The digestive system is an essential to the human body. It helps break down the foods we eat into smaller components that can be absorbed by the cells in our body for energy. This process is known as digestion. The process of digestion involves grinding of the food, moving food through the digestive tract, breaking down large molecules, absorption of nutrients into the blood and removing waste from the body. The digestive system is made up of the digestive tract and the digestive organs—a group of organs that each has its own job in the digestive process. The digestive tract is one long tube that starts at your mouth and ends at the anus. In a fully grown adult, the digestive tract is about 23 feet long.
This one long tube of the digestive system—the digestive tract—includes the esophagus, stomach, small intestine and large intestine along with accessory organs like the liver, gallbladder, pancreas, and salivary glands. The digestive tract of a normal adult human being is about 30 feet long. The accessory organs make enzymes and chemicals that help digest food. It takes several days for food to pass all the way through the digestive tract.
The process of digesting foods is done in 2 parts—one part is mechanical the other is chemical. Mechanical digestion is done in the mouth—your teeth break the food down into smaller pieces—and stomach—where the food is churned and broken down to even smaller pieces. Chemical digestion happens all along the digestive tract when enzymes and other chemicals dissolve and break down the food releasing the nutrients in the food.

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 to break it down into smaller pieces. The food is mixed with enzymes in the saliva secreted by the salivary glands located below the tongue, near the lower jaw. The saliva softens the food and allows the food to be compacted into a soft mass that is easily swallowed. Saliva also provides enzymes that start the digestive process by breaking down complex carbohydrates into simple sugars. The break down of proteins and fats does not begin in the mouth.
The tongue and mouth push the soft food, now called a bolus, to the back of the mouth where it is swallowed  down the esophagus and into the stomach. The soft palate, a tissue flap at the back of the mouth, presses upward to keep food from going up into you nose. The epiglottis, a flap-like covering over the trachea (the windpipe), automatically closes when the food bolus enters the esophagus to make sure that the food bolus doesn’t get into the windpipe (trachea) causing choking.

Esophagus

The bolus passes from the mouth through a long muscular tube called the esophagus. The esophagus is about 10 inches long and connects the throat to the stomach. The process by which the food or bolus is pushed through the esophagus and into the stomach is known as peristalsis—wavelike contractions of the muscles in the lining of the esophagus. Where the esophagus meets the stomach there is a ring-like muscle known as the cardiac sphincter. This sphincter muscle opens to let the food pass into the stomach and closes to keep the food from going back into the esophagus.

Stomach

The connection between the esophagus and the small intestine is a sac-like pear-shaped muscular bag with walls of muscles called the stomach. The stomach is about 12 inches long and 6 inches wide at the widest point. However, because of its elastic nature, its size and shape can change depending on the food inside. The stomach is made up of five layers. The innermost layer is the mucosa  which is protected by mucus so it doesn’t digest itself and produces acids and digestive juices. The next layer, the submucosa, is 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 folded lining of the stomach allows it to expand when filled with food.
Enzymes and stomach juices begin the digestion of fats and proteins by separating them into their basic parts of amino acids and fatty acids. Only a small amount of carbohydrate digestion happens in the stomach because the stomach acids are so strong. However, substances like water and alcohol are absorbed directly from the stomach. The stomach takes up to five hours to mix and digest solid food.
The food passed from the esophagus into the stomach is mixed and churned with  gastric juices (mucus, enzymes and acids secreted by the mucosa) and converted into a semifluid state (called chyme) which then passes into the small intestine. Once in the small intestine, carbohydrate digestion starts again and your body starts to absorb the nutrients from the food.
A thick, ring-like smooth muscle, the pyloric sphincter, separates the stomach from the duodenum of the small intestine. This muscle is usually closed, but relaxes and opens to let the acid chyme into the small intestine, then closes to keep the chyme from going back into the stomach.

Small Intestine

The small intestine is divided into three parts—the duodenum, jejunum and ileum. The small intestine is about 20 feet long and 1 inch wide. By putting your flattened palm on your belly button, you are covering most of the small space where the small intestine is coiled up. The small intestine is lined with protective mucus to prevent it from digesting itself. The lining has thousands of tiny folds and projections called villi. There are tinier projects on each villus called microvilli. These folds make a huge area for absorbing food.
The small intestine absorbs and passes along 90% of all the protein, fat and carbohydrates that you eat, along with a small amount of vitamins and minerals. Amino acids, sugars, vitamin C, the B vitamins, iron, calcium and magnesium are carried through the blood stream to your liver where they are processed and sent to the rest of the body. Fatty acid, cholesterol, and vitamins A, D, E, and K go into the lymph system and then into the blood. These also go to the liver, get processed and sent out to other cells in the body.

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 right after the stomach. The food, now 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 iron, calcium and magnesium are absorbed here. The rest of the food is passed into the jejunum.

Jejunum

The second (middle) section of the small intestine is a coiled tube which is thicker and more vascular than the ileum. It lies in the belly button area of the abdomen. There are small fingerlike projections in the wall of the jejunum called villi. These villi are covered with smaller projections called microvilli. The villi increase the surface area of the jejunum and allows much more absorption of nutrients in this part of the small intestine—most of the food absorption is done in this part of the digestive tract. Simple sugars, water soluble vitamins (except vitamin C and some Bs) and amino acids made from the food is passed from the villi into the blood stream while the fat is passed into the lymph capillaries. The rest of the food passes into the ileum.

Ileum

The last part of the small intestine is mainly the pelvic region. It looks very similar to the jejunum. Also there is no specific “line” between the jejunum and the ileum. However, the nature of the small intestine gradually changes. It is thinner and has fewer blood vessels as compared to the jejunum.
The last absorption of nutrients from the food takes place here—amino acids (the end products of protein digestion), fat-soluble vitamins (A, D, E, and K), fatty acids (the end products of fat digestion), cholesterol, sodium, potassium alcohol, and B12. The terminal ileum is an important part as this is where vitamin B12 is absorbed into the blood capillaries. The unabsorbed and undigested food then passes from the ileum into the cecum, the beginning of the large intestine. This food residue is full of bacteria.

Large Intestine

The large intestine forms the last part of the digestive tract, which is about 5 feet long and wider than the small intestine. The surface area on the inside of the large intestine is smaller than the small intestine. The large intestine can be divided into the cecum, colon and rectum. The undigestible food waste 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. The undigested food moves up the ascending colon, across the transverse colon, down the descending colon and into the rectum. The colon soaks up to 50 fluid ounces of water every day.
After absorption, the remaining undigested matter is squeezed into a bundle called feces. Feces is made of fiber, undigested food, cells that slough off the lining of the intestines and bacteria. About 30% of the weight of feces is bacteria. These bacteria are “good” bacteria and billions of them live in your colon all the time. They make vitamin K and B12 which is absorbed by the colon wall, break down amino acids and make nitrogen, live off of fiber which makes gas. These bacteria are harmless as long as they don’t spread to the rest of your body.
When the bacteria finish with the feces, it is passed into the rectum, where it is stored until it is passed out through the anus as a bowel movement. The anus has voluntary and involuntary sphincter muscles which can tell the difference between gas and solid contents.
A vestigial organ (an organ that had a purpose in the past but is now useless or close to it), the appendix, is attached to the large intestine at the cecum. Though this organ is potentially of no use, it can cause pains and complications once it gets inflamed, a disorder called appendicitis.

 Accessory Digestive Organs and Glands

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

Salivary Glands

There are three pairs of salivary glands:

• 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 on the floor of the mouth)
• and sublingual glands (in front of the submandibular glands under the tongue)

All three pairs of glands secrete saliva, a mixture of mucus and serous fluids that have enzymes needed to moisten and lubricate the food during mastication and ingestion. It also helps break down starches in the food.

Pancreas

A carrot-shaped gland 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 enzymes amylase and lipase which pass through the pancreatic duct into the small intestine (the duodenum). The pancreatic duct joins the bile duct. These enzymes aid in the further breakdown of food, mainly the carbohydrate, protein and lipid part of the food. From the endocrine part it secretes insulin and glucagon. Insulin enables you to digest and metabolize carbohydrates. The pancreas also secretes an antacid to help settle an upset stomach.

Liver

The liver is the body’s chemical-processing center. It is the largest organ of the human body and is 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 glucose, iron and vitamins A, B12, D etc. The liver also sends out the nutrients and substances digested from the food to the cells of the body.

Gallbladder

The gallbladder is a small organ located just below the liver. It is about 3 inches long and shaped like a hollow balloon. Its main function is to store bile produced by the liver and release it into the duodenum when food that contains fat needs to be broken down and absorbed. The bile in the gallbladder becomes more concentrated and more effective in breaking down the fat. Gallstones are a common disorder of the gallbladder. Gallstones are formed when there is too much cholesterol in the bile and often need removing the gallbladder. Gallstones can get as big as a golf ball.
The teeth and tongue also aid in digestion and are very much a part of the digestive system.

Digestive System Problems and Diseases

Disorders of the digestive system 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 during a short period is called diarrhea. It is a very common problem and most often gets better on its own. It can be caused by bacteria or a virus or can be an intestinal/functional disorder, which needs specific treatment by a doctor (gastroenterologist). 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 causing 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 doctor. The abdominal pains caused by diverticulitis can be very severe and require hospitalization. Rare cases may require surgery to correct the problem.

Gastroenteritis

Also called stomach flu is a temporary illness caused by a virus which mainly affects the stomach and intestines with diarrhea, nausea and vomiting. Gastroenteritis can be treated by your regular doctor.

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 a 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 your doctor. 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 air or by the breakdown of foods particularly in the colon by bacteria. This collection of gas can cause bloating, pain and discomfort in the abdomen. Gas is often released either by belching (burping) or flatulence (farting). Some situations may need medicines to release the gas and/or a change in diet to cut down the formation of gas. Aerophagia or air swallowing can be reduced by removing the causes, namely, eating and drinking too fast, 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 eating food or drinking water that is infected with feces. Hepatitis B is spread from an infected person by coming in contact with body fluids 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 from hepatitis B or hepatitis C are more severe and have a higher death rate as compared to hepatitis A. Effective vaccinations are available for hepatitis A and hepatitis B. Hepatitis D happens in people affected with hepatitis B. 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 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 be affected. The symptoms caused in this condition, like diarrhea and cramping, are relieved with medicine that soothe the inflammation. However, you can need hospitalization to treat malnutrition and/or the loss of blood, fluid and salts. In very few cases, a patient might need to have surgery, especially where there is risk of excessive bleeding, a perforated colon or a risk of cancer.

Crohn’s Disease

This 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 cure for Crohn’s disease, medicines are given to lessen the inflammation and supplements may correct nutritional deficiencies. Surgeries that remove part 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 risk that surgery 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. Doctors usually treat this disorder with changes in diet and medicine 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 which can have a major effect on their lifestyle.

Lactose intolerance

Lactase is an enzyme produced in the small intestine which help breakdown lactose (found in milk products) into a form that can be easily absorbed by the blood. When there is lack of lactase, the body can’t digest lactose—called lactose intolerance. This is generally caused from injuries to the small intestine. There is no treatment to improve the body’s ability to produce lactase but doctors may suggest more lactase enzymes. Lactose intolerance is controlled with changes in diet.

Peptic Ulcers

An open sore or lesion on the skin or mucous membrane is called an ulcer. A peptic ulcer is an ulcer 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 used to be thought of as the cause of this disease; later it was found that hydrochloric acid and pepsin (the stomach acids) were contributing to this disease. However, recent research has 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 (a hole) or even narrowing and obstruction. These can be treated by lifestyle changes and medications prescribed by your doctor. Patients who do not respond to medications or develop complications may need surgery such as a vagotomy, antrectomy or pyloroplasty performed at the site of the ulcer.

Cancers

Like any other part of the body, the digestive tract can also be infected with cancer. The common ones are stomach cancer and colorectal cancer. What causes these cancers in unknown but 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. Cancers are treated by an oncologist.