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Small intestine is the major site of digestion and absorption of nutrients. Small intestine is divided into three parts — proximal duodenum, middle jejunum and distal ileum. Duodenum is the shortest and wider part.

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It is of cm long. It is somewhat C-shaped. It has ampulla of vater which receives both bile duct from liver and main pancreatic duct from pancreas and whose opening is guarded by sphincter of oddi. Ileum is thinner than jejunum and less vascular.


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It is the longest 3. Its distal end is dilated into a small bulb like sacculus rotundus. It is about 1. Its diameter varies from one region to another but it is always larger than that small intestine. Hence it is so named. It is divisible into 3 parts — caecum, colon and rectum.

Caecum is a pouch like small junction between ileum and colon. The junction of the ileum and colon is called ileocaecal junction which is guarded by ileocaecal valve. Caecum is 6 cm long and externally it bears a blind tube having lymphoid tissue called vermiform appendix as an outgrowth of caecum and considered as vestigial in human beings. It is slightly coiled and thought to be vestigial as these are not involved in the cellulose digestion.

Inflammation of vermiform appendix due to decay of food or worm infection is called appendicitis and rupture of appendix leads to spilling of faecal contents onto the peritoneum leading to its infection and inflammation known as peritonitis. Caecum is well developed in herbivores mammals like rabbit, ass, horse and involved in cellulose digestion.

Colon is the largest part of large intestine which is thicker than small intestine but thinner than caecum. Colon has longitudinal bands composed of longitudinal muscle fibre called taeniae coli. The taeniae coli contracts and draw the remainder of the wall into small pouches called haustra. Large globules of adipose tissue called appendices epiploicae hang down from the colon. These three components taenia coli, haustra and appendices epiploicae help to distinguish large intestine from small intestine especially in X-rays. Colon is divided into four parts —. Moderate quantities of vitamin K and B complex are manufactured by bacteria E.

Rectum is the thinnest and last part of the alimentary canal. It is wider than colon and bear longitudinal folds. It terminates into a 3 cm long anal canal whose mucosa is folded into several vertical folds, called anal column supplied arteries and veins. The opening of anal canal is called anus which is guarded by two anal sphincter muscles — internal anal sphincter composed of smooth muscle fibre and external anal sphincter composed.

Three pairs of salivary glands open into the mouth cavity. These glands secrete saliva containing digestive enzymes. Types of three glands are. Gastric glands are numerous simple branched or unbranched tubular glands which are formed by infolding of the epithelium.

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These glands are lined with cells which secrete digestive juice containing digestive enzymes, acids HCI , mucous, inorganic salts etc. It is estimated that human stomach has about 25 million gastric glands. Gastric glands are of three types — cardiac, pyloric and fundic. Fundic gland has different types of secreting cells which secretes acids and digestive enzymes.

It converts proenzyme prorennin and pepsinogen into active rennin and pepsin respectively by lowering the pH of stomach between 1. Intrinsic factor is a glycoprotein secreted by parietal humans or chief rodents cells of the gastric mucosa. In humans, it has an important role in the absorption of vitamin B 12 cobalamin in the intestine, and failure to produce or utilize intrinsic factor results in the condition known as pernicious anaemia. Dietary vitamin B 12 is released from ingested proteins in the stomach through the action of pepsin and acid.

In the small intestine, pancreatic proteases digest the binding proteins, releasing vitamin B 12 which then becomes bound to intrinsic factor.

Why is digestion important?

Finally, there are receptors for intrinsic factor on the ileal mucosa which bind the complex, allowing vitamin B 12 to be absorbed into portal blood. Beside stomach, they are also found in the intestinal epithelium. Secretions of this gland are called succus entericus or intestinal juice. Largest digestive gland. In man it has four lobes. It is situated just below the diaphragm in the abdomen and makes upto 3.


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It is basically an organ of homeostasis. A common problem associated with the stomach is peptic ulcers, a type of ulcer formed by a disturbance in the regulation of the hormone gastrin, which causes to much gastric acid to be produced, which causes damage to the mucus membrane and the stomach lining. After food has been digested enough, it moves out of the stomach and into the small intestine.

Another round of digestion occurs in the small intestine, even more than in the stomach. Absorption also begins to occur at this stage in the digestive system. The small intestine is broken down into three parts, the duodenum, the jejunum, and the ileum. The duodenum is the first part of the small intestine, and it is responsible for most of the food digested in the small intestine. The duodenum also regulates the rate of emptying of the stomach via hormonal pathways.

The next part of the small intestine is the jejunum, which is the longest portion of the small intestine, but it is also responsible for very little digestion, but is primarily responsible for the first round of absorption, where it takes most of the nutrients out of the food matter. The final portion of the small intestine is the ileum, which is where the final bit of absorption in the small intestine takes place.

Once the now digested material finishes its path through the small intestine, it enters the large intestine. The large intestines primary purpose is to remove most of the water from the now indigestible mass, and move it to the anus, where it will be removed from the body. The large intestine is made up of the cecum, the rectum and the colon. The cecum is not very large in humans, and is barely existent at all. It serves a much higher purpose in herbivores, where it digests tough leaves with the help of symbiotic bacteria. In humans this is not necessary, so it has become very unimportant.

The colon is where the water is taken from the digested food matter. The rectum is where the waste is temporarily stored before it is excreted from the body, which takes place in the anus. The functions of the stomach include:. Most of these functions are achieved by the secretion of stomach juices by gastric glands in the body and fundus. Some cells are responsible for secreting acid and others secrete enzymes to break down proteins. The small intestine is composed of the duodenum, jejunum, and ileum. It averages approximately 6m in length, extending from the pyloric sphincter of the stomach to the ileo-caecal valve separating the ileum from the caecum.

The small intestine is compressed into numerous folds and occupies a large proportion of the abdominal cavity. The duodenum is the proximal C-shaped section that curves around the head of the pancreas. The duodenum serves a mixing function as it combines digestive secretions from the pancreas and liver with the contents expelled from the stomach. The start of the jejunum is marked by a sharp bend, the duodenojejunal flexure. It is in the jejunum where the majority of digestion and absorption occurs. The final portion, the ileum, is the longest segment and empties into the caecum at the ileocaecal junction.

The small intestine performs the majority of digestion and absorption of nutrients. Partly digested food from the stomach is further broken down by enzymes from the pancreas and bile salts from the liver and gallbladder. These secretions enter the duodenum at the Ampulla of Vater. The lining of the small intestine is made up of numerous permanent folds called plicae circulares. Each plica has numerous villi folds of mucosa and each villus is covered by epithelium with projecting microvilli brush border. This increases the surface area for absorption by a factor of several hundred.

The mucosa of the small intestine contains several specialised cells. Some are responsible for absorption, whilst others secrete digestive enzymes and mucous to protect the intestinal lining from digestive actions.

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The large intestine is horse-shoe shaped and extends around the small intestine like a frame. It consists of the appendix, caecum, ascending, transverse, descending and sigmoid colon, and the rectum. It has a length of approximately 1. The caecum is the expanded pouch that receives material from the ileum and starts to compress food products into faecal material.

Food then travels along the colon. The wall of the colon is made up of several pouches haustra that are held under tension by three thick bands of muscle taenia coli. The rectum is the final 15cm of the large intestine. It expands to hold faecal matter before it passes through the anorectal canal to the anus. Thick bands of muscle, known as sphincters, control the passage of faeces. The mucosa of the large intestine lacks villi seen in the small intestine.

The mucosal surface is flat with several deep intestinal glands. Numerous goblet cells line the glands that secrete mucous to lubricate faecal matter as it solidifies. The functions of the large intestine can be summarised as:. The liver is a large, reddish-brown organ situated in the right upper quadrant of the abdomen.

It is surrounded by a strong capsule and divided into four lobes namely the right, left, caudate and quadrate lobes. The liver has several important functions. It acts as a mechanical filter by filtering blood that travels from the intestinal system. It detoxifies several metabolites including the breakdown of bilirubin and oestrogen. In addition, the liver has synthetic functions, producing albumin and blood clotting factors.

However, its main roles in digestion are in the production of bile and metabolism of nutrients. All nutrients absorbed by the intestines pass through the liver and are processed before traveling to the rest of the body. The bile produced by cells of the liver, enters the intestines at the duodenum.

Here, bile salts break down lipids into smaller particles so there is a greater surface area for digestive enzymes to act. It consists of a fundus, body and neck. It empties via the cystic duct into the biliary duct system. The main functions of the gall bladder are storage and concentration of bile. Bile is a thick fluid that contains enzymes to help dissolve fat in the intestines. Bile is produced by the liver but stored in the gallbladder until it is needed.

Bile is released from the gall bladder by contraction of its muscular walls in response to hormone signals from the duodenum in the presence of food. Finally, the pancreas is a lobular, pinkish-grey organ that lies behind the stomach.

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Its head communicates with the duodenum and its tail extends to the spleen. The organ is approximately 15cm in length with a long, slender body connecting the head and tail segments. The pancreas has both exocrine and endocrine functions.