Digestive System and its Physiology PDF

DIGESTIVE SYSTEM, ITS ACCESSORY ORGANS AND ITS PHYSIOLOGY Alimentary canal/Gastro-Intestinal tract/Digestive tract is tube extending from mouth, pharynx, oesophagus, stomach, intestine and ends at anus. Tongue, Salivary glands, Teeth, Liver, Spleen and Pancreas are the accessory organs of this syste...

DIGESTIVE SYSTEM, ITS ACCESSORY ORGANS AND ITS PHYSIOLOGY Alimentary canal/Gastro-Intestinal tract/Digestive tract is tube extending from mouth, pharynx, oesophagus, stomach, intestine and ends at anus. Tongue, Salivary glands, Teeth, Liver, Spleen and Pancreas are the accessory organs of this system MOUTH Mouth of Cattle:  It is cavity at beginning of alimentary canal bounded externally by lips and cheeks and internally by gums and teeth. Lips are thick and rigid musculo-membranous structures, situated at opening of mouth.  The area between nostrils and above upper lip is generally black in colour called muzzle. It remains moist. Frenum labii is fold of mucous membrane between lower lip and lower gum. Cheeks are composed of skin, muscles, glands and form lateral walls of mouth and has many cornified pointed papillae on cheeks.  The gingiva or gums is composed of a thick layer of dense fibrous tissue, which is closely attached to the periosteum of the alveoli and surrounds the necks of the teeth.  The palate is divided into an anterior (hard palate) and posterior (soft palate).  The hard palate forms the roof of the mouth. There is dental pad in front of hard palate as there are no incisor teeth at the upper jaw of cattle. These dental pads occupy their positions.  Linear groove divides hard palate into two equal halves with 15-19 transverse ridges in each half of hard palate.  Soft palate acts as a partition between the mouth and pharynx. It is attached to the root of the tongue. Mouth of horse: Lips are thin and more mobile. Muzzle has groove called Philtrum. Whole hard palate has ridges. Soft palate is more extensive and embraces base of epiglottis and for that reason oral breathing is not possible and it vomits through nose. Dental pads are absent due to presence of upper incisor teeth. Mouth of dog: The opening of mouth is more extensive. The upper lips covers the lower lip. The lower lip is partially denticulate in appearance. Dental pads are absent. Mouth of pig: Upper lip is close to snout. Palate Ridges are 20 in number and are more prominent. Mouth of fowl: It is cone shaped. Cheek, lips and teeth absent. It has upper and lower horny pointed beak.  TONGUE Tongue of cattle:  It is highly movable muscular structure and main organ for prehension and mastication. It is organ for taste.  It is situated on the floor of the mouth, between the two rami of mandible. It has a root, a body and an apex.  The root is very thick.  Dorsal surface of body has a raised area known as torus linguae. When the mouth is closed, this surface remains in contact with the hard palate.  The apex is thin with round border.  Fold of mucous membrane that connect floor of mouth and ventral surface of tongue is Frenum linguae.  Various papillae are found on tongue for taste. These are filiform, fungiform, lenticular and vallate. Filiform papillae are numerous and are distributed all over the tongue. Fungiform papillae are larger in size and are scattered around filiform papillae. These papillae bear the taste buds. Torus linguae has some rounded lenticular papillae. The vallate papillae are large in size and 7 to 8 in number. Tongue of horse: Apex is spatula shaped. Torus linguae is absent. Vallate papillae are two in number. Tongue of dog: It is highly mobile. Dorsal surface presents a median groove. The vallet papillae are 2 to 3 in number. Thick fibromuscular cord situated at ventral surface of the tip of the tongue is known as Lyssa. Tongue of pig: Torus linguae is absent. Vallet papillae are 2-3 in number There are two frenum linguae. Tongue of fowl: It is a triangular rigid structure comprises of a fixed base and a considerable free part.  TEETH Teeth of cattle:  Teeth are the hardest structures in the body.  They are implanted into the alveolar sockets of the upper and lower jaw bones.  The exposed portion of tooth covered by very hard material (enamel) is known body or crown.  The implanted narrow part which is covered by a thin yellowish material (cement) is known as root.  Most of tooth is formed by dentin. Within dentin there is central canal called pulp cavity. It is lined by a layer of odontoblast cells. The pulp cavity contains connective tissue, arterioles and nerve fibers.  Teeth are classified according to their placement in oral cavity. The incisors (I) are most anterior in position. Next to them on either side are the canines (C). Canines are followed by pre molars (PM) and then by Molars (M). Premolars and molars are placed laterally and therefore also called cheek teeth.  Deciduous or milk teeth: This is a temporary set of tooth and is replaced by permanent set. These milk teeth are found at birth or within few weeks after birth. Temporary teeth donot have molars  Permanent teeth: The permanent teeth starts from age of one year and is completed by 3 to 4 years. In cattle, the incisors at upper jaw are absent. Their positions are occupied by dental pads. The space between teeth is called Diastemą. This space is between canine tooth and first premolar tooth. Upper jaw I 0-0 C 0-0 PM 3-3 M 3-3 Lower jaw 3-3 1-1 3-3 3-3 Teeth of horse: Upper jaw I 3-3 C 1-1 PM 3-3 M 3-3 Lower jaw 3-3 1-1 3-3 3-3 Canine tooth is absent in mare (female horse) Teeth of dog: Upper jaw I 3-3 C 1-1 PM 4-4 M 2-2 Lower jaw 3-3 1-1 4-4 3-3 Canine teeth are large and pointed Teeth of pig: Upper jaw I 3-3 C 1-1 PM 4-4 M 3-3 Lower jaw 3-3 1-1 4-4 3-3  PHARYNX  The part behind palate to start of oesophagus is pharynx.  This tube is common to the digestive and respiratory passages.  The cavity of the pharynx communicates with other structures by seven openings - 2 for eustachian tubes, 2 for posterior nares, 1 for esophagus, 1 for mouth and 1 for larynx.  The part of the pharynx above the soft palate is known as Nasopharynx.  The part of the pharynx between the soft palate and root of the tongue is known as Oropharynx.  The part dorsal and lateral to the larynx is known as Laryngo-pharynx. Pharynx of horse: It is comparatively longer and narrower. Superiorly it is related to Guttural pouches. These are a pair of large mucous sacs situated on either side of the midline above pharynx. These are downward diverticulum from the auditory tubes. Pharynx of fowl: Soft palate is absent. OESOPHAGUS Oesophagus of cattle:  It is a muscular tube extends from pharynx to stomach. The average length is about 90 cm.  It starts as the continuation of the pharynx.  It is dorsal to trachea upto 6th cervical vertebrae. Then it goes little upward and enters the thoracic inlet.  It then goes upward and backward and enters the abdominal cavity through esophageal hiatus.  After entering the abdomen it opens into the stomach at a dome shaped area between the dorsal sac of rumen and reticulum, known as atrium ventriculi. Oesophagus of horse: The average length is about 140 cm. Oesophagus of dog: It is comparatively wider. Length is variable. There is a constriction at the beginning of the esophagus due to the presence of a thick submucous gland at the ventral wall. Oesophagus of pig: The tube is comparatively short and straight.  Oesophagus of fowl: The wall is thin and the lumen is wide. Just before passing into the thoracic cavity, the tube is enlarged ventrally to a great extent to form the Crop. ABDOMINAL CAVITY  This is the largest cavity of the body from the diaphragm to the pelvic inlet.  The roof is formed by lumbar vertebrae.  The floor is formed by muscles and xiphoid cartilage of sternum.  The lateral walls are formed by abdominal muscles and few ribs.  The abdominal cavity is pierced by three openings at the diaphragm, namely aortic apertur e for aorta of heart, oesophageal aperture for oesophagus and vena caval aperture for vena cava vessel.  A scar formed by the remnant of the root of the umbilical cord is present at the mid ventral region of the abdomen termed umbilicus.  The triangular depressed area below the lumbar transverse processes is known as para-lumbar fossa.  The area close to the paralumbar fossa at the upper abdominal wall is known as flank region. PERITONIUM  It is membrane which lines the abdominal walls and is reflected over the viscera.  The lining portion of abdominal part is parietal part and reflected portion is visceral part of the membrane.  It secrets a small amount of serous fluid. This is helpful for free movement of viscera.  This peritoneal fluid is composed of water, electrolytes, proteins and lymphocytes.  The space between the parietal and visceral layers is known as peritoneal cavity.  The fold of peritoneum that attaches the intestine to the dorsal wall of the abdomen is known as mesentery.  The fold of peritoneum which is attached to the stomach is known as Omentum. Functions of peritoneum: 1. It provides a slippery surface for free movement of abdominal organs. 2. The phagocytic cells of peritoneum protects the viscera against infection. 3. The mesothelium absorb fluid and solutes, as it acts as a semipermeable membrane. 4. It helps in healing and adhesion by transformation of mesothelium into fibroblasts. 5. It acts as storage of fat. STOMACH Stomach of cattle:  Nearly three fourth of the abdominal cavity is occupied by stomach with capacity of 100-230 litres.  It has four compartments - Rumen, Reticulum, Omasum and Abomasum.  Stomach fill up the whole of the left side and a part of the right side of the abdomen.  In the left side only small part is left by the stomach for spleen and intestine.  Out of total volume, rumen occupies 80%, reticulum 5% and omasum and abomasum 7-8% each.  In new born calves, abomasum is the largest of all compartments of stomach because they live only on milk. At the age of one and half years, all the compartments of stomach gain normal size. In calves, the reticular groove starts from junction of oesophagus with rumen, passes to reticulum and ends at reticulo- omasal opening. This groove has two lips (right and left) and floor and thus encloses a tubular bypass to the rumen. Omasal groove is second part of gastric groove which passes from reticulum to abomasum. These combined grooves are functional in suckling calves only by reflex closure of lips so that milk can pass from esophagus to reticulo-omasal orifice and to abomasum. In adult animals such functions are not observed. But some chemicals like copper sulphate, sodium salt, etc. may produce reflex closure of the groove even in adult animals. Rumen:  It is a big muscular elongated sac placed in the abdominal cavity from 8th rib to the pelvic inlet and palpable at left paralumbar fossa. It occupies most part of left side and small part of right side of abdominal cavity.  Left surface is related to diaphragm and spleen and has left longitudinal groove.  Right surface is related to omasum, abomasum, liver, intestine, pancreas, adrenal gland, aorta etc and has right longitudinal groove. Sometimes the right longitudinal groove is divided into dorsal and ventral branch and encloses an elongated swelling termed Insula ruminis.  Cranial (anterior) end is divided into two blind sacs by transverse groove and is related to diaphragm.  Caudal (posterior) end is divided into two blind sacs by transverse groove and is related to intestine and urinary bladder.  A dome shaped area called Atrium ventriculi is formed between Rumen and reticulum at which oesophagus joins the stomach.  Inside of rumen is incompletely divided into a dorsal and a ventral sac.  The internal surface of the ruminal wall has a number of infoldings called pillars. They correspond to the external grooves, responsible for dividing the cavity of the rumen into dorsal and ventral sacs.  The mucous membrane is brown in colour and has large number of papillae except at pillars. Reticulum:  This sac is the smallest of all other compartments and situated between the diaphragm and liver in front and dorsal sac of rumen and omasum behind.  The interior of reticulum is honey comb shaped due to the foldings of mucous membrane.  Plenty of 4 to 6 sided cells (small chambers) are enclosed by the mucous folds.  These cells are subdivided into smaller areas by smaller folds which are covered by pointed horny papillae. Omasum:  It is roughly oval in shape and laterally compressed. It is situated at 7 th to 11th rib on right side.  Left surface is related to rumen, reticulum and abomasum.  The reticulo-omasal and omaso-abomasal orifices are at the upper side.  There are about 100 crecent shaped folds (laminae) in the cavity of omasum.  On the basis of the height, these folds are four or five varieties, arranged one after another in a cronological manner. The largest variety of folds are about 12 in number.  The surfaces are studded with numerous papillae. Abomasum:  It is the true stomach. It is placed more towards the left side.  Its two parts - fundus and pylorus separated incompletely by a constriction.  The fundus part is related to the ventral sac of rumen, reticulum and omasum.  The pylorus turns upward and becomes continuous with the duodenum.  There are about twelve oblique folds within the fundus and are thick glossy and reddish in colour.  The mucous membrane of pyloric part is devoid of any fold but wrinkled in appearance.  In the interior of the abomasum, three glandular areas (branched tubular gastric glands) are seen.  Cardiac glands are small in number and present close to the omaso-abomasal opening.  The fundic (highest in number) and pyloric glands are found at fundus and pylorus parts. Functions of gastric glands:  Cardiac glands are mucous secreting cells. Some cells called Argentaffin cells do not produce any secretion in the lumen but their secretions are distributed into the lamina propria and therefore called gastrointestinal endocrine cells and they secrete serotonin, gastrin etc.  The fundic glands has Chief or peptic cells that secrete pepsinogen and Oxyntic or parietal cells that secrete Hydrochloric acid.  Stomach of horse:  The stomach is in form of a simple saccular curved and J shaped structure with capacity of about 12 liters.  It is placed at the left side behind liver and diaphragm.  Close to the entrance of oesophagus, there is a rounded cul-de-sac known as saccus cecus.  The mucous membrane is divided into the esophagial (non-glandular) and a glandular parts separated by a rough line termed margoplicatus.  Glandular part is divided into cardiac, fundic and pyloric gland regions. Stomach of dog:  It is simple in form with capacity of about 2 liters.  The pyloric part is narrow and the fundic part is large and rounded.  There is a marked constriction between the fundic and pyloric parts.  There is no non-glandular area in the stomach of dog. Stomach of pig:  The stomach is simple with capacity of 6 liters.  Stomach has a conical diverticulum at the top. Stomach of fowl:  It has two distinct parts - glandular stomach and muscular stomach.  Glandular stomach is spindle shaped elongated structure connected with muscular stomach by constriction.  Muscular stomach resembles a biconvex disc which is popularly known as gizzard.  Internal surface of muscular stomach is covered by yellowish-brown membrane that can be pilled off by finger after dissection.  INTESTINE Intestine is a very long tube which begins at the pyloric end of stomach and ends at the anus. Most of its parts are situated in the right side of the abdominal cavity. It is divided into two parts: Small intestine (Duodenum, Jejunum, Ileum) and Large Intestine (Caecum, Colon, Rectum) Small intestine of cattle (Total length is 40 meters and Diameter is 5-6 cm): Small intestine starts from pylorus of stomach and ends at the ileum and caecum junction. Duodenum: It begins at the pylorus and presents S shaped curve. The first part of duodenum is attached to liver by omentum. The bile duct from liver opens into duodenum. The pancreatic duct enters the duodenum behind the entry of the bile duct. But in sheep and goat, bile duct and pancreatic duct join together and open into duodenum. Jejunum: It is longest part of small intestine. It is about 38 metres and has large number of close coils which are constricted and dilated to formed U shaped tubular loops by the attachment of mesentery. Jejunum is mobile part of intestine. It occupies space between right side of rumen and right abdominal wall. Ileum: It is the terminal part of small intestine and a thick tube. It is attached with cecum. Function of small Intestine: Small intestine performs two functions (a) digestive function and (b) absorptive function. Digestion in small intestine is done by various enzymes produced by Liver and pancreas that secretes enzymes in small intestine. For the digestive function it needs enzymes. Mucus is also secreted by Goblet cells. The absorptive function is done by its great length, mucosal fold, presence of villi and microvilli. The villi are finger like projections distributed all over the mucous membrane of small intestine. They are more in the Jejunum. Villi contains blood vessels, connective tissue, muscle fibers and lymphatic vessels. These lymphatic vessels are called lacteals. Large Intestine of Cattle (Total length is 11 meters and Diameter is 10 cm): The diameter of large intestine is greater at starting and afterward it reduces. Most of large intestine is situated at the dorsal aspect of the right side of abdominal cavity. It comprises of caecum, colon and rectum. Caecum: It is a blind sac and has capacity of about 7 to 8 liters. It is extended upto the right side of the pelvic inlet. It is attached to the proximal loop of colon at its dorsal aspect and with ileum at the ventral aspect. Colon: Colon is divided into ascending colon, transverse colon and descending colon. Ascending colon consists of proximal loop, spiral loop and distal loop. The spiral loop is continuous with the proximal loop and consists of two centripetal coils, a central flexure and two centrifugal coils. These coils are adhered to form a disc. The distal loop is continuous with the transverse colon. Transverse colon passes from the right to left. The descending colon runs caudally and curves near the pelvic inlet and becomes continuous with the rectum. The body of the colon do not have any sacculations and villi. Some Pyer's patches are present in the mucosa close to ileum. Rectum: It is the terminal portion of the intestine which is thick and presents constrictions and dilatations. Anus is the terminal opening of the G.I. tract situated below tail. Anal canal is about 2.5 cm in length and the mucous membrane is continuous with the skin. The mucous folds at the cranial part in the lumen are known as anal columns. Caudal part of the lumen is covered by skin. Intestine of Horse: (Small intestine is 21 meters and Large intestine is 7-8 meters) Duodenum- Pancreatic and hepatic duct open together in duodenum in pouch called hepato-pancreatic ampulla (Ampulla of Vater). Jejunum and ileum- Length is 20 meter. Practically no distinct demarcation is observed between them. Caecum: It is a very big comma shaped sac having capacity of about 30 liters. Colon- It is a very big tube and have been named separately on the basis of their presence in abdominal cavity. These are Right ventral, Sternal flexure, Left ventral, Pelvic flexure, Left dorsal, Diaphragmatic flexure and Right dorsal. The wall has bands and sacculations known as teniae coli. Intestine of Dog: (Small intestine is 4 meters and Large intestine is 0.5 to 1 meter) Duodenum- Hepatic and small pancreatic duct open in duodenum and large pancreatic duct opens 3-5 cm behind the preceding one. Jejunum- It has 7 to 8 coils and is the longest part. Ileum- Instead of opening into the cecum, the ileum opens into the colon directly. Caecum- It is closed tubular and curved structure. Rectum- The anal glands are situated at the submucous layer of the recto-anal junction. There are two lateral anal sacs, the ducts of which open on either side of its lumen. The sacs contain grey coloured fatty substance having offensive smell. Intestine of fowl: Duodenum- It has ascending and descending parts. The pancreas are placed between these two parts. Jejunum- At middle of jejunum, a short closed remnant of yolk sac is present called Meckel's diverticulum. Caecum- It is two in number, left and right. Colo-rectum-The straight portion of intestine which extend from ileum to the cloaca is termed colorectum. Cloaca- It comprises of three compartments - Coprodeum, Urodeum and Proctodeum. Cloaca ends and open to the outside by opening called vent. Coprodeum is the first compartment and its lining contains villi. Urodeum is the smallest in which urogenital ducts open. Proctodeum is also a short compartment in which the cloacal bursa called Bursa of Fabricius opens. Stomach and Intestine of fowl  Salivary Glands In Cattle: There are three pairs of salivary glands - Parotid, Mandibular and Sublingual.  Parotid Gland: It is triangular long reddish brown gland situated on the vertical part of ramus of mandible. Dorsal end is broad and placed close to the base of the ear. The Stenson's (parotid) duct leaves the gland at its medial aspect of the ventral part and goes forward on the medial side of the ventral border of mandible and opens at base opposite to second molar tooth of upper jaw.  Mandibular Gland: It is an elongated pale yellow lobulated gland situated along the medial side of the mandible. It is longer than parotid. The Wharton's (mandibular) duct leaves the gland from dorsal border and open at the caruncula sublingualis.  Sublingual Glands: There are two separate parts in each gland - superior and inferior. The superior (polystomatic) is a lobulated pale yellow gland situated under the mucous membrane of the floor of the mouth. Its many ducts opens on either side of the frenum linguae at the floor of the mouth. The inferior (monostomatic) is a pink coloured, thicker and shorter gland than the superior gland. There is only one duct for this gland which either joins the mandibular duct or opens separately at the base of the caruncula sublingualis, close to the opening of the mandibular duct. Salivary glands of horse: Parotid gland is the largest of all the salivary glands. The parotid duct opens opposite to last upper premolar tooth. Sublingual gland is single. Salivary glands of dog: Parotid is triangular in shape and its dorsal border is notched. Parotid duct opens opposite to upper 3rd premolar tooth. Mandibular gland is larger than parotid. The sublingual glands are two in either side and the ducts are arranged similar to those of ox. Zygomatic salivary gland is an extra gland found in dogs and its ducts open into the mouth opposite to last upper molar tooth. Salivary glands of fowl: There are number of salivary glands in fowl. Ducts of some glands (Maxillary, Palatine, Cranial lingual, Submandibular and Angular) open into the mouth. Others salivary glands (spheno pterygoid, caudal lingual, caudal submandibular, laryngeal) open into the cavity of pharynx.  Liver In Cattle:  It is the largest solid gland of the body with weight of 3-5 Kg  In fresh condition it is reddish brown in colour. It is placed in the right side of the abdominal cavity.  It is attached in the abdomen with five ligaments. T  The falciform ligament attaches the liver with the diaphragm.  Visceral surface presents impressions of different organs like pancreas, Omasum, reticulum, Abomasum.  Portal fissure is in form of linear depression that serves as hilus. It transmits portal vein, hepatic artery, bile duct and nerve fibers.  The dorsal border is thick. It has caudate lobe and renal impression.  Caudate lobe is connected to right kidney by caudate ligament and helps in formation of renal impression. Ventral border is thin and marked by the umbilical fissure.  The round ligament extends from this fissure to the umbilicus. Umblicus is the vestigial umbilical vein.  Hepatic duct comes out of liver through portal fissure.  Just after exit of hepatic duct, smaller duct arises from it upto neck of gall bladder known as cystic duct.  Gall bladder is long pear shaped musculo-membranous sac situated at the visceral surface of the liver.  Gall bladder not only functions as storage of bile but helps in concentration of bile.  The bile duct is formed by the union of hepatic duct and the cystic duct that opens into duodenum. Structure of liver:  Liver consists of number of small lobules separated by connective tissue.  The hepatic cells are arranged in cords in a radiating manner from the central vein.  Groups of three tubes i.e. hepatic artery, portal vein and bile duct are found in spaces in between the lobules known as portal triads.  Liver receives about 80% blood from portal vein and about 20% from hepatic artery.  Liver has Kupffer cells which are phagocytic. Liver of horse: The organ is extensive and thin having weight of 5 kg. It has three main lobes - right lobe, left lobe and middle lobe. Gall bladder is absent. Liver of dog: It has five lobes - Left lateral, left central, right lateral, right central and caudate. Gall bladder is present in a depression at the right central lobe. Caudate lobe has two parts- one on the right side called the caudate process and other on the left side called papillary process. Liver of fowl: It has two separate lobes- left and right Right lobe has a large gall bladder. The left lobe has no gall bladder.  Pancreas  It is soft gland situated mostly at the right side of abdominal cavity. Shape is flat and irregularly quadrilateral.  The colour of gland is reddish yellow and the weight is about 350 to 500 gm.  Dorsal surface is attached to diaphragm and liver.  Ventral surface is attached to dorsal sac of rumen, colon. Structure:  This is a compound gland composed of both exocrine and endocrine parts.  Gland has number of lobules inside.  Each lobule has numerous acini which are ramifications of pancreatic duct.  Each acinus has many pyramid shaped cells having round nuclei at base and zymogen granules at apical side.  Some small cells are located at the center called centro-acinar cells.  Ilets of Langerhans cells are endocrine parts of gland, are smaller than acinar cells and arranged in groups. Ilets of Langerhans cells contain Glucagon secreting Alpha cells and Insulin secreting Beta cells and somatostatin secreting Delta cells.  Pancreas has enzymes like Trypsinogen and Chymo-tryosinogen. Trypsingoen is converted to active trypsin by enzyme enterokinase produced by small intestine. Teypsin trhen activates Chymo-tryosinogen.  Pancreatic amylase acts on starch and produces oligosacharides and maltose.  Pancreatic lipase splits fats into free fatty acids and glycerol. Spleen  Spleen is hemo-lymph organ situated on left part of abdomen between left side of rumen and diaphragm.  It filters bood and lymph. Spleen forms erythrocytes in fetal life and lymphocytes after birth.  It is elliptical in shape with average weight of about 900 gms. Structure of spleen:  The spleen is surrounded by fibrous capsule containing smooth muscle cells.  From capsule fibromuscular trabeculae extend towards parenchyma of the gland.  From the hilus, many trabeculae carrying blood vessels radiate into the organ.  Arteries leave trabeculae and become surrounded by lymphocytes and constitutes White pulp.  Splenic tissue containing sinusoids are infiltrated with circulating blood and constitutes Red pulp.  From white pulp, the arterioles enter the red pulp and subdivide into few radiating capillaries.  Before formation of terminal capillaries, each artery become sheathed by reticular cells.  These capillaries open in splenic tissue of red pulp and from there blood cells percolates into blood circulation. Functions of spleen are phagocytosis, hemopoiesis, immune response and storage of RBC.  PHYSIOLOGY OF DIGESTIVE SYSTEM  Digestion is the process of breakdown of complex food into simpler form by alimentary tract and glandular secretions by glands for absorption of nutrients.  Herbivorous animals (Cattle, sheep, horse, goat) eat plant sources.  Carnivorous animals (dog, cat) eat animal sources.  Omnivorous animals (pig) eat both animals and plants sources.  In ruminants (cattle, sheep and goat) the stomach is large and complex whereas the large intestine is relatively small hence they are called as fore-gut digesters.  The simple stomach herbivores (horse) are called as hind-gut digesters because they have simple stomach but the large intestine is much more complex and voluminous. Functions of GI tract  To prepare nutrients for digestion.  To digest the nutrients.  To felicitate absorption of products of digestion.  Absorption of water.  As excretory organs to help in elimination of waste products.  As an endocrine gland to influence digestion and other metabolic functions.  Motility of the GI tract. Prehension of food  Prehension is the seizing and conveying of food into the mouth.  In cattle, large and strong tongue and incisor teeth of the lower jaw are prehensile organs.  Dog and Cat hold food with forelimbs or by tongue which is passed into mouth by head and jaw movements.  In horse, upper lip, tongue and incisor teeth are the main prehensile organs to collect food.  The clefted upper lip in sheep favours close grazing as compared to unclefted upper lip in goat.  The pointed lower lip functions as a prehensile organ in swine. Factors of digestion: Mechanical factors  Mastication (Grinding the food): It reduces size of food to provide greater surface area for enzymatic action and proper mixing with saliva. For drinking water, Horse, Bovine, Sheep and Goat do suction of water by keeping closed mouth beneath the fluid and tongue. In Dogs and cats, water is drawn into oral cavity by free end of spatula shaped tongue. Thus by mastication, initial digestion of carbohydrates is done.  Deglutition (Swallowing): It is act of passage of food from mouth to stomach through pharynx and oesophagus. It starts as voluntary action later on it becomes an involuntary after passing food. Soft palate is elevated to cut off communication with nasal passage. Closure of larynx by epiglottis make the bolus to enter directly into the oesophagus.  Regurgitation (Return of cud from stomach to mouth by ruminants like cattle, buffalo, sheep and goat): Drop in intrathoracic and intra-esophageal pressure will open cardia (Point of opening of oesophagus in stomach) and extra reticular contraction will push rumino-reticular cud to the mouth via esophagus.  Rumination (Chewing of cud): Reverse peristalsis/antiperistalsis is involved in bringing the gastric contents into the oesophagus during belching and regurgitation.  Remastication (Regrinding): In ruminants, only during remastication the food is properly chewed.  Redeglutition (Reswallowing)  Intestinal motility  Eructation (Elimination of gases)  Defecation (Elimination of fecal matters) Chemical factors: Digestive enzymes and Non-enzymatic chemicals: HCl (Hydrochloric acid), HCO3 Microbiological factors: Bacteria and Protozoa  How digestion occurs in rumen in ruminants:  Omasum is absent in Camel. So, camel is called as Pseudo-ruminant.  In ruminants, Microbial fermentation occurs in rumen by Anaerobic Bacteria and Protozoa.  Ruminants does not produce enzymes to digest cellulose of plants and depends on metabolic activity of microbes to utilize fibrous food. The pH of the rumen liquid ranges from 5.8 to 7.0.  Development of bacteria and protozoa in young ruminants occurs at very early age of about 6th week.  Rumen microbes digest all major carbohydrates of ruminant diet such as cellulose and hemicellulose.  Microbes take energy for their growth by fermenting cellulose, hemicellulose, sugars, starch.  Rumen microbes are responsible for the digestibility of about 70-85% dry matter of the diet resulting in the production of volatile fatty acids (Acetic, Butyric and propionic acids), gases like CO2, methane (CH4), ammonia (NH3) with small amounts of N2, H2, and O2.  From carbohydrates, microbes can synthesize good quality microbial proteins and B complex vitamins required for their growth and metabolic activities.  They also hydrolyse lipids, unsaturated fatty acids.  Proteins and NPN substances of dietary and salivary sources get degraded by the microbes and the released amino acids, NH3 are utilized for microbial growth and microbial protein synthesis.  Gases produced during fermentation are expelled by eructation. Blockage of eructation of gases leads to bloating of rumen, called as TYMPANY.  High grain feeding and lush pasture results in trapping of gases in rumen called “feedlot bloat”, “legume bloat” or acidosis. This is due to formation of foam and lactic acid in rumen which traps many small gas bubbles. Increased lactic acid formation in the rumen reduces rumen pH, leading to lactic acidosis.  Only the abomasum called true stomach secretes gastric enzymes and HCl. Significance of volatile fatty acids in ruminants:  Propionic acid is glucogenic and provides glucose through gluconeogenesis.  Acetic and butyric acids contribute the energy needs of animals as they are ketogenic acids and cause the formation of ketone bodies like acetone, acetoacetic acid and beta hydroxy butyric acid.  The ketone bodies serve as energy source in tissues like CNS and heart.  The acetic acid is also the precursor for milk fat synthesis. Protein digestion in ruminants:  Rumen microbes has proteolytic activity. The diet of ruminants contains proteins and non-protein nitrogenous (NPN) substances like ammonia, nitrates and urea.  20-100% protein in rumen will be degraded to ammonia called Rumen degradable protein (RDP).  The fraction which is not degraded by the microbes in the rumen escape and by-pass the microbial digestion, called as Rumen undegradable protein (UDP) and reaches small intestine for enzymatic digestion.  When the dietary proteins enter the rumen, the RDP fraction is hydrolysed by microbial proteases and produce amino acids as end products which are absorbed into the microbial cell bodies.  Within microbes, these amino acids are used for formation of microbial protein or for energy production. Fate of rumen lipids:  Rumen microbes hydrolyse dietary lipids to glycerol and free fatty acids with help of enzyme lipase secreted by gastric glands.  Glycerol is converted to propionic acid by the microbes. Digestion in Abomasum: Functions of HCl:  Activates pepsinogen into active pepsin for hydrolysing proteins  Also function as antiseptic in the stomach.  Secretion of acid by the parietal cell. Carbonic anhydrase enzyme with help of CO 2 and H2 O generates Carbonic acid (H2CO 3) which ionizes and generates Hydrogen ion (H+) and Bicarbonate ion (HCO3−). HCO3− from parietal cell diffuses in lumen in exchange for Cl−. H+ is also transported to the mucosal side alongwith Cl−. Rennin (Chymosin, Rennet)  It is a milk coagulating enzyme, present in young animals (Calf, Lamb)  Secreted as prorennin (inactive form of rennin)  Prorennin is activated by HCl  Rennin reacts with Milk protein called Casein in presence of calcium ion to form calcium para-caseinate to delay the passage of milk through stomach for prolonged action of pepsin enzyme on casein. Small Intestine: Concerned with digestion and absorption. It has various enzymes like Enterokinase to activate trypsinogen to trypsin, Dipeptidase to convert proteases to peptones and amino acids, Maltases to convert maltose to glucose, Lactases to convert lactase to glucose and galactose, Sucrase to covert sucrose to glucose and fructose. Action of Liver in digestion: Liver is the largest gland in the body and has number of important functions.  Secretion of bile  Formation and storage of glycogen and Regulation of glucose level in blood.  Destruction of uric acid  Synthesis of fatty acids from carbohydrates and proteins and Oxidation of fatty acids and to form ketone bodies.  Detoxification of poisonous substances brought to it by blood.  Destruction of old erythrocytes  Formation of fibrinogen and other plasma factors like prothrombin  Metabolism of hormones. Bile: Secretion of bile is necessary for fat digestion and its absorption. Gall Bladder is a storage organ of the bile. Horses, rat, deer, giraffe, camel, elephant do not have gall bladder. Removal of gall bladder does not result in any physiological disturbance of digestion. Bile is viscid, greenish or greenish yellow liquid with bitter taste. The constituents of the bile are:  Bile salts: Sodium or Potassium salts of cholic acids, deoxycholic and chenodeoxy cholic acid. They are formed by liver and found in bile. Main bile acid is cholic acid. They have powerful surface tension reducing ability (Detergent effect) and thereby act as a strong emulsifying agent and assist in fat emulsification. This allows to break the fat globules into small sizes. Bile salts are reabsorbed again in blood and reaches liver to be secreted again in bile so that these salts can circulate about 18 times before being voided in the feces.  Bile pigments: Bile pigments are the waste products of RBC’s Haemoglobin (Hb) breakdown (after life span of 120 days) that give yellow/green colour to bile. Bilirubin pigment is readily oxidized into biliverdin which gives green colour to the bile of the herbivores. Bilirubin is normally present in blood in low concentration. Haemoglobin splits into haem and globin parts. Globin is transported as protein for reutilization. Haem gives free iron that is transported in the blood by transferrin. The first pigment formed is biliverdin that becomes free bilirubin and is released into the plasma. Free bilirubin strongly combines with plasm albumin and is transported through the blood that is insoluble and can’t pass through kidney. Within hours, this free bilirubin reaches liver and is released from the plasma albumin and combines with other proteins inside the liver. It is then conjugated  with glucuronic acid to form bilirubin glucuronide. In intestine, ½ the conjugated bilirubin is converted into highly soluble urobilinogen and stercobilinogen by bacterial action. Some of the urobilinogen gets absorbed again into the blood and after exposure to air the urobilinogen is oxidized to urobilin and excreted through urine, giving yellow colour to urine. Stercobilinogen is converted into stercobilin and excreted through the faeces.  Bile acids:  Cholesterol  Mucin  Inorganic substances like Na, K, Ca, Cl and HCO3 Function of bile as whole  Bile acids brings about the absorption of fat and fat soluble vitamins like Vit. A, D, E and K.  Mucin and pseudomucin in the bile act as a stabilizers of fat emulsion in the intestine.  Bile has a marked antiseptic property and regulates the bacterial growth in intestine.  Bile has a mild laxative effect.  In absence of bile in intestine, fat absorption is reduced and other food particles like proteins are coated with fat that escape enzymatic digestion and leads to putrification of protein that gives offensive odour to feces.