DIGESTIVE SYSTEM

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DIGESTIVE SYSTEM

• Anatomy and Physiology of the Digestive Tract

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DIGESTIVE ANATOMY

• Mouth and tongue
• Salivary Glands
• Pharynx
• Esophagus
• Stomach
• Liver
• Pancreas
• Small intestine
• Large intestine

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LAYERS OF THE GI TRACT

• (1) SEROSA-
• OUTER TOUGH CONNECTIVE TISSUE MEMBRANE FOR
  PROTECTION.
• (2) MUSCULARIS EXTERNA-
• LONGITUDENAL AND CIRCULAR MUSCLE LAYERS FOR
  CONTRACTION
• (3) SUBMUCOSA-
• LOOSE CONNECTIVE TISSUE, BLOOD VESSELS AND
  GLANDS.

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LAYERS OF THE GI TRACT

• (4) MUCOSA- MADE OF THREE LAYERS
• (A) MUSCULARIS MUCOSA
• (B) LAMINA PROPRIA
• (C) EPITHELIUM LINNING
• FUNCTION FOR DIGESTION AND ABSORPTION OF
  NUTRIENTS.

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LAYERS OF THE GI TRACT
Muscularis
Mucosa
Submucosa
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FUNCTIONS OF THE GI TRACT

• INGESTION
• MECHANICAL DIGESTION
• CHEMICAL DIGESTION
• SECRETION
• ABSORPTION
• EXCRETION

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DIGESTIVE ENZYMES

• Enzymes are protein catalysts
• Enzymes are not altered themselves
• Enzymes speed up reactions at body temperatures
• Digestive enzymes are called hydrolytic enzymes.
  Water is used to split food molecules

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DIGESTIVE ENZYMES

• Enzymes are sensitive to such things as
  temperature and pH
• The names of enzymes usually end in ase For
  example, sucrase is the enzyme that catalyzes the
  hydrolysis of the sugar sucrose

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DIGESTION IN THE MOUTH

• Mastication
• Another name for chewing
• Breaks-up and lubricates the food
• Swallowing is easier
• Increases surface area of food so enzymes can
  work more efficiently

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DIGESTION IN THE MOUTH

• Salivary amylase, secreted by salivary glands,
  digests starch into smaller molecules, the
  smallest being the disaccharides.

amylase
Starch
Disaccharides
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SWALLOWING
Tongue pushes bolus of food from oral cavity into
oropharynx
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SWALLOWING
Soft palate closes nasopharynx and epiglottis
closes glottis
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SWALLOWING
ESOPHAGEAL PHASE Upper esophageal sphincter
open Peristalsis propels bolus down esophagus
toward stomach
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SWALLOWING
Cardiac (lower esophageal) sphincter opens and
bolus enters stomach
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STOMACH HISTOLOGY
Rugae---------------------
Gastric Pit-----------------
Gastric Gland------------
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GASTRIC GLANDS

• Gastric glands
• Mucous neck cells secrete protective mucus
• Parietal cells secrete hydrochloric acid and
  intrinsic factor
• Chief cells secrete pepsinogen and gastric lipase
• G cells secrete hormone gastrin which stimulates
  gastric secretions

Mucous Neck cells-----------
Parietal cell-------------------
Chief cell----------------------
G cell----------------------------
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FUNCTIONS OF THE STOMACH

• (1) Storage- can eat lots of food at one sitting
• (2) Mechanical Digestion
• Mixing waves every 15-20 seconds
• Reduce food to liquid acid chyme
• Force small amounts of chyme from stomach into
  the small intestine
• (3) Chemical digestion
• Protein digestion begins
• Inactive enzyme pepsinogen secreted by chief
  cells of gastric glands

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CHEMICAL DIGESTION

• Pepsinogen Converted to active enzyme pepsin in
  stomach lumen (cavity) by hydrochloric acid (HCl)
  
• Pepsin digests proteins to smaller polypeptides

HCl
Pepsinogen
Pepsin
Pepsin
Protein
Polypeptides
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FUNCTIONS OF THE STOMACH

• (4) Limited absorption
• Aspirin and some other drugs
• Alcohol
• Some water
• Electrolytes

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SMALL INTESTINE HISTOLOGY

• Lined with about 4.5 million villi (villus)
• Small finger like extensions
• Covered with a simple columnar mucous membrane
• Blood capillaries inside for absorbing most
  substances
• Single lymph capillary called a lacteal for
  absorbing most fat

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THE VILLUS
---------Absorptive Cell
Simple Columnar Cells-------
---------Goblet Cell
Blood Capillaries-------------
Lacteal--------------------------
---------Endocrine Cell
-----------Paneth Cell of intestinal crypt
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SMALL INTESTINE FUNCTIONS

• (1) Mechanical digestion
• Peristalsis propels chyme along intestine
• Segmentation move chymes back and forth to mix it
  thoroughly
• (2) Chemical digestion
• Enzymes from pancreas and small intestine
  complete digestion of protein, starch,
  disaccharide sugars and fat
• Gallbladder empties bile into small intestine to
  aid in fat digestion
• (3) Absorption of most substances

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THE PANCREAS

• Head, neck ,body and tail
• Head into duodenum
• Tail to spleen
• Pancreatic duct joins bile duct and connect to
  duodenum

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HISTOLOGY OF PANCREAS

• Acini are exocrine cells that secrete digestive
  enzymes into ducts
• Duct cells secrete bicarbonate to buffer the acid
  chyme from stomach and raise its pH from 2-3 to
  7-8

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SECRETION OF PANCREATIC JUICE

• Enzymes are secreted by Acinar cells
• Bicarbonate solution is secreted by Ductal cells
• About 1 liter of pancreatic juice is secreted by
  the pancreas per day
• Bicarbonate solution neutralize the acidity
• Enzymes digest proteins, starch and fat

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DIGESTION BY PANCREATIC ENZYMES

• Proteins Digestion
• Four proteolytic enzymes secreted as inactive
  proenzymes
• Proenzymes sequentially activated in duodenum
• These enzymes digest proteins polypeptides to
  smaller peptides.

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DIGESTION BY PANCREATIC ENZYMES

• Starch digestion
• Remaining starch is digested in intestine by
  pancreatic amylase
• Digestion same as in the mouth
• STARCH gt AMYLASE gt DISACCHARIDES

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DIGESTION BY PANCREATIC ENZYMES

• Fats digestion
• Triglycerides (fat molecules made of glycerol and
  three fatty acids) digested in small intestine by
  pancreatic lipase
• Digestion of each triglyceride yields a
  monoglyceride molecule and two fatty acid
  molecules

lipase
Monoglyceride
Triglyceride

Two fatty acids
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ROLE OF BILE

• Bile from the gallbladder is required for lipase
  to digest fat more efficiently.
• Bile flows from gallbladder down the bile duct
  into duodenum to mix with and emulsify the fat.
• Emulsification is breaking fat drops into very
  small droplets

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THE LIVER Anterior View
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THE LIVER Posterior View
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LIVER HISTOLOGY

• Lobes contain microscopic lobules
• Lobules consist of rows (plates) of liver cells
  (hepatocytes) and rich blood supply
• Blood supplied by branches of the hepatic artery
  and portal vein at six corners of lobule

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LIVER HISTOLOGY

• Blood flows toward the center of each lobule
  through liver capillaries (sinusoids)
• Rows of liver cells surround the capillaries
• Blood flows from the capillaries into the central
  vein in the center of the lobule
• Liver macrophages called Kupffer cells are in the
  capillaries

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LIVER HISTOLOGY
central vein--------------------------------------
---
Sinusoid---------------------------- (liver
capillary)
Hepatocyte-------------------
Kupffer cell------------------------
portal vein----------------------
bile duct------------------
hepatic artery-------------------------
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THE BILIARY SYSTEM

• Liver secretes bile
• Bile flows from liver through hepatic ducts into
  the ballbladder
• Gallbladder stores and concentrates bile
• Common hepatic duct and cystic duct from GB unite
  to form common bile duct
• Common bile duct unites with pancreatic duct
• Bile and pancreatic juices enter the duodenum

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THE BILIARY SYSTEM

• One-half to one liter of bile secreted by the
  liver each day
• Functions of bile
• Emulsification of fat in small intestine
• Aids in fat absorption
• Excretion of bilirubin and cholesterol

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FUNCTIONS OF THE LIVER

• (1) Carbohydrate, lipid and amino acid metabolism
  
• (2) Removal of waste products
• (3) Storage of glycogen, vitamins and iron
• (4) Phagocytosis by Kupffer cells
• (5) Detoxification
• (6) Bile secretion
• (7) Plasma protein synthesis

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DIGESTION BY INTESTINAL ENZYMES

• Called brush-border enzymes
• Located in microvilli of intestinal absorptive
  cells

Peptidases digest peptides to amino
acids Intestinal lipase digest fats to glycerol
fatty acids
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DIGESTION BY INTESTINAL ENZYMES

• Disaccharidases digest disaccharides to
  monosaccharides

sucrase
Sucrose
Glucose Fructose
maltase
Maltose
Glucose Glucose
lactase
Lactose
Glucose Galactose
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ABSORPTION IN THE SMALL INTESTINE

• Absorption is the transfer (uptake) of substances
  into absorptive cells then into blood or lymph
• Villi and microvilli of absorptive cells provide
  a very large surface area for absorption
• Most digested foods, water, electrolytes,
  vitamins are absorbed in the small intestine

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ABSORPTION IN THE SMALL INTESTINE

• Absorption into the blood
• Monosaccharides
• Amino acids
• Water
• Electrolytes
• Water soluble vitamins

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ABSORPTION IN THE SMALL INTESTINE

• Absorption into the lacteals (lymph capillaries
  in villi)
• Fat soluble vitamins
• Fats (triglycerides)
• Bile aids in absorption of fat digestion products
  (fatty acids and monoglycerides)
• Fat digestion products converted back to
  triglycerides in absorptive cells

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ABSORPTION IN THE SMALL INTESTINE

• Triglycerides and other lipids combine with
  protein to form small water soluble particles
  called chylomicrons
• Chylomicrons absorbed into lymph of lacteals

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INTESTINAL ABSORPTION
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LARGE INTESTINE HISTOLOGY

• Simple columnar mucosa
• Deep crypts with intestinal glands
• Glands secrete lots of mucus
• No villi

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FUNCTIONS OF THE LARGE INTESTINE

• (1) Feces formation by bacterial action
• (2) Limited digestion of undigested food by
  bacteria
• (3) Formation of vitamin K and some B vitamins
  by bacteria
• (4) Absorption of some water, electrolytes,
  vitamins and bile salts

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CLINICAL TERMS

• Gingivitis- Inflammation of the gums
• Periodontal disease- Inflammations of the teeth,
  ligaments and alveolar bones
• Stomatitis- Inflammation of the mouth
• Esophagitis- Inflammation of esophagus

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CLINICAL TERMS

• Gastritis- Inflammation of the stomach
• Enteritis- Inflammation of small intestine
• Diverticulitis- inflammation of the colon
• Hepatitis- inflammation of the liver
• Pancreatitis- Inflammation of the pancreas