Digestive System of Frog

Digestion is the process of breaking down of complex nutrients in the food into their simpler subunits which can be easily absorbed and utilised for energy production. Frog’s digestion begins with ingestion of the food into the mouth followed by chewing, swallowing, digestion, absorption, assimilation and gets completed with expulsion of the waste materials.

To facilitate this process, the Frog’s Digestive system consists of the tube like alimentary canal for the passage of food and the associated digestive glands which secrete the digestive enzymes for the chemical break down.

Alimentary canal

Frog has a complete alimentary canal consisting of a tube and two openings at each end of the tube. The tube opens with the mouth and ends in the cloaca which is analogous to the anus in human beings. Based on the function they perform, the tube varies in diameter and is long and coiled which increases the surface area to facilitate absorption. 

Alimentary Canal – Parts And Functions

Parts	Function
Mouth and Buccal cavity	Ingestion of food
Pharynx	Sound production and food passage
Oesophagus	Passage of food and digestion begins
Stomach	Digestion
Small Intestine	Absorption
Large Intestine	Passage of waste products with minimal absorption
Cloaca	Expulsion of waste products

Mouth and Buccal Cavity

The mouth is a wide opening that extends from either side of the snout. It is bound by the fixed upper jaw bone and the flexible lower jaw bone. 

The mouth opens into the buccal cavity which is large, wide and shallow. It is lined with ciliated columnar epithelium interspersed with mucus glands which secretes mucus to lubricate the food to facilitate easy passage in the alimentary tract.

The tongue of frog secretes a saliva like substance that makes it sticky to trap the prey. Due to lack of true salivary glands digestion does not begin in the oral cavity of frog.

The Buccal cavity consist of:

1. Teeth
2. Tongue
3. Internal Nares
4. Bulging or orbit

Teeth

The upper jaw is made up of 2 bones namely the maxilla and premaxilla. Both of them has a row of teeth which are pointed backwards. The Vomer bones which form the roof of the buccal cavity has two groups of vomerine teeth. Teeth are absent in the lower jaw. Unlike human beings, the teeth of the frog is not used for chewing. It used only to hold its prey within the mouth to prevent it from escaping.

All the teeth are similar in morphology (homodont). The teeth are acrodont i.e instead of being set in sockets they are attached to the jaw by a broad base made of a bone-like substance. The teeth are replaced numerous times in their life which is known as Polyphyodont

The protruding free segment of tooth is called crown. It is made up of a hard ivory like substance called dentine which is covered in the tip by enamel. Enamel is the hardest part of tooth and has a smooth, glistening surface.

The tooth is traversed by the central pulp cavity which opens at the side. The pulp cavity is filled with pulp containing connective tissues, blood vessels, nerve and odontoblast cells that are progenitors for the tooth.

Tongue

Frogs have a large and muscular tongue which lies in the floor of the mouth. The anterior end is attached to the inner aspect of lower jaw and posterior end is bifid and lies freely coiled in the buccal cavity. When the frog catches its prey, the tongue is flicked outside, snapping on the prey and retracting it inside the mouth. The tongue secretes a slimy substance which makes the prey to stick to the tongue. 

Internal nostrils

The external nares on the snout opens into the buccal cavity through the internal nares. The pair of small openings open in front of the vomerine teeth. They are part of the respiratory system

Bulging of orbits

After retracting of the prey into the buccal cavity, it is held down by the vomerine teeth. During the process of swallowing, the eyes of the frog sinks into the orbit. This causes a bulge on the roof of the buccal cavity behind the vomerine teeth. The bulging of orbits facilitates the passage of food into the pharynx

Pharynx

The buccal cavity continues posteriorly as pharynx, forming a diffuse bucco pharyngeal cavity. There is a midline elevation on the floor with a longitudinal slit like aperture forming the glottis. The glottis opens into the laryngo tracheal chamber which then reaches the lungs. On either side of the roof lies the Eustachian aperture that leads to the middle ear cavity. The pharynx extends posteriorly to the oesophagus through the gullet.

There are a pair of special apertures on either side on the floor of the pharynx that leads to the vocal sac in male frogs. The main function is to amplify the mating calls of the male frogs while some frogs use it to carry the tadpoles to the water sources. 

Oesophagus

The Oesophagus forms a short and wide, muscular conduit for the food to reach the stomach. The epithelial lining of oesophagus has scattered mucus glands which secrete alkaline digestive juices. The entire epithelial lining is thrown into multiple longitudinal folds which allow the oesophagus to expand during passage of food. The food moves through it by a wave of muscular contraction called peristalsis.

Stomach

Stomach is a curved muscular bag present on the left side of the peritoneal cavity. It is approximately 4 cm long with a large and broad anterior part called cardiac stomach and short and narrow posterior part called the pyloric stomach. The stomach is maintained in its position by its attachment to the posterior body wall by a mesentery called mesogaster.

The inner mucous layer of the stomach has oxyntic cells which secrete hydrochloric acid and gastric glands which secrete digestive enzymes like pepsinogen. The mucous layer is thrown into longitudinal rugae which increase the surface area of the stomach and makes it distensible to store the food.

The pyloric end of the stomach has a circular ring like musculature called the pyloric valve which prevents the retrograde movement of food from small intestine to stomach and also provides the ample time for digestion before the food gets emptied to the small intestine. 

Small Intestine

Small intestine is a long (approximately 30 cm), coiled, narrow tube attached to the midline of dorsal body wall by a mesentery similar to the stomach.

It is divided into: 

• Duodenum
• Ileum

Duodenum is a small tube that starts from the pyloric end of the stomach and runs upwards to form a U shaped conduit.  The common hepato pancreatic duct opens into duodenum and empties the digestive enzymes from liver and pancreas which acts on the food in the duodenum. The mucosal lining has transverse folds.

Ileum is the longest part of the alimentary canal which is highly coiled to loops which open into the rectum. Although there are many longitudinal folds in the mucosal lining there are no true villi or secretary glands.

The small intestine is the major organ of digestion and absorption.

Other than glandular cells, there are Goblet cells in the small intestine. They are mucus producing cells with oval vacuoles and basal nucleus.

The intestinal juices are collectively called Succus entericus which contains the following enzymes

• Enterokinase – Converts the inactive Trypsinogen to Trypsin 
• Erepsin  – group of proteolytic enzymes that convert polypeptide to amino acids.
• Maltase – converts maltose to glucose.
• Sucrase or invertase – converts sucrose to glucose and fructose
• Lactase – converts lactose to glucose and galactose.
• Lipase – converts emulsified into fatty acid and glycerol

Large Intestine Or Rectum

Large intestine is a short and wide tube for storage of waste products at the end stage of digestion and reabsorption of water. It opens into cloaca through anus. The muscles form a circular anal sphincter which guards the anus. 

Cloaca

Cloaca is the common sac-like structure through which both anus and urogenital apertures open. Cloaca opens to the exterior by cloacal aperture in the posterior end of the body.

Digestive Glands of Frog

The major digestive glands of frogs are liver and pancreas.

Liver

Liver is the largest gland in the body. It is a reddish brown organ lying close to the heart and lungs. It has 3 lobes namely right lobe, left lobe and median lobe. Microscopically, the hepatic cells are polygonal in shape and secrete a yellowish green alkaline fluid called Bile. The colour is due to a pigment called Bilirubin. 

The bile secreted by the liver is stored in a thin walled sac called Gallbladder and is secreted through a cystic duct. The cystic duct joins with the hepatic ducts of liver to form the common bile duct which passes through the pancreas where it is joined by pancreatic ducts to form the hepato pancreatic duct that opens into the duodenum. 

Bile

Bile is alkaline in nature and contains bile salts like sodium bicarbonate, sodium glycocholate, sodium perocholate, etc. It is secreted into the duodenum by hepatopancreatic duct where it neutralises the acidic chime and activates the lipase enzyme in pancreatic juice. The main function of bile is emulsification of fat which is then acted upon by the pancreatic lipase. 

Pancreas

Pancreas of frog is an irregular flattened and branched organ which is pale yellow in colour. It lies in the peritoneal cavity between the stomach and duodenum. Similar to human pancreas, they have islets of Langerhans which produce the hormone Insulin which is essential for carbohydrate metabolism.

The digestive juices of pancreas contain enzymes like pepsinogen for protein metabolism. All the pancreatic secretions are emptied through hepatopancreatic duct into the duodenum.

Pancreatic Juice

The pancreatic juice is a mixture of several enzymes that enables the digestion of carbohydrates, proteins and lipids. It’s alkaline nature neutralises the acidic chyme in duodenum. Pancreatic juice contains the following

• Inactive Trypsinogen – activated to Trypsin by the Intestinal enterokinase. Trypsin breaks down the proteoses, peptones and polypeptides to amino acids. They are second step proteolytic enzymes while the first step enzyme is Pepsin which breaks the complex protein to peptones and polypeptides
• Amylase or amylopsin – This is the first digestive enzyme to act on carbohydrates as frogs lack salivary amylase. It reduces starch (polysaccharides) to maltose (disaccharides).
• Lipase or steapsin – It converts the emulsified fats into fatty acids and glycerol.

Physiology of digestion in frog

Although most of the frog species are carnivorous vertebrates which feed on small insects, worms, molluscs and fish, a few species are omnivorous and feed on plant matter. Once the prey is caught by the flick of tongue, it is trapped in the buccal cavity. Due to lack of salivary glands, no digestion takes place in the mouth

From the buccal cavity the food enters the oesophagus. A wave of contraction is generated in the muscular wall of the oesophagus from above downwards called peristalsis. This facilitates the churning of food and its movement to the stomach.

Once the food reaches the stomach, a hormone called Gastrin is secreted by the stomach which in turn activates the oxyntic cells to secrete HCl. The acid catalyses the conversion of the inactive Pepsinogen to active Pepsin which breaks down the proteins to peptones and peptides.

The partly digested semi solid food is called Chyme. The gastric emptying time for frogs is approximately 2 – 3 hours after which the Chyme reaches the duodenum.

Gastric Digestion

The presence of Chyme in the duodenum stimulates the secretion of several intestinal hormones. Enterogastrone is a hormone which inhibits the secretion of HCl thereby preventing it from damaging the empty stomach. Cholecystokinin causes the contraction of gall bladder and release of bile.

Other hormones like Secretin and Pancreozymin stimulate the pancreatic secretions and Enterocrinin stimulates the secretion of digestive juices in the Small intestine collectively called Succus entericus.

Intestinal Digestion 

Chyme in Duodenum Stimulates secretion of the following hormones	Functions
Enterogastrone	Inhibits HCl secretion
Cholecystokinin	Gall Bladder contraction causing bile secretion
Secretin and Pancreozymin	Secretion of Pancreatic Juices
Enterocrinin	Secretion of Succus entericus
Bile	Emulsifies fat
Enterokinase  of Succus entericus	Converts Pancreatic Trypsinogen to Trypsin
Trypsin (Pancreatic)Erepsin in Succus entericus	Breaks down Peptones and Polypeptides to Amino acids
Pancreatic LipaseLipase in Succus entericus	Breaks down emulsified fat to fatty acid and glycerol
Pancreatic amylase	Breaks down starch to maltose
Maltase in Succus entericus	Converts maltose to glucose

Along with digestion, maximum absorption takes place in the small intestine which has a large surface area owing to the villi-like mucosal folds. After passing into the large intestine, the undigested waste material is excreted through the cloacal aperture. 

The absorbed nutrients are the simplest subunits like glucose, amino acids and fatty acids. They are carried in hepatic circulation to the liver where it is utilised for energy production or storage. The fatty acids are taken through the lymphatics and the by the veins.

Most of the energy production is by utilisation of glucose and excess glucose is stored as glycogen in liver and skeletal muscles. Amino acids are used in repair and building of tissues. Thus the process of digestion gets completed with absorption, assimilation and egestion.

Frequently Asked Questions

References

1. Comparative morphology of the digestive tract of two Neotropical tree frogs. Bruno S.L.Valverde^aLara ZácariFanali^aLilianFranco-Belussi^bcClassiusde Oliveira^c
2. https://www.bioscience.com.pk/topics/zoology/item/325-comparative-anatomy-rabbit-and-frog-digestive-systems
3. https://en.wikipedia.org/wiki/Frog 
4. Tesler, P. (1999). “The amazing adaptable frog”. Exploratorium:: The museum of science, art and human perception. 
5. The Anatomy of Frog – Alexander Ecker
6. A laboratory guide to Frog’s Anatomy – Eli C.Minkoff
7. Liver regeneration observed across the different classes of vertebrates from an evolutionary perspective- Blanca Delgado -Coello