DIGESTIVE SYSTEM 2

INTRODUCTION TO THE DIGESTIVE SYSTEM.

 

 

Learning Objectives

 

By the end of the lecture, the student should be able to:

 

•            Understand the functional significance of the gastrointestinal system

•            Describe the structure of the gastrointestinal tract, the glands that drain into it, and its subdivision.

•            List the major gastrointestinal secretions.

•            Identify the major hormones of the gastrointestinal system.

 

The Digestive System

Introduction.

•            Primary function of digestive system is to transfer nutrients , water and electrolytes from food into body’s internal environment.

 

•            Food first must be digested, or biochemically broken down, into small, simple molecules that can be absorbed from digestive tract into  circulatory system for distribution to cells.

 

 

 

 

 

GIT

 

•            The gastrointestinal tract is a continuous tube that stretches from mouth to the anus.

 

•            Meal is mixed with a variety of secretions that arise from both the gastrointestinal tract itself and organs that drain into it, such as  pancreas, gallbladder, and salivary glands.

 

 

 

 

 Basic Digestive Processes.

 

The digestive system performs four basic digestive processes:

•            Motility,

•            Secretion,

•            Digestion and

•            Absorption.

 

 

 

 

 

 

Motility.

 

•            Motility refers to muscular contractions that mix and move forward the contents of digestive tract.

 

•            Two basic types of digestive motility are superimposed on the ongoing tonic activity:

–      Propulsive Movements.

–      Mixing Movements. 

 

 

Secretion

•           Number of digestive juices are secreted into digestive tract lumen by exocrine glands located along the route.

•           Each digestive secretion consists of water, electrolytes, and specific organic constituents that are important in digestive processess such as enzymes, bile salts or mucous.

 

 

Digestion

Biochemical breakdown of structurally complex foodstuffs ( carbohydrates, proteins and fats ) of the diet into smaller, absorbable units by enzymes produced within digestive system.

 

 

 

Absorption

•            In the small intestine , digestion is completed and most absorption occurs.

•            Small Absorbable units that result from digestion, along with water, vitamins, and electrolytes , are transferred from the digestive tract lumen into the blood or lymph.

 

 

 

 

Digestive tract and accessory digestive organs.

•             The digestive system consists of digestive ( gastrointestinal ) tract plus the accessory digestive organs ( gastro means stomach).

•             Accessory digestive organs include :

–        Salivary Glands,

–        Exocrine Pancreas.

–        Biliary System ( composed of liver and gallbladder).

 

 

 

 

 

 

Digestive tract wall has four layers

 

From the innermost layer outward they are :

 

•           The Mucosa.

•           The submucosa,

•           The Muscularis Externa, and

•            The serosa.

 

 

 

 

 

 

 

 

 

 

 

 

Mucosa

•             The mucosa lines the luminal surface of the digestive tract.

•             It is divided into 3 layers:

–         A mucous membrane ( inner epithelial layer).

–        Lamina Propria ( middle layer of connective tissue)

–        Muscularis Mucosa ( outermost smooth muscle layer).

 

NOTE: LAMINA PROPRIA HOUSES THE Gut ASSOCIATED LYMPHOID TISSUE ( GALT) 

 

 

 

 

 

SUBMUCOSA

 

•            A thick layer of connective tissue.

•            Provides the digestive tract with its distensibility and elasticity.

•            Submucosal plexus lies within the submucosa.

 

 

 

 

 

 

MUSCULARIS EXTERNA

 

•             Major smooth muscle coat of the digestive tube.

•              surrounds the submucosa.

•             Contractile activity of these smooth muscle layers produces the propulsive and mixing movements.

•             Myenteric plexus lies between the two muscle layers. 

 

 

 

 

SEROSA

•            Outer connective tissue covering of the digestive tract is the serosa.

•            Secretes a watery serous fluid that lubricates and prevents friction between the digestive organs and the surrounding viscera.

 

 

 

 

 

The oral cavity or mouth

•             The oral cavity or mouth  is the enterance to the digestive tract.

•             Opening is formed by muscular lips , which help procure, guide and contain the food in the mouth.

•             Palate allows breathing  and chewing or sucking to take place simultaneously.

•             Uvula seals off the nasal passages during swallowing.

 

 

 

The oral cavity or mouth

 

•             Tongue : Movements of the tongue are important in guiding food within the mouth during chewing and swallowing.

•             Major tastebuds are embedded in tongue.

•             Pharynx acts as a common passage way for both the digestive system and the respiratory system.

•             Tonsils are part of the body’s defence mechanisms.

 

 

 

 

 

 

Teeth responsible for chewing

•            First step in the digestive process is mastication or chewing , the motility of the mouth that involves the slicing, tearing, grinding, and mixing of injested food by the teeth.

 

 

 

 

Saliva

 

•             Saliva begins carbohydrate digestion .

•             Is very important in oral hygiene, and

•             Facilitates speech.

•              serves to lubricate the food bolus (aided by mucins).

•              hypotonic compared with plasma and alkaline.

 

 

•         Digestion in the mouth is minimal ;

•         No absorption of foodstuffs occurs from mouth.

 

 

Imp: Some therapeutic agents can be absorbed by the oral mucosa.

•           Prime example being a vasodilator drug , nitroglycerin to relieve angina attacks.

 

Pharynx and oesophagus

•             The motility associated with the pharynx and oesophagus is SWALLOWING.

•             During the oropharyngeal stage of swallowing , food is prevented from entering the wrong passageways.

•             Peristaltic waves push food through the oesophagus.

•             Gastroesophageal sphinctor prevents reflux of gastric contents.

 

 

 

 

 

 

 

 

 

 

STOMACH

Anatomic Considerations:

 

•             The Stomach is a bag like structure.

•             The gastric mucosa contains many deep glands.

•              In the cardia and the pyloric region, the glands secrete mucus. In the body of the stomach, including the fundus, the glands also contain parietal (oxyntic) cells, which secrete hydrochloric acid and intrinsic factor, and chief (zymogen, peptic) cells, which secrete pepsinogens .

 

 

 

 

STOMACH

•           These secretions mix with mucus secreted by the cells in the necks of the glands.

•            Several of the glands open on a common chamber (gastric pit) that opens in turn on the surface of the mucosa.

•            Mucus is also secreted along with HCO₃^– by mucus cells on the surface of the epithelium between glands.

 

 

 

 

STOMACH

•           Carbohydrate digestion continues in the body of the stomach ;

•           Protein digestion begins in the antrum.

•           The stomach absorbs alcohol and aspirin but no food.

•           No food or water is absorbed into the blood through the stomach mucosa.

 

GASTRIC PITS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

GASTRIC SECRETION

•            The gastric mucosa secretes 1.2 to 1.5 litres of gastric juice per day.

•            Gastric juice renders food particles soluble, initiates digestion (particularly of proteins), and converts the gastric contents to a semiliquid mass called chyme, thus preparing it for further digestion in the small intestine.

•            Gastric juice is a variable mixture of water, hydrochloric acid, electrolytes (sodium, potassium, calcium, phosphate, sulfate, and bicarbonate), and organic substances (mucus, pepsins, and protein).

 

Small intestine

•           site where most digestion and absorption takes place.

•           consisting of duodenum, jejunum, and ileum.

•           No further digestion is accomplished after luminal contents pass beyond the small intestine.

•           No further absorption of nutrient occurs , although large intestine does absorb small amounts of salt and water. 

 

 

Large intestine

•           Large intestine is primarily a drying and storage organ.

•           Consists of colon, ceacum, appendix and rectum. 

•           Colon normally receives about 500 ml of chyme from small intestine each day.

•           Contents delivered to the colon consists of indigestible food residues ( such as cellulose), unabsorbed biliary components and remaining fluid.

 

Overview of gastrointestinal hormones.

 

•            Gastrin: Stimulates secretion by the parietal and chief cells.

•            Secretin.and gastric secretion.  Inhibits gastric emptying

•            Cholecystokinin ( CCK): inhibits gastric emptying and gastric secretion.  .

•            Gastric Inhibitary peptide (GIP): promotes metabolic processing of the nutrients once they are absorbed.

There are other functions also  performed by above hormones.

 

BILIARY SECRETIONS

 

•             The bile acids contained therein are important in the digestion and absorption of fats.

•              Bile serves as a  excretory fluid by which the body disposes of lipid soluble end products of metabolism .

•              Bile is the only route by which the body can dispose of CHOLESTEROL.

 

 

BILIARY SECRETIONS

•             Bile is made up of the bile acids, bile pigments, and other substances dissolved in an alkaline electrolyte solution .

•             About 500 mL is secreted per day. Some of the components of the bile are reabsorbed in the intestine and then excreted again by the liver (enterohepatic circulation).

 

 

 

 

PANCREAS

Anatomic Considerations:

 

•             The portion of the pancreas that secretes pancreatic juice is a compound alveolar gland resembling the salivary glands. Granules containing the digestive enzymes (zymogen granules) are formed in the cell and discharged by exocytosis

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PANCREAS

 

•             The small duct radicles coalesce into a single duct (pancreatic duct of Wirsung), which usually joins the common bile duct to form the ampulla of Vater.

•              The ampulla opens through the duodenal papilla, and its orifice is encircled by the sphincter of Oddi.

 

 

 

References

•           Ganong’s Review of Medical Physiology   - 23rd Edition.

 

•           Human Physiology . From Cells to Systems.    Fifth Edition.