GI physiology cram

mechanoreceptors sense _______ and ________; and trigger _______ and _______

stretch and pressure; peristalsis and segmentation

Describe segmentation and what type of control its under.

• contractions on either side of the bolus and mixing in the recieving segments to move chyme down the tract

• intrinsic (enteric nerve control) , can still operate even if PNS is down

The GI tract is composed of smooth muscle, except for which places?

the mouth, upper esophagus, and external anal sphincter

What is the primary nervous control of the GI tract, and what type of plexuses composes it?

• The enteric NS

  • Myenteric plexus = muscle movement; Submucosal plexus = secretions​

Describe the pathway of vitamin B12 from ingestion to absorption.

• HCl in the stomach releases B12 from food proteins → TC1 binds B12 in the mouth/stomach to protect it from acid → In the duodenum, trypsin cleaves TC1 → B12 binds intrinsic factor → The IF–B12 complex (dimerization) travels to the terminal ileum → binds receptors and is absorbed.

Describe where pepsinogen comes from, where it goes, and what its released to. Role?

• from chief cells into the gastric lumen

• pepsinogen gets cleaved into pepsin (via hcl & acidic environment) , active protease. Pepsin can break proteins into oligopeptides

Gastrin: where its from, where it goes, and its role?

• Comes from g cells in antrum of stomach—→ blood (hormone)

• role: circulate to increase HCL and increase GI motility

Lipase: where its from, where it goes, and its role?

• from chief cells—→lumen

• Role: hydrolyze lipids

What is the Others in Gastric secretions? Where are they from and what are their roles

-Somatostatin: from parietal cells, role is to decrease HCL (modulate secretion)

-Histamine: from parietal cells, role is to increase HCL release

What is the role of mucus as a gastric secretion?

• Protect bicarbonate so all the H+ (acid) cannot damage stomach epithelium

Chronic vomiting causes an acid base disturbance which can lead to what?

• hypokalemia

What stimulates contraction of the gall bladder, and what does the contraction of the gallbladder do?

• the horomone CCK and vagal stimulation

• it releases bile into the duodenum, facilitating lipid absorption. Vagal stimulation relaxes the sphincter of oddi (into the duodenum) and later in digestion (intestinal phase), CCK and vagal stimulation witll stimulate gallbladder contraction

Characterize what happens to surface cells in GI tract due to its high metabolic nature

they are sloughed off and replaced every 2 to 3 days by stem cells from the crypts

Secretions are into the _______of the GI tract but hormones are _______

• lumen

• secreted into blood

describe gastric accommodation

When food enters the stomach vagal stimulation results in relaxation of the stomach (receptive relaxation) which allows a larger intake of food before “feeling full”.

Is saliva hypo or hypertonic to plasma?

ALWAYS hypotonic to plasma, more hypertonic foods are ingested

Characterize what contributes to daily salvilary output

• Parotid serous fluid w amylase-25%

• Submandibular glands mixed serous/mucous fluid- 70%

• Sublingual glands w mucous fluid- 5%

• high amt of Na+

How does the PNS control salvilary glands

• though Facial nerves (submax and sublingual glands)

• through glossopharyngeal nerves (parotid glands)

• NOT through vagus nerve

How does PNS stimulation affect salvation vs SNS.

• PNS- increase salvation, increasing blood flow to the glands and let more ultrafiltrate and primary secretions to occur

• SNS- decreases salvation (nervous—>dry mouth)

What increases Salivary flow?

• Incr: PNS, Ach, CNS (cephalic phase—>anticipatory effect) , nausea, esophageal distention—>lubricate stuck food, chewy/ flavorful foods, dry acidic alkaline food, meats sweets bitter foods

• Decr: SNS, NE, hormone (ADH, aldo), sleep, dehydration, drugs+ aging (can atrophy glands)

What does saliva do?

• Taste, coagulation factors, antimicrobial action, protection, digestion, lubrication, oral hygiene

ECF is secreted into the ______and absorbed back further down in the tract

GI tract

chemoreceptors sense ______ and _______; and trigger _____ and ______ ________

nutrients and pH; hormone and enzyme secretion

osmoreceptors sense _______ ________ and trigger ______ secretion or absorption

fluid osmolarity; water

What are the gastric secretions?

Hcl, IF, Pepsinogen, gastrin, lipase, others (histamines, somatostatin), Mucus (HIPGLOM)

excitatory neurotransmitters (2)

ACh and substance P

inhibitory neurotransmitters (2)

NO and VIP

modulatory neurotransmitters (2)

serotonin and dopamine

the BER is influenced by the ______ and _____

SNS; PNS

normal BER of stomach

3/min

normal BER of small intestine

12/min

why is BER faster in duodenum than in the ileum

duodenum needs mixing; ileum needs absorption

normal BER of colon

3-6/min

increased metabolic rate; _______ BER; ________ motility

increases; increases

MMC: happens during _______ and is active when

fasting, 3-4 hours after you finish eating and is active from mid stomach through the terminal ileum

MMC purpose is to clear ____________ and _________

undigested food and bacteria into the colon

phase 3 contractions are stimulated by _____ and _______

gastrin and motilin

purpose of enterogastric reflex

slow down digestion to prevent duodenal damage

describe how prostaglandins decrease stomach acid vs mucous layer

p: decrease acid and increase mucus & bicarb production to protect stomach lining

m: sequesters bicarb that can neutralize acid

the enterogastric reflex is triggered by

low pH, fat and protein, and hyperosmolarity and distention in duodenum

Explain the 3 phases in neural control of gastric acid secretion

1. cephallic= anticipatory phase, smelling food can activate PNS stimulation thru vagus

2. gastric phase= food in stomach, vagal effects, gastrin, acid, enzymes, pH changes

3. Intestinal secretions= secretion (mucus, hormones, enzymes) and secretions from the pancreas, liver, and indirectly gallbladder—> travel down the hepatic duct and through the sphincter of oddi to enter the duodenum

the enterogastric reflex causes the release of _____ and ________; causing _____ and _______ _______

CCK and secretin; motility and acid secretion

describe the first pass effect

blood flows from intestines (splanchnic circulation)—→ portal vein——> liver——> gets metabolized before going to systemic circulation

Redirect blood from periphery to GI tract increases blood flow to that area by how much, and why?

• increases it by 30%

• metabolic organs need it more

what effect does NPY have on hunger . and what happens to NPY when you eat

• stimulates hunger

• NPY is suppressed as you eat and food is coming into duodenum

When you eat several peptides, incl ________ and _______ are secreted into circulation, and they have what effect on hypothalamic NPY and appetite

• Peptide YY and GLP1, supressing NPY and supressing appetite

List what stimulates vs inhibits acid secretion

• Stimulates: PNS: Ach, gastrin, histamine

• Inhibits: Secretin, Somastostatin, Peptide YY, prostaglandins

Describe how LEPTIN is the counterpart to ghrelin

• Ghrelin stimulates hunger and increases NPY. Released from parietal cells in the stomach

• Leptin is released from adipose tissues, and after eating, glucose and insulin increase circulating leptin, supressing NPY and hunger

What are the functions of the GI tract?

• Digestion– Enzymes and HCl

• Endocrine– LOTS of hormones which act on GI tract and other tissues

• Elimination– rids body of undigested waste

• Protection– HCl, IgAs, opsonins, and other immune cells

• Motility– propels and mixes chyme

• Absorption– of almost everything you eat

• Secretion– buffers mucus, hormones, enzymes

• Storage– stomach and colon

You may only ingest ______ of fluid a day, but the GI tract

SECRETES__________. This means it has to absorb______ of fluid each day!

• 2 L

• 7-8 L

• 9-10 L

What effect do secretions have on the GI tract

help lubricate, digest (HCl, enzymes), and buffer (HCO3 buffers the acid) the chyme

Characterize ENS vs PNS, SNS and CNS regulation differences

• ENS- myenteric plexus and submucosal plexus

• PNS- vagal to transverse colon, then pelvic nerves to anus- PRO digestion

• SNS- post ganglionic adrenergic fibers from celiac, superior and inferior mesenteric and hypogastric plexus- SLOW

• CNS- anticipatory, through PNS, primarily vagal also glossopharyngeal

What is chyme?

Semi-liquid mixture of food + gastric secretions in stomach/intestine

Why is pH important in the GI tract?

Different enzymes require specific pH environments

Stomach: acidic

Small intestine: more neutral/basic

Blood flow to the intestines increases for _________ after eating, to facilitate ________. 

3-6 hr

absorption

define borborygmi

The audible movement of fluids and gases in and out of the intestines

what are the phases of MMC and which is the most important phase? What happens in that phase?

• 4 phases and a full cycle lasts 75-120 mins

• phase 3 has the main propulsive movements- but the series of contractions sweep material lower in the tract.

  • is stimulated by hormone Motilin that gets released into the blood by M cells of the small intestine

mechanism of receptive relaxation

stretch receptors tell vagus

release of VIP and NO

stomach expansion

peristalsis action

propels food down from stomach to colon

peristalsis contraction description

as you swallow a bolus of food, you have contraction of circular muscle behind the bolus (tachykinin) and relaxation ahead of it (vip motor neuron)

peristalsis is under what type of control?

vagal/extrinsic

peristalsis occurs when

after a meal

segmentation description

mixes chyme and facilitates absorption

segmentation contraction desciption

localized circular contractions

segmentation occurs when

continuously

segmentation occurs where

small and large intestine

gastrocolic reflex purpose

clears colon to make room for food

gastrocolic is triggered by, and what role does haustration have in this process

distension of the stomach

• haustration is the process of the taneia coli and circular muscle contracting to make sacs in the colon—→ occurs slowing for storage, absorption, and dehydration of chyme to feces.

gastrocolic reflex effect

mass movements in colon, stimulated by PNS hormones (gastrin and CCK) elevated during digestion

defecation reflex/rectosphincteric desciption

stretch receptors in colon causes internal anal sphincter to relax

• stimulated by haustrae smoothing out into a tube and mass movement contractions forces the feces into the descending colon

mass movements are located where and describe them

colon

• peristaltic contractions stimulated by GI hormones and the vagus nerve in response to chyme in the upper GI tract

Describe what happens in Gastric mucosa?

• Gastrin is released from G cells in stomach antrum and duodenum.

• Gastrin role is to stimulate H+ secretion from parietal cells

Discuss parietal cell HCL production and what stimulates it ?

• parietal cells make stomach acid.

• It’s stimulated by histamine (released by ECL), gastrin (directly), and ACh (vagus stimulation).

• Carbonic anhydrase

  There’s an HCO₃⁻/Cl⁻ exchanger which takes bicarbonate into the blood and brings Cl⁻ into the cell (responsible for alkaline tide) .

  Cl⁻ then diffuses into the lumen.

  The H⁺/K⁺ ATPase (proton pump) moves K⁺ into the cell and H⁺ into the lumen, which is how stomach acid is secreted into the lumen.

  Then H⁺ and Cl⁻ combine in the lumen to form HCl.

achalasia is

when LES cannot relax

List the site of secretion, primary stimuli, and general actions for: Ghrelin

• oxnytic cells(parietal cells) of the stomach

• fasting

• stimulate hunger and increase hypothalamic NPY

What is the role of brush border villus cells vs the Crypts of liberkuhn

• bb: final digestion of carbs and proteins via hydrolases

• crypts: have CFTR that secretes CL- into the gut, and Na+ and water follows

List the duodenal hormones released into the small intestine

• Gastrin

• secretin

• cck

• GIP

Hormone or lumen? What does secretin respond to and what are its effects

• hormone

• Responds to acidic chyme

• Acts on pancreas

• ↑ bicarbonate + electrolyte secretion → buffers chyme (raises pH)

• Helps neutralize acid entering the duodenum

Hormone or lumen? What does cck respond to and what are its effects

• hormone

• Responds to fats & proteins (bile acids)

• Acts on pancreas + gallbladder

• ↑ pancreatic enzymes

• ↑ bile release (gallbladder contraction)

• ↑ bile production in liver

• ↓ gastric emptying

• Synergizes with secretin → ↑ enzyme secretion

Hormone or lumen? What does GIP respond to and what are its effects

• Hormone

• Responds to glucose (carbs)

• Acts on pancreas

• ↑ insulin secretion (major function)

• ↓ HCl secretion (minor)

Hormone or lumen? What does gastrin respond to and what are its effects

• hormone

• stimulate HCL at parietal cell and motility in lower intestine (gastrocolic reflex), and increase intestinal motility

  Responds to stretch

Pancreatic exocrine secretion – structure and regulation

Pancreas and salivary = exocrine gland → secretes into lumen of duodenum

Acini (acinar cells) = exocrine units → produce digestive enzymes

Secretions flow → pancreatic duct → common bile duct → duodenum

Centroacinar cells = stimulated by secretin → secrete bicarbonate (HCO₃⁻)

Feeding → ↑ pancreatic secretions

How are pancreatic proteases secreted and activated?

• Secreted as inactive zymogens (e.g., trypsinogen)

• Activated in duodenum by enterokinase → trypsin

• Trypsin autoactivates + activates other proteases

• Trypsin inhibitor prevents premature activation

What are the main pancreatic enzymes and their functions?

• Proteases → proteins (trypsinogen → trypsin)

• Lipase → fats

• Amylase → starch (~75% digestion before brush border)

What stimulates vs inhibits digestive secretions?

• Stimulate (food present):

Vagus (ACh), gastrin, CCK, secretin

• Inhibit (chyme gone):

↓ gastrin, ↓ vagus, ↓ duodenal hormones

Sphincter of Oddi closes → ↓ bile & pancreatic secretion

How does the jejunum absorb Na⁺ and water?

• Na⁺/H⁺ antiporter: Na⁺ in, H⁺ out

• CO₂ → H⁺ + HCO₃⁻ (carbonic anhydrase)

• HCO₃⁻/Cl⁻ exchanger: HCO₃⁻ out, Cl⁻ in

• Na⁺ pumped to blood (Na⁺/K⁺ ATPase)

• Cl⁻ follows to blood → water follows

What is the function of the ileum?

• Continues NaCl and water reabsorption

• Absorbs electrolytes into ECF

How does the colon “dry out” feces?

ENaC: Na⁺ absorbed

Na⁺/K⁺ ATPase: drives water absorption

K⁺ secreted into lumen

Cl⁻/HCO₃⁻ exchanger: Cl⁻ absorbed, HCO₃⁻ secreted

What is found in feces vs blood (ECF)?

Reabsorbed (ECF): Na⁺, Cl⁻, water

In feces: K⁺, HCO₃⁻, some water

hirschsprung disease is

aganglionic region in lower colon

hirschsprung disease can cause/lead to

toxic megacolon and constipation

List the site of secretion, primary stimuli, and general actions for: GASTRIN

• G cells in antrum of stomach and duodenum

• stretch, peptides, amino acids, vagus(through GRP)

• Stimulate gastric H+, increase lower GI motility, increase gastric mixing

hormones that inhibit or slow motility and emptying

CCK, secretin, GIP

duodenum inhibits gastric emptying when ______ ______

its full

What is found in saliva and what do the components do?

• a-amylase—> startch breakdown and digest alpha linkages to smaller maltose and isomaltose

• lingual lipase—> digest/hydrolyze lipids

• mucous —> lubricate food and get it down esphagous

• TC1- protects vit B12 from stomach acidity. binds in stomach

SNS influence on saliva makes it

thicker

PNS influence on saliva makes it

watery and enzyme rich

what PNS nerves influence saliva

glossopharyngeal and facial