Control of Digestive Processes

how does bile move between meals?

between meals, bile is sent to the gallbladder for storage

• the sphincter of Oddi is closed, directing liver-produced bile to the gallbladder for storage and exocrine pancreas is largely inactive

• during a meal, CCK and secretin are released to enhance secretions and cause their release into the duodenal lumen thorugh a relaxed sphincter of Oddi

how are carbohydrates digested?

digested and absorbed early in the small intestine

• Salivary and pancreatic carbohydrases degrade starch and

glycogen, which are broken down into glucose and fructose by enterocyte apical membrane carbohydrases

• Glucose is absorbed with Na+ in secondary active transport

• Fructose enters by intependent Na+ transporter (GLUT5)

• Both carried by GLUT2

how are proteins digested?

absorbed as single AA or small peptides

• broken down via pepsin/acidification

• then proteases

zymogens secreted after digestion to prevent self-digestion

how do lluminal free AA enter cells?

Na+ or H+ cotransporters, facilitated diffusion

chylomicrons

protein-coated fat droplets that are absorbed by villi lacteals

role of bile salts

• emulsify pancreatic-lipase digested lipids

• lipid insolubility bakes their digestion complex

Protonated long-chain fatty acids are transported across the apical membranes of enterocytes, then bound by fatty acid binding proteins (FABPs) to avoid membrane

accumulation, and delivered to the SER

how does water absorption work at the small intestine?

Most water abssorption is coupled with nutrient absorption

how is water absorbed in the large intestine?

water absorption is coupled with ion absorption

how is water secreted at the small and large intestines?

Water secretion at the small and large intestines occurs via electrogenic secondary active transport of Cl

function of the hepatic portal vein

• All nutrient-rich blood exiting stomach and intestine capillaries, as well as the spleen and pancreas, is channeled into the hepatic portal vein

• Any circulatory arrangement like this, in which venous blood leaving one organ (or structure) is sent directly to another, is called a portal system

why does the gut epithelium “sample” lumen contents?

to continually stimulate immune responses

• innate mucose-associated lymphoid tissues (MALT) cells include neutrophils,

dendritic cells, and macrophages that present pattern recognition receptors

luminal particulates bind B and T lymphocytes (Peyer’s patches) for an immune response

what do B-cells provide as an immune defense?

B cells exposed to antigen turn into plasma cells and secrete IgA

• Within the gut, sIgA binds microbial antigens, viruses, and toxins to prevent their uptake by and interaction with the epithelium