Duodenal Functions in Intestinal Phase

The duodenum receives digestive secretions from the liver, pancreas, and intestine, including enzymes, bile, bicarbonate, mucus, and isotonic NaCl solution.

The pancreas produces digestive enzymes (zymogens) and a bicarbonate solution.

Enzymes anchored to the small intestine's lining, crucial for the final stages of digestion.

Non-enzymatic solution from the liver/gallbladder assisting in fat digestion.

Neutralizes stomach acid in the small intestine, crucial for enzyme activity.

Pancreatic enzymes initially secreted as inactive forms, activated later in the intestines.

Contraction and relaxation cycles in slow waves occurring in the small intestine, generated by Interstitial Cells of Cajal (ICCs).

Hormones like insulin and glucagon secreted via clusters of cells (Islets).

Enteropeptidase, previously called enterokinase, activates trypsinogen to trypsin.

Trypsin activates other pancreatic enzymes.

Distension, food presence, neural signals, and CCK trigger pancreatic enzyme release.

Chloride-bicarbonate exchanger, and Na+-H+ exchangers are used.

Chloride channel that facilitates bicarbonate secretion.

Emulsifies fats, making them easier to digest.

Bile salts are steroid bile acids conjugated with amino acids.

Triglycerides combine with cholesterol and proteins in enterocytes to form chylomicrons.

Bile salts are reabsorbed and reused in digestion.

Bile pigments (like bilirubin) are excreted in feces.

Bile salts emulsify fats, lipases digest them, colipase aids lipase access.

Fatty acids, bile salts, monoglycerides, phospholipids, and cholesterol come together to form micelles.

Fatty acids and monoglycerides are absorbed by simple diffusion across enterocyte membranes.

Digestion of proteins into smaller peptides eventually stops with pepsin in the small intestine.

Emulsification by bile, enzymatic action of lipases, and assistance from colipase.

Large fat particles (chylomicrons) enter the lymphatic system (lacteals) instead of directly into the bloodstream; shorter fatty acids bypass this process.

Amylase breaks down complex sugars, disaccharidases break down disaccharides into monosaccharides (glucose, galactose, fructose).

Absorbed via a sodium-glucose symporter (SGLT) and a glucose transporter (GLUT2).

Absorbed via facilitated diffusion (GLUT5) and a glucose transporter (GLUT2).

Endopeptidases (proteases) break down polypeptide chains into smaller fragments, exopeptidases break off amino acids from the ends.

Begin as inactive proenzymes, then become active in the GI tract.

Most amino acids are absorbed through Na+-dependent cotransport proteins; some use H+-dependent processes.

Di- and tripeptides are absorbed, digested into amino acids, or transported intact across the membrane.

Enzymes digest DNA and RNA into nucleotides, then bases and sugars.

Bicarbonate from pancreas and duodenum neutralizes stomach acid, creating optimal pH for enzyme function.

Pancreatic amylase continues breaking down carbohydrates into simpler sugars.

Humans have enzymes to digest starch and glycogen, but not cellulose, resulting in dietary fiber.

Absorptive epithelial cells in the small intestine that do not preferentially use glucose as an energy source; obtain this from glutamine.

Processed as dietary fiber in the intestines, stimulating peristalsis.

The duodenum is the first part of the small intestine where chyme from the stomach mixes with digestive secretions from the liver, pancreas, and the intestinal lining itself. This is where the final stages of digestion begin.

Slow waves are rhythmic contractions and relaxations that occur in the smooth muscle of the small intestine. These waves are generated by specialized cells called Interstitial Cells of Cajal (ICCs) and help propel food along the gut.

The pancreas releases digestive enzymes, which break down proteins, carbohydrates, and fats, and also secretes bicarbonate, a solution that neutralizes the acidic chyme coming from the stomach.

Zymogens are inactive forms of digestive enzymes. They are released by the pancreas and become activated within the small intestine, where they can safely break down food molecules.

The pancreas secretes bicarbonate in response to secretin. Bicarbonate solution is crucial for neutralizing the acidic chyme entering from the stomach, creating an ideal pH for digestive enzymes to work.

Bile, produced by the liver and stored in the gallbladder, helps emulsify fats. It breaks down large fat globules into smaller ones, making them easier for lipases, enzymes that break down fats, to digest.

These are enzymes anchored to the luminal cell membranes of the small intestine. They are crucial for the final stages of digestion, breaking down carbohydrates and proteins into smaller units that can be absorbed by the body.

The exocrine portion of the pancreas secretes digestive enzymes, including proteases (protein digestion), amylases (carbohydrate digestion), and lipases (fat digestion), along with a watery solution of NaHCO3 (sodium bicarbonate).

The final absorbable products of carbohydrate digestion are glucose, galactose, and fructose. These simple sugars are ready to be absorbed into the bloodstream.

Dietary fiber, primarily cellulose, is undigested plant matter that adds bulk to stool and promotes peristalsis, aiding in bowel movements.

Endopeptidases (proteases) break down long peptide chains into smaller fragments, while exopeptidases remove individual amino acids from the ends.

The digestion of proteins primarily takes place in the small intestine, where enzymes from the pancreas and the intestinal brush border break them down into smaller units.

Nucleic acids, like DNA and RNA, are broken down by pancreatic and intestinal enzymes into nucleotides, then into nitrogenous bases and monosaccharides.

Bicarbonate secreted by the pancreas and duodenal cells neutralizes stomach acid, creating an optimal pH for digestive enzymes to function.

Glucose absorption in the small intestine occurs via a sodium-glucose symporter (SGLT) and a glucose transporter (GLUT2).

Fructose doesn't rely on sodium for absorption. It uses facilitated diffusion through GLUT5 and GLUT2 transporters.

Protein digestion starts in the stomach with pepsin, an enzyme that breaks down proteins into smaller polypeptides. The process continues in the small intestine.

Dipeptides and tripeptides can be digested into individual amino acids by cytoplasmic peptidases or be transported intact across the basolateral membrane.

Most free amino acids are carried by Na+-dependent cotransport proteins. Some amino acids use H+-dependent transport.

Enteropeptidase (formerly called enterokinase) activates trypsinogen, an inactive precursor, into active trypsin in the small intestine.

Enterocytes, the absorptive cells in the small intestine, prioritize using glutamine as their energy source, allowing absorbed glucose to pass into the bloodstream unchanged.

Brush border enteropeptidase (previously called enterokinase) converts inactive trypsinogen to active trypsin.

Trypsin converts other inactive pancreatic zymogens (precursor enzymes) to their active forms, starting a cascade of activation.

Distension of the small intestine, food presence, neural signals, and the hormone CCK (cholecystokinin) stimulate pancreatic enzyme release.

Pancreatic enzymes are released in a watery fluid containing bicarbonate, which neutralizes stomach acid and creates optimal pH for enzyme activity.

Bicarbonate from the pancreas and duodenal cells neutralizes stomach acid, creating the ideal environment for digestive enzymes.

Bicarbonate production relies on carbonic anhydrase. It is secreted via a chloride-bicarbonate exchanger, with hydrogen ions removed through Na+-H+ exchangers. Chloride enters cells through NKCC transporters and exits through CFTR channels.

CFTR (cystic fibrosis transmembrane conductance regulator) is a chloride channel crucial for bicarbonate secretion. Defects in CFTR, like in cystic fibrosis, disrupt pancreatic secretion.

Bile is a nonenzymatic solution secreted by hepatocytes (liver cells). It contains bile salts (for fat digestion), bile pigments (waste products of hemoglobin degradation), and cholesterol (excreted in feces).

Bile salts, which act as detergents to make fats soluble during digestion, are made from steroid bile acids combined (conjugated) with amino acids.

Bile secreted by hepatocytes travels in hepatic ducts to the gallbladder, where it is stored and concentrated.

During a meal containing fats, gallbladder contraction sends bile into the duodenum through the common bile duct.

Bile pigments, such as bilirubin, cannot be reabsorbed and are excreted in the feces.

Most lipids aren't water-soluble, forming large fat droplets in the chyme leaving the stomach. This reduces surface area for enzymatic digestion.

Bile salts, secreted by the liver, help break down large fat droplets into smaller, more stable particles, increasing surface area for enzymatic digestion.

Pancreatic lipases are responsible for the enzymatic breakdown of triglycerides, removing two fatty acids to form a monoglyceride and two free fatty acids.