digestive system anatomy and physiology of the digestive tract
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DIGESTIVE SYSTEM
Anatomy and Physiology of the Digestive Tract
DIGESTIVE ANATOMY
• Mouth and tongue
• Salivary Glands
• Pharynx
• Esophagus
• Stomach
• Liver
• Pancreas
• Small intestine
• Large intestine
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.
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.
LAYERS OF THE GI TRACT
Muscularis
Mucosa
Submucosa
FUNCTIONS OF THE GI TRACT
• INGESTION
• MECHANICAL DIGESTION
• CHEMICAL DIGESTION
• SECRETION
• ABSORPTION
• EXCRETION
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
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
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
DIGESTION IN THE MOUTH
• Salivary amylase, secreted by salivary glands, digests starch into smaller molecules, the smallest being the disaccharides.
Starch Disaccharidesamylase
SWALLOWING
Tongue pushes bolus of food from oral cavity into oropharynx
SWALLOWING
Soft palate closes nasopharynx and epiglottis closes glottis
SWALLOWING
ESOPHAGEAL PHASE
Upper esophageal sphincter open
Peristalsis propels bolus down esophagus toward stomach
SWALLOWING
Cardiac (lower esophageal) sphincter opens and bolus enters stomach
STOMACH HISTOLOGY
Gastric Pit-----------------
Gastric Gland------------
Rugae---------------------
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----------------------------
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
CHEMICAL DIGESTION
• Pepsinogen Converted to active enzyme pepsin in stomach lumen (cavity) by hydrochloric acid (HCl)
• Pepsin digests proteins to smaller polypeptides
Pepsinogen HCl
Pepsin
Pepsin
Protein Polypeptides
FUNCTIONS OF THE STOMACH
(4) Limited absorption– Aspirin and some other drugs– Alcohol– Some water – Electrolytes
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
THE VILLUS
Blood Capillaries-------------
Simple Columnar Cells-------
Lacteal--------------------------
---------Absorptive Cell
---------Goblet Cell
---------Endocrine Cell
-----------Paneth Cell of intestinal crypt
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
THE PANCREAS
• Head, neck ,body and tail
• Head into duodenum
• Tail to spleen
• Pancreatic duct joins bile duct and connect to duodenum
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
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
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.
DIGESTION BY PANCREATIC ENZYMES
• Starch digestion– Remaining starch is digested in intestine by
pancreatic amylase – Digestion same as in the mouth
STARCH > AMYLASE > DISACCHARIDES
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
MonoglyceridelipaseTriglyceride + Two fatty acids
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
THE LIVER – Anterior View
THE LIVER – Posterior View
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
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
LIVER HISTOLOGY
central vein-----------------------------------------
Sinusoid----------------------------(liver capillary)
portal vein----------------------
hepatic artery-------------------------
Kupffer cell------------------------
Hepatocyte-------------------
bile duct------------------
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
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
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
DIGESTION BY INTESTINAL ENZYMES
• Called brush-border enzymes– Located in microvilli of intestinal absorptive cells
Peptidases digest peptides to amino acidsIntestinal lipase digest fats to glycerol & fatty acids
DIGESTION BY INTESTINAL ENZYMES
• Disaccharidases digest disaccharides to monosaccharides
Sucrose Glucose + Fructosesucrase
Maltose maltase Glucose + Glucose
Lactose lactase Glucose + Galactose
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
ABSORPTION IN THE SMALL INTESTINE
• Absorption into the blood– Monosaccharides – Amino acids – Water– Electrolytes– Water soluble vitamins
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
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
INTESTINAL ABSORPTION
LARGE INTESTINE HISTOLOGY
• Simple columnar mucosa
• Deep crypts with intestinal glands
• Glands secrete lots of mucus
• No villi
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
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
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
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