Lecture # 13: The Digestive System 2 The Digestive System II (Chapter 25)

Objectives

2- To describe 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.

1- To describe the gross and microscopic anatomy of the liver and the pancreas.

Anatomy of Liver and Pancreas.Chemical Digestion and Absorption.

Gross Anatomy of Liver

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).It has a variety of functions, but only the secretion of bile contributes to digestion.

Right lobe Left lobe

Falciformligament

(a) Anterior view (b) Posterior view

Caudate lobe

Quadrate lobeRound

ligament Gallbladder

Inferior vena cava

Microscopic Anatomy of Liver

Bile ductule

Branch of proper hepatic artery

Hepatic Triad:

Central vein

Blood from the intestine and

stomach

To the hepatic vein To the inferior

vena cava

Branch of hepatic portal vein

Hepaticsinusoid

Hepatocytes

Hepatic Lobule

Bile canaliculum

To the right and left hepatic ducts

They are blood-filled channels that fill spaces between the plates of hepatocytes.

Hepatic sinusoids:

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.

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 productsBetween meals, they breaks down stored glycogen and releases glucose into the blood.

Functions of the Hepatocytes:

Bile canaliculum

Bile ductule

Right hepatic ducts

Left hepatic ducts

Common hepatic duct

Cystic duct

Bile duct Pancreatic ductAccessorypancreatic duct

Minor duodenal papilla

Hepatopancreatic sphincter

Major duodenal papilla

Hepatopancreatic ampulla

Pancreas

Gallbladder

Duodenum

Jejunum

It stores and concentrates bile

Pancreas It is a spongy retroperitoneal gland posterior to the greater curvature of the stomach.

The head of the pancreas is encircled by the duodenum.

Accessorypancreatic duct

Head

Body

Tail

It is both an endocrine and exocrine gland.

The endocrine portion consists of the pancreatic islets that secrete insulin and glucagon.

The exocrine portion is about 99% of pancreas and secretes 1200 to 1500 mL of pancreatic juice per day.

Pancreatic duct

Endocrine pancreas (produces hormones)

Exocrine pancreas (produces enzymes)

Pancreatic aciniThey secrete large quantities of an alkaline, enzyme rich fluid.1- Beta cells:

2- Alpha cells:

3- Delta cells:

4- F cells:

Insulin

Glucagon

Somatostatin

Pancreatic polypeptide

Islets of Langerhans

H2O9

Enzyme

H2O9

Enzyme

Starch (Polymer of Glucose) Glucose

Aminoacids

Protein (Polymer of Aminoacids)

All digestion reactions consists of hydrolysis reactions:

Digestion

Carbohydrate DigestionStarch is the most digestible carbohydrate.

Cellulose and chitin are indi-gestible.

Polysaccharides

Monosaccharides

Disaccharides

Starch

Salivary amylase

Oligosaccharides

Pancreatic amylase

Brush borderof microvilli

Amylase quickly denatured on contact with stomach acid and digested by pepsin.

Dextrinase, glucoamylase, maltase, sucrase, and

lactase

Salivary amylase hydrolyzes starch into oligosaccharides (up to 8 glucose residues long). It works best at pH of 6.8 – 7.0 of oral cavity.

1 When reaching the small intestine, pancreatic amylase converts the remaining starch to oligosaccharides and maltose within 10 minutes.

2

Oligosaccharides and maltose contacts brush border enzymes (dextrinase, glucoamylase, maltase, sucrase, and lactase).

3

Monosaccharides are absorbed immediately.

4

Carbohydrate Digestion in the Small Intestine In the oral cavity, 50% of dietary starch is digested.Salivary amylase stops working in stomach at pH less than 4.5

1

When reaching the small intestine, pancreatic amylase converts starch to oligo-saccharides and maltose within 10 minutes.

2 2

Oligosaccharides and maltose contacts brush border enzymes (dextrinase, glucoamylase, maltase, sucrase, and lactase) act upon oligosaccharides, maltose, sucrose, lactose, and fructose to glucose.

3

3 4

Monosaccharides are absorbed immediately. 4

Mouth

Protein DigestionThe amino acids absorbed by the small intestine come from three sources:

3- Sloughed epithelial cells digested by enzymes

1- Dietary proteins2- Digestive enzymes digested by each other

Enzymes that digest proteins are called proteases or peptidases

Peptidases are absent from the saliva.

No chemical digestion of proteins occurs in the oral cavity.

Stomach

Polypeptides

Pepsin ( ) hydrolyzes certain peptide bonds, breaking protein down into smaller polypeptides.

Small intestine Actions of pancreatic enzymesTrypsin ( ) 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.

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 8).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.

Brush border enzymes (contact digestion) also remove one aminoacid at a time.

Dipeptidases split dipeptides in the middle and release two free amino acids.

Aminopeptidase removes them from the –NH2 end.

Actions of brush border enzymes

Carboxypeptidase removes amino acids from –COOH end of the chain.

Carboxypeptidase ( ) of the brush border continues to remove amino acids from the carboxyl (–COOH) end.

Aminopeptidase ( ) of the brush border removes one amino acid at a time from the amino (–NH2) end.

Dipeptidase ( ) splits dipeptides ( )into separate amino acids ( ).

Hydrophobic quality of lipids makes their digestion and absorption more complicated that carbohydrates and proteins.

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.

1- EmulsificationComponents of the bile

Emulsificationdroplets

Fat globule is broken up and coated by lecithin and bile acids.

1- Emulsification 2- Fat Hydrolysis 3- Lipid uptake by micelles4- Chylomicron Formation 5- Chylomicron Exocytosis

Lipid Digestion and Absorption

2- Fat HydrolysisPancreatic lipase acts on triglycerides. It removes the first and third fatty acids from glycerol backbone and leaves the middle one.The product of lipase action are two free fatty acids (FFAs) and a mono-glyceride.

Pancreatic lipase Emulsification droplets are acted upon by pancreatic lipase, which hydrolyzes the first and third fatty acids from triglycerides usually leaving the middle fatty acid.

3- Lipid uptake by micelles

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 are made in the liver and they are very small droplets in the bile.

Micelles in the bile pass to the small intestine and pick up several types of dietary and semidigested lipids.Micelles

4- Chylomicron Formation

Intestinal cells absorb lipids from micelles, resynthesize triglycerides, and package triglycerides, cholesterol, and phospholipids into protein-coated chylomicrons.

Lipoprotein

5- Chylomicron Exocytosis Golgi complex packages chylomicrons into secretory vesicles. 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.