Digestive System of Frog

The digestive system of frog consists of organs which are concerned with capturing of food, its digestion with the help of certain enzymes, absorption of the digested food and removal of the waste food. It consists of different parts.they are:

• Alimentary canal
• Digestive glands
• Physiology of digestion and absorption

Alimentary canal:

The alimentary canal is a long coiled tube of variable diameter starting from the mouth and ends into choca(aperture). Mouth, buccopharyngeal cavity, oesophagus, stomach, small intestine, large intestine are the parts of the alimentary canal.

Mouth:

The mouth is wide transverse slit-like aperture present on snout which is bounded by bony jaw having immovable lips. The mouth leads into a wide cavity i.e. buccopharyngeal cavity which is bounded by upper and lower jaw and it is lined with columnar ciliated epithelium having mucous glands.The following structures are seen in the upper jaw.

Internal nostril:

A pair of internal nostril lies anteriorly on the roof of the buccal cavity through which fresh air enters inside into the buccopharyngeal cavity.

Inbulging of eyeball:

Dorsally on a roof of the buccal cavity behind vomerine teeth, there is a socket for eye and the frog depress the eye while swallowing the food.

Teeth:

Teeth are premaxillary, maxillary which is arranged in a row and a pair of a patch of vomerine teeth which are attached to the premaxillary, maxillary, and vomer bone. The teeth are identical so called isodont. The teeth are polyphyodont i.e. they can replace for many times. The teeth are called acrodont because they are directly attached to a bone.

Function:

Teeth are used to hold the prey and paralyze it.

Structure:

Teeth consists of two parts; the base which is directly attached to the bone. So, it is called bony base and the crown consist of a hard substance called dentine which is covered by the shiny substance called enamel. At the inner side of teeth, there is a pulp cavity which is soft and vascular and consists of connective tissue, blood vessels, nerve fibre and layer of odontoblast cells.

Crown:

The lower jaw contains a large fleshy, sticky, muscular tongue which is bifid in nature.

Buccopharyngeal cavity:

The posterior or narrow part of the buccopharyngeal cavity is called pharynx. Dorsally it bears a large transverse opening called gullet which leads to oesophagus ventral to it a median slit-like aperture called glottis which leads to lungs through the laryngotracheal chamber on either side dorsally. The pharynx bears a spindle shape opening of eustachian tube which leads to the middle ear ventrally on either side. In case of male, opening of vocal sac acts as a resonator.

Oesophagus:

The gullet leads to a short wide muscular distensible tube called oesophagus. It has the inner lining of mucous glands which helps in lubrication of food. Its wall contains highly developed muscles called muscularis which play the role in peristalsis.

Stomach:

The stomach is large sac-like structure about 5-7 cm long.It is located on left side of the abdominal cavity. It has two parts. The upper part is larger and the lower part is nanmopyloric having sphincter. It is kept in its position by the help of mesentery called mesogaster. The stomach plays an important role in the storage of food, mixing of food with digestive enzymes and it also helps in digestion.

Small intestine:

The small intestine is situated in the loops supported by the mesentery. It consists of duodenum and ileum.

Duodenum:

It is U-shaped. It is 3-5 cm long.It runs parallel to the stomach and receives hepatopancreatic duct at the anterior end. It plays the role in digestion as its wall contain digestive glands secreting intestinal juice.

Ileum:

The ileum is very long coiled structure about 25 cm long held in position by mesentery. It plays an important role in the absorption of digested food which is facilitated by very long narrow tube internally having finger-like projections called villi which are richly supplied with blood vessels.

Large intestine:

The large intestine includes rectum which is short, wide, flask-shaped structure.It is 4 cm long with highly developed circular muscle fibre which helps in defaecation.The mucosa layer is thick and contains mucous gland.

Cloaca:

The posterior sac-like structure is called cloaca.Its wall contains opening of the urinary bladder, ureter, and ovisac. Cloaca opens outside by an aperture called cloacal aperture or vent.

Digestive glands:

There are two large digestive glands.They are liver and pancreas.

Liver:

The liver is a largest gland reddish brown in color with two lobe right and left lobe is subdivided into further sub-lobe. In between right and left lobe ventrally gull bladder lies which have the greenish sac-like structure which store bile. The duct of gull bladder is cystic duct. It combines with a duct of lobes of the liver called hepatic duct forming common bile duct which unites with pancreatic duct forming hepatopancreatic ducts which open into the abdomen.

Function of liver:

• The liver produces bile juice containing NaHCO₃ which neutralizes acidic chyme so that further digestion occurs. It has sodium salts like sodium-taurocholate, glycocholate which emulsify fats.
• Excess of glucose is converted in the liver to glycogen and stored.
• Clotting factors like prothrombin, fibrinogen is produced by the liver.
• Anti-clotting factor-heparin produced which prevent clotting of blood in blood vessels.
• Vitamin A is synthesized and stored.
• It stores Fe and Cu.
• Urea is produced by combining ammonia to CO₂ and removed.
• Kupffer cell engulfs bacteria; so, protective in nature.
• Albumin is produced which play the role in osmoregulation.
• Heat is produced in the liver.
• Toxic substances from blood are removed through the liver.

Pancreas:

It is the second largest, light yellow flattened and irregular. It lies in between stomach and duodenum. It is a heterocrine gland. The aim secrete digestive enzymes whereas islets secrete hormones(insulin, glucagon). It is held in place by mesentery.

Function:

Pancreas is heterocrine gland and the exocrine part secrete digestive enzymes like trypsinogen, amylase, lipase etc which play an important role in the digestion of all food substances.

The endocrine part secrete hormones like insulin and glucagon which playrole in the metabolism of carbohydrate.

Physiology of digestion:

Ingestion:

The frog is carnivorous animal feeding on insect, worms, arachnids, crustaceans and molluscs. Sometimes they may be cannibals. The tongue is bifid sticky ptotrusebal and the prey attached to it and is withdrawn into the buccopharyngeal cavity. The hook-like teeth prevent the escape of prey and paralized the prey. The inbulging of eyeball depressed and the food is propelled towards gullet. The pharyngeal muscle contracts which help in the passing of food towards the oesophagus. The mucus secreted by mucous gland helps in lubrication of food.

Digestion in oesophagus:

There is only physical digestion or mechanical digestion. As there is no any enzymatic activity, only the larger food substance breaks into smaller form due to peristalsis of the wall of the oesophagus. The mucus gland secretes mucus which helps in easy passage of food. Its wall also secretes pepsinogen which only becomes active after reaching the stomach.

When food enters into stomach its wall secrete gastrin hormone which stimulates the gastric gland to secrete gastric juice. The gastric juice contain little amount of HCl but it has very important function: it kills the bacteria

• It kills the bacteria and fungus present in food.
• It dissolves the inorganic salt and makes the food soft.
• It activates pepsinogen.

Pepsinogen + HCl → pepsin

Pepsin + protein → peptones and proteases

The mucus lubricates the food and protects the mucosa layer. The food is mixed with gastric juice and remain in the stomach for 3 to 4 hour and there is vigorous mixing called churning process. The food is converted to the acidic paste-like food substance called chyme.

Digestion in intestine:

When acidic chyme enters duodenum through the pyloric constriction. Its wall secrete different types of hormones:

• Enterogastrone
• Cholecystokinin
• Secretin
• Pancreozymin
• Enterocrinin

Enterogastronstops the activity of gastric gland.The cholecystokinin stimulates the gall bladder to release bile juice.The secretin and pancreozymins stimulate the pancreas to release pancreatic juice.The enterocrinin stimulate the intestinal gland.

Bile juice:

• The bile juice neutralizes the acidic food.
• It emulsify fats.
• It prepares medium for pancreatic juice.

Pancreatic juice:

The pancreatic juice is the alkaline substance which contains different type of enzymes.

• The trypsinogen is activated by enterokinase enzyme of intestinal gland called succus entericus.Trypsinogen is converted to trypsin which acts on peptones and proteoses and converts into the polypeptide.
• The amylase acts on starch and converts into maltose.
• The lipase acts on emulsified fats and converts into fatty acid and glycerol.

Intestinal juice:

The intestinal gland secretes intestinal juice which contains different enzymes like enterokinase and enteropeptide.

• Peptidase acts on polypeptides and converts into the amino acid.
• The lipase acts on emulsified fats and converts into glycerol and fatty acid.
• The maltase acts on maltose and converts into glucose.
• Sucrase acts on sucrose and converts into glucose and fructose.
• Deoxyribonuclease acts on DNA and ribonuclease acts on RNA to convert into nucleotide.
• Lactase and lactose convert into one mole ule of glucose and galactose.

The digested food is absorbed in the intestine villi. The monosaccharide, glucose, minerals and amino acid directly passed to capillaries whereas fatty acid and glycerol are absorbed in lymph vessels called lackedls. The food is used to liberate energy and excess of it are converted and stored in the form of glycogen and fats. The undigested food is discarded out which is called egestion.