Anatomy of the Digestive System

The digestive system is an essential to the human body. It helps in break down of the foods we eat into smaller components that can be absorbed by the body for its nourishment. This process is known as digestion. The process of digestion involves grinding of the food, foods movement through the digestive tract, breaking down of larger molecules, absorption of nutrients into the blood and removing waste from the body.

The digestive system is made up of the digestive tract including the esophagus, stomach, small intestine and large intestine along with accessory organs like the liver, gallbladder, pancreas, etc. The digestive tract of a normal adult human being is about 30 feet long.

Mouth

Though the esophagus is the first part of the digestive tract, the process of digestion begins with the mouth where our teeth (premolars and molars) chew (masticate) the food and the food is mixed with enzymes in the saliva secreted by the salivary glands located below the tongue, near the lower jaw. The food is then a soft mass that is easily swallowed and later digested. The tongue and mouth push the soft food, now called a bolus, to the back of the mouth where it is swallowed. The epiglottis, a flap-like covering over the trachea (the windpipe), closes automatically when the bolus enters the esophagus to make sure that the food bolus doesn’t get into the windpipe (trachea) causing choking.

Esophagus

The bolus passes through a long muscular tube called the esophagus. The esophagus is about 10 inches long and connects the throat and the stomach. The process by which the food or bolus is pushed through the esophagus and into the stomach is known as peristalsis, that is, wavelike contractions in the muscle. At the junction of the esophagus and the stomach there is a ring like muscle known as the cardiac sphincter. This allows the food to pass into the stomach but does not permit the food to go back into the esophagus.

Stomach

The connection between the esophagus and the small intestine is a sac-like pear-shaped muscular bag called the stomach. The stomach is approximately 12 inches long and 6 inches broad at the broadest point. However, due to its elastic nature, its size and shape can change depending on the food content. The stomach is made up of five layers. The innermost layer is the mucosa (which produces the acids and digestive juices). The next layer is the submucosa covered by muscularis (which moves and helps in the mixing of the food). Then comes the two layers of covering called subserosa and serosa (the outermost layer).

The food from the esophagus passes into the stomach where it is mixed and churned with the gastric juices (enzymes and acids secreted by the mucosa) and converted into a semifluid state (called chyme) which then passes into the small intestine. However, substances like water and alcohol are absorbed directly from the stomach. A ring-like musculature called the pyloric sphincter separates the stomach from the small intestine.

Small Intestine

The small intestine is divided into three parts, namely, the duodenum, jejunum and ileum.

Duodenum

It is a 10-inch long C-shaped tube found around the head of the pancreas which forms the first part of the small intestine. The food, now converted to chyme, enters from the stomach into the duodenum where it is mixed together with the bile and other digestive juices produced by the accessory digestive organs and drained into the duodenum. Absorption of food also begins here with the absorption of vitamins, minerals and other nutrients. In particular, before the food passes into the next part of the small intestine, that is the jejunum, iron and calcium contained in the food is absorbed here. The remaining of the food is passed into the jejunum.

Jejunum

This second part or the midsection of the small intestine is a coiled tube which is thicker and vascular than the ileum. It lies in the umbilical region of the abdomen. There are small fingerlike projections in the jejunal wall called villi. These villi are covered with smaller projections called microvilli. These increase the surface area and thus aids in the greater absorption of nutrients in this part of the small intestine. Thus majority of absorption of food is done in this portion of the digestive tract. The glucose and amino acids produced in the food is passed from here into the blood stream while the fat is passed into the lymph capillaries. The remainder of food passes into the ileum.

Ileum

The last portion of the small intestine occupies mainly the pelvic region. It looks very similar to the jejunum. Also there is no specific demarcation between the jejunum and the ileum. However, the nature of the intestine gradually changes. It is thinner and less vascular as compared to the jejunum. The final absorption of nutrients from the food takes place here. The terminal ileum is an important part as this is the portion where vitamin B12 is absorbed into the blood capillaries. The unabsorbed and undigested food then passes from the ileum into the cecum, which is the beginning of the large intestine. This residue food is full of bacteria.

Large Intestine

The large intestine forms the last portion of the digestive tract, which is about 5 feet long. The large intestine can be further divided into cecum, colon and rectum. The food passes from the small intestine into the cecum which then passes into the colon (further divided into ascending colon, transverse colon, descending colon and sigmoid colon) where the fluids and salts are absorbed. After absorption, the remaining feces (undigested matter) pass into the rectum, where it is stored until it is passed out through the anus as bowel movement. The anus has voluntary and involuntary sphincter muscles which help to differentiate between gas and solid contents. A vestigial organ, the appendix, is found attached to the large intestine at the cecum. Though this organ is potentially of no use, yet it can cause pains and complications once it gets inflamed, a disorder called appendicitis.

Accessory Digestive Organs

Though not directly part of the digestive tract, the accessory digestive organs play a major role in the digestion process. The accessory digestive organs include the salivary glands, pancreas, liver and gallbladder.

Salivary Glands

There are three pairs of salivary glands, namely, parotid glands (the largest of the salivary glands is located one in each cheek between the ear and the lower jaw), submandibular glands (also called submaxillary glands located on the floor of the mouth) and sublingual glands (located in front of the submandibular glands under the tongue). All three pairs of glands secrete saliva, which is a mixture of mucus and serous fluids that contains enzymes which is necessary to moisten and lubricate the food during mastication and ingestion. It also aids in the breaking down of the starch content of the food.

Pancreas

Located behind and under the stomach, the pancreas acts both as an endocrine gland and an exocrine gland. From the exocrine part it secretes pancreatic juices containing enzymes which pass through the pancreatic duct into the small intestine (the duodenum). These enzymes aid in the further breakdown of the food, mainly the carbohydrate, protein and lipid part of the food. From the endocrine part it secretes insulin and glucagon.

Liver

The liver is the largest organ of the human body located below the diaphragm in the upper epigastric region of the abdomen. It has many functions including production of chemicals necessary for digestion, synthesis of protein and detoxification. The major function of the liver is to produce bile (yellowish-green fluid) which aids in the digestion and absorption of fats. It also stores substances like glucose, iron and vitamins A, B12, D etc.

Gallbladder

The gallbladder is a small organ located just below the liver. Its main function is to store the bile produced by the liver and release it into the duodenum when food containing fat needs to be broken down and absorbed. The bile contained in the gallbladder becomes more concentrated and thus is more effective in burning down the fat.

The teeth and tongue also aid in the digestion process and thus are very much a part of the digestive system.

Digestive System Problems and Diseases

Digestive system disorders can range from common digestive diseases to inflammatory bowel diseases, irritable bowel syndrome, lactose intolerance, ulcers or even cancers of the stomach, colon and/or rectum.

Diarrhea

The condition of watery stools at frequent intervals is called diarrhea. It is a very common problem and most often it resolves on its own. It may be caused by bacteria or virus, which is treated by antibiotics, or can be an intestinal/functional disorder, which needs specific treatment by a physician. Dehydration is a major side effect of this problem and thus the fluid lost should be replaced with constant intake of saline water.

Diverticular Disease

In some people, especially the elderly, the colon begins to have sac like protrusions called diverticula (singular diverticulum). This condition is known as diverticular disease. It is generally caused due to constipation where there is increased pressure to pass stool that is too hard. The pressure causes weak parts of the colon to bulge and thus cause diverticula. This condition occurs in almost 15% of people; however, this rarely causes any symptoms or complications. In cases where the diverticula get infected, a condition called diverticulitis; it needs thorough treatment by a physician or healthcare provider. The abdominal pains caused by diverticulitis may become very severe and may require hospitalization. Rare cases may require surgery to rectify the situation.

Heartburn

Heartburn or GERD (gastroesophageal reflux disease) is a condition where the gastric juices and/or food and fluid from the stomach flow back into the esophagus. This can be caused either by overeating or eating certain foods like citrus or fatty and spicy foods, or even can be a result of some serious underlying medical conditions like hiatal hernia. Hiatal hernia is a condition where the stomach pushes up into the chest via some opening in the diaphragm. In most cases, heartburn is relieved by over-the-counter antacid and/or diet and lifestyle modifications as recommended by the physician. However, heartburn may mimic more serious underlying conditions like heart diseases. In such cases, the chest pains are accompanied by sweating, light-headedness and/or nausea and over-the-counter antacids do not seem to help. In such a condition, immediate medical care should be sought.

Gas in the Digestive Tract

Gas goes into the digestive tract either by swallowing or by the breakdown of certain food particularly in the colon area due to the presence of certain bacteria there. This collection of gas can cause bloating, pain and discomfort in the abdomen. Gas is often released either by belching or flatulence. Some situations may require medications to release the gas and/or diet modification to reduce the formation of gas. Aerophagia or air swallowing can be reduced by removing the causes, namely, rapid eating and drinking, chewing gums, smoking and wearing loose dentures.

Hepatitis

The inflammation of the liver mainly due to viral infections is known as hepatitis. There are six types of hepatitis, namely, hepatitis A, hepatitis B, hepatitis C, hepatitis D, hepatitis E and hepatitis G. Hepatitis A is caused by fecal-oral contact that is by ingestion of fecal infected food or water. Hepatitis B is spread from an infected person by exposure to body fluid like saliva, blood, semen or vaginal secretions. It can also be transmitted to a baby born of an infected mother. Hepatitis C is primarily transmitted by contact with infected blood but can also be transmitted via sexual contact or to a baby born of an infected mother. Infections caused due to hepatitis B or hepatitis C are more severe and has a higher mortality rate as compared to hepatitis A. Effective vaccinations are available for hepatitis A and hepatitis B. Hepatitis D occurs to patients already affected with hepatitis B, either simultaneously or later. Similar to hepatitis A, hepatitis E is also caused by fecal-oral contact. However, it is less common than hepatitis A and is only found in poorly developed countries. The latest form of hepatitis is hepatitis G. Though very little is known about this form, it is believed to spread through the blood especially found in IV drug users. It generally shows no clinical symptoms.

Inflammatory bowel diseases

Ulcerative colitis

In this type of inflammatory bowel disease, the inner lining of the colon and the rectum gets inflamed. This is a chronic disease with an unknown cause. It generally does not affect the small intestine, though at times the ileum (the part of the small intestine that joins with the large intestine) may get affected. The symptoms caused in this condition, like diarrhea and cramping are relieved with medication that soothe the inflammation. However, the patient might need hospitalization in order to treat malnutrition and/or loss of blood, fluid and salts. In very few cases, a patient might need to undergo surgery, especially where there is risk of excessive bleeding, perforated colon or risk of cancer.

Crohn’s Disease

This type of inflammatory bowel disease, of unknown cause, affects the deeper layers of the bowel, mainly the terminal ileum, but may extend to the other parts of the digestive tract. Though there are no sure cures for Crohn’s disease, medications are given to lessen the inflammation and supplements are suggested to correct nutritional deficiencies. Surgeries like removal of a section of the bowel, ileostomy, colostomy etc. are done in certain cases which might help though it cannot be completely cured by surgery. There is a possibility that it might affect the area next to the removed section of the bowel.

Irritable Bowel Syndrome

This is a functional disorder of the intestine, mainly the colon. When closely monitored the disease cannot be seen but causes symptoms like pains, bloating, changes in the bowel habits etc. The exact cause of this disorder is unknown but is believed to be caused due to emotional stress and/or improper diet. Physicians generally treat this disorder with diet modifications and medications like antidepressants, laxatives, tranquilizers etc. They may also include a fiber supplement. However, in using these medications the patient may tend to become dependent on laxatives or tranquilizers and it can have a major effect on their lifestyle.

Lactose intolerance

Lactase is an enzyme produced in the small intestine which helps in the breakdown of lactose (found in milk products) into a form that can be easily absorbed by the blood. When there is lack of lactase, the body is unable to digest lactose. This condition is called lactose intolerance. This is generally caused due to injuries to the small intestine. There is no treatment to improve the body’s ability to produce lactase but physicians may suggest additional lactase enzymes. The patient’s lactose intolerance is controlled with diet modifications.

Peptic Ulcers

An open sore or lesion on the skin or mucous membrane is called an ulcer. A peptic ulcer is an ulcer found in the lining of the stomach and duodenum. In particular, the ulcer of the stomach is known as gastric ulcer while that of the duodenum is called duodenal ulcer. Stress and diet were earlier thought to be the cause of this disease, while later it was found that hydrochloric acid and pepsin (the stomach acids) were contributing to this disease. However, recent researches have shown that the primary cause is an infection with a bacterium known as Helicobacter pylori. These ulcers can lead to serious complications like bleeding, perforation or even narrowing and obstruction. These can be treated by lifestyle changes and medications as prescribed by the physician. For patients who do not respond to medications or develop complications, surgeries like vagotomy, antrectomy or pyloroplasty are performed depending on the site of the ulcer.

Cancers

Like any other parts of the body, the digestive tract can also be infected with cancer. The common ones are the stomach cancer and colorectal cancer. What causes these cancers in unknown but it is believed that the cells in the stomach or colon and rectum become cancerous due to the risk factors like diet, tobacco, alcohol, H. pylori in case of stomach cancer and age, diet, polyps, ulcerative colitis, personal/family history in case of colorectal cancer. Treatment includes surgery to remove the cancerous tissues (like gastrectomy or segmental resection of the colon), radiation therapy and chemotherapy.