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1. Definition:
It is a process where the breaking down of intake food into small particles happens inside the human body. The human digestive system is also called the alimentary canal or the gastrointestinal tract.
2. Stages involved:
There are three stages involved in this human digestive system. They are cephalic stage, gastric stage and intestinal stage.
Cephalic stage:
It includes the breakdown of food into smaller particles in a mechanical way by the process known as chewing. It also involves the breakdown of food into smaller particles through chemical processes with the help of saliva.
Gastric stage:
The food particles which are broken down are further broken down to smaller particles with the help of the gastric juice in the stomach. It is known as partial digestion.
Intestinal stage:
The partially digested food particles are allowed to mix with the digestive enzymes of the pancreas in the intestine. This results in the completion of the digestion.
3. Accessory organs:
The accessory organs involved in the human digestive system include mouth, tongue, the glandular organs such as salivary glands, liver, gall bladder and pancreas. The additional components are teeth, epiglottis and sphincter (esophageal in esophagus and pyloric in stomach).
Mouth:
The teeth and the tongue are present in the mouth. They are divided into two major parts namely, vestibule and oral cavity. The vestibule is the space between the lips, cheeks and teeth and the remaining space is the oral cavity. The mucus is secreted by the oral cavity which contains the mucin, a glycoprotein that plays a major role in digestion. The food particles which are lubricated with mucus are termed bolus which are swallowed down further into the gastric stage for partial digestion and it will ultimately lead to the intestinal stage for complete digestion.
Tongue:
This organ is involved in the first stage of the digestion which secretes the saliva in the mouth as soon as the papillae buds on the tongue senses the food particles. Thus the tongue is known as a sensory organ. Frenum is involved in the attachment of the tongue to the base of the mouth. The intrinsic and extrinsic muscles present in the tongue are four in number each and are involved in the shaping and movement of the tongue respectively.
Salivary glands:
Three pairs of salivary glands are found in the mouth and they are the parotid glands, the submandibular glands and the sublingual glands. The first two pairs of the salivary glands produce 70 percent of the saliva and the third pair accounts for only a small percentage of saliva. There are certain glands involved in the secretion of lipase, a digestive enzyme for the digestion of lipids and these are known as the Von Ebner's glands. They also secrete histatins, a component involved in the defense against the microbes that enter the mouth.
Liver:
The second largest organ right after the skin is found to be involved in the digestion process through the detoxification of the metabolites and the production of the biochemicals required for digestion. It is involved in the protein, carbohydrate and lipid metabolism. It is also involved in the synthesis of fat, lipoproteins and glucose. It also synthesises lecithin and bile acids for the digestion of the fat.
Gall bladder:
It is the organ which stores the bile juices produced in the liver. The release of the bile juices take place after the induction of Cholecystokinin (CCK), a peptide hormone secreted in the duodenum. The induction of CCK takes place only in the presence of fat at the duodenum. The gall bladder is divided into three parts namely, fundus, body and neck. The inner lining of the gall bladder secrete the hydrogen ions which play a major role in maintaining the acidic nature of the bile juice. The addition of electrolytes and water takes place to dilute the bile juice. The attachment of salts to cholesterol also takes place to prevent the crystallization of the bile juice.
Pancreas:
It functions both as an exocrine and endocrine gland. The endocrine gland of the pancreas releases insulin and glucagon when the blood glucose levels are high and low, respectively. The pancreas is located at the back and below part of the stomach. It is connected to the duodenum through the pancreatic duct. The secretion of alkaline bicarbonate ions by the pancreatic duct cells takes place to neutralise the acidic bile juice secreted by the liver.
Teeth:
The teeth are the major part in the mechanical digestion of the food particles which is known as mastication. There are different kinds of teeth with certain shapes to ensure the complete mechanical digestion of the food particles and also after that they are mixed with mucus and saliva to form the bolus. The different kinds of teeth involved are incisors, canines, pre-molars and molars. The incisors are involved in biting or cutting of food particles, canines are involved in tearing , premolars and molars are involved in grinding and chewing.
Epiglottis:
It is a piece of elastic cartilage located at the larynx entrance. It is present at the glottis entrance to prevent the bolus from entering the larynx which is connected to the trachea of the lungs. The inappropriate swallowing of bolus into the larynx results in a cough after which the food comes out back to the mouth to be travelled down to the esophagus.
Esophageal sphincter:
The esophageal sphincters are located at the upper and lower ends of the esophagus, which is also known as gullet or foodpipe. The food is swallowed down from the mouth through the pharynx into the esophagus. The average length of the esophagus is 25 cm with its length varying according to the individual's height. The Upper Esophageal Sphincter (UES) results in the prevention of air from entering into the digestive tract and the Lower Esophageal Sphincter (LES) prevents the backflow of the contents of the stomach into the esophagus. The LES does not have similar protection just like stomach from acid, so any malfunctioning of this sphincter will cause heartburn.
Pyloric sphincter:
It is present at both the ends of the stomach. The upper portion is known as pyloric antrum and the lower portion is known as pyloric canal. It is involved in the regulation of the chyme which is the partially digested food particle from the stomach. The chyme passes through the pyloric sphincter into the small intestine.
