Lecture 29 Flashcards
Where is the main area of absorption?
Duodenum
Where is the main area of watery absorption?
Colon
Anatomy of Small Intestine
Water, Sodium Chloride and Nutrient absorption
Secretors of bicarbonate
-done by increase SA
Surface of small intestine is amplified (folded) at 3 levels
Folds of Kerchring
Microvilli and Crypts of Lieberkuhn
Submicroscopic microvilli (facing lumen)
Anatomy of Large Intestine
Reabsorption of Water
Active secretion of Potassium and Bicarbonate
Surface of Large intestine is amplifies (folded) at 3 levels
-Semilunar folds
-Crypts, but no villi
-Microvilli
What is the daily volume of secretions into the GI tract?
Food 2.0 L/day Saliva 1.5 L/day Gastric Secretion 2.0L/day Pancreatic secretion 1.5L/day =8.5 L of secretions into GI tract daily
How much water is Reabsorbed and Secreted by the Small Intestine?
Reabsorbed= 6.5 L (of the daily 8.5L secreted-presentations of secretions to the small intestine is 8.5L/day
-main, along with nutrient, NaCl
Secreted= 1L daily
How much water is Reabsorbed by the Large Intestine?
Reabsorbed= 1.9L/day
-presentation of 2.0L of fluid/day
-primary secretion of K and Bicarbonates
(99% reabsorbed, and 0.1L lost as feces)
Increased Fluid loss in Feces
Change in (increase) secretion or (decreased) absorption
- will increase in fluid loss in feces
- diarrohea
Features of Small Intestine
6m Length ~200 m2 Area of apical Plasma Membrane Present: Folds, Villi, Crypts or glands, Microvilli -Yes Nutrient absorption -Yes Active Na+ absorption Absent Active K+ secretion
Features of Large Intestine
2.4m Length
~25 m2 area of apical plasma membrane
Present: Folds, Crypts or glands, Microvilli
No: Villi or Nutrient Absorption
-Yes Active Na+ absorption
-Yes Active K+ secretion (main functional difference, alongside bicarbonate and water reabsorption)
Comparing Structural and Functional Differences between the small and large intestine
Absorption of non-electrolyte nutrients occurs mainly in small intestine. where as both the small and large intestine absorb water and electrolytes (Na+, Cl- etc)
The small intestine absorbs net amounts of water, Na+, Cl- and K+ secretions and secretes HCO3-
Whereas the Large intestine absorbs net amounts of Water, Na+, Cl- and secretes both K+ and HCO3-
Intestinal Epithelial cells
Intestinal epithelial cells are Polar
Basolateral and Apical surface
Basal surface:
1. NaK ATPases- have alpha and beta units, remove 3x Na+ out of cell in exchange for 2x K+ = always have a deficiency of Na+ inside of the cell. (maintains low intracellular Na+) Channel has to work against Na+ gradient therefore active. always need deficiency of Na+ in the cell, therefore diffusive conc gradient for Na lumen –> cell through Apical 2Na+ glucose channel (co-transporter/symporter) (on submicroscopic villi)
-driven be high extracellular (Na+)
Therefore: -If you block, then no absorption of Na+ and glucose into the cell
Lastly: Glucose uniporter GluT2 facilitates downhill efflux (into blood stream)
Transepithelial movement of water and solutes
Either absorptive or secretory
Transcellular or paracellular
Transcellular= must move across 2 membranes (apical and basal) in series. Solutes involved, across at-least one membrane is active
Paracellular= movement passive via tight junctions
Absorption of water
Entirely of osmosis
Coupled by solute movement
Occurs via transcellular or paracellular routes
Paracellular predominates mode of absorption
Primarily in the jejunum
“solvent drag” responsible for considerable Na+ and urea absorption in jejunum
Na+ absorption
Occurs in villus epithelial cells of the small intestine and surface epithelial cells of the large intestine
All transcellular Na absorption mediated by Na-K pump (Na-K ATPase) on basolateral membrane
Maintains a low intracellular Na concentration
Provides force for Na movement from diffusion from lumen across apical membrane
Apical transport mediated by Na+ coupled transporters (e.g. Na/glucose transporters) or Na+ channels …. dependant on fasting/postprandial state and GI region (amount of glucose in GI tract)
-postprandial: Na-glucose co-transporters
-fasting: Na+ channels
4x types of Na+ reabsorption
- Na glucose co-transporters. Primarily on Jejunum (high) and (small a) Ileum, where most glucose reabsorption occurs.
-occurs due to activity of Na+K+ATPase maintaining low sodium in cell, concentration gradient - Na+H+ exchangers. Primarily in Duodenum and Jejunum (high a). Apical surface. Amiluroide drug blocks H+ ions and Na+H+ channel, NHe3
- Parallel Na+H+ x Cl-HCO3- exchangers. Ileum and Proximal Colon
ensure electrical charge balance, Cl- enters cell. HCO3- and H+ combine to form H2O and CO2 - Epithelial Na+ channel. Distal Colon(High a). Apical surface specific sodium channels which reabsorb Na+ in large colon