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== Functional organization ==
== Functional organization ==
<p style="text-align:justify;">The digestive system comprises of gastrointestinal tract (GIT) and accessory organs of digestion like teeth (danta), tongue (jihwa), salivary glands (praseka granthi), liver (yakrut) and exocrine part of pancreas (agnyasaya). <br/>Gastrointestinal tract, also known as alimentary canal, is basically a muscular tube extending from the mouth to the anus. At either end, the lumen is continuous with external environment. It measures about 10 m (30 feet) and comprises of following parts:</p>
<p style="text-align:justify;">The digestive system comprises of gastrointestinal tract (GIT) and accessory organs of digestion like teeth (danta), tongue (jihwa), salivary glands (praseka granthi), liver (yakrut) and exocrine part of pancreas (agnyasaya). <br/>Gastrointestinal tract, also known as alimentary canal, is basically a muscular tube extending from the mouth to the anus. At either end, the lumen is continuous with external environment. It measures about 10 m (30 feet) and comprises of following parts:</p>
; Mouth (mukha): : Mouth is a loosely used term to denote the external opening and for the cavity it leads to. Strictly speaking, the term mouth should be applied only for the external opening which is also called oral fissure. The cavity containing anterior two-third of tongue and teeth is the mouth cavity or oral cavity or buccal cavity. The oral cavity extends from the lips to the oropharyngeal isthmus, i.e., junction of the mouth with the pharynx. The oral cavity is subdivided into two parts: the vestibule and oral cavity proper.
<ul style="list-style-type:none; text-align:justify;"><li style="font-weight: bold;">Mouth (mukha) : <span style="font-weight: normal;">Mouth is a loosely used term to denote the external opening and for the cavity it leads to. Strictly speaking, the term mouth should be applied only for the external opening which is also called oral fissure. The cavity containing anterior two-third of tongue and teeth is the mouth cavity or oral cavity or buccal cavity. The oral cavity extends from the lips to the oropharyngeal isthmus, i.e., junction of the mouth with the pharynx. The oral cavity is subdivided into two parts: the vestibule and oral cavity proper.</span></li>
* Vestibule lies between the lips and cheeks externally, and the gums and teeth internally.
* Vestibule lies between the lips and cheeks externally, and the gums and teeth internally.
* Oral cavity proper lies within the alveolar arches, gums and teeth.
* Oral cavity proper lies within the alveolar arches, gums and teeth.
<li style="font-weight: bold;">Tongue (jihwa): <span style="font-weight: normal;">In the digestive system, tongue plays two important roles:</span></li>
* Tells the taste of food and
* Helps in chewing and swallowing of food.
<li style="font-weight: bold;">Teeth (danta): <span style="font-weight: normal;">Functions of different types of teeth in chewing are:</span></li>
* Incisors provide strong cutting action.
* Canines are responsible for tearing action.
* Premolars and molars have grinding action.
<li style="font-weight: bold;">Pharynx (kantha): <span style="font-weight: normal;">The pharynx is a median passage that is common to the gastrointestinal and respiratory systems. It is divisible, from above downwards, into three parts:</span></li>
* Nasal part or nasopharynx into which nasal cavities open,
* Oral part or oropharynx which is contiguous with the posterior end of oral cavity and
* Laryngeal part or laryngopharynx which is contiguous in front with larynx, and below with oesophagus.
<li style="font-weight: bold;">Oesophagus (anna nalika): <span style="font-weight: normal;">It is a fibromuscular tube about 25 cm long. At its junction to the pharynx, upper oesophageal sphincter is present and at its junction with the stomach, lower oesophageal sphincter is present. During swallowing, the upper oesophageal sphincter opens and food passes into the oesophagus. The peristaltic movements of the oesophagus propel the food into stomach.</span></li>
<li style="font-weight: bold;">Stomach (amashaya): <span style="font-weight: normal;">It is a hollow muscular bag connected to the oesophagus at its upper end and to the duodenum at the lower end. It serves the following motor functions:</span></li>
* Storage of food till it can be accommodated in the duodenum,
* Mixing of food with gastric secretions to form a semiliquid mixture
called chyme and
* Slow emptying of food into the small intestine.
<li style="font-weight: bold;">Small intestine (kshudrantra): <span style="font-weight: normal;">It is a long tubular structure which can be divided into three parts:</span></li>
* Duodenum is the first part of small intestine. It is C shaped and measures about 25 cm in length.
* Jejunum, the middle part of the small intestine is about 25 m long and
* Ileum, the last part of small intestine, is about 3.5 m long.
Gastric chyme enters the duodenum where it meets with pancreatic juice, bile and secretions of the small intestine (succus entericus). The partially digested foodstuff in the gastric chyme is digested further and the final products of digestion are absorbed by the villi of small intestine. The movements of small intestine help in mixing, digestion and absorption of the food. The peristaltic activity of small intestine also helps in moving the undigested and unabsorbed food material to the large intestine.
<li style="font-weight: bold;">Large intestine (pakvashaya): <span style="font-weight: normal;">It arches around and encloses the coils of the small intestine and tends to be more fixed than the small intestine. It is divided into following parts:</span></li>
* Caecum is a blind-ended sac into which opens the lower end of ileum. The ileocecal junction is guarded by the ileocecal valve which allows on flow but prevents backflow of intestinal contents.
* Appendix is a worm-shaped tube that arises from the medial side of caecum which in humans is a vestigial organ.
* Ascending colon extends upward from the caecum along the right side of the abdomen up to the liver. On reaching the liver, it bends to the left, forming the right hepatic flexure.
* Transverse colon extends from the right hepatic flexure to the left splenic flexure. It forms a wide U-shaped curve.
* Descending colon extends from the left splenic flexure to the pelvic inlet below.
* Sigmoid colon begins at the pelvic inlet as continuation down to the descending colon and joins the rectum in front of the sacrum.
* Rectum descends in front of the sacrum to leave the pelvis by piercing the pelvic floor. Here it becomes continuous with the anal canal in the perineum.
* Anal canal opens to the exterior through the anus, the opening which is guarded by two sphincters.
<p style="text-align:justify;">In large intestine, absorption of water and electrolytes occurs from the intestinal contents. The remaining material is called faecal matter. The mucus secreted from the wall of large intestine lubricates the faecal matter. The faecal matter is stored in the sigmoid colon (pelvic colon) till expelled by the process of defaecation which is assisted by the movements of large intestine, activity of anal canal and anal sphincters.<ref name="ref12">Khurana I. Textbook of Medical Physiology. 2nd ed. Khurana A, editor. New Delhi: Reed Elsevier India Pvt. Ltd.; 2015.</ref> <br/>The gastrointestinal system is the portal through which nutritive substances, vitamins, minerals, and fluids enter the body. Proteins, fats, and complex carbohydrates are broken down into absorbable units (digested), principally, although not exclusively, in the small intestine. The products of digestion and the vitamins, minerals, and water cross the mucosa and enter the lymph or the blood (absorption). <br/>Most substances pass from the intestinal lumen into the enterocytes and then out of the enterocytes to the interstitial fluid. The processes responsible for movement across the luminal cell membrane are often quite different from those responsible for movement across the basal and lateral cell membranes to the interstitial fluid.<ref name="ref13">Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong’s Review of Medical Physiology. 25th ed. New York: McGraw-Hill Education; 2016.</ref> </p>
[[File:Different parts of mahasrotas.jpg|center]]
<center>Annavaha srotas in relation with mahasrotas (gastrointestinal tract)<ref name="ref14">Dash B. Concept of Agni in Ayurveda with special reference to Agnibala Pariksha. Revised. Varanasi: Chaukhamba Amrabharati Prakashan; 2015.</ref> [Source: Dash B. Concept of Agni in Ayurveda]</center>
== Deglutition reflex ==
<p style="text-align:justify;">Swallowing or deglutition is the process by which the food material from oral cavity is transported into the oesophagus. Though it is initiated voluntarily, most part of it is involuntary or reflexive. Therefore, this is also called the deglutition reflex.</p>
# During the reflex act of deglutition, respiration is inhibited, which prevents the entry of food into the trachea.
# The receptors for the deglutition reflex are present near the opening of the pharynx.
# The afferent impulses are transmitted to deglutition centres in medulla and pons.
# The efferent impulses are directed to the muscles of pharynx and upper oesophagus via cranial nerves.<ref name="ref15">Pal GK, Pal P, Nanda N. Comprehensive Textbook oof Medical Physiology. Vol. 1. New Delhi: Jaypee Brothers Medical Publishers (P) Ltd; 2017.</ref>
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