Human digestive system involves mechanical as well as chemical changes in the food taken. It involves breaking down of the complex organic food materials such as carbohydrates, proteins,lipids, nucleic acids, into simpler , smaller , soluble molecules.
☆ Mechanical changes involves breaking food into smaller pieces increases the surface area exposed to the enzymes of digestive juices.
☆ Chemical changes involves in cleaving of complex insoluble macromolecules into simpler , soluble units and are brought about by action of enzymes.
☆ ☆ :- Definition -:☆ ☆
” The systems of organs responsible for getting food into and out of the body and for making use of food to keep the body healthy is called digestion.”
Digestion is three types : intercellular,extra cellular and mixed.
Intercellular digestion occurs within the cell. It involves secretion of digestive enzymes by the surrounding cytoplasm into the food vacuoles. Digestion of food within food vacuoles in cells is called intercellular digestion.
The Lysosomes carry out intracellular digestion to breakdown the complex molecules of worn out cell organelles into simple one for reuse. Certain white blood corpuscles carry on intracellular digestion of ingested cells and bacteria . This process is called phagocytosis.
Protozoans and sponges food in membrane – bound sacs, called food vacuoles. Lysosomes containing digestive enzymes fuse with the food vacuoles is called a digestive vacuoles .
Extracellular digestion -:
Extracellular digestion occurs outside the cells in the cavity of tha alimentary canal. It involves secretion of digestive enzymes by special cells into the cavity of alimentary canal by ducts. Digestion of food outside the cells in a cavity is known as extracellular digestion.
In other animals, food is digested in the cavity , the lumen ,of a tube called alimentary canal.
Mixed digestion -:
They have a simple digestive tract which has a single opening ,the mouth ,for intake food and elimination of indigestible remains . This digestive tract is called to be incomplete .
Example – In Coelenterates ,such as ample Hydra ,carry on both types of digestion.
Flatworm , such as planarian and liverfluke also have incomplete digestive tract.
Parts of the body concerned with the uptake and digestion of food and elimination of indigestable remains form the digestive system, also called alimentary system. The human digestive system consists of an alimentary canal and many digestive glands.
The alimentary canal is a long tube with muscular walls, glandular epithelial lining, and varying diameter. It opens out at the upper and lower ends, and most of it lies in the abdomen in a much coiled form.
The wall of alimentary canal consists of 4 main concentric coats.
1. Visceral Peritoneum :-
It is a simple squamous epithelium attached to the underlying muscular coat by a thin sheet of loose connective tissue.
2.Muscular Coat -:
It is composed of unstriped muscle fibres arranged in two layers : the outer of longitudinal muscle fibres and inner of circular muscle fibres. Between the two layers is a network of nerve cells and parasympathetic nerve fibers named plexus of Auerback or myoenteric plexus . It controls the muscular contractions.
3. Submucosa -:
It consists of highly vascular, dense connective tissue. The intestinal glands sink into the submucosa. Another network of nerve cells and sympathetic nerve fibers, called plexus of Meissner or submucosal plexus , lies between the muscular coat and the submucosa. It controls the pharynx secretion of intestinal juice.
4. Mucosa -:
It further consists of 3 layers –
(i) outer thin muscularis mucosae of outer and longitudinal and inner circular muscle fibers, both is unstriped.
(ii) middle lamina propria of vascular reticular connective tissue containing lymphatic nodules.
(iii) inner simple columnar epithelium that forms gastric glands in the stomach and intestinal glands in the intestine.
☆Parts of alimentary canal
It consists of several organs ;-
The mouth is a transverse slit . It is bounded by two soft , movable lips : upper and lower .The lips are covered with skin on the outer side and lined with mucous membrane on inner side. Mouth leads into the vestibule.
It is narrow space enclosed between the lips and cheeks externally and the gums and teeth internally. The vestibule leads into the oral cavity.
3- Oral ( Buccal ) Cavity
It is a large space bounded above by the palate, below by the throat and on the sides by the jaws. The throat supports the tongue. The jaws bear teeth. The buccal cavity is lined by stratified squamous epithelium.
i. Palate – Anterior part of the palate is arched and strong. It is called hard palate. It is supported by bones. It bears transverse ridges called palatine rugae. The rugae help in keeping the food in place during mastication. The posterior part of the palate is smooth and fleshy. It is termed soft palate. Its smooth surface makes swallowing casy. The middle of the hind free end of the soft palate hangs down as a small, conical flap, the uvula.
ii. Tongue – The tongue is a large , muscular , highly mobile organ. It lies mainly in the oral cavity and partly in the pharynx. Its base is fixed but apex is free and greatly protrusible.
Attachment – The middle of the undersurface vascular of the tongue is attached to the floor of the oral cavity by a median fold of mucous membrane called frenulum linguae ,or frenulum of Tongue.
Furrows – An inverted v – shaped furrow, named sulcus terminalis . extends across the tongue between it’s pharyngeal and oral parts .At the angle of this furrows is a small pits ,termed as foramen caecum.
Papillae – Upper surface of the tongue bears numerous papillae that contain taste buds. Such papillae are of 3 kinds —
Vallate , fungiform and filiform .
Oral /Buccal cavity
The pharynx is about 12 cm long vertical canal beyond the soft palate. The food and air passages cross here . The Pharynx may be divided into 3 parts: nasopharynx, oropharynx and laryngopharynx.
The function of the pharynx as a part of the digestive tract is merely to serve as a passage way for the food from the oral cavity to the oesophagus. It has in its walls the voluntary muscles which start swallowing movements.
Pharynx divided into three part showing in diagram –
The oesophagus is a 25cm ,long, narrow , muscular , straight tube lined by stratified squamous epithelium containing mucous glands. It runs downward through the neck behind the trachea and through the thorax behind the heart, and passes through the diaphragm into the abdomen stomach. Here ,it sharply bends to opens into the stomach. This bend is one of the devices to check the backflow of the stomach. contents into the oesophagus. The upper part of the oesophagus has striated muscle muscle, and lower part purely smooth muscle.
The oesophagus serves to convey the food by peristalsis from the pharynx to the stomach.
The stomach is a wide, J – shaped, distensible, muscular sac placed obliquely on the left side in the upper part of the abdomen just below the diaphragm. It is about 30 cm long and 15 cm wide. It has a greater curvature anda lesser curvature along its lower and upper sides. Unlike other parts of the digestive tract, the stomach wall contains three smooth muscle layers: –
Outer of longitudinal, middle of circular and inner of oblique fibres, to churn the food and to mix it with the gastric secretion. The stomach has 4 regions:- Cardiac part , fundus , body and pyloric part.
It is the left broad upper part. The oesophagus opens into it. This opening is called the cardiac aperture , or cardia. It is guarded by a valve, the cardiac sphincter, which checks the regurgitation of food. The cardiac sphincter is much less effective in infants. Therefore, regurgitation is more common in infants than in adults.
It is the small, dome – like upper part that projects above the cardiac aperture. It often contains gas or air.
It is the middle main region of the stomach.
It is the right narrow lower portion of the stomach. It comprises a wider pyrolic antrum and a narrow canal . The pyloric canal leads into the small intestine by pyrolic aperture or pylorus guarded by a valve called pyloric sphincter. It regulates the passage of food into the intestine.
The stomach serves 4 main functions:
Storage of food , mechanical churning of food , partial digestion and regulation of flow of food into the small intestine . Storage of food in the stomach enables us to take food at intervals as a have meal instead of continuously. Stomach secretes the hormone gastrin. It also secretes a protein called Castle’s intrinsic gastric factor helps in the absorption of vitamin B12. Some absorption also occurs in the stomach.
The small intestine is a narrow tube, about 6 meters long in a living adult. It is the longest part of the alimentary canal. It comprises three parts: duodenum, jejunum and ileum.
It follows the stomach. It is proteins somewhat shaped and about 25 cm long. It a receives the hepatopancreatic ampulla of the is hepatopancreatic duct formed by the union bile duct and pancreatic duct .
Jejunum is the middle part of the small intestine. It follows the duodenum, and is about 2-4 meters long. Its wall is thicker and more vascular than that of the ileum.
The ileum forms the lower part of the small intestine. It is about 3.6 meters long, and opens into the large intestine. Its wall is thinner and less vascular than that of the jejunum.
The jejunum and ileum are greatly coiled and largely fill the abdominal cavity below the liver and lacks stomach. The inner surface of the small intestine is thrown into a series of permanent circular folds called plicae circulares or valvulate conniventes . The plicae are best developed in the jejunum. The mucosa is raised into numerous microscopic projections called the villi . The villi are leaf – shaped in the duodenum and tongue – like in the jejunum but gradually become finger – like as the ileum is reached. The villi contain blood vessels and lacteals the which receive the products of digestion after they as a have been absorbed . The free surfaces the of the cells covering the villi bear innumerable microvilli. The villi and microvilli greatly increase the the absorptive surface.
Small white patches of lymphoid tissue, called Peyer’s patches, occur on the mucous membrane of the small intestine. They fight infection.
The small intestine serve 2 main functions completion of digestion and absorption of digested food. It also secretes some hormones such as cholecystokinin, secretin, duocrinin, villikinin, enterocrinin and enterogastrone.
8- Large intestine
The large intestine is shorter than the small intestine. It is called large intestine as it is wider than the small intestine
(1)It is about 1.5 meters long .
(2)It lacks villi and microvilli.
(3)It shows three regions: Caecum, colon, rectum.
The caecum is small, blind sac coming off the colon at the latter’s junction with the ileum. It is just 6 cm wide. At the ileocaecal junction is an ileocolle (sometimes called ileocaecolic) valve that regulates the passage of materials from the small to the large intestine. The caecum bears a short, slender, worm – like projection called the vermiform appendix .The latter is about 8 cm long, and has lymphoid tissue in its wall. The appendix is a vestigial organ of no use. It may have helped our nonhuman primate ancestors digest fibrous plant matter. The appendix may prove harmful. Food may collect and decay in it or intestinal worms may settle in it and cause inflammation, or appendicitis. In such cases, it has to be removed immediately so that it does not burst and spill infection in the abdominal cavity. Its removal is called appendictomy .
The caecum is very large in herbivores such as rabbit, ass and horse. It serves to digest cellulose by bacterial action.
It is wider than small intestine. It to appearance due to a series of constrictions. The colon shows four regions:
Ascending colon that passes upward on the right side of the abdominal cavity, transverse colon that bends to the left and runs across the abdominal cavity, – descending colon that extends downward on the left side of the abdominal cavity, and sigmold, or pelvic, colon the that turns to the right and joins the rectum. The bend between the ascending and transverse colon is called right colic flexure or hepatic flexure, and that between transverse and descending colon left colic flexure or splenic flexure.
It follows the colon. It is about 15 to 20 cm. long. The rectum has longitudinal folds and large blood vessels. The rectum leads through 2.5 cm. long anal canal to the exterior at the anus. A painful condition, called haemorrhoids or piles, sometimes develops in the anal canal due to enlargement of rectal veins.
The large intestine mainly aids in the absorption of water: formation, temporary storage and elimination of faeces; and production of mucus for lubrication of mucosa. It also plays some role in digestion, absorption and excretion. The colon bacteria vitamins Band K which are absorbed.