The digestive system is related with the digestive processes i.e. intake of food material, its digestion, absorption, assimilation and excretion of the undigested waste material. Digestive system also related with alimentary canal and its associated organs and digestive glands.
The digestive system
The five main activities of the digestive system are as follows:-
Alimentary canal: It is also known as the gastrointestinal (GI) tract, it is a long tube like structure through which food passes. It starts from mouth and terminates at the anus, the alimentary canal contains following parts:-
The digestive glands secrete various types of secretions that are poured into the alimentary tract, some secreted by glands e.g. gastric juice secreted by glands in the lining of the stomach, and some secreted by the glands situated outside the tract. These secretions pass through ducts to enter the tract:
• The salivary glands
• The pancreas
• The liver and biliary tract.
The alimentary canal:-
Buccal cavity: The next part of alimentary canal is made by bones and muscles:
Anteriorly – by the lips
Posteriorly – it is continuous with the oropharynx
Laterally – by the muscles of the cheeks
Superiorly – by the hard palate and soft palate
Inferiorly – by the tongue and the soft tissues
Diagram of buccal cavity
Diagram of tongue
Blood supply: The main artery that supplies the blood to the tongue is the lingual branch og external carotid artery. Venous drainage is by the lingual vein, which joins the internal jugular vein.
The nerves involved are:
• The hypoglossal nerves (12th cranial nerves), it supplies the voluntary muscle.
• The lingual branch of the mandibular nerves, that arise from the 5th cranial nerves, these are the nerves of somatic (ordinary) sensation, i.e. pain, temperature and touch
• The facial and glossopharyngeal nerves (7th and 9th cranial nerves), the nerves of taste.
Functions of the tongue:
The functions of the tongue are as follows:
1. Chewing (mastication)
2. Swallowing (deglutition)
Arterial blood supply by branches of the maxillary arteries
The venous drainage is by a number of veins which empty into the internal jugular veins.
The nerve supply to the upper teeth is by branches of the maxillary nerves and to the lower teeth by branches of the mandibular nerves. These are both branches of the trigeminal nerves (5th cranial nerves).
Salivary glands secretes into ducts that lead to mouth.
The three main pairs are:
There are also various smaller salivary glands scattered around the mouth.
Arterial supply is by various branches from the external carotid arteries and venous drainage is into the external jugular veins.
Saliva consist of
Saliva is secreted by submandibular glands.
Functions of saliva
Structure of Pharynx:
Pharynx contains five layers form inside to outside which are as following:
Muscles of pharynx
Diagram of pharynx and esophagus
Arterial: by oesophageal arteries, and abdominal region is supplied by branches from the inferior phrenic arteries and the left gastric branch of the coeliac artery.
Venous drainage: by azygos and hemiazygos veins.
Functions of oesophagus:
Structure of the stomach
The stomach is divided into three regions:
At the distal end of the pylorus is the pyloric sphincter, guarding the opening between the stomach and the duodenum. When the stomach is not active the pyloric sphincter is relaxed and open, and when the stomach has food the sphincter is closed.
Diagram of stomach
Walls of the stomach
There are two types of walls present in the stomach are as follows:
Muscle layer: This consists of three layers of smooth muscle fibers:
• An outer layer of longitudinal fibers
• A middle layer of circular fibers
• An inner layer of oblique fibers
Mucosa: it consist some special cells that secrete gastric juice.
Blood supply: Arterial supply to the stomach is by the left gastric artery, a branch of the coeliac artery, the right gastric artery and the gastroepiploic arteries.
Venous drainage is by the portal vein.
Function of stomach:
About 2 liters of gastric juice is produced by the stomach daily, it consist of:
Functions of gastric juice
• Water helps to liquefy the swallowed food.
• Hydrochloric acid: it acidifies the food and stop the action of salivary amylase
– kills ingested microorganisms
– provides the acid environment, important for the action of pepsins.
• By hydrochloric acid pepsinogens are activated into pepsins and by the pepsins which is already present in the stomach. These enzymes start the digestion of proteins by breaking them into smaller molecules. Pepsins are most effectively at a low pH i.e. in between 1.5 and 3.5. 12.7
• Intrinsic factor (a protein) is necessary for the absorption of vitamin B12 from the ileum. (Deficiency leads to pernicious anemia)
• Mucus lubricates the stomach contents to prevent mechanical injury to the stomach. It also acts as barrier between the stomach wall and the corrosive gastric juice to prevent chemical injury.
Functions of the stomach
• Temporary storage for the food to digestive enzymes, pepsins, to act
• Chemical digestion – pepsins break proteins into polypeptides
• mechanical breakdown – the three smooth muscle layers enable the stomach to act as a churn, gastric juice is added and the food contents are liquefied to chyme.
• Limited for absorbing water, alcohol and some lipid-soluble drugs
• Non-specific defense against microbes – provided by hydrochloric acid in gastric juice.
• Secretion of the hormone gastrin.
5) Small intestine
It has following parts:
Duodenum: this is the proximal fixed part of the small intestine and it is about 25cm in length.
Jejunum: it is about 8feet in length.
Ileum: this is about 12 feet in length
Structure of small intestine
Anatomy of small intestine:
The upper part of the small intestine is called jejunum lower part it called ileum.
Jejunum forms the 2/5th of the small intestine. Ileum 3/5th of small intestine
The ileum is the mobile part of small intestine.
The difference between jejunum and ileum:
|Circular folds||Large and closely present||Small and not closely set|
|Mesentery||Thinner, near the upper part, contain wide arteries||Thicker, arteries are narrow|
|Villi||More in number, large, thicker & leaf like structure||Lesser, thinner & shorter|
Arterial supply: by superior mesenteric artery.
Venous drainage: by portal vein.
Nerve supply: by parasympathetic branches of Vagus nerve.
Caecum: it is about 6cm in length and 7.5cm in width. Ileum opens into the caecum, which is guarded by the ileocaecal valve which prevents the reflux from caecum to ilium.
Ascending colon: It starts from caecum and turns at the hepatic flexure.
Transverse colon: This is present in the abdomen.
Descending colon: Starts from the splenic flexure to the lower part of the left pelvic brim bone.
Sigmoid colon: It starts from the left pelvic brim bone & ends in front of sacrum-3 bone.
Diagram of large intestine
Rectum: This is the lower dilated part of the large intestine. It is situated in the pelvic cavity.
Anal canal: This is the last part of alimentary canal. It starts from the anorectal junction to the anus. It contains the internal & external sphincters.
Anus: This is the posterior opening of alimentary canal by which the undigested material is given out of the body.
Diagram of liver
The liver is located in right hypochondrium region.
Structure of liver:
Arterial supply: hepatic artery & portal vein
Venous drainage: inferior vena cava
Functions of liver:
Diagram of gall bladder
It has three parts
The neck is connected with cystic duct. It contains three layers. The outer covering is perineal, middle is the muscular covering and internal is mucus layer. The neck is connected with the cystic duct, which is 4 cm in length. It joins with the right and left hepatic duct and make the common bile duct. It takes bile juice to the duodenum.
Function of gall bladder:
Composition of bile juice: The bile is an alkaline secreted by the liver cells. It is secrete about 500ml per day. The bile juice contains about 86% water and bile salts + bike pigments + cholesterol + mucin.
Bile juice helps in fat digestion.
Diagram of pancreas
The pancreas has four parts:-
The head of pancreas present in duodenum.
The exocrine portion contains the acini cells which secrete pancreatic enzymes.
The exocrine portion contains the pancreatic islets cells which secrete pancreatic hormones like insulin and glucagon.
Arterial supply: by splenic artery
Venous drainage: by portal vein
1. Alfa amylase: It digests the starch.
2. Pancreatic: It digests the protein.
3. Phospholipase: It functions on the phosphor lipids.
Within a few minutes of taking the food the pancreatic juice secretion increases under the effect of Vagus nerve. When the gastric content passes in the duodenum the two hormones secretion and pancrcoxgymic are produced in mucosa which further stimulates the secretion of the pancreatic juice.
The endocrine portion the pancreatic islets cells are present in the small group in the epithetical cells.
The pancreatic islet cell secretes the different hormones one of the most important hormone is insulin. This is the anti-diabetic hormone.
The insulin is a protein which can be affected by the protein digestive ferments.
Therefore the insulin is not given by the mouth but it is given by the substances (under the skin).
The insulin contents and absorbs the glucose and fats.
Hyperglycemia: This is the high bland slyer, loss of weight, fatigue, polyuria, mouth dryness and tongue thirst and also increased berating.
Important pancreatic hormones:
The physiology of digestion starts from the mouth.
The food is taken from the mouth into the buccal Carty. The buccal cavity contains the tongue and teeth.
The food is tasted by the tongue. The buccal cavity receives the saliva from the salivary gland which makes the food soft.
The saliva is secreted by the parotic submandibular and sub lingual.
The food is chewed or masticated the teeth. The starch dextrin and the glycogen are converted into the limit dextrin’s maltose and isomaltose.
Now, the food passes in the esophagus which doesn’t contain any enzyme. It passes the food material into the stomach by the cardiac sphincter.
The wall of the stomach undergoes the macular contraction 15-20 times per minute. By this action the food is properly mixed with the gastric juice. Gastric juice contains the mucin + HCL + pepsin + renin.
Secretion of gastric juice in the stomach is controlled by the nervous system.
The food material remains in the stomach for the average meal is about 3-5 hours.
It is different for the different food material
Food starts coming into duodenum. The food material receives the bile juice from the gall bladder and pancreatic juice from the pancreas. No action of these enzymes takes place in duodenum.
They pass with the food material to small intestine, the small villi (0.5-1.5mm long) it also contains maximum intestinal glands.
Action of bile juice in the lower part of the small intestine
Action of pancreatic juice starts in the upper part of small intestine.
Now food enters in upper part small int.
Food passes is lower part of the small intestine: It is mixed with the intestinal juice it is also called as (succus-entericus).
The intestine juice is secreted about 3 little in 24 liter in 24 hours with PH value 7.8-8. It contains the following enzymes- Amino peptidase, maltose, sucrose, lactase and lipase.
The maximum food is absorbed by the villi which are mixed with the blood and goes in the blood circulation the unabsorbed part of the food material passes in the large intestine which simply reabsorbs the water and some salts.
The unabsorbed material passes into the rectum which stores it temporary and finally passes out of the body by the anus.
Disaccharides: Sucrose, lactose and maltase
Monosaccharaides: Glucose and fructose
The carbohydrates are not digested by the stomach. The digestion starts in the mouth (buccal cavity) by the action of salivary glands and then in small intestine by the action of pancreatic amylose this results in the small chains of polymers- oligosaccharides and disaccharides.
The enzymes in the small intestine alfa dextrinase maltose, sucrose, lactose complete the process of digestion of carbohydrates and changes to glucose and fructose.
The glucose is very quickly absorbed from the small intestine. Absorption of glucose depends on Na+ the high sodium ion concentration in the lumen helps in maxM absorption of glucose.
The maximum rate of glucose absorption in small intestine is 120gm/hm.
The pepsin is secreted by the cells of mucosa. Pepsin breaks peptide bonds.
The best action of pepsin is all PH= 1.6 – 3.2 and the action finishes as the food passes in the small intestine.
The further digestion of proteins takes place in duodenum by the photolytic enzyme.
Proteolytic enzyme (Tripsin + chemotripsin etc.)
In the smalls intestine the compute digestion of the polypeptides takes place at three sites.
The digestion of fats starts in the stomach by the action of the lingual lipase and gastric lipase.
In the duodenum by the action of pancreatic lipase the digestion of fats takes place.
The presence of the bile salts is very important because it helps in the fat digestion which is secreted by bile juice.
Bile slats + lipids help to convert the H2O soluble molecules. Therefore very well absorb.
The HCL produced in the stomach is so strong that it can even dissolve the zinc but the wall of the stomach is protected due to very quick regeneration of cells so that the HCL cannot get time to dissolve the cells of the wall of the stomach.
Water soluble vitamins: CB, B6, B12, B, B2 biotin and niacin.
Essential amino acids: Beans, Pea, Com
1. Due to bacterial or viral infection in the large intestine.
2. Intake of toxic materials or poisonous substance.
3. Drugs like ampicillin
Type A: Spread by hands, food and feces.
Type B: Spread by blood or blood product
Type NANB: This is also spread by infected blood.
Chronic hepatitis: Any form of the hepatitis which persists more than 6 months may be due to viruses or drugs.
Gastritis: The inflammation of the gastric wall due to imbalance between the gastric juice and protective effect of mucus on the epithelial gastric surface.
Pyloric stenosis: This is the condition in which spasmodic (pain in stomach) constriction of the pyloric sphincters. The main symptom is the vomiting or weight loss. This is more common in the males.
Peptic reflux esophagitis: This is due to the acid juice in the esophagitis causing the irritation. The reflux of gastric contents is due to the: