Its major role in digestion is the production of bile, which is concentrated and stored in the gall bladder Fig 2. In a healthy person, the liver releases around ml of bile into the duodenum each day. This is done in small amounts as the gall bladder contracts in response to CCK Hundt et al, Bile salts play an important role in chemical digestion in the duodenum. They lower the surface tension of large fat globules, causing them to collapse into smaller droplets Fig 3.
This process is called emulsification, as it creates an emulsion of tiny fat droplets with a larger surface area for subsequent breakdown by fat-digesting enzymes lipases. The digestion of fats by bile and lipases also allows the efficient absorption of fat-soluble vitamins such as vitamins A, D, E and K.
The pancreas is a small, elongated gland around 15cm long and weighing around g; it is located in the loop of the duodenum and is covered by the lower portion of the stomach Fig 2. The pancreas is an important organ in the endocrine and digestive systems, playing key roles in both the regulation of blood-glucose levels and digestion.
The endocrine portion of the pancreas consists of tiny clusters of cells called islets of Langerhans, which produce several key hormones involved in controlling blood-sugar levels and regulating appetite. It consists of thousands of acini, small berry-like structures that secrete pancreatic juice into tiny ducts. These connect and eventually fuse before discharging their secretions into the central pancreatic duct, which itself fuses with the common bile duct before entering the duodenum at the major duodenal papilla Fig 2.
Around As explained by Agrawal and Aoun, , pancreatic juice consists mostly of:. One of its key roles is to neutralise acidity in the chyme coming through from the stomach. This is essential to prevent autodigestion and ulceration of subsequent sections of the GI tract. Beyond pancreatic juice, the pancreas also produces several enzymes that continue, in the duodenum, the chemical digestion started in the mouth and stomach.
Pancreatic lipase Pancreatic lipase is the most active of the gut lipases. Like salivary and gastric lipases see part 1 and part 2 , it breaks down triglyceride fats into fatty acids and glycerol, working mostly on the emulsified fat droplets created by bile salts. For maximal activity on fat droplets, pancreatic lipase requires the presence of the small protein co-enzyme colipase, which is itself produced by the pancreas Ross et al, Weight-loss drugs such as orlistat inhibit pancreatic lipase, thereby reducing the digestion and absorption of fats.
However, the use of such drugs is often associated with side-effects such as abdominal bloating and diarrhoea, as undigested fats collect in the colon and irritate it Qi, ; Al-Suwailem et al, Pancreatic amylase Carbohydrate digestion starts in the mouth with salivary amylase see part 1.
Like salivary amylase, pancreatic amylase attacks the glycosidic bonds between adjacent glucose molecules in polysaccharides, breaking down starch into maltose. As it catalyses starch digestion in the duodenum lumen inside space of the duodenum , it is also called luminal amylase Williams, Pancreatic amylase can function over a broad neutral-to-alkaline pH range.
Amylases are most efficient in an environment with a neutral-to-alkaline pH. The activity of salivary amylase slows down in the acidic environment of the stomach, but the enzymatic digestion of carbohydrates resumes in the alkaline environment of the duodenum. Protein digestion starts in the stomach, where pepsin attacks the peptide bonds of large molecules, breaking down proteins into smaller chains of amino acids termed polypeptides see part 2.
The next stage of protein digestion relies on several pancreatic proteases:. Trypsin is the major protease present in pancreatic juice. To avoid autodigestion and damage to the pancreatic acini and ducts, it is initially secreted as the inactive precursor or zymogen trypsinogen.
Once in the duodenal lumen, trypsinogen is converted into trypsin by the action of the enzyme enteropeptidase or enterokinase , which is produced by the mucosal cells of the duodenum and jejunum. Trypsin then catalyses the activation of the other pancreatic zymogens into their active forms chymotrypsin, carboxypeptidase and elastase Goodman, Carboxypeptidase — an exopeptidase — catalyses the removal of single amino acids from the ends of protein and polypeptide molecules, gradually reducing their length.
Trypsin, chymotrypsin and elastase — all endopeptidases — attack the peptide bonds in the central portions of proteins and polypeptides. This results in the generation of smaller chains of amino acids called peptides Fig 4 , which are subsequently digested in the jejunum and ileum by the action of intestinal peptidase see part 4. Pancreatic nucleases All food consumed by humans comes from plant, animal, fungal or bacterial sources. As all cells, whatever their origin, contain deoxyribonucleic acid DNA and ribonucleic acid RNA , the human body can digest these by breaking them down into their building blocks, which are called nucleotides.
DNA is composed of four major nucleotide bases: adenine, cytosine, guanine and thymine; RNA has the same bases, except that thymine is replaced by uracil Knight and Andrade, The digestion of nucleic acids allows some of the nucleotide bases to be recycled and used as building blocks for human DNA synthesis during cell division, and for RNA during the process of transcription that precedes protein synthesis.
The duodenum is primarily dedicated to digestion, but the presence of short villi in relatively small numbers indicates that some nutrient absorption does also take place here. Small amounts of sugars, amino acids and fats, but large amounts of minerals iron, calcium, phosphorus and trace elements of zinc and copper are absorbed in the duodenum Kiela and Ghishan, Patients who have had extensive ulceration or cancer that has required the removal of all or part of the duodenum may need to take mineral supplements to compensate for reduced mineral absorption.
Gastric and duodenal ulcers used to be attributed to stress and poor diet — particularly the consumption of greasy and spicy foods — but it is now accepted that infection with Helicobacter pylori is the major precipitating factor see part 2. The use of non-steroidal anti-inflammatory drugs significantly increases the risk of peptic ulcers Kaur et al, Symptoms of gastric and duodenal ulcers are very similar.
Some with duodenal ulcers experience pain either on an empty stomach or occurring two to three hours after food consumption, which tallies with gastric emptying times. However, the only way to locate a peptic ulcer with absolute certainty is to visualise it, usually via endoscopy.
Duodenal ulcers must be treated because of the risk of perforation and life-threatening complications such as peritonitis and sepsis. As with gastric ulcers see part 2 , treatment relies on the combination of a proton pump inhibitor to reduce stomach acid secretion and two antibiotics to eradicate H.
Gallstones are one of the most common pathologies of the GI tract, usually forming as a result of the consolidation of bile salts and cholesterol. Many people have, in their gall bladder, a collection of gritty deposits resembling wet sand, known as biliary sludge. Over time, the particulates can aggregate to form a gallstone. However, being female, being overweight and being middle-aged are recognised risk factors.
Many people who have gallstones are unaware of them, as they do not usually cause symptoms unless they start to travel out of the gall bladder. Biliary colic — the pain associated with gallstones — usually occurs when gallstones leave the gall bladder and begin to scrape their way along the highly innervated bile ducts. It is usually experienced in the upper right quadrant and commonly radiates to the back, particularly around the right shoulder blade.
The stomach varies considerably in size, shape and position but lies in the upper central part of the abdomen behind the lower ribs. Although it is a single organ, several different parts of the stomach exist: the fundus, body, antrum and pylorus.
Food passes through the pylorus and into the duodenum where further digestion occurs. The stomach wall is composed of four layers. The inner lining mucosa consists of millions of microscopic glands which secrete gastric juices. Beneath this is a supporting layer submucosa and beneath this is the muscle layer. This is responsible for stomach contractions and emptying.
Finally there is a thin outer covering known as the serosa. The stomach and duodenum have a rich blood supply, derived from the aorta the main artery in the body and are also supplied by nerves from the spinal cord. The stomach carries out several different functions. It acts as the major store for food during a meal and can hold up to 1. Special cells parietal cells in the glands of the inner lining of the stomach secrete powerful hydrochloric acid that help break down food in the stomach.
Other special cells release protein-digesting enzymes pepsinogens which become active in the acid environment and begin digesting protein. The stomach secretes a number of other important substances including hormones to regulate the functions of the stomach, mucus to protect the gastric lining from damage by acid, and a substance intrinsic factor which is necessary for the body to absorb vitamin B12 from the diet.
Coordinated contractions of the stomach are important for grinding and mixing ingested food with the gastric secretions. This ensures good mixing of stomach contents and also helps to filter out partially digested food to prevent large pieces from entering the duodenum.
Lastly, partially digested food and liquids are carefully emptied from the stomach, through the pylorus, into the duodenum. These processes of secreting gastric juices, mixing food and gastric emptying are normally carefully regulated and involve the coordinated action of hormones, nerves, and muscles. Once food enters the duodenum, its acidity is neutralized by mixing with alkaline juices from the pancreas and bile in preparation for further digestion and absorption lower down the small intestine.
Disorders of the stomach and the duodenum are extremely common and a considerable source of suffering in the population. Problems may arise from a number of different mechanisms and lead to a variety of symptoms. The integrity of the inner lining mucosa of the stomach depends on a careful balance between the "aggressive" factors such as acid which tend to damage the lining and "defensive" factors such as mucus which help to protect the delicate surface lining.
In other patients, symptoms may result from problems with stomach emptying. The primary function of the small intestine is to facilitate the breakdown and absorption of nutrients needed by the body. The duodenum begins this process by preparing the chyme to be further broken down so that nutrients can easily be absorbed. The process of breaking down food and absorbing nutrients is known as digestion.
The food that is swallowed moves from the esophagus the muscular tube lined with mucous membrane that connects the throat with the stomach , then travels into the stomach through a valve called the pyloric sphincter. The pyloric sphincter's primary job is to open and shut in order to selectively allow only very small particles into the duodenum. Chemical digestion begins in the mouth, as saliva starts to break down the food that is ingested. This initial process of digestion called chemical digestion continues in the stomach via gastric stomach acid, and continues in the duodenum by the use of enzymes and other chemicals such as bile from the liver.
The duodenum receives undigested food from the stomach—called chyme—and mixes it with digestive juices and enzymes from the intestinal wall and pancreas as well as with bile from the gallbladder. This mixing process, called chemical digestion, prepares the stomach contents for the breakdown of food and the absorption of vitamins, minerals, and other nutrients.
The process of chemical digestion begins in the stomach. Chemical digestion continues in the duodenum as pancreatic enzymes and bile are mixed with the chyme. Absorption of nutrients begins in the duodenum and continues throughout the organs of the small intestine.
Nutrient absorption primarily occurs in the second portion of the small intestine called the jejunum , but some nutrients are absorbed in the duodenum. The duodenum is considered the mixing pot of the small intestine because of the churning process that takes place there: it mixes the chyme with enzymes to break down food; adds bicarbonate to neutralize acids, preparing the chyme for the breakdown of fats and proteins in the jejunum; and incorporates bile from the liver to enable the breakdown and absorption of fats.
Specific functions of the duodenum include:. In addition to the function of enzymes, intestinal juices, and bile, certain hormones also play a role in digestion. These include:. Another important function of the duodenum is immune support. The duodenum acts as a barrier to prevent harmful microbes from entering the body. The friendly bacteria in the duodenum and other parts of the small intestine take up space and compete for food inside the duodenum. As a result, pathogens disease-causing germs have a difficult time multiplying there.
Conditions of the duodenum are prevalent in people of any age. Maladies of the duodenum are a common source of abdominal discomfort for many people. Due to a complex connection between the duodenum and the accessory organs of digestion such as the liver and pancreas , malignancies cancerous cells are often seen concurrently in the duodenum and pancreas as well as the bile duct of the liver.
Duodenitis is an inflammation of the lining of the duodenum. Common conditions of the duodenum, such as duodenitis, may be acute short term and severe or chronic long term. The condition may not result in any symptoms at all; it may be diagnosed when a person is being examined for another type of digestive disorder.
In other instances, symptoms such as discomfort or a burning sensation in the abdominal region may be present. Other symptoms may include:. Several tests are commonly used to diagnose conditions of the duodenum, including duodenitis. Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Collins J, Badireddy M.
Anatomy, abdomen and pelvis, small intestine. Updated April 5,
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