Gut 3 Flashcards
What is the site where most digestion and absorption takes place?
What are the segements of this organ?
The small intestine
–Segments of this organ: duodenum (joined to stomach by the pyloric sphincter), jejunum, and ileum
Segmentation and contractions mix contenst of small intestine and slowly propel the chyme
What is it initiated by?
What is it controlled by?
What are the functions?
–Initiated by basal electrical rhythm (BER) from pacemaker cells
–Controlled by gastrin, level of distension and extrinsic nerve activity
–Functions:
(1) mixing the chyme with the digestive juices secreted into the small-intestine lumen (small intestine does not produce any digestive enzymes)
(2) exposing all the chyme to the absorptive surfaces of the small-intestine mucosa.
Describe the process of mixing and propulsion of the small intestine contents
From the diagram:
- First line- two food types separated by ring like contractings
- Second line- as ring like contractions alternate to mix the contents of each segment if the small intestine
- Third line- through repeated contractions you make a homogenous mix of chyme
Also propelling the contents forward whilst ensuring all surfaces of the chyme is exposed to the small intestine
The migrating motility complex sweeps the intestine clean between meals
How many phases are there?
–Phases I, II, III and IV (different levels of electrical activity associated with each one)
•MMC cycles in a repetitive pattern about every 1.5 hours as long as a person is fasting
Squence of contrile events that sweep (migrate) the small intestine contents forward after a meal (doesnt happens when stomch is still emptying contents into the small intestine)
Forces contents from small intesting to large intestine which are separated by the ileocecal valve which controls the egress (exit) of chyme from the small intestine
What prevents contamination of the small intestine by colonic bacteria?
•The ileocecal juncture prevents contamination of the small intestine by colonic bacteria
–Ileocecal valve and sphincter
Does small intestine secretions contain digestive enzymes?
•Small-intestine secretions do not contain any digestive enzymes
–Small intestine synthesizes digestive enzymes that act intracellularly within the brush-border membrane of the epithelial cells that line the lumen
What do small intestine enzymes do?
•The small-intestine enzymes complete digestion within the brush-border membrane
–Microvilli: hairlike projections on epithelial cells
villi-folds in the mucosal layer that increase the surface area
Describe digestive processes for the three major categories of nutrients
How is the small intestine remarkably well adapted for its primary role in absorption
–Adaptations that increase the small intestine’s surface area include:
- Circular folds that increase s.a. x 3
- Microscopic, finger-like projections known as villi, which increase the s.a. another 10 x
- Even smaller hairlike projections (microvilli or brush border), on the luminal surface of epithelial cells on villi, increasing the s.a. 20x. Each epithelial cell has as many as 3000 to 6000 of these microvilli
- The mucosal lining experiences rapid turnover
–Crypts of Lieberkühn
Compare the structure of a normal small intestine to one with gluten enteropathy
The image at the bottom has celiac disease, there is an allergic reaction to gluten.
This leads to an autoimmune mediated destruction of the brush boarder which means that the small intestine is relatively less able to absorb nutrients across
Intestinal absorption: Define paracellular
between cells via tight junctions
Intestinal absorption: Define transcellular
Through cells
Chyme may be hyper- or hypo-tonic, define both of these terms
Duodenum adjusts tonicity:
If hypertonic, water enters duodenal lumen from blood down osmotic gradient
And Na+ and Cl- diffuse out of duodenum down concentration gradient
What drives passive H2O absorption?
•Energy-dependent Na+ absorption drives passive H2O absorption
–Na+ may be absorbed passively (electrochemical gradient, between cells) and actively (Na+–Cl- symporter, Na+–H+ antiporter, or Na+–glucose transporter, through cells)
What are digested carbohydrates and proteins absorbed by?
•Digested carbohydrates and proteins are both absorbed by secondary active transport and enter the blood
–Carbohydrate absorption
–Protein absorption
Intestinal absorption: Enterocytes
Describe the basolateral membrane, including what it is permeable to
Describe the luminal membrane, including what it can transport
• Basolateral membrane
- has Na+ K+ ATPase pump
- is impermeable to passive Na+ movement
- is permeable to passive K+ movement
• Luminal (brush border) membrane
- can transport Na+ into enterocyte down its concentration gradient
- does not transport K+
Intestinal absorption:
Describe the ion concentrations across the basolateral membrane
- The intracellular Na+ conc is 15-20 mEq/L lower than outside
- The Na+ K+ ATPase pump is electrogenic – pumps Na+ out faster than K+ in (ratio 2:1).
- Inside cell electronegative (-40 mV) vs outside
- Cl- enters across luminal border carried by Na+, then out via basolateral membrane down conc gradient
- Net cycling of Na+ and Cl-, depending on conc gradient.
- Movement of NaCl affects osmotic water movement
Explain carbohydrate digesiton
How do carbohydrates enter the blood?
- The dietary polysaccharides starch and glycogen are converted into the disaccharide maltose through the action of salivary and pancreatic amylase.
- Maltose and the dietary disaccharides lactose and sucrose are converted to their respective monosaccharides by the disaccharidases (maltase, lactase, and sucrase-isomaltase) located in the brush borders of the small-intestine epithelial cells
- The monosaccharides glucose and galactose are absorbed into the epithelial cells by Na+- and energy-dependent secondary active transport (via the symporter SGLT) located at the luminal membrane.
- 4The monosaccharide fructose enters the cell by passive facilitated diffusion via GLUT-5.
- 4Glucose, galactose, and fructose exit the cell at the basal membrane by passive facilitated diffusion via GLUT-2.
- These monosaccharides enter the blood by simple diffusion.
Explain protein digestion
How do amino acids enter the blood?
- Dietary and endogenous proteins are hydrolyzed into their constituent amino acids and a few small peptide fragments by gastric pepsin and the pancreatic proteolytic enzymes.
- Many small peptides are converted into their respective amino acids by the aminopeptidases located in the brush borders of the small-intestine epithelial cells.
- Amino acids are absorbed into the epithelial cells by means of Na+- and energy-dependent secondary active transport via a symporter. Various amino acids are transported by carriers specific for them.
- Some small peptides are absorbed by a different type of symporter driven by H+, Na+-, and energy-dependent tertiary active transport.
- 5Most absorbed small peptides are broken down into their amino acids by intracellular peptidases.
- Amino acids exit the cell at the basal membrane via various passive carriers.
- Amino acids enter the blood by simple diffusion. (A small percentage of di- and tripeptides also enter the blood intact.)
Small Intestine: Digested fat is absorbed passively and enters the lymph
- Fat droplets suspended in aqueous solution.
- These dropslets get broken down into monoglycerideses which can diffuse into the cell without the need of transprter molecule
- –Fat absorption
Is vitamin absorption largely active?
No, it is largely passive
Is absorption of calcium and ion regulated?
Yes
Explain lipid (triglyceride) digestion
Do chylomicrons directly enter the bloodstream?
- Dietary fat in the form of large fat globules com- posed of triglycerides is emulsified by the detergent action of bile salts into a suspension of smaller fat droplets. This lipid emulsion prevents the fat droplets from coalescing and thereby increases the surface area available for attack by pancreatic lipase.
- Lipase hydrolyzes the triglycerides into monoglycerides and free fatty acids
- These water-insoluble products are carried to the luminal surface of the small-intestine epithelial cells within water-soluble micelles, which are formed by bile salts and other bile constituents.
- When a micelle approaches the absorptive epithelial surface, the monoglycerides and fatty acids leave the micelle and passively diffuse through the lipid bilayer of the luminal membranes.
- The monoglycerides and free fatty acids are resyn- thesized into triglycerides inside the epithelial cells.
- These triglycerides aggregate and are coated with a layer of lipoprotein from the endoplasmic reticulum to form water-soluble chylomicrons.
- Chylomicrons are extruded through the basal membrane of the cells by exocytosis.
- Chylomicrons are unable to cross the basement membrane of capillaries, so instead they enter the lymphatic vessels, the central lacteals then enter the bloodstream through the thoracic lymph duct.
Explain iron absorption
How does iron enter the blood?
- Only a portion of ingested iron is in a form that can be absorbed, either heme iron or ferrous iron (Fe2+).
- Iron is absorbed across the luminal membrane of small-intestine epithelial cells by different energy-dependent carriers for heme and Fe2+.
- Dietary iron that is absorbed into the small-intestine epithelial cells and is immediately needed for red blood cell production is transferred into the blood by the membrane iron transporter ferroportin.
- In the blood, the absorbed iron is carried to the bone marrow bound to transferrin, a plasma protein carrier.
- Absorbed dietary iron that is not immediately needed is stored in the epithelial cells as ferritin, which cannot be transferred into the blood.
- This unused iron is lost in the feces as the ferritin-containing epithelial cells are sloughed.
- Dietary iron that was not absorbed is also lost in the feces
