1. Gastro-intestinal tract 1
Physiology
232 BMS
Dr/Nahla Yacout
2016/2017

2. Objectives
Enumerate the organs of the digestive system & explain the digestive processes
Explain types of salivary glands, & their secretions
Explain composition & functions of saliva
Explain the control of salivary secretion
Explain digestion in mouth, pharynx & esophagus
Enumerate the glands present in stomach & their secretions
Explain functions of HCl & mucus
Explain the three phases of gastric juice secretion & digestion in stomach
Know a brief account on gastric ulcers

4. Digestive system
The digestive system takes in food, breaks it down into nutrient molecules, absorb these molecules into the blood stream, & then gets rid of the undesirable molecules
The organs of the digestive system are divided into two groups:
Organs of the alimentary canal
(Gastrointestinal tract)
Mouth, Pharynx, Esophagus, Stomach,
Small intestine, & Large intestine
The large intestine leads to a terminal
opening called (Anus)
Accessory organs
Teeth, Tongue, Salivary glands,
Gall bladder, Liver, Pancreas

5. Digestive processes
Processing of food by the digestive system involves six activities: Ingestion, Propulsion, Mechanical digestion, Chemical digestion, Absorption & Defecation Ingestion: Is taking food into the digestive system through the mouth Propulsion: Moving food through the GIT Includes: Swallowing & Peristalsis (Is the contraction & relaxation of muscles, in order to squeeze food along the tract) Mechanical digestion: Includes: Chewing, Mixing of food with saliva by the tongue, Segmentation (Mixing the food with digestive juices & increasing the efficiency of absorption by moving the food over the intestinal wall)

6. Chemical digestion: Is breaking down the complex food molecules by enzymes Absorption: Is the passage of digested end products (Plus vitamins, water) from the GIT to the blood Small intestine is the major site for absorption Defecation: Eliminates the indigestible substances from the body through the anus in the form of feces

8. Mucosa
Is the layer that lines the alimentary canal starting from the mouth to the anus Functions 1. Secretion of mucus, digestive enzymes & hormones 2. Absorption of digested food into the blood 3. Protection against diseases

9. Mouth Is the only part that is involved in ingestion,
But is involved also in other digestive processes, because in mouth food is chewed & mixed with saliva containing enzymes that begins the process of chemical digestion …. Also, it begins propulsion by swallowing & carrying food though the pharynx, esophagus to the stomach
Salivary glands
Are a number of glands within the oral cavity that secrete saliva
We have 3 types of salivary glands: Parotid glands – Sublingual glands – Submandibular glands
Salivary glands are composed of two types of cells: Mucous cells – Serous cells
Mucous cells: Secrete mucus Serous cells: Secrete watery
secretion containing enzymes & ions

10. Parotid glands: Contain only Serous cells
Sublingual glands: Contain mostly Mucous cells
Submandibular glands: Contain equal numbers of Mucous cells & Serous cells
Functions of saliva
Cleans the mouth
Moisten food & helps in chewing & swallowing
Contains enzymes that begin the chemical breakdown of starch (Starts digestion of starch)
Helps in tasting food
Protects the mouth from bacteria, & prevents tooth decay

11. Composition of saliva: 1. Electrolytes (Na+ , K+ , Cl+ ) 2
Composition of saliva: 1. Electrolytes (Na+ , K+ , Cl+ ) 2. Salivary amylase enzyme 3. Lingual lipase enzyme …. Secreted from Sublingual salivary glands (Fat – digesting enzyme) 4. Proteins 5. Antibodies 6. Metabolic wastes

12. Control of salivary secretion: Salivary secretion is controlled by sympathetic & parasympathetic nervous system
Sympathetic
Parasympathetic
Causes vasoconstriction & secretion of small amounts of thick saliva
It may inhibits completely salivary secretion causing a dry mouth
….. (Xerostomia)
Causes vasodilatation & secretion of large amount of watery saliva
Reflex secretion occurs when there is food in mouth, or even the smell, thought of food stimulates the flow of saliva

13. Pharynx Is divided into three parts: Nasopharynx Oropharynx
Laryngopharynx
Nasopharynx …. Important in respiration
Oropharynx & Laryngopharynx …. Are common passages for respiratory & digestive systems
Food passes from the oral cavity into the pharynx & then esophagus

15. Esophagus Is a muscular tube about 25cm long
After food moves through the laryngopharynx, it passes into the esophagus (There is a flap called Epiglottis that closes over the larynx to force food to enter esophagus not the larynx & trachea)
Esophagus passes between the muscles of the diaphragm, just it has passed the diaphragm, it curves upwards before opening into the stomach
The lower end of esophagus is closed by sphincter called (Esophageal sphincter …. Between esophagus & stomach) …. To prevent the reflux of gastric contents into the esophagus

17. Digestion in mouth, pharynx & esophagus
The mouth is involved in most digestive processes, it:
Ingests
Begins mechanical digestion by chewing
Begins propulsion by chewing
Once the food enters the mouth, saliva will be produced containing Salivary amylase enzyme
Salivary amylase will start digestion of carbohydrates (Starch)
Then by swallowing, food will enter pharynx & then esophagus by peristalsis which act only as pathways for the food
Once swallowing occurs, Epiglottis will cover the larynx to prevent the food entry in it
Then food passes from esophagus to the stomach
Once the food enters the stomach, esophageal sphincter closes to prevent the back flow of the food into the esophagus again

18. Stomach Below the esophagus, GIT expands to form the stomach
Inside the stomach, digestion of proteins begins, & food is converted to a creamy paste called (Chyme)
At the lower part of stomach (Between stomach & duodenum …. 1st part of small intestine), there is a sphincter called (Pyloric sphincter)
Once the food passes from stomach to the duodenum, pyloric sphincter closes to prevent the back flow of food from duodenum to stomach again

19. Stomach
There are glands present in the stomach that secrete the gastric juice
These glands contain 4 types of cells:
Mucous cells ….. Secrete mucus
Parietal cells ….. Secrete hydrochloric acid (HCl) & intrinsic factor (Is required for vitamin B12 absorption in the small intestine)
Chief cells ….. Secrete pepsinogen (Inactive form of the enzyme that is converted to pepsin to digest proteins)
Enteroendocrine cells ….. Secrete some chemical messengers as histamine, serotonin & hormones as gastrin
Gastric juice is highly acidic (Because of the presence of high concentration of HCl) …. This high acidity can digest the stomach itself
For the stomach to protect itself from digestion: It secretes mucus that forms a thick coat on the wall & prevents HCl from reaching the stomach wall

20. Functions of HCl:
Converts pepsinogen (Inactive form) into pepsin (Active form) to digest the proteins
Kill many bacteria ingested with food
Functions of mucus:
Prevents injury of stomach wall by lubricating the stomach contents
Acts as a barrier between the stomach wall & gastric juice to prevent formation of peptic ulcer

21. There are 3 phases for gastric juice secretion:
Cephalic phase:
It occurs before the food enters the stomach
Taste, sight or thought of food will stimulate the (Vagus nerve)
Vagus nerve will cause gastric secretion
Gastric phase:
It occurs when the food enters the stomach
Presence of food in the stomach will stimulate the entero-endocrine cells in gastric glands to secrete Gastrin
Gastrin: Is a hormone that stimulates gastric glands to secrete more gastric juice

22. Intestinal phase:
When the partially digested contents of the stomach reaches the small intestine, a complex called (Enterogastrone …. Consists of two hormones Secretin & Cholecystokinin) is produced by the cells present in the intestine
This complex will decrease the secretion of gastric juice & also decrease the gastric motility

24. Digestion in stomach
Stomach is involved in most of the digestive processes
Once food enters the stomach, HCl & pepsinogen are secreted
HCl activates pepsinogen & converts it into pepsin to digest the proteins
(Protein digestion starts in the stomach)
Lingual lipase enzyme may digest some lipids in the stomach

25. Gastric ulcers
Damage to the underlying tissue, which causes erosions of the stomach wall
The most important symptom of gastric ulcer is: Epigastric pain that occurs 1 – 3 hours after eating & is often relieved by eating again
Causes of gastric ulcers:
High HCl production in stomach
Low mucus secretion in stomach
Infection with Helicobacter pylori bacteria, which destroys the mucosa of stomach
Complications of gastric ulcers:
Hemorrhage – Shock
Perforation
May lead to development of malignant tumor