Organs of digestive tract


 Organs of digestive tract


Mouth or oral cavity- The mouth is known as the main door of the digestive tract. The part from the mouth to the initial part of pharynx is called mouth cavity. The mouth has been divided into two sections-

  1. Vestibule or buccal cavity, that is the limited area of lips, cheeks from the outside and teeth, gums from inside.
  2. Oral cavity proper that is the area from the rear of the teeth and gums to fauces and is being opened in the pharynx. The ends of sensory nerves are found in the fauces that takes part in involuntary action- swallowing.   

         The mouth or oral cavity is surrounded by lips and cheeks from outside and is ended in the pharynx. The upper part of it is made up of soft and hard palates and the lower one is made up of tongue and soft tissues. Its walls are made up of cheeks’ muscles. The mucus membrane making the layer of mouth remains connected with the skin of lips and the mucus membrane of pharynx. The internal hole of the oral cavity is known as fauces. In the middle on the upper part of it, a fleshy piece remains hanged, called uvula. In both the lateral walls near the pharynx in the oral cavity, 1-1 gland is present inside the muscles, called tonsil.  

Lips- The lips are two fleshy (sarcous) muscular layers, that keeps the mouth opening surrounded. The lips are mainly made up of fibro elastic connective tissue and skeletal muscular fibers. The lips shut (close) through arbicularis oris muscle and are being opened through liveter anguli oris muscle.

        The lips are very sensitive and have lots of sensory ends of blood vessels, lymphatic ducts and trigeminal nerve.

        The upper and lower lips are connected with each other at the corners of the mouth. The outer part of the lips is made up skin and the internal part of them is made up mucus membrane. Both the lips, in their middle lines, remain connected with gums through a layer of mucus membrane, called labial frenulum. The lower frenulum of the tongue is called lingual frenulum. It controls the backward movement of the tongue.     

Cheeks- The fleshy part found below the eyes, between nose and ears and towards both the sides of face is called cheek. This fleshy part is connected with lips. Buccinator muscle is present in it.

        The cheek and lips are layered with thick stratified squamous epithelium. Small glands secreting mucus are also present in it.

Palate- The palate is divided into two front and back part inside the oral cavity. It is also known as the roof of oral cavity. The front part of the palate is hard, so it is called hard palate. Its back part is soft, so it is called soft palate. The hard palate is surrounded by teeth and made up of palatine bones and maxial. Its upper surface makes the bottom of nasal cavity. The soft palate is located in the rear of the hard palate. Along with being muscular, it remains covered with mucus membrane. The soft palate lies between the mouth of pharynx and nasal part and in the middle of its free end, the uvula remains hanged. When any thing is swallowed by the mouth, the uvula acts to prevent it to enter in the nasal path. From the upper end of this uvula, 2-2 layers of mucus membrane grow towards both the sides and makes membranous arch and then goes downwards. The front layers are called palatoglossal arches and the back layers are called palatopharyngeal arches. In the middle of every arches, palatine tonsils are present. 

Tongue- The tongue is a voluntary or skeletal muscular structure present in the bottom of mouth that remains connected with hyoid bone at its base. It remains connected with the bottom of mouth through a layer of its mucus membrane (called lingual frenulum) below the opened part in the midline (called nose). The upper layer of the tongue remains covered from stratified squamous epithelium. There are many small raised parts present on the tongue, called papillae having taste buds and only due to it; the rear part of the tongue appears as rough. With the help of these taste buds, one feels tastes, because nerve endings of taste sensation are present on them.  

Teeth- The teeth remain fit in the teeth cavities of upper jaw (maxilla) and lower jaw (mandible. During the life of every people, teeth appear (come out) two times. The teeth appearing in the first and second year of life are called temporary teeth. They are also known as milk teeth. When these temporary teeth are fallen down, new teeth start coming out again in the sixth year of age, which are called permanent teeth.

Temporary or milk teeth- The temporary teeth start appearing after sixth month of birth and all the teeth are appeared till the age of second year. Such types of teeth are present 10-10 in both the jaws i.e. total 20 in number. In both the sides of midline of every jaw, 5-5 teeth are located. The names of such teeth from midline are as follow- two incisors, one canine and molar.

        In the beginning, this type of teeth appears in the midline of lower jaw, but sometimes they also appear in the midline of upper jaw. They grow as following ways-

First of all, two incisors appear in the midline of jaws and then two more teeth are appeared next to them. After 12 to 15 months of appearing these teeth, 1-1 molar teeth are appeared in both the sides. After that, till 18 months, canines are appeared. In the last, remaining molar teeth are appeared during 20 to 24 months. This process takes place in both the jaws. In this way, in the upper and lower jaws of one year old children, two incisors and 1-1 canines on the both sides i.e. total 8 milk teeth are appeared and total 20 milk teeth appear in a 2 years old healthy child. After six years, such type of milk teeth start falling down and new i.e. permanent teeth start appearing instead of them. Up to the age of 12 years, all the temporary teeth are fallen down and up to the age of 25 years, all the permanent teeth are appeared (come).       

Permanent teeth- The permanent teeth are total 32 in numbers. 16 teeth among them are located in the upper jaw and 16 of them are located in the lower jaw. There are 8-8 teeth present in both the sides of midline of each jaw. Its names starting from the midline of jaw are as follow- the center four teeth are cutting teeth, called incisors. On the both sides of these teeth, 1-1 pointed teeth are present used for tearing, called canines. Next to them 2-2 crushing teeth are present, called premolars and further next to them, 3-3 chewing teeth are present, called molors. The last three molars teeth are also known as wisdom teeth. These teeth appear in the age between 17 to 30 years.

Structure of teeth- Well, the shape of every teeth are different to each other, however their structure is same. There are three parts of tooth-

Crown- This part of the tooth remains outside the gums. The crown of incisors is same as chisel. The crown of incisors is pointed. There are two cusps found in the crown of premolars and 4-5 cusps found in the crown of molars.  

Neck- The neck is the part between the crown and root of tooth. It is surrounded by gums.

Root- This part of the tooth remains fit in the alveolus of the alveolar process of jaw’s bone. The incisors, canines and premolars have 1-1 root. Well in the beginning, premolars teeth of the upper jaw have two roots. In the molars teeth of lower jaw, two flat roots and in the molars teeth of upper jaw, 3-3 conical roots are present. On the apex of every root, there is a hole present, called apical foremen that is present to the root canal, root cavity or pulp cavity.

        All the teeth are made up of dentine, enamel, cementum and pulp. Too sensitive yellow substance found around the pulp is called dentine. It is known to be main substance of the tooth, because the most of teeth part are made of it. A thin layer of insensitive white substance covering the crown of teeth is called enamel. It is the hardest substance of the body. The cementum is bone like substance covering the neck and root of the teeth that keeps the teeth in tooth cavity. There is a cavity found inside the tooth, called root cavity. A substance like paste produced by soft connective tissue is filled in it, called pulp. In this pulp, nerves and blood vessels are present that reach inside each of the teeth through small holes found on the apex of every root.

Salivary glands- There are three pairs of salivary glands. It secretes saliva in the mouth cavity. The mucus glands found on the surface of oral cavity and salivary glands, the secretions of both are watery, transparent and tasteless. In these liquids (sections), mineral salt, enzymes named lysozyme and tyline are found.

There are three pairs of salivary glands found inside the mouth towards left and right sides-

Parotid glands- 1-1 gland is present downward inside each of the ear, called parotid glands and these are considered as the biggest glands. 1-1 long tube comes from each of these glands and opens near the second molar tooth inside the cheek’s skin, called parotid duct or stensen’s duct. Through these parotid ducts, the liquids, salt and secretions enriched with salivary amylase come in the mouth.

Submandibular glands- These glands are present in each side below the medulla or lower jaw in 1-1 number. These glands are in the shape of walnut and half in the length of parotid glands. Their tubes called submandibular ducts or Wharton’s ducts are being opened in papillae located on surface of mouth in both the sides of lingual frenulum behind lower incisors. These glands secrete liquid, salt, salivary amylase and mucin that come in the mouth through submandibular ducts.   

Sublingual glands- These glands are located in the bottom of the mouth below the tongue towards both the sides in 1-1 number. They are known to be very small glands. Many small tubes come from each of these glands that are being opened just behind the holes of submandibular ducts i.e. towards both the sides of lingual frenulum. These glands mostly secrete liquid, salt and mucin and are stickier than the secretions of other glands. In it, mucus is present in additional quantity and salivary amylase is present in less quantity. 

Saliva and its functions- Saliva has about 99% water and 1% electrolytes and proteins. Proteins consist of mucin (helpful in making mucus) and an enzyme named salivary amylase. The mucin reacts with liquid present in the saliva and produces sticky mucus in extra quantity. This sticky mucus helps in making foods moist and slippery, due to which foods go down in the pharynx and oesophagus easily while swallowing. The salivary amylase reacts on carbohydreates and converts them immediately into maltose and dextrin. This enzyme is become inactive (neutralize) just after secreting gastric juice, because gastric juice is acidic and the acid is neutralized alkali. There is no enzyme, responsible for decomposition of fat and protein, present in the saliva.       

        The saliva cleans the mouth and teeth and dose not allow the fluids gathered that harms teeth. It keeps the mouth’s tissues soft. The saliva also helps in producing taste sensation, because the taste buds present on the tongue are being activated only after mixing the foods with saliva.

Secretion of saliva- Saliva goes on secreting all the time from keeping the mouth and throat as moist. This type of secretion takes place after the activation (stimulation) of autonomic nerve endings present in the salivary glands. In addition to it, the secretion of saliva is often increased by putting foods in the mouth, by chewing and just after smelling, tasting, looking and thinking about foods.

        The secretion of saliva is under control of autonomic nerve endings completely while the controlling of remaining all the digestive secretions is neuralgic and hormonal. Most of the stimulations of all the salivary glands come from parasympathetic system. Bad stimulation related to foods i.e. smelly food stuffs, disturbs the activities of parasympathetic system, resulting the mouth and throat are dried up. In case of stress too, the mouth is dried up.  

Composition of saliva-

  Component

Function

Water (99%)

It acts as a solvent and keeps the mouth moist; helps the tongue and lips during talking (speaking)

Bicarbonates

Keeps the pH value of saliva in between 6.35 to 6.85.

Chlorides

Activates salivary amylase.

Immuno globin A (Ig-A)

It is a part of salivary antibacterial system.

Lysozyme

Destroys bacteria, prevents dental decay and infection of mucus membrane.

Mucin

A type of protein producing mucus.

Mucus

Makes foods as morsel after lubricating and helps in swallowing.

Phosphates

Helps in maintaining the pH value of saliva.

Salivary amylase (tyline)

Converts carbohydrates into maltose and dextrin.

Urea, uric acid

It does not play any role in digestion, but being excreted out in the form of waste products by the means of saliva.

        Changes go on taking place in the quantity and composition of saliva by different types of stimulations. Sticky (smooth) and thick secretion go on coming out by the stimulation of parasympathetic nerves fibers. Mucus is present in extra quantity in this secretion. By the stimulation of sympathetic nerve fibers, watery saliva secretes having enzymes and organic substances in extra quantity.

Pharynx- It is a wider structure like funnel posterior to the oral cavity. Oesophagus and trachea start from here. After the mouth, eaten foods reach to the oesophagus passing through pharynx. The pharynx acts both like a trachea during breathing and as alimentary canal during swallowing. The upper wide part of it remains connected with the base of skull while the lower part is fused in the oesophagus and front part is fused in nasal cavity.  The pharynx has divided into three parts-

  1. Nasopharynx- It is a part of pharynx situated in the rear of nose and in the front of soft palate. Eustachian tubes are being opened in the nasopharynx and adenoids are also found in it.
  2. Oropharynx- The part from soft palate to epiglottis is called oropharynx. In the posterior walls of this part, tonsils are found.
  3. Laryngopharynx- The posterior part of epiglottis of pharynx is called laryngopharynx. This part remains connected with oesophagus.        

There are following three differences found in the walls of pharynx-

        The mucus layer of stratified squamous epithelium is located inside the pharynx that remains connected with the epithelium of pharynx from lower side and with the mouth from upper side. The middle layer of it is made up of fibrous tissue. Blood vessels and nerves are present in it. The outer layer of the pharynx is muscular. Constrictor muscles are chiefly present in it that pushes the chewed food into the oesophagus.

Stomach- The widest part of digestive tract or alimentary canal is called stomach. It is located in the middle of initial part of small intestine and in the last of oesophagus. Inside the abdominal cavity, the stomach is located maximum in epigastric region and partially in umbilical region and in the left hypochondriac region, below the diaphragm and heart. Its shape goes on changing according to food and the activeness of its muscles.

        The stomach is like the English alphabet ‘J’ in shape. It has two curvatures-  

1.     Lesser curvature- The lesser curvature makes the right or back edge (boundary) of the stomach and small in shape.

2.     Greater curvature- The greater curvature is big in shape and makes an arch upwards. In upper section, it makes the fundus of stomach on the left side and in the lower section; it is connected with duodenum by curving rightwards.

The stomach has been divided in to four main regions-

  1. Cardiac region.
  2. Fundus.
  3. Body.
  4. Pyloric region.
  1. Cardiac region- It is a small region, situated near the hole towards oesophagus, called cardiac orifice.
  2. Fundus- It is a small round part situated on the surface of the cardiac orifice of the stomach. Normally, inhaled air is present in it.
  3. Body- It is the big central part of stomach.
  4. Pyloric region- It is the lower horizontal part of the stomach having a narrow pyloric antrum or canal. This canal is being opened in the pyloric orifice. On this orifice, pyloric sphincter is present that does not allow food particles present in the duodenum to enter in the stomach.  

Stomach walls- the stomach walls are made up of four layers-

2.     Muscular layer- This layer is made up of three layers of smooth or involuntary muscular fibers. In the outermost layer of it, fibers are present vertically that is situated in the continuity of oesophagus’s muscles and found more in the stomach’s curvatures. In the middle layer of it, circular fibers are present that remain spread everywhere in circular form in the body of stomach. These fibers make pyloric sphincter after becoming thick in the pyloric orifice. In the innermost layer of it, oblique fibers are present that keeps the fundus covered and found parallel to lesser curvature with the front and back walls of stomach.

4.     Mucosal layer- This layer is being covered with columnar epithelium. In this layer, thousands of gastric pits are present and 3-8 tubular gastric glands grow from each of these pits.    

        When the stomach remains empty completely, lots of contractions take place in its internal mucous membrane, called rugae, and are disappeared by their selves after filling the stomach.  

        There are three types of gastric glands found in the stomach that produce gastric juice. This gastric juice reaches in the stomach through tubes. The mouth of the tiny tubes of these glands is being opened on the mucus membrane. These gastric glands are as follow- 

1.     Cardiac glands- These glands are present near the pyloric orifice. Mucus goes on secreting from these glands, which is alkaline in reaction.

2.     Fundic glands- These glands are present in the fundus and body of the stomach. Mucous, peptic, oxtic or lateral cells are present in these glands that secrete mucin, pepsinogan or pepsin enzyme and hydrochloric acid respectively.

3.     Pyloric glands- These glands are found in pyloric canal or antrum and secrete mucus, which is alkaline in nature.

Functions of the stomach- The stomach do mainly three types of works-

  1. The stomach acts as a temporary storage for ingested food before reaching it into small intestine.
  2. The stomach also churns the ingested food, by that crop of food is broken into simpler forms and then it converts these food particles into soup like liquid mixture after mixing gastric juices with them, called chyme.  

       The stomach secretes hydrochloric acid and enzyme. It starts the digestion of proteins and kills most of the bacteria reaching inside the stomach with foods.