digestive SYSTEM

The mouth (Oral Cavity) (Buccal Cavity) is an opening through which food and air enter the body. The mouth is connected to outside through the lips and empties into the pharynx at the back. Mouth is divided into two portions: (1) the vestibule, which is the area between the cheeks and the teeth, and (2) the oral cavity proper, which is filled with tongue and teeth. In addition to the primary function of the mouth which the initial digestion of food, the mouth and its related structures are essential in humans to the formation of speech (Fig. 1). 

In the process of digestion, the function of mouth is related to the sense of olfaction which can identify the presence of food and consequently stimulate the production of various gastric juices by different digestive organs in preparation for digestion. There are three major salivary glands in the mouth: Parotid, submandibular and sublingual (Fig. 2). These glands produce saliva which acts as a lubricant and detergent through killing some of the microorganisms. This saliva also has a role in the chemical digestion through an enzyme called amylase that breaks starches down into sugars.

The mechanical digestion is done through the teeth, which are divided into incisors and molars that cut and grind the food, respectively into smaller pieces suitable for swallowing (Fig. 3). Incisors are located at the front of the mouth cutting and tearing food, while molars appear in the middle and back of the mouth. Visit Dental page for more detail.

The tongue is a muscular band covered by moist, pink tissue called mucosa. It also contains papillae, tiny bumps giving the tongue its rough texture (Fig. 4). The surface of these bumps includes the taste buds (sweet, sour, salty, savoury) (Fig. 5). However, different parts of the tongue are responsible for the bitter tastes. The decision to swallow starts as a voluntary movement, but once swallowing begins, the process becomes involuntary, and it is controlled by the nervous system. 

The pharynx is a hollow tube lying inside the neck behind the mouth. It is split into three main divisions. It starts with the nasopharynx, which is located above the soft palate and behind the nose. Then comes the oropharynx which is lined by a mucous membrane able to handle food as well as air. The oropharynx includes the two types of tonsils, which are composed of lymphoepithelial tissues. The last part of the pharynx is the laryngopharynx that acts as a connecting point through which air, water and food pass (Fig. 6). 

Bolus is a swallowed ball of food. The pharynx is responsible for carrying the bolus to the esophagus. Then it, through waves of peristalsis, reaches the stomach (Fig. 7). Bodies of the thoracic vertebrae, azygos veins, thoracic duct, right posterior intercostal arteries and descending thoracic aorta appear posterior to the esophagus, while trachea, left principal bronchus, left recurrent laryngeal nerve, pericardium and left atrium are seen anterior to this digestive organ.  

The stomach is a J-shaped organ with 25cm long in the digestive system. It is divided into four main parts: cardiac region (connected to esophagus), fundus (the upper part), body (the middle-lower) and pyloric region (the lower that is connected to duodenum) (Fig. 8). The stomach is responsible for storing the food and liquid, mixing them up with various digestive juices through the muscular action, and moving the resultant partially digested mixture into the small intestine. These actions refer to the mechanical processes undertaken by the stomach. However, some chemical digestive processes are conducted too in the stomach through the production of gastrin, a hormone, in the blood. A combination of acid gastric juices is release by the stomach to break down the food into chyme, a liquid. This high level of acidity plays a role in killing microorganisms. Also, hydrochloric acid, mucus and proteases (such as pepsin) are produced in the stomach. An empty adult stomach has a volume of about 45 mL but can expand to hold as much as 3 L, although a normally full stomach is about 1 L in volume.

The stomach is seen between the esophagus and the small intestine as a flexible part of the alimentary canal. It is surrounded by the lower ribs and covered by liver & diaphragm (Fig. 9). The stomach includes two sphincters: the lower esophageal sphincter (Fig. 10) and the pyloric sphincter (Fig. 11) at the junction of the stomach with the duodenum. The gastro-esophageal sphincter guards the entrance to the stomach. It is a ring-like valve, which defines the bolus, and then relaxes and opens to allow it into the stomach. When the sphincter shuts, it ensures that the food moves in one direction. The stomach has anterior and posterior surfaces with lesser and greater curvatures as borders.

The small intestine is divided into three main parts:  the duodenum, jejunum and ileum (Fig. 12). The first part is the duodenum (Fig. 13) which receives the chyme coming from the pyloric sphincter. Also, it receives enzymes from the pancreas and bile from the gallbladder to coat the chyme after mixing with the mucus. In the duodenum, the acids from the stomach are neutralized by alkalis, and the majority of proteins, fats and carbohydrates are absorbed into the bloodstream. The jejunum is the second part of the small intestine (Fig. 14), which, through its villi, is responsible for absorbing more carbohydrates and proteins to enter the bloodstream. However, the absorption of vitamin B12 and bile salts as well as any remaining nutrients are done by the ileum which is the distal and last part of the small intestine (Fig. 14). The waste products are moved in the ileocaecal valve through the peristaltic waves to the large intestine. The small intestine is the longest part of the intestinal tract as it is around 6 m in length. 

The large intestine is split into three main portions. It starts with the caecum which acts as a holding bay between the small intestine and the remainder of the large intestine (Fig. 15). The colon is the middle portion of the large intestine, and it has four parts, the ascending, transverse, descending and sigmoid colon (Fig. 15), which extract salts, residual vitamins, minerals and water from the remainder of the ingested materials. The waste product is mixed with mucus and bacteria forming  faeces, which are pushed through the retraction of the walls of the sigmoid colon into the rectum (Fig. 15) where it is stored. This intestine is large as it is 4 cm in width. However, its length is only 1 meter. 

The anus is the digestive organ, which is responsible for the process of defecation (Fig. 16). It is the sphincter, which ends the gastrointestinal tract. Through relaxation, it allows the stool stored in the rectum to be released to the external environment. 

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The liver is one of the accessories of the digestive system (Fig 17). The falciform ligament divides the liver into large right lobe and small left lobe (Fig. 18). The liver is a vital organ in human body because of its role in regulating metabolism, filtering blood before being transported around the body, removing toxins, and processing nutrients into more manageable products. Also, it is responsible for releasing the bile used to decompose fats in the duodenum (Fig. 19). In addition to its participation in making cholesterol and other fats, it works as a store for vitamin B12, iron and copper. Beside the small intestine, glucose is produced by the liver, storing it as glycogen, which is required during exercise. The glycogen is used in the muscles as energy, as it is turned into glucose and carried to the muscles via the bloodstream. The liver is sited on the upper right-hand side of the abdominal cavity in which it represents the largest visceral structure, while it is the second largest organ in the body after the skin (Fig 17). 

From the posteroinferior side, the liver is surrounded by the esophagus, the stomach, the duodenum, the right colic flexure, right kidney, suprarenal gland, and the gallbladder. From the anterior side, it is framed by the diaphragm, right and left costal margins, lower margins of both lungs and anterior abdominal wall at subcostal angle. The liver is partially surrounded by peritoneum, and it is completely covered by fibrous capsule (Fig 17).

The gallbladder is located in the upper right part of your tummy taking a pear shape (Fig. 20). It is linked, through the common bile duct and the hepatic duct, to the liver and duodenum (Fig. 19). The liver releases the bile which facilitates the task of digesting fats and neutralizing some acids by the small intestine. The gallbladder functions to store this bile making it more concentrated. 

The pancreas as an organ and a gland has two main functions: it acts as an exocrine and endocrine gland releasing digestive enzymes and insulin, respectively. While the digestive enzymes work to break down the proteins, fats, and carbohydrates and to neutralize the stomach acids, insulin, produced by a small clump of cells in the pancreas, it also organizes the levels of sugars in the blood. In addition, glucagon is produced by the pancreas, and it participates in the blood sugar regulation. The pancreas, sited behind the stomach, is split into head, neck, body and tail (Fig. 21). It is attached to the duodenum via the pancreatic duct (Fig. 22).  

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The digestion of food includes mechanical and chemical processes, and both of them start together once the food has entered the mouth. The mechanical process is achieved through chewing which breaks the food physically down into smaller pieces. The mouth is normally moistened by saliva, which is continuously secreted. The chemical process starts when bigger amounts of saliva is released as the food is placed into the mouth. At that moment, both food and saliva are mixed together, and consequently the chemical digestion of starch begins as it is separate into maltose via salivary amylase. It stands behind the sweety taste of a piece of bread, chewed for a few minutes before swallowing, as the sugars are already produced. 

The saliva is released as a result of physical or emotional stimuli. Any physical material put in the mouth can stimulate the execration of saliva. Also, salivation can be spurred through emotional factors such as thinking of a desired food. Whether the stimulus is physical or emotional, these reflexes are achieved by parasympathetic fibers in cranial nerves VII and IX. The process of digestion starts in the mouth. Then the food moves through pharynx and esophagus, which act as pass ways only with no digestive function, to the stomach, the next processing site.

Once the food reaches the stomach, the pH raises, and the stomach cells are stimulated to execrate the hormone gastrin, which in turn spurs the gastric glands to release more of the protein- digesting enzymes, hydrochloric acid and mucus. However, gastric juices are released prior to the arrival of food to the stomach when the senses of smell, sight, and taste work to induce parasympathetic nervous system reflexes to raise via the gastric glands the secretion of gastric juice. The quantity of gastric juice released daily is estimated with 2 to 3 liters under normal conditions (Fig. 23). 

The protein-digesting enzymes include pepsin that is produced as a result of the activation of pepsinogen through the extremely acidic environment provided by the hydro-chloric acid. Also, rennin, released by the stomach and acting to change the milk protein into a sour milk-liked substance is the second protein-digesting enzyme. It is released in infants in big amounts. In addition to protein digestion, little chemical digestion takes place in the stomach (Fig. 23). 

Chyme is not altogether digested in the stomach. The digestion of both carbohydrate and protein starts in the small intestine, but virtually no fats or nucleic acids have been digested up to this point. The food is digested chemically through the looping twists and coils of the small intestine. The so-called brush border enzymes execrated in the small intestine are responsible for breaking the double sugars into simple sugars and for completing protein digestion. With the completion of digestion taking 3 to 6 hours in this area, the food reaches the end of the small intestine to be absorbed.  The most important secretion of the intestinal gland is the protective mucus. However, the small intestine receives outside execrations such as enzyme-rich pancreatic juice from the pancreas and the bile from the liver, in which the food is deluged (Fig. 23). 

While the enzymes of pancreatic juice participate in the digestion of starch (along with brush border enzymes), and protein, they have full responsibility for fat digestion (as it is unique for being the only organ having lipases) and nucleic acids digestion. In addition to enzymes, pancreatic juice is rich with bicarbonate ions with pH 8 capable of neutralizing the acidic chyme sent from the stomach and providing the proper environment for activation and activity of intestinal and pancreatic digestive enzymes. But how do the pancreatic juice and bile are stimulated? It is the job of the vague nerve and hormones. The mucus cells in the small intestine are spurred by the chyme coming in from the stomach, and consequently several hormones are released. Secretin and cholecystokinin (CCK) are two of these hormones affecting the production of pancreatic juice and bile. However, the bile is not considered as an enzyme. Large fat globules are chemically broken down into thousands of tiny ones by the bile. It allows the pancreatic lipases to find a much greater surface to work on. In addition, the bile works as a detergent to emulsify, and it has a vital role in the absorption of fats and the fat-soluble vitamins (K, D, E, and A) absorbed from the intestinal tract (Fig. 24).

The large intestine receives few nutrients which stay in this area from 12 to 24 hours. Some vitamins (K&B), ions and most of the remaining water are the only substances absorbed in the large intestine. The “resident” bacteria in the colonel lumen are responsible for metabolizing the remaining nutrients, and it is due to the non-existence of digestive enzymes in the colon. As a result, gases, which increase with eating certain carbohydrate-rich foods and cause the odor of feces and flatulence, are released with an average about 500 ml daily. Some vitamins such as K and B are made by the resident bacteria in the large intestine. The large intestine sends the undigested food residue, millions of bacteria, mucus and enough water to the rectum. These things compose the feces, and they move in the colon through haustral contractions which occur for 1 minute every 30 minutes (Fig. 23). 

The distension which takes place for being the haustrum filled with food residue, spurs the contraction of the haustral muscles and so the contents are pushed to the next haustrum. Then strong contractile waves happen along the colon three or four times daily, forcing the contents toward the rectum, the walls of which are stretched causing the defecation reflex. It is a spinal reflex, leading to the contraction of the walls of the sigmoid colon and the rectum, and the relaxation of the anal sphincters. Messages sent to the brain giving us time to decide whether defecation will continue, or the passage of feces will stop through closing the external voluntary sphincter. If the texture of the feces is not smooth enough, a temporary delay in the bowel movement can happen. When the reflex contractions end, the relaxation of the rectal walls is resumed within few seconds, allowing the defecation reflex to start again. When the stomach and small intestine are filled with food, such contractions in the large intestine take place during or after eating (Fig. 23). 

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The mouth including the cheeks, lips gums, palate and tongue is lined with a mucous membrane. Stomatitis happens with the inflammation of the lining. Mouth breathing, cheek biting, poor oral hygiene, burns from hot foods and drinks stand behind this inflammation (Fig. 25).

It is a medical term that describes the swelling of one or both parotid glands. There are two types of parotitis: bacterial and viral. While paramyxovirus causes the viral parotitis leading to a severe swelling of the parotid glands, radiation therapy or immunodeficiency cause the bacterial infection leading to the acute bacterial parotitis. Also, chronic parotitis is defined as recurrent bouts of infection in patients with a blocked or narrowed salivary duct (Fig. 26).

Barrett’s Esophagus means that some cells in the tissue lining the esophagus have started to change and damaged by acid reflux. It is a pre-malignant condition. The Barrett’s lining always extends from the bottom of the esophagus towards the mouth. It is commonly found in people with gastro esophageal reflux disease (GORD). It can develop to adenocarcinoma of the esophagus (Fig. 27).

When dilated or enlarged veins appear in the upper part of the stomach or the lower part of the esophagus, this condition is called esophageal varices. Patients with liver diseases such as hepatitis are more susceptible to this condition due to the increased venous pressure. Esophageal varices are life threatening as rupture of veins with extreme bleeding are likely to occur (Fig. 28).

It is a disease taking place with the formation of malignant cells in the lining of the stomach. It is also called stomach cancer. Helicobacter pylori (H. pylori) infection, excessive consumption of alcohol and cigarette smoking are the risk factors. Also, excessive intake of foods and beverages containing nitrates and nitrites as in pickled vegetables, salted and smoked fish and meats helps in developing stomach cancer. Dysphagia, anorexia, indigestion, nausea, bloating and haematemesis are among the symptoms. Gastric carcinoma can be treated through surgery, radiotherapy and chemotherapy (Fig. 29).

When the mucous lining of the stomach is abnormally inflamed, this condition is called gastritis. Nonsteroidal anti-inflammatory drugs (NSAIDs) and prolonged use of alcohol are among the causes of gastritis. However, the bacterial infection of Helicobacter pylori is the most common cause, which must be rapidly treated as it may develop to gastric ulcer disease or cancer.  Dyspepsia, vomiting or nausea may indicate the occurrence of gastritis (Fig. 30).

Gastric and Duodenal Ulcers or peptic ulcers refer to erosions in the lining of the stomach, esophagus or intestinal tract. Different factors can lead to the occurrence of and worsening Gastric and Duodenal Ulcers:  the bacterium Helicobacter pylori (H. pylori), long term use of NSAIDs, such as ibuprofen and aspirin stress and diet. A burning pain in the stomach and duodenum is the most common symptom, which can be treated via a combination of antibiotics and acid-reducing medication (Fig. 31).

It is a common condition occurring when the cardiac sphincter between the stomach and oesophagus is weak allowing the acids from the stomach to flow back to the oesophagus. It is a form of chronic heartburn, which causes a severe burning pain in the oesophagus, and may develop to esophagitis or ulceration (Fig. 32).

When the healthy liver tissue is replaced by scar tissue, it is called cirrhosis. It is a chronic disease in which the blood flow in the liver is blocked, which badly affects the performance of the hepatic functions and leads to the deterioration of the liver.  Cirrhosis mainly takes place as a result of chronic hepatitis B and C, and excessive alcohol consumption as well as fatty liver disease. Treatment includes medications, nutrition therapy, and avoidance of alcohol and other drugs. The aim of the treatment is not the cure, but it is applied to treat causes of the diseases and specific complications in addition to preventing the disease from progressing. If the cirrhosis progresses and the liver fails, a liver transplant may be required (Fig. 33).

The liver as a vital organ in human body. It fights infections, filters blood and processes nutrients. When the cells in the liver are damaged, the hepatic inflammation occurs, and it is called hepatitis. Five viruses can cause different types of hepatitis: Hepatitis A &E which are transferred through ingestion of contaminated food or water, Hepatitis B & D which are transmitted through body fluids (blood, semen), and Hepatitis C which are transferred via blood and blood products. They have the same symptoms. Patients with hepatitis suffer from dark urine, clay-colored faeces, hepatomegaly, jaundice, abnormal liver function tests and generalized malaise. Hepatitis can develop cirrhosis or cancer of the liver (Fig. 34).   

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  a. Fructose

  b. Glycogen 

  c. Lactose

  d. Galactose

  a. Jejunum 

  b. Ileum

  c. Duodenum

  d. Caecum

  a. Carbohydrates

  b. Proteins

  c. Lipids

  d. Vitamins

  a. Secretin 

  b. Insulin 

  c. Pancreatic juice

  d. The bile

  a. Gastric ulcers

  b. Barrett’s esophagus

  c. Cirrhosis 

  d. Stomatitis