Small Intestine Anatomy & Physiology (Week 11) Flashcards
What are the three regions of the small intestine?
1) duodenum
2) jejunum
3) ileum
shortest and widest part of the small intestine, C-shaped, and closely associated with the head of the pancreas
duodenum
length of duodenum
~ 25cm
length of jejunum
~ 1m
length of ileum
~ 2m
True or False: The vast majority of the duodenum is retroperitoneal (behind the peritoneum)
True
The jejunum has larger circular folds called ______________
plicae circulares
The ileum has many large lymphoid nodules called ________________
Peyer’s patches
The first 2/3 of the duodenum is supplied by the ___________ artery off the celiac trunk
hepatic
The hepatic artery branches into the __________ and __________
gastroduodenal artery,
anterior superior pancreaticoduodenal artery
The superior mesenteric artery (SMA) branches into the _____________
inferior pancreaticoduodenal artery
The pancreas and duodenum receive arterial blood from both the ________ and _________
hepatic artery,
superor mesenteric artery (SMA)
Note: “from the top it’s from the hepatic artery and from the bottom it’s from the mesenteric artery”
The last 1/3 of the duodenum to the ileum + first part of the large intestine up until the transverse/descending colon receives arterial blood from the ________________
superior mesenteric artery
The superior mesenteric vein receives venous blood from?
- small intestine
- portions of the large intestine
- stomach
- pancreas
The splenic vein receives blood from?
- stomach
- spleen
- pancreas
- distal large intestine
The superior mesenteric vein and splenic vein join together to form the _____________, which drains almost all subdiaphragmatic fore-, mid-, and hindgut structures
hepatic portein vein
The hepatic portal vein drains into the __________
liver
Folds of mucosa and submucosa, characterized by permanent ridges about 10 mm tall that project into the lumen, encourages mixing, and enhances absorption by increasing surface area
plica circulares
Finger-like projections of mucosa that are 0.5-1mm long and vastly increase surface area of epithelium
villi
projections of apical membrane of absorptive cell that are 1 um long (super small), cylindrical, contain 20-30 actin filaments, and greatly increase surface area
microvilli
What type of cell of the small intestine can be characterized as simple columnar epithelium with microvilli and a short life span (~ a few days)?
surface absorptive cell/enterocytes
What type of cell of the small intestine can be characterized as:
- scattered among the absorptive cells
- specialized in mucus secretion
- facilitates passage of material through bowel
goblet cell
What type of cell of the small intestine can be characterized as:
- typical serous-secretory appearance, with basophilic basal cytoplasm and apical secretory vesicles
- basal portion of the intestinal crypts, below the stem cells, release lyzosyme, phospholipase A2, and defensins (note: these are all immunoproteins)
- regulate the microenvironment of the intestinal crypts and innate immune response
paneth cell
What type of cells in the small intestine secrete hormones?
1) enteroendocrine cells
2) mucosal cells
What do the following enteroendocrine cells in the small intestine secrete?
I cells:
S cells:
D cells:
K cells:
L cells:
Mo cells:
I cells: cholecystokinin
S cells: secretin
D cells: somatostatin
K cells: GIP
L cells: peptide YY, incretins
Mo cells: motilin
What do mucosal cells in the small intestine secrete
incretins (similar to L cells)
What is the main hormonal function of somatostatin?
Review: somatostatin is released by D cells
“turns down” the release of hormones from nearby cells
What is the main hormonal function of histamine?
If released from ECL –> stimulates acid secretion
If released from EC –> increased motility
What is the main hormonal function of gastrin?
Review: gastrin is secreted by G cells in the stomach (not small intestine)
increases secretion of stomach acid
What is the main hormonal function(s) of CCK (cholecystokinin)?
- pancreatic enzyme secretion*
- gallbladder contraction*
- satiety
- inhibits acid secretion
What is the main hormonal function of peptide YY?
- inhibits gastric secretion
- slows gastric emptying/motility
What is the main hormonal function of motilin?
migrating motor complex (stimulates contractions between meals)
What is the main hormonal function of secretin?
- bicarbonate and water secretion from pancreas
- inhibits gastric acid secretion
- slow gastric emptying
Stretch from chyme against intestinal wall elicits ______________ (local reflex)
concentric contractions
note: this is NOT the same as peristalsis
Concentric contractions in the small intestine are spaced 1 to 5cm and cause ___________
segmentation
Chyme propelled through the small intestine by propulsive waves that move towards the ansu is known as ____________
peristalsis
a powerful wave of contractile activity that travels long distances down the small intestine and is caused by intense irritation or unusual distension
peristaltic rush
distension in the alimentary canal causes distal parts of the canal to relax and proximal parts to contract (circular muscle)
“law of the gut”
Note: peristaltic reflex/”law of the gut” is mediated by the enteric nervous system (ENS) and various hormones
The hormones that enhance peristalsis include:
- gastrin
- CCK
- serotonin
The hormones that inhibit peristalsis include:
- secretin
- peptide YY
- epinephrine
What are some requirements for chyme to enter the duodenum?
- food particles must be very small (<2mm)
- small volumes of low pH fluid (large volumes of low pH/acidic fluid = damaging)
- gradual release (to allow absorption and chemical digestion)
The _____________ is the major organ that regulates rates of gastric emptying
duodenum
Both the sympathetic nervous system (@ spinal cord) and the enteric nervous system (submucosal and myenteric plexuses) will INHIBIT gastric emptying in response to what?
- increased or decreased osmolarity in the duodenum
- decreased pH (increased acidity) in the duodenum
- distention or any chemical irritation of the duodenum
- breakdown productions of proteins and fats (mostly proteins) in the duodenum
True or False: CCK is secreted in the small intestine, but mostly in the ileum
False
Mostly in the duodenum
What substances elicit CCK release?
FATS** > peptides > carbs
True or False: Secretin is secreted throughout both the duodenum and jejunum
True
What substances elicit secretin release?
low pH > fats, capsaicin, bile acids
a major hormone that performs the role of the “ileal brake”, secreted by L cells in the distal small intestine (ileum)
peptide YY
What substances elicit peptide YY release?
FATS > carbs, amino acids
True or False: If undigested food reaches the distal small intestine (ileum), then peptide YY “slows everything down”
True
What is an oleate meal?
A meal that is high in fat
True or False: Compared to acidic and saline meals, oleate meals tend to increase the rate of gastric emptying.
False
Oleate (fatty) meals tend to slow gastric emptying, whereby there is more food remaining in the stomach after a period of time, compared to an acidic and saline meals.
The _______________ valve forcefully closes when excess pressure builds up in the cecum
ileocecal
True or False: The ileocecal sphincter usually remains mildy constricted
True
Digestive juices (which includes large quantities of water and electrolytes) are secreted by _____________ at a rate of about _____________ mL/day
enterocytes,
1800mL/day (1.8L)
True or False: intestinal juices have a pH of 7.5-8.0 and are a little more alkaline that ECF, due to the presence of more bicarbonate
True
The ________________ are found proximal to the sphincter of Oddi and secrete alkaline mucous in response to:
- tactile stimuli or irritating stimuli of the overlying mucosa
- vagal stimulation
- GI hormones (esp. secretin)
Brunner’s Glands
Note: vagal stimulation can both increase acidic secretion in the stomach and alkaline mucous in the small intestine
What is the function of Brunner’s Glands?
protect duodenal wall from digestion by gastric juice
Brunner’s Glands are inhibited by ______________ (parasympathetic/sympathetic) stimulation
sympathetic
Chyme arrives in the duodenum via the ________ sphincter
pyloric
function of bile
emulsification of lipids
function of pancreatic enzymes
the hydrolysis of fats, proteins, and carbohydrates into smaller molecules
bile is secreted by the liver by the right and left _______________
hepatic ducts
the right and left hepatic ducts join to form the ___________________
common hepatic duct
the common bile duct is formed at the junction of the ___________ and _____________
cystic duct and common hepatic duct
the sphincter of Oddi regulates the release of bile into the _________ from the ampulla of Vater
duodenum
If the sphincter of Oddi is closed, then the bile is stored in the _____________
gallbladder
The main pancreatic duct meets the common bile duct at the __________
ampulla of Vater
True or False: The sphincter of Oddi regulates biliary secretions but not pancreatic secretions into the duodenum
False
The sphincter of Oddi regulate both biliary and pancreatic secretions into the duodenum
Enterokinase activates/cleaves trypsinogen to ____________
trypsin
Trypsin activates/cleaves:
chymotrypsinogen –> ?
proelastase –> ?
procarboxypeptidase –> ?
chymotrypsinogen –> chymotrypsin
proelastase –> elastase
procarboxypeptidase –> carboxypeptidase
Note: these are all enzymes relevant to PROTEIN digestion
What is the function of pancreatic amylase?
carbohydrate digestion (digests starch in particular)
What enzymes are used for lipid digestion?
- pancreatic lipase/co-lipase
- phospholipase A2
enzymes break macronutrients down into smaller and smaller particles through the process of hydrolysis
enzyme digestion
Note: all pancreatic, gastric, and brush border enzymes use hydrolysis as a means of breaking a macromolecule into smaller molecules
Food is first mixed with saliva in the mouth, which contains the enzyme __________ secreted mainly by the parotid glands
ptyalin (alpha-amylase or salivary amylase)
Pyalin hydrolyzes ___________ into the disaccharide maltose and other small polymers of glucose (3-9 glucose molecules, aka oligosaccharides)
starch
What blocks the activity of ptyalin?
the acid of gastric secretions (pH <4)
we’re swallowing our saliva as we eat, so once it reaches the stomach, that’s when it stops working (because it’s too acidic in the stomach)
What enzyme do we use for digestion of carbohydrates in the small intestine?
pancreatic amylase
Note: like salivary amylase, but “much much more powerful”… “the most important enzyme for digestion of starches”
What are the three major enzymes that digest (hydrolyze) the major disaccharide sugars in our diet?
1) lactase
2) maltase (isomaltase)
3) sucrase
glucose + galactose = ?
lactose
glucose + glucose = ?
maltose
fructose + glucose = ?
sucrose
Pepsin is most active between pH _________
1.5-3.5
True or False: In the mouth, there is only mechanical digestion of protein and no enzymatic digestion
True
True or False: In the stomach, HCL denatures protein but is not involved in hydrolysis
True
Note: denaturation improves the “exposure” of peptide bonds to digestive enzymes
Where does most protein digestion occur?
duodenum and upper jejunum
Upon entering the small intestine, polypeptides (protein) are attacked by the following enzymes: trypsin, chymotrypsin, carboxypolypeptidases, and proelastase
Which of these four enzymes activates the others?
trypsin
33% of protein is absorbed as ___________ and 67% is absorbed as ____________
free amino acids,
peptides
Why is the digestion of fats challenging?
not very water soluble (therefore tends to accumulate in large droplets) –> reduces surface area –> poor water solubility –> impairs the process of chemical digestion and absorption
How do we improve the digestion of fats?
emulsify the fat droplets into very small droplets that have an amphipathic molecule on the outside (formation of micelles)
Note: this increases surface area which improves interaction with water
________ contains a variety of amphipathic molecules that help with the formation of micelles…
- lecithins
- bile salts
- cholesterol
Bile
Outside of the micelle is ____________ (hydrophobic/hydrophilic)
hydrophilic
Inside the micelle is _____________ (hydrophobic/hydrophilic)
hydrophobic
Once micelles have been formed, _____________ acts on the triglycerides inside the micelle
pancreatic lipase
True or False: Pancreatic lipase works alone to digest lipids
False
Usually requires colipase to help it insert into the micelle and activate it
What does pancreatic lipase break triglycerides into?
2 free fatty acids and 2-monoacyl glycerol
When free fatty acids and 2-monoacyl glycerol diffuse across the brush border of the enterocyte, they reassemble into _______________
triglycerides
Phospholipase works similar to pancreatic lipase, except it breaks down _____________ into fatty acids and other components
phospholipids
After triglycerides are reassembled into the enterocyte, they are inserted into ________________
chylomicrons
a spherical lipoprotein, with a phospholipid bilayer and proteins on the outside and that absorbs triglycerides and cholesterol in the inside
chylomicron
Note: chylomicrons are necessary for transporting triglycerides and cholesterol esters (big molecules) throughout our circulation
Chylomicrons are secreted into the ____________ and eventually transported into the _____________, to join the venous blood
lymphatic vessels,
thoracic duct
True or False: Carbohydrates, amino acids, and small free fatty acids can diffuse across the basement membrane of the enterocyte, into the capillary loops within the villus, but larger lipids need to be carried by chylomicrons, which enter via openings of the lacteals (lymphatic capillaries within the villi)
True
When absorption at the small intestine is unsuccessful, nutreints remain int he lumen of the intestine, potentially leading to what?
- diarrhea
- micronutrient and macronutrient deficiencies
What are four major malabsorption problems that can occur?
1) disturbances in intraluminal digestion
2) disturbances in terminal digestion
3) disturbances in transepithelial transport
4) disturbances in lymphatic transport
when enzymes/bile secreted into the lumen are inadequate for the near-complete breakdown of proteins, carbs, or fats
disturbances in intraluminal digestion
when the brush border enzymes cannot break down small peptides or disaccharides
disturbances in terminal digestion
when nutrient, fluid, and/or electrolyte transport are disordered
disturbances in transepithelial transport
when lipid transport is impaired
disturbances in lymphatic transport
pathogenesis of celiac disease
damage to the villi and microvilli resulting in loss of surface area and overall absorptive enterocyte function
characterized by disturbances in:
- terminal digestion
- transepithelial transport
pathogenesis of chronic pancreatitis
lack of major digestive enzymes from the pancreas leads to major impairment in absorption, diarrhea, and nutrient deficiencies
characterized by disturbances in:
- intraluminal digestion
pathogenesis of disaccharidase deficiencies (e.g., lactose intolerance)
lack of disaccharide [digestion?] results in unabsorbed sugar in the lumen –> results in bacterial gas production and osmotic diarrhea
characterized by disturbances in:
- terminal digestion
pathogenesis of gastroenteritis (e.g., bacterial, viral, parasitic)
damage to the brush border or dysregulation of electrolyte transport results in impaired ability to absorb nutrients
characterized by disturbances in:
- terminal digestion
- transepithelial transport
