Lesson # 17

1. Lesson # 17 The Digestive System 3 Chapter 24 Objectives: 1- To describe the macroscopic and microscopic anatomy of liver. 2- To discuss how each major class of nutrient is chemically digested, and name the enzymes involved. 3- To describe how each nutrient is absorbed by the small intestine.

2. The Liver The liver is a reddish brown gland located immediately inferior to the diaphragm. It is the body’s largest gland weighing about 1.4 kg (3 pounds). Functions: Liver 1- It produces the bile to emulsify the fats. 2- It processes nutrients from the blood. It has many other metabolic and synthetic roles. Caudate lobe Inferior vena cava Right lobe Left lobe Falciform ligament Quadrate lobe Round ligament It separates the right and left lobes and attaches the liver to the diaphragm and the abdominal wall. Gallbladder (a) Anterior view (b) Posterior view

3. Hepatic veins Inferior vena cava Sinusoids of liver Hepatic portal vein Left gastric vein Splenic vein Superior mesenteric vein Inferior mesenteric vein

4. Histological Organization of the Liver To the hepatic vein To the inferior vena cava Hepatic Lobule Central vein Hepatic Triad: Branch of hepatic portal vein Branch of proper hepatic artery Bile ductule To the right and left hepatic ducts Blood from the intestine and stomach Hepatocytes Bile canaliculum Hepatic sinusoid

5. Hepatic sinusoids: • They are blood-filled channels that fill spaces between the plates of hepatocytes. • Hepatic sinusoids are lined by a fenestrated endothelium that separates hepatocytes from blood cells and allows plasma into the space between the hepatocytes and endothelium. • Hepatocytes have brush border of microvilli that project into this space. • Hepatic macrophages (Kupffer cells) are phagocytic cells in the sinusoids that remove bacteria and debris from the blood. Functions of the hepatocytes: After a meal, the hepatocytes absorb from the blood: glucose, amino acids, iron, vitamins, and other nutrients for metabolism or storage. They remove and degrades hormones, toxins, bile pigments, and drugs. They secrete into the blood: albumin, lipoproteins, clotting factors, angiotensinogen, and other products. Between meals, they breaks down stored glycogen and releases glucose into the blood.

6. Bile canaliculum Right hepatic ducts Bile ductule Left hepatic ducts Common hepatic duct Cystic duct Common bile duct Pancreatic duct Gallbladder Accessory pancreatic duct It stores and concentrates bile Pancreas Duodenum Minor duodenal papilla Hepatopancreatic sphincter Major duodenal papilla Jejunum Duodenal ampulla

7. Pancreaticduct Common bile duct Hepatopancreaticsphincter Duodenalampulla Duodenalpapilla Intestinal lumen Pancreas

8. Physiology of the Gallbladder The liver secretesbile continuously—roughly 1 literper day. As it remains in the gallbladder, bilebecomes moreconcentrated. Liver CCK: Cholecystokinin Duodenum The release of CCK by the duodenum triggers dilation of the hepatopancreatic sphincter andcontraction of the gallbladder. Thisejects bile into the duodenumthrough the duodenal ampulla. CCK In the lumen of thedigestive tract, bilesalts break the lipiddroplets apart byemulsification. Lipiddroplet

9. The Processing and Absorption of Nutrients 1- Carbohydrates Amylases Amylases Amylases Disaccharides Monosaccharides Starch is the most digestible carbohydrate 2- Proteins • They are absorbed immediately Oligosaccharides Proteases Peptidases Protein Aminoacids Polypeptides • They are absorbed immediately

10. 3- Lipids + Lipases Triglycerides Glycerol Fatty acids Lipases + Monoglycerides Fatty acids

11. Carbohydrate Digestion and Absorption • Salivary amylase hydrolyzes starch into oligosaccharides (up to 8 glucose residues long). It works best at pH of 6.7 – 7.5 of oral cavity. When reaching the duodenum, pancreatic amylase converts the remaining starch to oligosaccharides and disaccharides within 10 minutes. Oligosaccharides Lactose Sucrose Salivary amylase Pancreatic amylase • Amylase quickly denatured on contact with stomach acid and digested by pepsin. Maltose Starch Oligosaccharides and maltose contacts brush border enzymes (maltase, sucrase, and lactase). Brush border of microvilli • Monosaccharides are absorbed immediately. Contact Digestion

12. Carbohydrate Digestion in the Small Intestine Salivary amylase stops working in the stomach at pH less than 4.5 Only 50% of dietary starch digested before it reaches small intestine. When reaching the small intestine, pancreatic amylase converts starch to oligosaccharides and disaccharides (maltose, sucrose and lactose) within 10 minutes. Brush Border Enzymes: Oligosaccharides and disaccharides contacts the brush border of the mucosa and the enzymes (maltase, sucrase, and lactase) break down the disaccharides (maltose, sucrose, and lactose) in to monosaccharides.

13. Lipid Digestion and Absorption Enzymes that digest lipids (fats) are called lipases. • The lingual lipase secreted by the intrinsic salivary glands of the tongue is active in mouth, but more active in stomach along with gastric lipase. 10-15% of lipids digested before reaches duodenum. • Pancreatic lipase in the small intestine digest most of the fats. Digestion of lipids have five steps Hydrophobic quality of lipids makes their digestion and absorption more complicated that carbohydrates and proteins. 1- Emulsification 2- Fat hydrolysis 3- Lipid uptake by micelles 4- Chylomicron formation 5- Chylomicron exocytosis

14. 1- Emulsification Fat globule is broken up and coated by lecithin and bile acids. Emulsification droplets Pancreatic lipase 2- Fat Hydrolysis • Pancreatic lipase acts on triglycerides. It removes the first and third fatty acids from glycerol backbone and leaves the middle one. Components of the bile • The product of lipase action are two free fatty acids (FFAs) and a monoglyceride. Emulsification droplets are acted upon by pancreatic lipase, which hydrolyzes the first and third fatty acids from triglycerides usually leaving the middle fatty acid.

15. 3- Lipid uptake by micelles Micelles are made in the liver and they are very small droplets in the bile. • They consist of 20 to 40 bile acid molecules aggregated with their hydrophilic side groups facing outward and their hydrophobic steroid rings facing inward. Micelles in the bile pass to the small intestine and pick up several types of dietary and semidigested lipids. Micelles

16. 4- Chylomicron Formation Intestinal cells absorb lipids from micelles, resynthesize triglycerides, and package triglycerides, cholesterol, and phospholipids into protein-coated chylomicrons. Lipoprotein

17. 5- Chylomicron Exocytosis Golgi complex packages chylomicrons into secretoryvesicles. They are released from basal cell membrane by exocytosis and enter the lacteal (lymphatic capillary) of the villus. They enter the bloodstream when lymphatic fluid enters the subclavian vein via the thoracic duct.

18. Protein Digestion and Absorption The amino acids absorbed by the small intestine come from three sources: • - Dietary proteins. • - Digestive enzymes digested by each other. • - Sloughed epithelial cells digested by enzymes. Enzymes that digest proteins are called proteases or peptidases. Mouth Peptidases are absent from the saliva. No chemical digestion occurs. Polypeptides Pepsin ( ) hydrolyzes certain peptide bonds, breaking protein down into smaller polypeptides. Stomach

19. Digestion of proteins continues in the small intestine because pepsin is inactivated when it passes into the duodenum and mixes with the alkaline pancreatic juice (pH 7.0 – 8.0). • Pancreatic enzymes trypsin and chymotrypsin take over the process hydrolyzing polypeptides into even shorter oligopeptides. • Pancreatic carboxypeptidase removes amino acids from –COOH end of the chain. Actions of pancreatic enzymes Small intestine Trypsin ( ) and chymotrypsin ( ) hydrolyze other peptide bonds, breaking polypeptides down into smaller oligopeptides. Carboxypeptidase ( ) removes one amino acid at a time from the carboxyl (–COOH) end of an oligopeptide. • The epithelial surfaces of the small intestine contains dipeptidases that break dipeptides into individual aminoacids.

20. pH: 6.7 – 7.5 ORAL CAVITY Salivary Amylase Polysaccharides Oligosaccharides + Disaccharides + Trisaccharides Lingual Lipase Triglycerides Triglycerides + Monoglycerides + Fatty acids Proteins No digestion Salivary Amylase Oligosaccharides + Disaccharides + Trisaccharides Polysaccharides (1-2 hours) STOMACH Lingual Lipase Triglycerides Triglycerides + Monoglycerides + Fatty acids (a short variable time, 20% of TG are broken down) pH: 1.5 – 2.0 Polypeptides HCl Proteins Pepsinogen Pepsin pH: 7.0 – 8.0 SMALL INTESTINE Pancreatic alpha Amylase Polysaccharides Disaccharides + Trisaccharides Pancreatic Lipase Triglycerides Triglycerides + Monoglycerides Bile salts (emulsify lipid) Polypeptides Trypsin and Chymotrypsin Short peptides Carboxypeptidase Aminoacid

21. A Summary of the Chemical Digestion in the Digestive Tract

22. A Summary of the Chemical Digestion in the Intestinal Mucosa CARBOHYDRATES LIPIDS PROTEINS Disaccharides, Trisaccharides Monoglycerides, Fatty acids Short peptides, aminoacids Sucrase Lactase Brush border Maltase Dipeptidases Simple diffusion Monosaccharides Aminoacids Monoglycerides, Fatty acids Facilitated diffusion Facilitated diffusion Cell body Triglycerides Monosaccharides Aminoacids + Proteins Chylomicrons Facilitated diffusion Facilitated diffusion Exocytosis Interstitial fluid Aminoacids Monosaccharides Chylomicrons Capillary Capillary Lacteal