An overview of organs of the digestive system

Digestion is a complex process and the objective of the entire process is to provide energy for sustenance of the human body. The human digestive system consists of the digestive tract and other organs that are necessary for the digestion of food, absorption of nutrients, and elimination of waste.

The digestive tract consists of the following:

Mouth

The gastrointestinal tract is a long tube that begins at the mouth. Digestion also begins in the mouth. Morsels of food are chewed and ground into smaller pieces or a paste. The scientific word for chewing is mastication. The first digestive juice that helps break down food is saliva. Saliva is a watery liquid that is released in the mouth which moistens the food so that it becomes easier to swallow. Saliva also contains digestive enzymes that initiate the process of carbohydrate digestion.

The lump of chewed food is also termed a bolus. This lump of chewed food is carried and pushed into the esophagus from where it travels to the stomach.

Oral cancer, halitosis, tooth abscesses, gingivitis are some of the conditions that can affect the mouth.

Esophagus

The esophagus, sometimes called the gullet, is a tube that runs from the mouth to the stomach. Food from the mouth enters the esophagus via the pharynx. The pharynx is unique because it is a part of the digestive system, respiratory system, and essential for creating sound. The length of the esophagus is approximately 10 inches and it has an opening at each end – upper esophageal sphincter and lower esophageal sphincter.

Esophageal disease includes GERD, cancer, esophagitis, and achalasia.

Stomach

The stomach is a muscular, sac-like organ that receives chewed food from the esophagus. The stomach also acts as a storage space for food while the intestines are full and busy executing their functions. The process of chemical breakdown of food begins in the stomach.

The stomach releases enzymes to digest proteins and hydrochloric acid to kill bacteria and provide an acidic environment to aid digestion. The partially digested food that passes from the stomach into the duodenal region of the small intestine is termed chyme.

The churning action of the strong stomach muscles works alongside the action of enzymes to convert bolus into the paste-like chime. The four regions of the stomach, namely cardia, fundus, body, and pyloric region perform different gastric functions. The cardia receives contents from the esophagus. The gases produced during the chemical digestion of food are stored in the fundus. The chemical action of the enzymes and churning of the bolus happens in the body. The pylorus connects the stomach to the small intestine. The stomach is also the site for the absorption of iron and fat-soluble substances.

Diseases of the stomach include gastritis, constipation, and various cancers. Crohn's disease, although typically associated with the colon, may also affect the stomach.

Small Intestine

The small intestine is the longest segment of the gastrointestinal tract. The small intestine opens into the large intestine. It is named thus because its diameter is smaller than that of the large intestine. It receives chyme from the stomach.

Most of the digestion and absorption takes place in the small intestine. The digestion of chyme occurs with the help of bile, pancreatic juice, and enzymes present in the intestines.

Protein is broken into amino acids, carbohydrates are broken down to release sugars, and fats are digested into fatty acids and glycerol. Chyme typically takes around four to five hours to traverse from one end of the small intestine to the other.

The three structural sections of the small intestine, and the order in which partially digested food travels through them, are the duodenum, jejunum, and ileum.

Bile from the gallbladder and pancreatic juice are mixed with the chyme in the horseshoe-shaped duodenum. These help initiate fat digestion. The digestion process is almost completed in the jejunum where the pancreatic juices and enzymes from within the jejunum chemically break down the food to the fullest extent possible.

After food breakdown has been largely completed in the jejunum, the food reaches the ileum. The ileum is where most of the absorption takes place. The unabsorbed matter is passed into the large intestine via the ileocecal valve.

Disorders of the small intestine include Crohn's disease, infections, cancers, etc.

Large Intestine

The large intestine can be semantically divided many ways. The most common description includes the right and left colon. The cecum is the first part of the large intestine that digested food reaches. The cecum is part of the right colon and is where the appendix attaches. The chyme that reaches the cecum is mostly food that cannot be absorbed by the body and therefore can be categorized as undigested waste.

This material enters the cecum from the ileocecal valve. There is no breakdown of the food matter at this stage; it usually takes approximately 16 hours for the processes of the large intestine to be fully completed. Water, electrolytes, and Vitamin B12 are absorbed in the colon. The watery matter that enters the large intestine ends up as less watery fecal matter where it is stored and later expelled via the rectum.

It is worth noting that the large intestine is home to more than 700 species of bacteria. Bacteria in the large intestine generate Vitamin K which is important for blood coagulation and also biotin. Bacterial fermentation of undigested matter also results in the generation of flatus or gas. Colitis and colon cancer are common conditions to affect this region of the digestive system.

Rectum

The rectum receives stool from the large intestine and opens into the anus. It is a site for temporary storage of excrement and gas. Nervous system receptors are stimulated when the rectum stretches and the brain takes a call if evacuation can be initiated. If the contents of the rectum can be emptied, then the rectum contracts and the sphincters relax allowing the stool to pass.

If defecation does not occur; feces may re-enter the large intestine; here more water is absorbed, stool becomes hard and difficult to pass out of the system. This can lead to constipation.

Anus

The anus is the exit point of the digestive tract. The function of this organ is to control the release of feces from the body, which it does with the help of internal and external anal sphincters. Feces consist of indigestible matter, dead bacteria from the intestines, and digested food that has had almost all the nutrients extracted.

The upper portion of the anus is sensitive to the nature of content entering from the rectum. The sphincters respond differently to gaseous, liquid, and solid matter. Anal cancer, warts, fissures, hemorrhoids are common medical conditions affecting the anus.

Other organs that are a part of the digestive system:

Tongue

The tongue guides the food around in the mouth molding it while the teeth break down the food. It also crushes the food by pressing it against the palate. It helps to mix saliva with the food thereby making food chewable and easier to digest.

The end result is the bolus which is pushed to the back of the mouth by the tongue to be swallowed. By sensing the texture, taste, and temperature of the food, the tongue regulates the process of swallowing.

Salivary Glands

Saliva is a very important digestive aid. It is produced in the mouth by the major and minor salivary glands. The composition of saliva is mainly water with some amount of digestive enzymes – salivary amylase, lingual lipase - and salts.

Amylase breaks down carbohydrates and releases the sugars maltose and dextrin. Lingual lipase breaks down fat. The three major salivary glands are the parotid, submandibular, and the sublingual.

Pancreas

Along with the liver and the gallbladder, the pancreas lies outside the long tube of the gastrointestinal tract; yet it is considered an important organ for digestion. The exocrine glands of the pancreas release pancreatic juice into the small intestine. The juice contains digestive enzymes such as trypsinogen, elastase, pancreatic lipase, and amylase.

These enzymes are required for the breakdown of protein, carbohydrates, and fats. The pancreas releases a substance that helps neutralize stomach acidity. It also secretes insulin directly into the bloodstream; insulin is crucial for sugar metabolism. Conditions affecting the pancreas include pancreatitis, pancreatic cancers, and diabetic mellitus type I.

Liver

The liver is the largest internal organ and also the largest gland in the human body. It plays a very important role in metabolizing the nutrients released in the small intestine. Bile is produced in the liver and gets stored in the gallbladder from where it is released for lipid digestion in the small intestine.

Carbohydrate and fat metabolism occurs in the liver. During these processes glucose and triglycerides are respectively produced. The liver is the only organ of the human body that is capable of regeneration.

It stores iron, Vitamin B12 and copper.< Diseases of the liver include hepatitis, cancer, fatty liver, and cirrhosis.

Gall Bladder

Bile produced by the liver is stored in the gallbladder. This pear-shaped organ is connected to the liver and the duodenum. Bile stored in the gallbladder gets more concentrated and thus more powerful and effective. It is released into the duodenum to breakdown fatty foods. Symptomatic gallstones which can cause either pain known as biliary colic or infection known as cholecystitis are common conditions that may require gallbladder removal or cholecystectomy.