Small Intestine Flashcards
Chronic Intestinal Pseudo-Obstruction (CIPO)
- characterized by dilation of bowel on imaging
- major manifestation of small intestinal dysmotility
- bacterial overgrowth a complication
- sx: N/V, abd pain, distention, constipation, diarrhea
- causes: PD, autoimmune, chagas, diabetes, scleroderma, congenital in children
Duodenum Basic Electrical Rhythm (BER)
-12 cycles per minute
Key Hormones
- CCK
- secretin
- GIP
Types of Motility in Small Intestine
- peristaltic
- segmentation
- this alternates aspect of chime exposed to brush border
Gastroileal Reflex
-stomach activity stimulates movement of chyme through ileocecal sphincter
Gastrocolic Reflex
-food in stomach stimulates mass movement in colon
Ileocecal Valve
- normally closed to prevent reflux of bacteria from colon into ileum
- opened by distension of end of ileum (local reflex)
- closed by distension of proximal colon (local reflex)
Digestion
- some occurs in mouth and stomach
- most occurs in intestinal lumen or at surface or the absorptive enterocytes
Dietary Carbohydrates
- plant starch amylopectin is largest source of carbs in our diet
- amylase is major enzyme in saliva and pancreatic secretions
- only simple monomeric sugars can be absorbed
Isomaltase
-converts alpha-limit dextrins to glucose
Maltase
-converts maltose and maltotriose to glucose
Lactase
-converts lactose to glucose and galactose
Sucrase
-converts sucrose to gluvose and fructose
Trehalase
- converts trehalose to glucose
- shrimp
Lactose Intolerance
- missing brush border enzyme lactase
- causes gas and diarrhea due to colonic bacterial digestion of lactose
- areas where dairy is not part of the staple have higher prevalence
SGLT1 Transporter
- requires Na as a co transporter
- transports glucose and galactose across the apical membrane of the enterocyte
- can operate in the setting of secretory diarrhea (inc. cAMP/cholera) so is important for oral rehydration
GLUT5 Transporter
-fructose transport across the apical surface via GLUT5 is Na independent
GLUT2
- not Na dependent
- glucose and galactose use the same transporter as fructose on the basolateral surface
Regulation of Carbohydrate Absorption
- inc. carb consumption upregulates transporters and inc. the uptake of simple sugars-> obesity
- and vice versa
Protein Digestion
-pancreatic proteases like trypsin, chymotrypsin, carboxypeptidase & and elastase break down proteins to oligopeptides, di/tri-peptides and amino acids
Brush Border
-peptidases break down oligopeptides into amino acids, dipeptides, tripeptides
Protein Uptake Pathways
- Na dependent co-transporters that utilize the Na+/K+ ATPase gradient are major route for the different classes amino acids
- water follows
- Na independent transporters of amino acids
- specific carriers for small peptides
Pinocytosis
-infants
Dietary Fat
- fats provide 30-40% of caloric intake
- essential fatty acids: linoleic acid-> arachidonic acid, and alpha-linolenic acid
- triglycerides are the most abundant fat in our diet
- GI tract is water based so there are challenges to fat absorption
Bile Acids
- primary bile acids are produced in liver from cholesterol
- secondary bile acids are formed by bacteria in the intestines and colon
- bile acids are complexed with glycine or taurine to make bile salts
- bile is recycled during a meal by update in the distal ileum- enterohepatic circulation
Vitamin Absorption
- fate soluble vitamins (A, D, E, K) are absorbed along the length of the small intestines and are carried in micelles and form chylomicrons similar to dietary lipids
- water soluble vitamins either enter the enterocyte by simple diffusions of via spefic transporters
Secretion and Absorption of Fluids
- There is a net fluid secretion from cells in the intestinal crypts and a net fluid absorption from enterocytes on the villi
- Villi surface area > crypt surface area
- paracellular water permeability dec. from proximal to distal in the small intestines
- the colon has the lowest paracellular permeability to water bc it’s trying to solidify waste and it needs to link water movement to transcellular ion movement
Sodium Absorption
- absorbed along intestine with most absorbed in jejunum
- dependent on gradient established by Na+/K+ ATPase
- water absorption is critically linked to Na absorption
- mechanism is via Na+/glucose and galactose or Na+/amino acide cotransport, NaCl cotransport, Na+/H+ exchange or passive diffusion
Chloride Absorption
- passive in proximal intestines (due to loose TJs), offests Na+ charge in the intracellular space
- in the distal ileum and colon, with less leaky TJs, Cl- is exchanged for HCO3 that is offsetting the acids produced by bacteria
Potassium Absorption
- passive process
- paracellular movement in jejunum but transcellular in colon
- K+ normally high in cells due to Na+/K+ ATPase
- severe diarrhea can cause significant loss of K+
Calcium and Magnesium Absorption
- Ca++ and Mg++ compete for uptake by the cells
- Ca++ enters enterocyte passively down is electrochemical gradient in proximal intestines
- uptake of Ca++ in intracellular calcium stores maintains the gradient
- Ca++ ATPase pumps calcium out to the blood
- vitD stimluates uptake of Ca++ by inc. Ca++ binding proteins and Ca++ ATPase molecules
Iron Absorption
- transported across apical membrane as either heme or Fe++ (receptor mediated
- two possible fates: binds to apoferritin to form ferritin that stays in the cell and is lost when the cell dies or binds transferrin and enter the blood
Parietal Cells
- located in stomach
- secrete intrinsic factor and H+
- destruction of parietal cells -> chronic gastritis and pernicious anemia
Intrinsic Factor
- secreted by parietal cells in stomach
- action: vitamin B12 binding protein (required for B12 uptake in terminal ileum)
Goblet Cell
- column-shaped cell which secretes the main component of mucus
- located in small intestine
- ileum has largest number of goblet cells in small intestine
- secrete HCO3
Paneth Cells
- located in small intestine
- reside at the bottom of the intestinal crypts
- key effectors of innate mucosal defense
- produce large amounts of α-defensins and other antimicrobial peptides, such as lysozymes and secretory phospholipase A2
Cholecystokinin
- peptide hormone responsible for stimulating the digestion of fat and protein
- synthesized by I-cells in the mucosal epithelium of the small intestine and secreted by duodenum
- release stimulated by monitor peptide released from cells in duodenum
- causes release of digestive enzymes and bile from the pancreas and gallbladder
Bruner Gland
- characteristic of duodenum
- produce mucous rich alkaline secretion to protect duodenum from stomach acid
Meckel Diverticulum
-connects intestine to umbilicus
Enterokinasae
-secreted by enterocytes (in small intestine)
Enterocytes
-only in small intestine
-5 functions
1- activating zymogens from pancreas
2- reclaims bile salts
3- release of IgA
4- uptakes disacharrides and converts to monosacharrides
5- uptake glucose
Celiac Disease
- a gliadin peptide complexes with TTG-> autoantibody formation
- Class 2 HLA-DQ2 or HLA-DQ8
- associated with other autoimmune disease
- features: fatty diarrhea, gas, weight loss, anemia, dental enamel defects, arthritis, villous blunting, inc. intraepithelial lymphocytes
- dermatitis herpetiformis
- scalloping of mucosa
Campylobacter
- bacterial enterocolitis
- causes diarrhea
- most common stool isolate is US
- poultry, water, dairy
Shigella
- bacterial enterocolitis
- severe watery/bloody diarrhea
- contaminated water
- highest infectivity rate
Salmonella
- bacterial enterocolitis
- typhoid (bad diarrhea + risk of perforation) vs non-typhoid (mild, self-limiting)
Pseudomembranous Colitis
-most often caused by C. diff
Ischemic Colitis
- features: older pts, long distance runners, women on OCs, people with hernias or volvulus
- lack of blood flow to bowel -> acute transmural infarction
