The components of body’s fuelling system can be grouped into two categories, namely, primary organs and accessory organs, where the former are those components that form the basic structure of elementary canal, like mouth, esophagus, stomach, small intestine, large intestine, rectum and anus. The accessory or associated organs, on the other hand, are the secondary structures of gastrointestinal system that include gallbladder, liver, pancreas, tongue, salivary glands (in mouth), and so on. Here follows a bird-eye view of the parts of both these broad categories with their particular role in the whole biochemical procedure of food processing.

Primary Organs Of Digestive System

Mouth

Being the foremost of the parts of digestive system, it is located at the anterior end of alimentary canal and is involved in the mechanical breakdown of meal by a process, called mastication. For this purpose, there are two sets of teeth, one along the upper mandible and the other on the lower jawbone that occur in pairs (meeting end-to-end with each other). The arrangement of teeth from front to back is described as incisors, canines, premolars and molars with each group facing their similar counterparts on the opposite jawbone.

Here morsel is chewed and ground into very small particles thus increasing their surface area so that enzymes can act on them more efficiently. Here food also gets wetted and forms a soft mass called bolus which can easily pass through the next segment (i.e. esophagus or food pipe). The next on the line of major digestive system parts is esophagus.

Esophagus

Next to mouth, there comes esophageal portion of systema alimentarium that plays and important digestive system function. It assists in the transport of food, in the form of bolus, from mouth to stomach. As measured in cadaver, the length of esophagus in a human adult is about one foot or, more precisely, ranges from 25 to 30 cm. Delivery of bolus occurs with the help of esophageal peristalsis, which is a wave-like motion of gastrointestinal tract with coordinated contractions and relaxations of circular and longitudinal muscles of the digestive tract.

There is a valve at the lower end of esophagus, which is called GE junction, Gestro-Intestinal Junction or esophageal sphincter. However, according to some sources, it is just a structure (and not a valve). After crossing this junction, the bolus of food travelling through esophagus enters stomach for further digestion.

Stomach

Initiating and completing the second phase of digestion, it is a hollow and dilated organ enclosed by a muscular wall. This elastic pouch is responsible not only for acidic, hormonal and enzymatic secretions, but also protects the contents of stomach from getting out of it and harming other internal organs of the body. The wall of stomach is four-layers thick, namely, mucosa, sub-mucosa, muscularis externa and serosa where mucosa forms the innermost lining while serosa forms an outer most protective covering.

Hydrochloric acid present in gastric juice is very strong that not only kills the germs but also provides an ideal pH environment for the action of protein digesting enzymes, like pepsin. Churning of masticated and wetted food boluses is achieved through the coordinated contractions of stomach muscles resulting in a wave like motion, peristalsis, the same that was witnessed in esophagus.

The food here is converted into chyme, a mass of partly-digested food material, that is then emptied into duodenum of small intestine through the lower most section of the organ called pylorus or pyloric sphincter. There are different digestive system diseases that affect stomach.

Small Intestine (Small Bowel)

Ranging in length from 15 to 32 feet in total, it consists of three continuous hollow tubular parts, namely, duodenum, jejunum and ileum. These parts of digestive system play the most important role both in digestion and absorption of food.

Duodenum

Inserted between the pylorus of stomach and jejunum, duodenum is the smallest part of small intestine, measuring about 10 to 15 inches in length, and it is the place where most of the chemical and iron absorption takes place. Based on its curvature, structure and relative positioning, small bowel is further divided into four segments, namely, superior, descending, inferior (or horizontal) and ascending.

Jejunum

Next to duodenum, there comes second part of small intestine, jejunum, that measures about 8.5 feet in its average length, but there may be present some exceptional cases. The medium inside it varies from neutral to slightly alkaline with pH ranging from 7 to 9, and the longitudinal and circular smooth muscles present in it assist in the movement of food through peristalsis. The surface area for the absorption of foodstuff is increased in jejunum with the presence of finger-like projections of mucosal lining, generally known as villi with microvilli of epithelial cells that line these hair-like outgrowths.

Ileum

The third and last part of the small intestine is named as ileum which ranges in length from 7 to 13.5 feet and is separated from secum by ICV (IleoCecal Valve). As it is the last part of small bowel, remainder of the absorption takes place here that includes bile salts, vitamin B12 and left-over nutrients in the digested and undigested food material passing through it.

Large Intestine

Joined to the lower end of small intestine (through ileum), it is the last of the digestive system parts that measures about one-fifth of the entire length of the tube, and is equal to approximately 5 feet. Also known as colon or bowel, its cecum is separated from ileum by IleoCecal Valve (ICV) and, running through rectum, it ends at anal canal. The location of bowel’s parts can be described in terms of ascending colon, right colic flexure, transverse colon, transverse meso-colon, left colic flexure, descending colon and, finally, sigmoid colon.

Before the indigestible and waste matter is passed on to rectum, bowel is kept about fifteen hours for the absorption of watery fluid and any other left-over nutrients. Large bowel is also responsible for the absorption of vitamins which are synthesized by bacteria present in the colon, namely, vitamin B12 (cyanocobalamin), vitamin B2 (riboflavin), vitamin B1 (thiamine) and vitamin K (a fat-soluble oily substance). Attached to the inferior surface of cecum, there exists a finger-shaped out-growth resembling a blind-ended, hollow tube, called appendix, which has been found to play a significant role in immunity against disease causing agents.

Anus

The last of the digestive system parts, it is located at the extreme posterior end of alimentary canal resembles a ring-shaped opening and assists in the excretion of indigestible and waste matter out of the body through a process, called defecation. Two sphincters, namely, internal anal sphincter, and external anal sphincter, are located in the anal canal, just behind anus, which are under the voluntary control of humans and regulate the expulsion of feces out of the body.

Accessory Organs of Digestive System

Tongue

Involved in the even manipulation of food during mastication and regarded as the primary organ of gustation (or taste), it is a muscular hydrostatic organ placed at the bottom of mouth in humans and many other animals. There is a rich supply of blood vessels and nerves from the nervous system. The upper surface is covered with taste buds and papillae which are used for the selection of food through taste. It is not only a natural means of cleaning teeth, but also serves the purpose of phonetic articulation for the production of speech sounds in human beings.

Salivary Glands

The ducted or exocrine glands, in humans, are divided into three major types of parotid, submandibular and sublingual along with hundreds of others that are present in the oral cavity. Secretions produced by parotid glands (the largest of salivary glands) are drained into your mouth by salivary ducts present near the upper teeth. The submandibular and sublingual glands, as the name indicates, are located beneath the lower jaw and beneath the tongue, respectively. Saliva thus secreted by these glands is a mixture of water (about 99.5 %) and other substances, namely, enzymes, antibacterial compounds, mucus, glycoproteins and electrolytes that collectively make up the remaining 0.5 %. It is, actually, due to the secretions of these exocrine organs that the first phase of digestion takes place in the oral cavity, for example, amylase breaking down starch into maltose.

Pancreas

Playing a significant role both in digestive as well as endocrine system of an individual, the glandular structure is present in the left hypochondrium of abdominal cavity. On the basis of physical structure, it can be divided into four prominent parts, namely, head, uncinate process, neck, body (lying behind stomach) and tail (left end). Being a part of endocrine system, it synthesizes important hormones, namely, glucagon, insulin, and somatostatin, and as a digestive organ, it secretes digestive enzymes that assist the digestion and absorption food and nutrients in small intestine.

Liver

Weighing up to 1.7 kilogram in weight, it is not only the largest gland, but also the largest internal and the second largest organ of the human body, while the first one being skin that lies externally. Just below the diaphragm, the pinkish-brown, soft, triangular structure lies on the right side of stomach and above gallbladder. Two large blood vessels, namely, portal vein and hepatic artery ensure a rich supply of circulatory system fluids for the overall performance of a wide range of activities assigned to this vital part of the body, for example, detoxification, synthesis of plasma protein, production and excretion of hormones and other biochemical substances of the digestive system. Bile juice, produced by liver, and stored & concentrated in gallbladder, is an alkaline fluid containing bile salts, mucus, inorganic salts, fats, pigments and about 85% of watery content.

Gallbladder

Measuring about 8x4 centimeters in adult humans, the hollow system of biliary vesicle lies just below liver and is connected with the later through hepatic duct to receive hepatic juice containing pigments, salts, mucus and water. For the sake of convenience, the cholecyst (an alternative terms for biliary vesicle) can be divided into fundus, body and neck region. The bile juice, after substantial increase in its intensity through concentration, is emptied into the duodenum (first section of small bowel) for the breakdown of the lipid contents of chyme (semi-digested food) passing through it.

Do you know one of the important digestive system facts that the biliary vesicle is not as important as liver or other vital organs of the body because its surgical removal does not leave any adverse effects on the digestive system as, under such conditions, bile is directly poured into duodenum.