2Secretions of the Small Intestine (2000 ml/day) Crypts of Lieberkuhn: Small and Large IntestineContain Primarily Epithelial Cells, Mucus Goblet Cells, and Endocrine Cells.Water Secretion - VIP Activates Water Secretion by Elevating cAMP and Cl- conductance.Cell Growth and Differentiation: Cell Turnover at the Tip of a Villus is High (3-6 day Cycle).
7ABSORPTIONAbsorption of Nutrients: CarbohydratesGlucose and Galactose - Sodium Co-transporter Across Luminal MembranesFructose - Facilitated Transport (Na+ Independent)Xylose: Pentose; Uses the Fructose Carrier not Na+ Dependent.
11Nutrient Digestion and Absorption Only Monomers and small digestion products can be absorbed.For amino acids and carbohydrates, most absorption is Na+dependentLipids are digested before absorption, then re-synthesized in epithelia cells before packaging and secretion into lymphGlycerol and Monoglycerides diffuse into the blood.Complex fats are packaging into chylomicrons and secreted to the lymph.Lymphatic flow increased 10X during absorption of a high fat meal.
13Relations Between Salt /Water Secretion and Absorption Net Fluid and NaCl Villus Epithelia AbsorptionNet Fluid and NaCl Crypt Epithelia Secretion
14cAMP Activated Cl- Channel CFTR Crypt SecretionH2O2 Cl-Na+K+PDtecAMPCl-ATPADP + Pi3 Na+2 K+Key Player:cAMP Activated Cl- ChannelCFTRVIP – neurotransmitter Histamine also can activate formation of cAMP
15Concentration in Chyme: Na+ Conc. K+ Conc. Volume Absorption of Na+Concentration in Chyme: Na+ Conc K+ Conc. Volume5 mEq/L23 mEq/L80 mEq/L5 Liters1 Liter200 mLDuodenumIleumColon (Prox)140 mEq/L125 mEq/L40 mEq/L
16Absorption of Na+ Na+ is Absorbed Along the Entire Intestine Na+ Diffuses Down its Electrochemical Gradient into Epithelial CellsLuminal Membrane: Co-transport with Nutrients, Diffusion Through Na+ Channels and Na+/H+ ExchangeTransported Across the Basolateral Membrane by the Na-K ATPase.Cl- Moves to the Blood to Maintain Electroneutrality.
18Vitamins and Ileal Transport Fat Soluble – absorbed all along the intestine via normal fat absorptive pathways.(D, K, A)Water Soluble: Specific TransportMost all along the intestine.Several Ileal Specific- Vitamin: B12also Ileal Specific: Bile Salts.
19Where salt goes, so too does water. Absorption of Water90-95% of Water (re)absorption in the Small Intestine1-2 L/day is Absorbed in the Colon.A Normal Colon Can Reabsorb a Maximum of About 4.5 liters per day.Mechanism of H2O Absorption-Where salt goes, so too does water.
20Pathophysiology of Diarrhea The Causes of Diarrhea Delineate into 3 CategoriesMotility Based: Limiting Time for AbsorptionBacterial InfiltrationMalabsorption; Maldigestion: OsmoticLactase DeficiencyIleal Resection: Biles Salts also stimulate secretion.Celiac Disease (Sprue; Gluten Sensitive) – Xylose TestSecretory: Initiated by Effectors which Alter cAMP Levels in Enterocytes; Bile Salts