GI tract digestion and absorption Flashcards
What are the organs of the GI tract?
mouth
pharynx esophagus
stomach
small intestine
large intestine
rectum
anus
What are the accessory organs?
Salivary glands
liver
gallbladder
pancreas
What are the functions of the digestive system?
Ingestion - food enters mouth
Propulsion: moving food
Mechanical breakdown
Digestion
Secretion
Absorption
Defecation
What is the benefit of mechanical breakdown of food?
increases surface area of food particles
What is secreted in the digestive system and by what?
release of:
-water
-acids
-buffers
-enzymes
-salts
by epithelium and glands
Describe absorption by the digestive system?
Movement of organic substrates, electrolytes, vitamin and water across digestive epithelium into interstitial fluid
Occurs by active or passive transport to blood or lymph
What is the purpose of the salivary glands?
secrete saliva to moisten food and begin chemical digestion of carbohydrates
What are the 3 pairs of salivary glands that exist in the oral cavity?
Parotid glands (largest, most watery)
Submandibular (viscous serous)
Sublingual (mucus)
Describe the shape and location of the parotid glands?
Irregularly shaped
Inferior to zygomatic arch and deep to skin covering lateral and posterior surfaces of mandible
What is the parotid duct?
Also known as Stenson’s duct
Drains secretion of parotid glands
Empties into vestibule at 2nd upper molar
Describe the secretions of the parotid glands?
Serious and contain large amounts of salivary amylase
Breaks down complex carbohydrates
Produces 25% of total saliva
What could cause parotitis?
Mumps (paramyxovirus)
other viruses
parotid calculi
bacterial (S.a, gram -) most common in debilitated patients with poor oral intake
TB
autoimmune (Sjogren’s)
Drugs - PTU
Signs and symptoms of parotitis?
parotid tenderness
facial edema
erythema/edema/discharge of parotid duct
constitutional symptoms
Describe the anatomy of the sublingual salivary glands?
covered by mucous membranes of the floor of the mouth
Describe the sublingual ducts
open along either side of the lingual frenulum
Describe the secretions from the sublingual salivary glands?
mucus which buffers and lubricates
produces 5% of total saliva
Where are the submandibular salivary glands located?
in floor of the mouth along inner surface of mandible
Describe the submandibular ducts
open into the mouth on either side of the lingual frenulum
Immediately posterior to the teeth
Describe the secretions from the submandibular glands?
Mixture of buffers, glycoproteins, and salivary amylase
Produces 70% of total saliva
Describe the composition of saliva?
99.4% water
0.6%:
-electrolytes (Na, Cl, HCO3)
-buffers
-glycoproteins
-antibodies
-enzymes
-waste products
-mucins
What are mucins?
glycoproteins in the saliva which provide lubrication actions
What are the functions of saliva? (6)
- Flush and clean the oral cavity
- Buffer saliva (pH~7)
-helps prevent acid buildup from bacteria - Contains lysozymes and antibodies to help control oral bacteria
- Lubricates the mouth and food that enters the mouth
- Dissolves food chemicals which stimulate the taste buds
- Begins digestion of complex carbohydrates
-salivary amylase
What are causes and the result of decreased saliva production?
Leads to increased bacterial build-up in oral cavity
-radiation
-sleep
-anticholinergic drugs
Describe the salivary reflex?
stimulated by objects in the mouth
Stimulate receptors/taste buds
-mediated by cranial nerves V, VII, IX, X
Describe the innervation of the salivary glands?
Each has both sympathetic and parasympathetic innervation
Describe the parasympathetic innervation of the salivary glands?
-outflow originates in salivary nuclei of the medulla oblongata
-Stimulation increases secretion by all of the salivary glands
Describe the sympathetic innervation of the salivary glands?
Stimulation increases secretion of THICK saliva
What are the triggers that increase saliva production? (6)
- Chewing with empty mouth (chewing gum)
- Smelling food
- Thinking about food
- Irritating stimuli in esophagus, stomach, and intestines
- Nausea/pre-vomiting
- Unpleasant oral stimuli
Define deglutition?
swallowing
complex process that can be initiated voluntarily but then proceeds automatically
Outline the swallowing reflex?
- Initiated when tactile receptors on palatal arches and uvula are stimulated by the passage of a food bolus
-info relayed to swallowing center of medulla oblongata - motor commands from swallowing center signal pharyngeal musculature
-produces coordinated pattern of muscle contraction - When food bolus enters esophagus, respiratory centers are inhibited and breathing stops
What are the 4 phases of deglutition?
- Buccal phase
- Pharyngeal phase
- Esophageal phase
- Food bolus enters stomach
Describe the buccal phase?
bolus is compressed against the hard palate
the tongue retracts, forcing the bolus into the oropharynx which triggers reflex responses
Also elevates the soft palate which closes off the nasopharynx
Describe the pharyngeal phase?
Begins as food bolus enters palatopharyngeal/palatoglossal arches and posterior pharyngeal wall
larynx is elevated and the epiglottis folds
-directs food bolus past the closed glottis
Uvula and soft palate block passage into nasopharynx
Describe the esophageal phase?
pharyngeal muscles contract and food bolus enters esophagus
food bolus is pushed toward stomach via peristalsis
Describe what happens when the food bolus enters the stomach?
Bolus approaches stomach and triggers opening of LES
Once sphincter opens, food bolus enters stomach
Describe a medullary CVA?
infarct occurs in medullas at level of swallowing center
impacts swallowing reflex and results in dysphagia
Speech therapist performs swallowing evaluation on all patients before they are allowed to resume oral food/drink; ensures swallowing center/reflex has not been impacted
What are the two types of secretory cells in the stomach?
parietal and chief cells
Describe parietal cells?
Secrete intrinsic factor and hydrochloric acid
Intrinsic factor: helps absorb vit B12 in small intestine
Hydrochloric acid: highly acidic
Describe chief cells?
Secrete pepsinogen and renin/gastric lipase
Pepsinogen: inactive proenzyme which is converted to pepsin in gastric lumen; functions as proteolytic (digests proteins)
Renin and gastric lipase: enzymes produced in newborns which assist in digestion of milk; NOT PRESENT IN ADULTS
At what pH do the parietal cell secretions keep the stomach at?
1.5-2.0
What are 4 functions of the highly acidic environment of the stomach?
- Kills most microorganisms ingested in foods
- Denatures proteins in food and inactivates enzymes in foods
- Breaks down plant cell walls and connective tissue in meat
- Activates pepsin
Do pyloric glands produce enzymes or acids?
NO
What do pyloric glands secrete?
a mucus secretion containing hormones;
enteroendocrine cells that secrete:
-gastrin
-somatostatin
Describe gastrin
a hormone produced by enteroendocrine cells that stimulates secretion of parietal and chief cells
stimulates contraction of gastric wall (helps mix and churn chyme)
Describe somatostatin
Hormone produced by enteroendocrine cells that INHIBIT the secretion of gastrin
What 3 phases regulate gastric activity
- cephalic phase
- gastric phase
- intestinal phase
Describe the cephalic phase
begins when you see, smell, taste, or think of food
Vagus nerve is stimulated via parasympathetic nervous system
-triggers production of gastric juices
Several minutes in duration
Function of cephalic phase
prepare stomach for arrival of food
Describe mechanism of cephalic phase
neural, via preganglionic fibers in vagus nerve and synapses in submucosal plexus
Describe the gastric phase
Begins when food arrives in the stomach
Acids and enzymes process undigested food
3-4 hours in duration
What stimulates gastric phase
- distension of stomach
- increase in pH of gastric contents
- Presence of undigested substances in stomach
What is the function of the gastric phase?
enhances secretion started in cephalic stage; initiate digestion of proteins by pepsin
Describe the intestinal phase?
begins when chyme first enters small intestine
functions as control rate of gastric emptying to ensure functions of small intestine proceed efficiently: secretory, digestive, absorptive
Also triggers activities of pancreas, intestines, liver, and gallbladder
What triggers gastrin
vagus nerve; food in stomach, esp. undigested proteins
Where is gastrin released from?
G-cells within gastric glands and also the duodenum
What are the actions of gastrin?
Stimulates the stomach (production of acids and enzymes and also increase motility)
What triggers secretin release?
Chyme entering duodenum
Where is secretin released from?
duodenum
What are the actions of secretin?
stimulates the pancreas to secrete buffers; slows down digestion in stomach; increases production of bile
What triggers cholecystokinin (CCK)
chyme entering the duodenum containing lipids and partially digested proteins
Where is cholecystokinin (CCK) released from?
duodenum
What are the actions of cholecystokinin?
stimulate pancreas to secrete digestive enzymes; slows down digestion in stomach; causes gall bladder and liver to release contents; reduces feeling of hunger
What triggers gastric inhibitory peptide (GIP)?
chyme entering the duodenum containing many lipids and/or glucose
Where is gastric inhibitory peptide (GIP) secreted from?
duodenum
What are the actions of gastric inhibitory peptide? (GIP)
stimulates the release of insulin from pancreas, inhibits stomach
What is the function of the intestinal phase?
control rate of chyme entry into duodenum
What triggers central reflexes of gastric regulation?
stretch receptors in stomach
What is the gastroenteric reflex?
stimulates motility and secretion in small intestine
INCREASE motility/secretion:
-carbs
-alcohol
-caffeine
DECREASE motility/secretion:
-proteins
-lipids
Describe the gastroilieal reflex?
triggers opening of the ileocecal valve
-allows material to enter from small to large intestine
Where is the vomiting center located?
medulla oblongata
When does vomiting occur?
in response to irritation of the pharynx, stomach, esophagus, small intestine, or oral fauces
What happens during emesis?
motor response from the medulla
-pylorus relaxes
-contents from the duodenum regurgitate
General info about liver?
largest visceral organ
> 200 functions in the body
center for metabolic regulation in the body
Describe the metabolic regulation of the liver
regulates composition of circulating
-all blood leaving the GI tract enters liver via hepatic portal system
-Liver cells extract nutrients and toxins from blood prior to it entering back into systemic circulation via hepatic veins
What are the functions of metabolic regulation of the liver?
carbohydrate metabolism
lipid metabolism
amino acid metabolism
waste product removal
vitamin storage
mineral storage
drug inactivation
Describe carbohydrate metabolism by the liver and the different triggers?
stabilizes blood glucose
hypoglycemia triggers:
-glycogenolysis puts glucose into blood
-gluconeogenesis
hyperglycemia triggers:
-storage of glycogen in liver
-synthesis of lipids utilizing glucose for liver
describe lipid metabolism by the liver
regulates circulating levels of triglycerides, fatty acids, and cholesterol
high levels: lipids removed from bloodstream for storage
low levels: lipid reserves are broken down and released into blood
Describe amino acid metabolism by the liver
removal of excess amino acids from bloodstream for:
-protein synthesis
-conversion to lipids or glucose
Hepatic encephalopathy syndrome symptoms?
confusion
personality changes
intellectual impairment
depressed level of consciousness
Etiology of hepatic encephalopathy
Ammonia is unable to be metabolized by damaged hepatocytes, enters bloodstream, crosses blood-brain barrier and negatively impacts astrocytes
Treatment of hepatic encephalopathy
low-protein diet (amino acids are converted to ammonia)
Lactulose (decreases production of ammonia in intestinal tract)
What mineral storage does the liver do
converts iron to ferritin for storage
What vitamin storage does the liver do
B12
Fat-soluble vitamins A, D, E, K
What drug inactivation does the liver do
removes and breaks down circulating drugs
-limiting duration of their effects
How much cardiac output does the liver receive?
25%
What are the elements of hematologic regulation that the liver does?
phagocytosis/antigen presentation
synthesis of plasma proteins
removal of circulating hormones
removal of antibodies
removal/storage of toxins
What part of the liver does phagocytosis/antigen presentation?
Kupffer cells
Describe Kupffer cells in the liver
-antigen presenting cells
-capable of stimulating an immune response
-engulf old and damaged RBCs, cell debris, and pathogens to remove them from the bloodstream
What is an example of phagocytosis in the liver?
Kupffer cells engulf and phagocytize old RBCs –> RBC heme converted to bilirubin which travels to large intestine and is converted to urobilinogen
Describe the synthesis of plasma proteins that the liver does
-albumins, transport proteins, clotting proteins, and complement proteins
-ex) prolonged PT/PTT:
-In presence of liver damage/disease, liver is not able to adequately synthesize clotting proteins
Describe the removal of circulating hormones by the liver
absorb and recycle:
-epinephrine
-norepinephrine
-insulin
-thyroid hormones
-steroid hormones (sex hormones)
-corticosteroids
-cholecalciferol (VIt D3)
Does the liver remove antibodies
Yes
Describe the removal/storage of toxins by the liver?
Absorbs lipid soluble toxins in the diet (DDT) and stores them in lipid deposits where they do not disrupt cell functions
Also removes toxins for excretion in the bile
What is the function of the gall bladder
stores and concentrated bile prior to excretion into small intestine
What are the main structures of the gall bladder?
3 regions: neck, body, fundus
Cystic duct:
-extends from gallbladder
-Unites with common hepatic duct from common bile duct
Describe the anatomy of the common bile duct?
Common bile duct meets pancreatic duct then enters duodenal ampulla
Describe the duodenal ampulla
chamber that receives:
-buffers and enzymes from the pancreas
-bile from the liver and gall bladder
Ampulla of Vater
Describe the hepatopancreatic sphincter?
Muscular sphincter that encircles lumen of common bile duct
Sphincter of Oddi
Describe the process of bile release and the importance of cholecystokinin (CCK)
Released when chyme (especially lipid rich) enters the duodenum
-stimulates the relaxation of hepatopancreatic sphincter
-Triggers gallbladder contraction and bile release
-bile blows to duodenum via common bile duct
What is cholelithiasis
presence of gallstones in the gallbladder
-usually cholesterol or bile salts/calcium salts
-effects 20% of patients over age 40
What are the risk factors for cholelithiasis?
cirrhosis of the liver
female gender
Define cholecystitis?
acute inflammation of the gallbladder, usually caused by a gallstone which cannot pass through cystic duct
What are the risk factors for acute cholecystitis?
female
fertile
fat
forties
fair-skinned
What is bile
synthesized by liver and excreted into lumen of the duodenum
What are the components of bile?
-majority water
-ions: dilute and buffer acids prior to entrance into duodenum
-bilirubin: pigment derived from hemoglobin
-cholesterol
-bile salts: lipids synthesized from cholesterol in the liver
What is the function of bile?
bile salts emulsify ingested lipids and fat-soluble vitamins in stomach
-increase surface area to facilitate action of lipid-insoluble enzymes
Describe the enterohepatic circulation of bile?
after lipid digestion, 90% of bile salts are:
-reabsorbed into the ileum
-enter the hepatic portal circulation
-liver collects and recycles bile salts
What happens following a cholecystectomy?
-gallbladder no longer present to store bile produced by liver
-large amounts of bile enter the large intestine (lack of regulation)
-Triggers intestinal water secretion and increased motility
-leads to frequent loose stools/diarrhea
What is the function of the pancreas?
produces digestive enzymes and buffers
exocrine: secretes pancreatic juice:
-alkaline mixture of digestive enzymes, water, and ions
Describe the pancreatic duct (duct of Wirsung)
-delivers digestive enzymes and buffers to duodenum
-Also meets with common bile duct to empty into duodenum via the ampulla of Vater
Describe the Accessory pancreatic duct (duct of santorini)
-Present in 3-10% of the population
-directly branches from the pancreatic duct
-empties into the duodenum independently
How is the pancreas controlled?
- hormones in the duodenum
-control the secretory activities of the pancreas - stimulation of vagus nerve (CN X)
-during cephalic phase of gastric regulation
-allows pancreas to get a “head start” on digestive enzyme production
What type of cells produce pancreatic enzymes
acinar cells
What are the types of pancreatic enzymes?
Alpha-amylase
Lipase
Nucleases
Proteolytic enzymes
Describe proteolytic enzymes of the pancreas?
-70% of all pancreatic enzyme production
-Attack peptide bonds of amino acids
-Secreted as inactive proenzymes, then activated in the small intestine; protects the secretory cells from their own destructive effects
What is pancreatic insufficiency
Condition characterized by inadequate production and secretion of pancreatic enzymes and/or hormones
What is the etiology of pancreatic insufficiency
disease process which is destructive to pancreatic tissues
-chronic alcohol use syndrome
-cystic fibrosis
-pancreatic cancer obstructing duct
What are the treatment options for pancreatic insufficiency?
pancreatic enzyme replacement
Dietary modification
vitamin supplementation
What is the function of the small intestine?
90% of nutrient absorption
What is peristalsis?
movement through small intestine:
increased via reflexes
1. gastroenteric
2. gastroilial
Describe the anatomy and function of the duodenum
receive chyme from stomach and neutralize acids prior to damaging the small intestine
Describe the anatomy and function of the jejunum
performs majority of nutrient absorption via plicae circulares of villi
What is the function of the ileum
immune function
What are Peyer’s patches
-masses of lymphoid tissues within lamina propria of small intestine
-protect small intestine from commensal bacteria of large intestine
-also respond to GI infection
Describe the intestinal villi
-fingerlike projections within the mucosa of small intestine
-covered by simple columnar epithelium and microvilli (brush border)
-move substances through the small intestine
Describe the intestinal cells of the small intestine
glandular pockets which extend deep into the lamina propria
Describe brush border enzymes
-integral membrane proteins on the surfaces of intestinal microvilli
-function in breaking down material that contact microvilli; then, epithelial cells absorb the breakdown products
Describe the duodenal glands (brunner’s glands)
-Produce copious amounts of mucus when chyme arrives from stomach
-protects epithelium from acidity of chyme
-Contains bicarb which raises pH of chyme
What are the duodenal hormones?
secretin
CCK
Gastrin
gastric inhibitory peptide
vasoactive inhibitory peptide
When is secretin released
when chyme enters the duodenum
What does secretin trigger
pancreatic secretion of a watery buffer solution that helps to raise pH of chyme
How does CCK effect pancreatic enzymes
stimulates their release
What cells secrete gastrin
G cells in stomach
What triggers gastrin secretion
exposure to undigested proteins
What does gastrin do
promotes stomach motility and secretion of enzymes/gastric acid
When is gastric inhibitory peptide (GIP) secreted?
when fats and carbs enter the small intestine
What does gastric inhibitory peptide (GIP) do
inhibit gastric activity so that insulin is released and lipids synthesized
What are the effects of vasoactive inhibitory peptide (VIP)?
-stimulates secretion of intestinal glands
-inhibits stomach acid production
-dilates intestinal capillaries to allow for nutrient absorption
What is the dominant feature of glands in the large intestines? why?
dominated by mucous glands
-mucus lubricates fecal material as it becomes dry due to water reabsorption
Do large intestines have villi
No; little nutrient absorption (10%)
What is the reabsorbed in the large intestine?
water
bile salts
vitamins
organic waste products
Why is vitamin K reabsorbed in the large intestine?
required for liver synthesis of clotting factors
Why is biotin reabsorbed in the large intestine?
needed for glucose metabolism
Why is vitamin B5 reabsorbed in the large intestine?
required to manufacture steroid hormones and neurotransmitters
What are the organic waste products reabsorbed by the large intestines?
bilirubin
peptides
indole/skatole
hydrogen sulfide
toxins
Describe bilirubin as a waste product in large intestine?
breakdown product of heme conversion to urobilinogen by bacteria in large intestine
some urobilinogen reabsorbed into bloodstream and excreted in urine
remaining urobilinogen stays in large intestine and provide feces with brown color
Describe breakdown by peptides in large intestines?
breakdown by normal flora in large intestine yields:
-ammonia: enters hepatic portal circulation, removed by liver, and converted to urea for excretion
What is indole/skatole
nitrogen compounds responsible for odor of feces
What is hydrogen sulfide
gas with rotten egg odor
How do the large intestines move
Slowly:
-peristalsis
-haustral churning (segmental)
- mass movement
Describe mass movements in the large intestine
-occurs several times a day
-move contents from transverse colon through rest of large intestine
-forces feces into rectum and provides the urge to defecate
What is the defecation reflex?
occurs when rectal wall distends and consists of 2 positive feedback loops
Describe the long defecation reflex
-parasympathetic innervation
-internal anal sphincter relaxes, allowing feces to move into anal canal
Describe the short defecation reflex?
somatic reflex (involuntary response to a stimulus
stimulates contraction of external anal sphincter
Does the release of feces require conscious effort
Yes
What coordinates the movements of the GI tract?
enteric nervous system (ENS)
What are the components of the enteric nervous system?
sensory neurons
interneurons
motor neurons
What part of the nervous system is the enteric nervous system?
Part of the ANS
-primary innervation is PARASYMPATHETIC
-sympathetic fibers also have synapses here
Many fibers continue and innervate the myenteric plexus
Describe the myenteric plexus
Contains both sympathetic and parasympathetic innervation
What elements of the GI tract does the parasympathetic portion of the myenteric plexus control?
increase:
-muscle tone
-GI activity
-glandular secretion
What elements of the GI tract does the sympathetic portion of the myenteric plexus control?
decreases:
-muscle tone
-GI activity
-glandular secretion
