Digestive System

1
Digestive System

• Anatomy and Physiology of the Digestive Tract

2
Digestive Anatomy Overview

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

3
Functions of the Digestive Tract

• Ingestion intake of food (eating)
• Mechanical Digestion chewing, swallowing,
  peristalsis and other muscular actions of GI
  tract
• Chemical Digestion chemical break-down of food
  molecules by digestive enzymes
• Secretion release of useful chemicals such as
  enzymes and mucus by cells and glands
• Absorption uptake of substances, such as
  nutrients, into circulation
• Excretion elimination of digestive wastes

4
Digestive Enzymes

• Enzymes are protein catalysts
• Enzymes are not altered themselves
• Enzymes speed up chemical reactions, and chemical
  digestion involves a series of chemical reactions

5
Digestive Enzymes Continued

• Digestive enzymes are called hydrolytic enzymes.
  
• Water is used to split food molecules

6
Digestive Enzymes Continued

• 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

7
Digestion in Mouth

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

8
Digestion in Mouth Continued

• Salivary amylase, secreted by salivary glands,
  digests starch into smaller molecules, the
  smallest being the disaccharide sugar maltose

amylase
Starch
Maltose
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Swallowing - Deglutition
Tongue pushes bolus of food from oral cavity into
oropharynx
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Swallowing Continued
Soft palate closes nasopharynx and epiglottis
closes glottis
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Swallowing Continued
Peristalsis propels bolus down esophagus toward
stomach
12
Swallowing Continued
Cardiac (lower esophageal) sphincter opens and
bolus enters stomach
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Stomach Histology section of stomach lining
Stomach Lumen
Rugae---------------------
Mucosa
Gastric Pit-----------------
Gastric Gland------------
Submucosa
Muscularis 3 layers
------------Serosa
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Gastric Glands

• Gastric glands
• Mucous neck cells secrete protective mucus
• Chief cells secrete pepsinogen
• Parietal cells secrete hydrochloric acid
• 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

• Storage-can eat lots of food at one sitting
• Chemical digestion
• Protein digestion begins, but not completed
• Inactive enzyme pepsinogen secreted by chief
  cells of gastric glands. Why must it be inactive
  when secreted?

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Digestion in the Stomach Continued

• Pepsinogen is converted to active enzyme pepsin
  in stomach lumen (cavity) by pepsin and
  hydrochloric acid (HCl)
• Pepsin digests proteins to smaller chains of
  amino acids called peptides

HCl pepsin
Pepsinogen
Pepsin
Pepsin
Protein
Peptides
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Functions of Stomach Continued

• Mechanical Digestion in Stomach
• Mixing waves every 15-20 seconds
• Reduce food to liquid acid chyme
• Force small amounts of chyme from stomach into
  the small intestine

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Functions of the Stomach Continued

• Limited absorption in stomach
• Aspirin and some other drugs
• Alcohol
• Some water
• Electrolytes

19
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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Small Intestine Histology - Villus
---------Absorptive Cell
Simple Columnar Cells-------
---------Goblet Cell
Blood Capillaries-------------
Lacteal--------------------------
---------Endocrine Cell
Intestinal crypt--------
-----------Paneth Cell of intestinal crypt
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Small Intestine Physiology

• Responsible for most 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
• Absorbs most substances

22
Small Intestine Physiology Continued

• Mechanical digestion
• Peristalsis propels chyme along intestine
• Segmentation moves chyme back and forth to mix it
  thoroughly

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Pancreas

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

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Histology of Pancreas

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

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Secretion of Pancreatic Juices

• Secreted by acini and ducts
• About 1 liter secreted per day
• Contains enzymes to digest protein, starch fat
  and nucleic acids (DNA, RNA)

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Digestion by Pancreatic Enzymes

• Protein Digestion
• Four proteolytic enzymes secreted as inactive
  proenzymes
• Proenzymes sequentially activated in duodenum
  (first part of small intestine)
• These enzymes digest protein and polypeptides to
  smaller peptides and amino acids

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Pancreatic Digestion Continued

• Starch digestion
• Starch not digested by salivary amylase in the
  mouth and stomach is digested in intestine by
  pancreatic amylase
• Digestion same as in mouth

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Pancreatic Digestion Continued

• Fat 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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Pancreatic Digestion Continued

• Bile from the gallbladder required for lipase to
  digest fat
• When secreted, bile flows down bile duct into
  duodenum to mix with and emulsify the fat
• What does emulsify mean?

30
Digestion by Intestinal Enzymes

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

• Peptidases digest peptides to amino acids

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Intestinal Enzymes Continued

• 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 uptake of substances into
  absorptive cells then into blood and lymph
• Villi and microvilli of absorptive cells provide
  large surface area for absorption
• Most digested foods, water, electrolytes,
  vitamins and other substances absorbed in small
  intestine

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Absorption Continued

• Absorption into blood
• Monosaccharides
• Amino acids
• Water
• Electrolytes such as sodium, potassium.
• Water soluble vitamins such as vitamin C and the
  B vitamins

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Absorption Continued

• Into lacteals (lymph capillaries in villi)
• Fat soluble vitamins
• Large fatty acids and monoglycerides
• Bile salts help with absorption into intestinal
  cells
• Triglycerides (fat) reformed in intestinal cells
• Triglycerides (fat) transported in lymph and
  blood by combining with protein

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Summary of Intestinal Absorption
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Functions of Large Intestine

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

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Functions of the Liver

• Carbohydrate, lipid and amino acid metabolism
• Removal of waste products
• Storage of glycogen, vitamins and iron
• Phagocytosis by Kupffer cells
• Bile secretion
• Plasma protein synthesis
• Detoxification

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Bile

• One-half liter to a liter of bile each day
• Functions of bile
• Emulsification of fat in small intestine
• Excretion of bilirubin and cholesterol
• Micelles for fat absorption

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Clinical Terms

• Gingivitis-inflammation of gums
• Periodontal disease plaque causes destruction
  of periodontal membrane and jaw bone with
  possible loss of teeth
• Stomatitis inflammation of mouth
• Dental caries - cavities

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Clinical Terms Continued

• Esophagitis and GERDS acid reflux from stomach
  into esophagus
• Gastritis inflammation of stomach
• GI neoplasms cancers of GI tract
• Hepatitis inflammation of liver, several types
• Peptic ulcer erosion of stomach/duodenal
  lining, usually caused by a bacterium

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Acid Blocking Drugs

• Histamine (H2) blockers
• Tagamet
• Zantac
• Pepcid AC
• Proton pump blockers
• Prilosec
• Nexium
• Prevacid