Gastrointestinal Tract 6 - Small Intestine Flashcards
Small intestine divided into three sections
- Duodenum (closest to stomach) - 30cm
- Jejunum-~1 m
- Ileum-~1.7 m (closest to large intestine)
Does the small intestine do absorption AND digestion?
YES
Duodenum functions
- Mixing of pancreatic digestive enzymes and bile with food
- Absorption of nutrients, iron and calcium.
- Release of secretin and CCK
Jejunum function
- Digestion and absorption
Ileum function
- Digestion and absorption
- Bile acids, vitamin B12
How is SA increased in the small intestine?
By Folds of Kerckring/ Circular folds
Structure of a villus in the small intestine contain:
- cells enterocyte (absorptive cell)
- goblet cell (mucus)
- endocrine cell
- paneth cell
Potential cell types derived from stem cells
- Absorptive cells: absorption, brush-border enzymes
- goblet cell: secrete mucus
- endocrine cell: release hormones
- paneth cell: secrete antibacterial proteins
Brush border
small projections (microvilli) of epithelial cells covering the villi of the small intestine, major absorptive surface of the small intestine
Brush border enzyme
enzyme anchored to the brush border with catalytic activity in the lumen
Brush border enzymes are Important for breaking down
carbohydrates and peptides into sugars and amino acids prior to transport across the enterocyte
Digestion of carbohydrates
- Starch (amylose and amylopectin) is broken down into maltose, maltotriose, and α-limit dextrins by salivary and pancreatic amylases
- These are broken down to the monosaccharide glucose by brush border enzymes
Other sugars broken down by brush border enzymes
- Sucrose broken by sucrase into (glucose + fructose)
- lactose broken by lactase into
glucose +galactose
Intestinal absorption of glucose/galactose
Glucose/galactose from intestinal lumen goes into epithelial cells (low Na+, high K+) via transport protein (SGLT) and leaves through another one (GLUT2) into the blood (high Na+, low K+)
Intestinal absorption of fructose
Fructose from intestinal lumen goes into epithelial cells (low Na+, high K+) via transport protein (GLUT5) and leaves through another one (GLUT2) into the blood (high Na+, low K+)
When lactose cannot be completely digested:
Results in decreased water absorption in the gut (presence of lactose produces an osmotic gradient)
Lactose containing fluid passes on to large intestine and bacteria digest lactose
- Gas (colon distension and pain), diarrhea
Proteins are broken down by
pepsin in the stomach, and in the small intestine by pancreatic proteases (major ones are trypsin and chymotrypsin)
Once proteins are broken down in the stomach or small intestine, they are then further broken down by:
- Carboxypeptidase (pancreatic protease)
- Aminopeptidase (brush border enzyme in the small intestine)
- Other brush border enzymes (>20 different peptidases)
Once digested, free amino acids are absorbed by
secondary active transport coupled to Na+ (multiple transport proteins)
Small peptides can also be absorbed by different secondary active transport proteins coupled to H+, after …
Peptidases within the cytosol then hydrolyze peptides to amino acids
After the Peptidases hydrolyze peptides to amino acids, the amino acids then undergo
facilitated diffusion across the basolateral surface of the enterocyte
- different facilities transporters between the two surfaces, each for a specific amino acid.
Products of lipase are incorporated into
micelles which are in a dynamic state- breaking down and reforming
As micelles breakdown they will release
fatty acids and monoglycerides that can then diffuse across the small intestine epithelium
Once fatty acids and monoglycerides are absorbed by the enterocyte, they are
Processed by the ER back into triglycerides.
- This maintains diffusion gradient
Within the ER triglycerides aggregate into
lipid droplets coated with amphipathic proteins
The lipid droplets are packaged into the ______ and secreted via exocytosis
Golgi
Extracellular fat droplets secreted are known as
“chylomicrons”
Chylomicron contains
triglycerides, phospholipids, fat soluble vitamins and cholesterol
The large chylomicrons enter into the
lymphatic system (rather than the capillaries in the intestine - Lacteals are leakier
Lymphatics eventually enter into the systemic circulation through the thoracic
duct
Lipoprotein lipase on endothelial cells of blood vessels release
triglycerides from chylomicrons as monoglycerides and free fatty acids which can be taken up by tissues
Iron (as Fe2+) is actively transported into the enterocyte and incorporated into the protein
ferritin, a protein iron complex that acts as a storage form of iron
Iron that is not stored is released on the blood side of the enterocyte and transported through the blood attached to the plasma protein
transferrin
Iron that remains in the enterocyte bound to ferritin is excreted from the body when the enterocytes
slough off of the villi tips
When iron stores are ample in the body expression of ferritin protein is
upregulated resulting in a reduced absorption of iron
When iron stores are depleted the production of
intestinal ferritin decreases resulting in increased absorption
Is there a mechanism for the excretion of iron, if so what’s it called?
NO, it accumulates in the tissue = toxicity = skin pigmentation and hear failure.
Causes for iron accumulating in tissue
- Genetic defects in absorption control pathways
- Adult males/post-menopausal women excessively supplementing
- Poisoning-often children accidentally consuming supplements
Iron-deficiency anemia
- Reduced number and/or size of red blood cells
- Symptoms: tiredness, light-headedness, headaches,
- Causes: not enough iron in diet, iron loss via blood loss, poor iron absorption, intestinal disease
Water and electrolyte absorption and
secretion in the intestine is critical for GI function
- Permits contact between food and digestive enzymes
- Diffusion of digested nutrients to absorption site
- Fluidity provides for transit without damage to the epithelium
Intestine handles large quantities of fluids each day
- Some from diet
- Majority from intestine and organs that drain into the intestine (e.g., liver and pancreas)
- Majority is “reclaimed” or reabsorbed by the small intestine
How much fluid is handled by the GI tract each day ?
- 8-9 L
- Majority is reabsorbed in the small intestine.
- 100 ml lost in feces.
Villi does what and crypts do what?
absorption at the villi, secretion from the crypts
Intestinal epithelium establishes an osmotic gradient and water
follows through tight junctions (paracellular transport).
Na+, Cl-, HCO3- are important electrolytes for
water transport
Absorption of water in the small intestine Predominantly depends on
Na+ gradients generated during secondary active nutrient uptake (e.g., glucose and amino acids)
Secretion of water in the small intestine Predominantly depends on
Cl- gradients generated by secondary active Na+/K+/2Cl- transporter (NKCC1)
Cholera Occurs after eating food or drinking water contaminated with
Vibrio cholerae bacteria, which produces a toxin that INCREASES cAMP in crypts of small intestine = activation of the Cl- channel and excessive secretion of Cl- into the gut lumen, water follows (diarrhea)
Treatment to cholera
- Consumption of clean water containing salt and glucose (fluid
replacement) - Intravenous fluids
Most common motion in small intestine during digestion is
“segmentation”
Segmentation is continuous
division and subdivision of intestinal contents
- Mechanical breakdown of food (increases surface area)
- Mixing of food with digestive enzymes
During segmentation, Frequency of contraction set by
basic electrical rhythm
- 12 contractions/min in duodenum
- 9 contractions/min in ileum
Contraction force during segmentation is determined by
neurohormonal input
- Slow net migration towards the large intestine
- Allows digestion and absorption of food
Segmentation occurs in the small intestine
during digestion
After digestion, segmentation stops and is replaced by
a pattern of peristaltic activity called the migrating myoelectric complex (MMC)
MMC Begins in the
lower portion of the stomach travels about 2 feet along the small intestine and then dies out, next wave starts further down SI, this is repeated
(takes -2hrs, then repeats)
Purpose of MMC
- Pushes any undigested material from the small to the large intestine
- Prevents bacteria from remaining in the small intestine
Regulation of MMC
- Intestinal hormone motilin (released by cells in the small intestine) thought to initiate MMC
- Feeding inhibits the release of motilin
- MMC ceases when next meal is consumed
