Digestive system is one of the major system of human body that helps in digestion and absorption of all nutrients along with removal of unnecessary substances from the body.
Digestion starts from the mouth and ends in the intestine. The organs involved in this process are mouth, esophagus, stomach and intestine. These are some of the major organs, along with this there are some of the accessory organs that are involved in process of digestion such as gall bladder, pancreas, liver, teeth, tongue and salivary gland.
Mouth: it is also called oral cavity that involves teeth, tongue and cheeks. Inside the mouth are two types of palate, hard and soft palate that forms the roof of mouth. Hard palate forms the anterior portion of the mouth and is made of maxillary and palatine bones whereas soft palate forms the posterior portion of the mouth. Hanging from the end of soft palate is uvula. The palates helps to chew and shallow foods along with breathing process at the same moment whereas uvula closes nasopharynx and prevents food from entering nasal cavity. Likewise, cheeks helps to keep the food between the upper and lower teeth.
Salivary glands: it secretes saliva that has different functions in the mouth. Some of the main functions of saliva are:
There are different types of salivary glands including labial, buccal and parietal gland in lips, cheeks and palate respectively and lingual gland in the tongue, all these contribute in production of saliva at a small amount. Beside this, the major glands that contribute in secretion of maximum saliva are: parotid, sub mandible and sub lingual glands. On one hand parotid gland are located anterior and inferior to the ear and opens secrete saliva via parotid duct. Secondly, sub mandible are found in the floor of mouth and open through sub mandibular duct. The sublingual gland are beneath the tongue thus, opening via sublingual duct. All these ducts helps in the secretion of saliva. The parotid gland secrete a watery liquid that contains salivary amylase whereas sub mandibular consist of mucous along with salivary amylase and sublingual gland consist mostly mucous. Saliva consist of water, some solutes, dissolved gases and some organic substances like urea, uric acid, mucous and immunoglobulin A, lysozymes and salivary amylase.
Water in saliva helps to dissolve food, chloride ions activates salivary amylase, thus breaking down starch, mucous lubricates the food, immunoglobulin A prevents attachment of microbes, lysozymes kill’s bacteria. Hence, overall salivary gland in mouth helps in removal of waste and breaking down of starch.
Pharynx: it is the funnel shaped tube that is divided into three parts, nasopharynx, laryngopharynx and oropharynx. Nasopharynx is involved in the process of respiration whereas rest two are involved in both the process of respiration and digestion. Therefore, epiglottis present in the pharynx prevents food from entering the windpipe and rest parts propels the food from pharynx to the esophagus.
Esophagus: it lies posterior to trachea, starting from inferior end of the laryngopharynx and ending in superior portion of the stomach. It secrets mucous thus transporting the food to the stomach.
Stomach: it is present at the end of the esophagus and lies left to the liver. Stomach is divided into different parts: cardiac, fundus, body and pyloric. Cardiac surrounds the superior portion of the stomach whereas fundus lies at the superior and to the left of cardiac. Body is the largest portion of the stomach; the part that connects to the duodenum is the pyloric. When the stomach is empty, mucosa forms folds called rugae. Different sphincter are present in the stomach that controls the passage of food from and out of the stomach.
Pancreas: it lies posterior to the greater curvature of the stomach and consist of head, body and tail. Exocrine cells of pancreas secrete juice that unite to forms pancreatic duct and accessory ducts. The pancreatic duct also called duct of Wirsung is larger among the two.
Liver and gallbladder: liver is the heaviest gland whereas gallbladder is the pear shaped organ that is located in the depression of the posterior surface of the liver. It is divided into two lobes: left lobes and right lobes whereas gallbladder is divided into three parts: fundus body and neck.
Small intestine: it is a long tube that helps in absorption of digested food material. The area of small intestine is increased by folds, villi and microvillus. It is divided into three regions: duodenum, ileum and jejunum. Duodenum is a long tube that is equal to width of 12 fingers, whereas jejunum extends up to ileum and is called empty, finally comes the ileum that joins large intestine and has a sphincter called ileocaecal sphincter. Therefore, beside absorption, all the digestion that has not been completed in earlier compartment is successfully done in small intestine.
Large intestine: it is the last part of digestive system where final absorption, production of vitamins, and excretion of faeces takes place. It is divided into different parts: caecum, colon, rectum and anal canal. Firstly, caecum hangs inferior to the ileocaecal valve that is a small pouch. Secondly, its long end merges with forming a tube called colon that is divided into ascending, descending and sigmoid colon. Rectum is the last portion of the GI tract that lies anterior to sacrum and coccyx. Finally lays the anal canal that helps in the excretion of faces.
As the food is taken in the mouth, the action of teeth, tongue and cheeks breaks the food into smaller particles, reducing it to soft and flexible form. The process is called mastication and here food is also mixed with saliva. Now, two enzymes, salivary amylase and lingual lipase comes in play where salivary amylase helps to break down the starch in food and on the other hand, lingual lipase gets activated at the acidic environment of the stomach thus breaking down triglycerides into smaller forms. Therefore, the food is now called bolus by the mechanical action of the mouth. From the mouth bolus goes to the pharynx where its muscular action pushes it to the esophagus. There is a sphincter at the beginning of esophagus that pushes food from pharynx to the esophagus. When the bolus reaches the esophagus, the peristalsis movement of esophageal muscles pushes the bolus further downwards towards the stomach. Mucous in the esophagus lubricates the bolus and hence reduce the friction. Peristalsis movement involves circular and longitudinal muscle, where they contract pushing down the bolus and walls of esophagus outwards, so as to allow easy passage of food. The relaxation of the sphincter present at the end of esophagus called esophageal sphincter prevents back flow of the bolus and moves its way to reach the stomach. As the bolus reaches the stomach, the peristalsis movements also called mixing waves masticates the food which is now called chyme. The waves starts slowly but then it becomes more vigorous, thus mixing the food. At this stage, pyloric sphincter remains closed, and after every few minutes, some food is forced to the duodenum and at that time pyloric sphincter opens for the passage of chyme and rest of the chyme is send back to the stomach again where the process of mixing, digestion and absorption continues. When food remain in stomach without being mixed with gastric juices, at that moment action of salivary amylase continues but as soon as chyme gets mixed with the gastric juices, action of salivary amylase stops and lingual lipase starts. Many other cells are present in the wall of stomach that helps in digestion of one or more of the substances. Chief cell present in the stomach secrete pepsinogen that is activated and hence breaks protein into peptides. On the other hand parietal cells secrete HCL that has many functions in the stomach. The acidic environment created by HCl kills microbes, denatures protein and converts the inactive form pepsinogen and pepsin. Parietal cells also secrete some intrinsic factor that helps in absorption of B12. Beside these cells, mucous cells present in the stomach secrete mucous that prevents digestion of stomach wall and its walls absorbs water, ions and many other important substance that are released in the bloodstream. G cells are also present in the stomach that secrets gastric having many functions in the stomach. It stimulates parietal cell and chief cell to secrete HCL and pepsinogen respectively. The sphincter present above and below the stomach prevents back flow of food and passes it to the further compartment for the digestion process. Then the food enters small intestine where all the essential components are absorbed and hence the completion of digestion takes place here.
Pancreatic amylase: breaks starch into smaller fragments such as monosaccharides.
Alpha-dextrinase: acts on dexrins thus reducing it to the simplest form.
Sucrose: sucrose is broken into glucose and fructose.
Lactase: breaks lactose into glucose and galactose.
Maltase: maltose breaks into molecules of glucose.
Tyrosin, chemotrypsin and elastase: breaks peptide bond between specific amino acids.
Carboxypeptidase: cleaves of amino acid at the carboxyl terminal.
Aminopeptidase: cleaves off amino acid at the amino end of the peptide.
Dipeptidase: splits dipeptide into single amino acids.
Pancreatic lipase: it breaks triglycerides ( that has been emulsified by the bile salt) into fatty acid and monosaccharide.
Ribonuclease: breaks ribonucleotide into nucleotides
Deoxyribonuclease: breaks dioxyribonucleotide into nucleotide.
This way, different important molecules are broken down into their simplest form in small intestine and hence, with the help of villi and microvilli, they gets absorbed and thus are secreted in the blood stream to reach different parts of the body.
In the large intestine no enzymes are secreted, only mucous is secreted and the bacteria present their helps to break any remaining substances into simpler form and in excretion of unwanted substances. Overall, toxic and unwanted compounds are converted to less toxic substance and excreted from large intestine through faeces, thus completing the process of digestion.