, acidity, osmolarity, digestive products

Mediated by short and long reflexes, hormones secretin, CCK, GIP

What are the phases of food as it is digested in the body

On plate/in cup → food, nutrients, liquid

in mouth/esophagus → bolus

In stomach/small intestine → chyme

In large intestine/rectum → chyme/feces

Describe motility of mouth and pharynx

Chewing (mastication) - grinding of food to increase surface area, mixing saliva to dissolve it and aid taste

Voluntary skeletal muscles and complex brainstem patterns

What are the functions of the 6 components of saliva

Water - moistens and dissolves molecules

Mucus - lubricates food, fascilitates formation of bolus for swallowing

Bicarbonate - neutralizes acids in food

Lysozyme - kills bacteria

Amylase - begins digestion of polysaccharides

Lipase - begins digestion of triglycerides

How does ANS affect secretion of saliva

Parasympathetic - lots of secretion, thin, watery

Sympathetic - little secretion, thick, viscous

Explain motility of esophagus

Swallowing (deglutition) - propels good to stomach, has two phases

Oropharyngeal phase - voluntary closing of lips and elevation of tongue (irreversible) → soft palate and uvulate elevate to block off nasal passage → larynx elevates and bolus flattens epiglottis over glottis → UES opens and smooth muscle relaxes → UES closes rapidly

Esophageal phase - peristaltic wave, propulsion

Explain the role of the esophagus

Secretes mucus to make to make GI slippery and buffers acidity to protect GI ract wall

Doesn’t digest or absorb

Describe the three functional regions of the stomach

Fundus (top) - flexible size, thin, weak smooth muscle to acommodate bolus

Body (middle) - thicker, stronger smooth muscle, has rugae (ridges and folds), secretes mucus, pepsinogen, HCl

Antrum (bottom) - mixing and grinding, secretes mucus, pepsinogen, and gastrin

List the 6 secreting cells in the stomach

Mucous, parietal, ECL, chief, D, G

What do mucous cells secrete

Secrete mucus and bicarbonate

Exocrine

What do parietal cells secrete

Secrete HCl acids and intrinsic factors

Secretions go into the duct/pit (exocrine)

What do ECL cells secrete

Paracrine

Histamine secretion that acts on parietal cells

What do chief cells secrete

Exocrine

Secretes pepsinogen

What is pepsinogen

An inactive enzyme that when activated turns into pepsin and breaks down proteins

What do D cells do

Endocrine

Secretes somatostatin

Act on parietal cells

What do G cells do

Secrete gastrin

Act on parietal cells

Explain gastric pits

Where glands are secreted in the stomach. Lined with epithelial layer

How is HCl produced in the stomach by parietal cells

CO2 + H2O makes HCO3- and H+ via carbonic anhydrase

• H+ → stomach lumen via K+/H+ ATPase

• HCO3- → capillary via bicarb/Cl- exchanger

• Apical has K+ and Cl- leaking out

• H+ and Cl- meet up in the lumen to make HCl

Explain alkaline shift

The shift in pH to be more alkaline in the capillaries of the stomach during eating

Due to the entry of HCO3- into capillaries during HCl production, increasing the capillary’s buffering capacity

Which hormones have a positive effect on HCl production and how

Hormone starts second messenger cascade → vesicle containing K+/H+ ATPase pumps to fuse with apical membrane → more H+ pumping and HCl production

Gastrin, histamine, and ACh (from ENS + para, binds to cholinergic receptors)

What hormone has an inhibitory effect on HCl production?

Somatostatin - inhibits the second messenger cascade that fuses vesicle to apical membrane

What are the types of drugs used to treat stomach acid problems

OTC antacid tablets/solution - neutralizes acids

Histamine blockers - blocks acid stimulation from histamine

Proton pump inhibitors - blocks protons from pumping

What are drugs used to treat stomach acid for?

Excess acid production, heartburn, GERD (gastroesophageal reflux disease, a condition where stomach acid flows back into esophagus)

What are the 5 ways of regulating parietal cell secretion

Increases secretion

1. Cephalic stimuli → increases enteric neural activity

2. Increase histamine secretion

3. Increase gastrin secretion (acts on both histamine and parietal cells)

4. Gastric phase stimuli increases amino acids, peptides, luminal distension → increases gastrin

Decreases secretion

5. Increases somatostatin

How do intestinal contents affect HCl secretion

Intestinal contents means the stomach wants to slow gastric emptying

• Stimulated by distension/swelling, increases nutrient concentrations, decreases HCl

• Intestinal nutrients decreases both stomach secretion and motility

What pathways do cephalic phase, gastric contents, and intestinal contents make to influence HCl secretion

Cephalic → parasympathetic nerves to ENS → ↑ HCl

Gastric → long and short neural reflexes, gastrin → ↑ HCl

Intestinal → long and short neural reflexes, secretin, CCK → ↓ HCl

Describe the secretion of pepsin in the stomach, inactivation, and if it’s essential or not

Pepsinogen is produced by chief cells → activated by HCl to make pepsin (can autocatalyze itself) → breaks down proteins into peptides

Inactivated by alkaline pH in small intestine and isn’t essential, pancreas can produce proteases that do the same thing

Define intrinsic factors

Essential proteins that bind to vitamin B12 and chaperones it to the ileum to be absorbed, preventing its breakdown and degradation

List the 4 motilities of the stomach

Receptive relaxation, peristaltic waves, retropulsion, gastric emptying

Describe receptive relaxation motility in stomach

Occurs from cephalic into gastric phases, in fundus

Relaxation and weakening of smooth muscle that increases the lumen volume of the stomach to store bolus

What neurotransmitters are involved in receptive relaxation

Serotonin and nitric oxide from the ENS (stomach wall), which come from submucosal plexus and myenteric plexus to increase lumen size

Describe peristaltic wave motility in stomach

Wave-like contraction beginning in the body and approaches the pyloric sphincter

Closes and squirts out a little chyme

Describe retropulsion motility in stomach

Backward splashing that occurs in antrum

Pulverizes bolus and mechanically digests it

Explain slow waves motility in stomach

Pacemaker potentials that develop tension when it goes above threshold that sets rhythm for gastric motility

Depolarized and hyperpolarized by location of bolus (mechano and stretch receptors), ENS, hormones, and ANS

Describe gastric emptying

Chyme moving into the small intestine

Stimulated by stomach stretching and chyme fluidity

Inhibited by enterogastric reflex

What is the enterogastric reflex

A reflex that slows gastric emptying

Senses acidity, lipids, amino acids, hypertonicity, and distention in the duodenum

• Stimulate long and short neural reflexes

• Increase secretion of enterogastrones (CCK and secretin)

How much absorption occurs in the stomach

Not a lot, only weak acids and alcohols

How does eating greasy, fatty foods help with alcohol consumption

Prevents BAC spike and its problems

Greasy fatty foods (has proteins, lipids, acidity) that inhibit gastric emptying and slows the rate that alcohol is emptied into the intestine where absorption is rapid

Describe the anatomy of the small intestine

Begins at pyloric sphincter, ends at ileocecal sphincter/valve

Has three parts, duodenum, jejunum, ileum

Longest portion of GI

Components of SI that increase surface area

Coiled length, circular folds, villi (epithelial layer), and microvilli (plasma membrane)

Explain digestion and absorption in the SI

Occurs at the microvilli, where nutrients move from GI lumen into epithelial cells (outside to inside body).

This is where chyme actually moves across the epithelial barrier

Describe amount of secretions from SI

1500 ml/day of water, mucus, NaCl (for secondary active transport)

500 ml/day of bile from liver and gall bladder to digest and absorb fats

1500 ml/day of pancreatic secretions (bicarb to neutralize pH and enzymes to digest nutrients

All exocrine secretions

Where does the liver and pancreas connect?

At the duodenum (small intestine)

Explain duct connections of SI with pancreas and liver

Right and left hepatic ducts of liver merge with bile duct from gall bladder - transports bile

Bile duct merges with pancreatic duct - adds secretions to bile at the sphincter of Oddi

Explain the exocrine parts of the pancreas

Acinar cells - secretes pancreatic enzymes

Duct cells - secretes bicarb

Explain endocrine parts of the pancreas

Islet cells - insulin and glucagon

How does the pancreas secrete bicarbonate through duct cells

H2O + CO2 → H+ and bicarb in pancreatic cells

• Bicarb → duct lumen via bicarb/Cl- exchanger into the bile

• H+ → capillary through a Na+/H+ exchanger

  • Na+ moves into the cell, then out through Na/K ATPase, then K moves out through a leak channel

• Apical has a CFTR channel for Cl-

• Basolateral has K+/Na+ ATPase and K+ leak channel

Overall makes the capillaries more acidic and the duct less acidic

Explain the process of enzymes secreted by acinar cells in the pancreas

Acinar cells secrete inactive enzymes and trypsinogen into the intestine → trypsinogen is activated by enterokinase into trypsin → trypsin activates the inactive enzymes

How does secretin regulate pancreatic duct secretion

When there’s stomach in the acid, secretin binds to duct cells and increases bicarbonate secretion to further neutralize intestinal acid

How does CCK regulate acinar cell secretion

Fatty acids and amino acids in the intestine stimulates CCK to bind to acinar cells → increases enzyme secretion, further digesting fats and proteins

Inhibiting gastric emptying by decreasing motility

How does CCK regulate bile flow

The duodenum senses fatty acids → CCK is secreted

• Gallbladder contracts to increase bile flow into common bile duct → flow into duodenum

• Sphincter of Oddi relaxes → bile flow into duodenum

How does the liver mainly help with digestion

It produces bile salts that emulsify (evenly distributes) fats

• BIle salts are made from cholesterol and contains bilirubin

• Are reabsorbed in distal ileum

How does the SI digest and absorb carbohydrates

Polysaccharides go into SI lumen → broken down by amylase into disaccharides → turned into monosaccharides by brush border enzymes

• Fructose → in cell via GLUT → in capillary via GLUT

• Glucose and galactose cotransport with Na into cell via SGLT → out to capillary via GLUT

• All go to the hepatic portal vein to the liver

• Basolateral has K+/Na+ ATPase and K+ channel

How does the SI digest and absorb proteins

Proteins are broken down by proteases and peptidases into small peptides and amino acids

• Small peptides → amino acids by brush border peptidases OR enter cell via H+ cotransporter → turn into amino acids via peptidases in cell

• Amino acids cotransport into cell with Na+ → go into ISF via amino acid transports into capillary to the liver

Needs Na/K ATPase and K channel

How does the SI digest and absorb lipids

Fat-soluble vitamins (A, D, E, K) are emulsified into bile salts →

Pancreatic lipases breakdown triglycerides →

Turned into fatty acids and monoglycerides that diffuse into epithelial cells to be absorbed →

Combine with amphipathic proteins to form chylomicrons (triglycerides) that exocytose into lacteals →

Bypasses liver to go to lymph vessels, entering through veins near thoracic duct near heart

What capillaries in the SI all feed into the hepatic portal vein

Stomach, pancreas, small and large intestines

Explain absorption of hydrophilic substances

Capillaries all go through the hepatic portal vein which goes straight to the liver and another capillary bed (bypassing heart), where it’s processed at liver

Then liver capillaries drain from hepatic veins → inferior vena cava for circulation

What are the motilities of the small intestine

Segmentation and migrating myoelectric complex (MMC)

What occurs in segmentation motility in small intestine

An alternating contraction-relaxation, fastest at duodenum and slowest at ileum

Mixes chyme with enzymes and bicarb for more enzyme exposure and neutralization of acid

What occurs in migrating myoelectric complex (MMC) motility (SI)

A housekeeping motility that occurs after a meal during post-absorptive, intestinal phases

Short distance peristaltic wave that sweeps everything left behind after eating to clear the intestine

Pathway of large intestine

Cecum (w appendix) → ascending colon → transverse colon → descending colon → sigmoid colon → rectum

What is the main function of the LI

Store and dry feces

Maintains a healthy gut bacterial environment to repopulate gut in case of antibiotics wiping them out

Secretes alkaline mucus with bicarb and K+

Bacterial breakdown of complex polysaccharides

Absorbs mainly water, some NaCl, vitamin K, and products of bacterial metabolism

What are the motilities of the LI

Segmentation (Haustrations), mass movement, and defecation

Explain segmentation/haustrations motility in the LI

Occurs every 30 minutes

Shuffling back and forth motility where haustral sacs shrink and expand to dry chyme

Explain mass movement motility in LI

Long range peristaltic wave stimulated by gastrin that packs rectum with feces and leads to defecation reflex

Begins at ascending colon to rectum

Explain defecation motility of the LI

Rectal stretch relaxes internal anal sphincter, contracts external anal sphincter (which relaxes when ready to defecate)

Valsalva maneuver can increase pressure in abdomen and aid with defecation

Reduced motility → constipation

Enterohepatic circulation of bile salts

Begins in the liver → goes down bile duct → duodenum (emulsifies fats) → reabsorbed at ileum through hepatic portal vein back to liver

Exocrine vs paracrine vs endocrine

Exo - secreted into the digestive tract (lumen)

Para - affect nearby cells only, don’t enter bloodstream

Endo - released into the bloodstream