Lecture 9 : Gastrointestinal System III : Intestines and Physiology of Digestion Flashcards
main location of digestion and absorption =
Small intestine
Small intestine : Single convoluted tube running from ____ to ______ – divided into 3 sections:
stomach
large intestine
3 sections of small intestine
Duodenum
Jejunum
Ileum
Duodenum
(12 in)
Most digestion
Alkaline and mucous secretions neutralize acidic chyme
Retroperitoneal position
Jejunum
(8 ft)
A lot of absorption
Intraperitoneal and suspended by mesentery
Ileum
(12 ft)
Absorption of vitamin B12 and bile salts
Peyer’s patches – immune tissue in wall
Intraperitoneal and suspended by mesentery
Histology of the Small Intestine : 4 layered wall
mucosa
submucosa
muscularis externa
serosa
has both extensions into lumen _____ and extensions into lamina propria ______
(villi)
(intestinal crypts)
Specializations of Small Intestine :
- Circular Folds
- Villi
- Microvilli
Circular Folds
Extensions of mucosa & submucosa into lumen to increase surface area for absorption
Visible at the gross anatomical level (to the naked eye)
Villi
Finger-like projections of the mucosa into the lumen: increase surface area for absorption
Contain blood capillaries and lacteals (lymph capillaries) that collect absorbed nutrients
Villi=plural; individual=villus
Microvilli
Apical surface extensions off of individual absorptive cells (=enterocytes)
Make apical surface appear fuzzy = “brush border”
Two types of proteins in apical surface membrane:
Transport proteins : more surface area for each enterocyte to absorb nutrients
Brush border enzymes hang from microvilli to complete carbohydrate & protein digestion in lumen
4 Features of Small Intestine to Maximize Absorption
Circular Folds
Villi off circular folds
Microvilli off individual cells of villi
Small intestine is 32 feet long
More surface area =
more absorption
The ____ absorbs most all nutrients passing through
intestine
Absorption is not regulated –
absorbs nutrients whether we need them or not
Only way to reduce absorption is to remove surface area –
bypass surgeries remove or detour part of intestine
Cell Types of the Small Intestine Mucosa
On the Villi:
Epithelial cells
Enterocytes
Goblet cells
Epithelial cells at the tip of the villus
constantly shed – new epithelium every 3-5 days
Enterocytes
form the bulk of the epithelium; simple columnar absorptive cells with microvilli bound to each other by tight junctions
Goblet cells
mucous secreting cells
Cell Types of the Small Intestine Mucosa
In the Intestinal Crypts:
Enteroendocrine cells
Paneth cells
Stem cells
Enteroendocrine cells
secrete hormones (enterogastrones) like CCK and secretin, sense food in the lumen
Paneth cells
release antimicrobial agents determining which bacteria can colonize the intestine
Stem cells
continuously divide with daughter cells differentiating into the 4 other cell types
Two motility patterns in small intestine
segmentation
peristalsis
After a meal, ____ is principal form of motility
segmentation
segmentation
Ensures mixing and absorption
Intrinsic pacemaker cells in duodenum depolarize more frequently than ileum
Can be altered by ANS
Between meals, a type a peristalsis called ____ is observed
migrating motor complex
Waves of peristalsis begin in the _____ and sweep distally
proximal duodenum
Peristalsis
Controlled by the hormone motilin
Every few hours, sweeps all material into large intestine
Large Intestine : Gross Anatomy
Subdivisions :
cecum
ascending colon
> hepatic (right colic) flexure
transverse colon
> splenic (left colic) flexure
descending colon
sigmoid colon
rectum
anal canal
Special features of the Large Intestine
“Valve” at each end:
Ileocecal valve – prevents backflow of chyme into SI
Internal/External anal sphincters at anal canal
Teniae coli -
Longitudinal layer of muscularis reduced to 3 strips
Haustra -
pockets in wall due to smooth muscle tone
Appendix –
worm-like appendage
contains immune cells and stores of bacteria to recolonize gut; vulnerable to blockage
Appendicitis =
inflammation of appendix; risk of rupture and bacterial spread throughout peritoneal cavity
Histology of Large Intestine:
Mucosa
simple columnar through most of colon, smooth surface – no circular folds or villi
enterocytes - absorptive cells to absorb water, electrolytes
Intestinal glands (crypts) – contain many more goblet cells to help lubricate and protect epithelium as feces move through
Histology of Large Intestine:
Muscularis Externa
incomplete outer longitudinal layer forms teniae coli
The Microbiome (gut bacteria) is important for health
Thousands of different types of bacteria
– weigh several pounds
Help us recover energy from indigestible foods and synthesize some vitamins
Fermentation of _____ produce short chain fatty acids our cells use for energy – also produce mixture of gases (flatulence)
indigestible carbohydrates
Gut bacteria synthesize _____used by liver to produce clotting proteins
B complex vitamins and vitamin K
Microbiome =
Interaction between immune system & microbiome
Beneficial gut bacteria help control _____ like Clostrium difficile
pathogenic bacteria
Gut bacteria vulnerable to long term ____
antibiotic treatment
Main digestive functions of large intestine :
Absorb remaining water
Absorb metabolites from bacteria
Package, store and release feces
Diarrhea =
transit time too fast : too little water absorbed
Constipation =
transit time too slow : too much water
Common Disorders of the Intestinal Mucosa =
Irritable bowel syndrome (IBS)
Inflammatory bowel disease (IBD)
Irritable bowel syndrome (IBS)
diarrhea, constipation or both
abnormal motility, no damage to wall
due to hypersensitivity or reactivity of enteric nervous system
Inflammatory bowel disease (IBD)
Crohn’s, ulcerative colitis - chronic inflammation and ulcerations of intestinal wall
treat with anti-inflammatory or immunosuppression drugs, monoclonal Abs
____ is the last section before anal canal
Rectum
Anal canal =
~ 3 cm long
opens to exterior at anus
lined by stratified squamous epithelium that merges with surrounding skin
2 muscular sphincters control anus:
Internal anal sphincter
involuntary smooth muscle
External anal sphincter
voluntary skeletal muscle; part of pelvic floor
Hemorrhoids =
dilated and inflamed varicose veins of anal canal/anal opening
Defecation reflex =
Stretching of rectum wall initiates reflex
Parasympathetic spinal reflex causes:
contraction of sigmoid colon & rectum wall
relaxation of internal anal sphincter
Sensory signals to brain – conscious decision to voluntarily relax external anal sphincter
Valsalva maneuver –
Glottis (larynx) is closed, diaphragm and abdominal muscles are contracted to increase intra-abdominal pressure
Defecation reflex
Steps:
1) feces move into and distend the rectum = stimulating stretch receptors there = receptors transmit signals along afferent fibers to spinal cord neurons
2) spinal reflex is initiated = parasympathetic motor (efferent) fibers stimulate contraction of rectum and sigmoid colon = relaxation of internal anal sphincter
3) if it is convenient to defecate, voluntary motor neurons are inhibited = external anal sphincter relaxes so feces may pass
Digestion =
catabolic process
breakdown foods into chemical building blocks (monomers)
most digestive enzymes require neutral pH for optimum function – exception: stomach
Most of absorption occurs in ____
small intestine
Nutrients, ions and water are absorbed by moving across the ____ and through the _____
brush border
enterocytes
Transporter proteins are involved in absorption except for______, and water follows the movement of solutes by ____.
lipid soluble substances
osmosis
Most nutrients diffuse into blood capillaries, travel to liver through _____
portal vein system
some fat digestion products enter ______ and reach bloodstream at subclavian vein
lacteals (lymphatic capillaries),
_______ diffuse across plasma membrane by simple diffusion
Lipid soluble molecules (fats)
Many breakdown products of ______ and _____ are co-transported with Na+ across apical membrane secondary active transport
carbohydrates (monosaccharides)
proteins (amino acids)
______ are transported across basolateral membrane by facilitated diffusion
Monosaccharides, amino acids
Most dietary carbohydrates in form of ____
starch (plant storage form of glucose)
____ breaks down starch and glycogen (polysaccharides) into oligosaccharides.
Amylase
Salivary amylase in mouth
Pancreatic amylase in small intestine
Oligosaccharides and Disaccharides must be broken down into _____ by brush border enzymes
Sucrose (table sugar) -
Maltose (grain sugar) –
Lactose (milk sugar) –
monosaccharides
sucrase
maltase
lactase
Only monosaccharides can be absorbed by _____
enterocytes of small intestine
Lactose intolerance -
lactase deficiency
lactose sugars stay in lumen creating osmotic pressure and attracts water
gut bacteria metabolize lactose creating gas, cramping, diarrhea
Cellulose –
indigestible polysaccharide in humans (we lack the enzyme)
acts as dietary fiber
Protease =
enzyme that breaks down protein
Proteins to be digested include:
dietary protein
enzyme proteins
proteins from dead cells
Protein digestion begins in ____ : pepsinogen from chief cells activated to ____ in stomach acid breaks proteins down into polypeptides and amino acids.
stomach
pepsin
In small intestine more ____ from pancreas break polypeptides down further: _______ break proteins into smaller pieces.
proteases
trypsin, chymotrypsin, carboxypeptidase
______ finish digestion by breaking oligopeptides and dipeptides into individual amino acids.
Brush border enzymes
Individual ______ are absorbed into enterocytes of small intestine.
amino acids
Lipase
enzyme that digests fat
Fats –
hydrophobic, emulsified by bile salts from liver (and gall bladder) – form small fat droplets
Lipase hydrolyzes triglycerides into _____
monoglycerides and free fatty acids
Form ____ with bile salts and lecithin – transport products to microvilli.
micelles
At _____, lipids leave micelles and diffuse into enterocyte
apical membrane
In smooth ER of cell, converted back to triglycerides, combined with proteins to form ____ which are released at basal surface by ____
chylomicrons
exocytosis
_____ are too big for blood capillaries, so are picked up by ____ (lymph capillaries) and eventually enter bloodstream near heart – not picked up into portal vein
Chylomicrons
lacteals
_____ don’t have to go through this process and can just diffuse across membrane of enterocyte.
Short chain fatty acids
Enterohepatic Circulation recycles bile salts
Steps:
1) bile salts are secreted into the duodenum
2) as bile salts travel through the small intestine = they allow lipid digestion and absorption to occur
3) 95% of bile salts are reabsorbed by the ileum
4) reabsorbed bile salts travel via the hepatic portal vein back to the liver where they are recycled
(only 5% of bile salts are newly synthesized each time)
The ____ does first processing of absorbed substances
liver
Any substance picked up by intestine ______ travels through portal vein to liver
excludes most fats, which are picked up into _______
blood capillaries
lymphatic capillaries
Liver processes :
nutrients
bile salts
absorbed drugs
Liver may detoxify drugs and ______ their effects
decrease or eliminate
“first pass metabolism” =
how much of an orally-administered drug’s action is lost on its first pass through the liver, before it even reaches the target of the drug
