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