In this frog digestive system in order post we have briefly explained about frog’s alimentary canal, physiology of digestion, and food absorption in frog digestive system.
Frog Digestive System
The main components are the alimentary canal and the frog digestive system glands. Mastication, digestion, and absorption occur in the alimentary canal, while the frog digestive system glands secrete enzymes that aid in the digestion of the ingested food.
The frog’s alimentary canal is essentially a coiled tube of varying diameter that extends from the mouth to the cloacal aperture. It is made up of the mouth, the buccal cavity, the pharynx, the oesophagus, the stomach, the intestine, and the cloaca.
Frog Digestive System
The alimentary canal begins with an aperture known as the mouth opening. The mouth has a very wide gape that is bounded by upper and lower jaws and is covered by immovable lips.
2. Buccal Cavity
The mouth opens into a large and broad oral or buccal cavity that lies between the two jaws the upper and lower jaws. The upper jaw is fixed or immovable, whereas the lower jaw is hinged and can move up and down in the vertical plane freely. The ciliated, columnar epithelium that lines the buccal cavity contains mucus secreting glands. Mucus acts as a lubricant for the food. Salivary glands are absent in frogs.
The upper jaw has a row of closely set, small, uniform, hook-like pointed teeth over the premaxillae and maxillae, whereas the lower jaw does not. Aside from these, two more patches of teeth can be found on either side of the median line of the roof of the buccal cavity; these are known as vomerine teeth because they are born on the vomer. These teeth are not used for chewing, but rather to keep the captured prey from escaping.
The teeth are homodont, which means they are all the same shape. Instead of being set in sockets, they are curved backwards and attached to the bones (acrodont). Each tooth has a conical shape and is made up of two parts: the base and the crown. The base is made of a bone-like substance and is attached to the jaw bone.
Frog Digestive System: Buccal Cavity
The crown is the free end of the tooth and is made of dentine that is crossed by numerous fine branching canals or canaliculi that lead from the interior of the tooth. The crown’s tip is coated with a very hard, resistant, and gleaming white layer of enamel substance. Each tooth has a cavity called the pulp cavity in the interior.
It is filled with a highly vascular and nutritive tissue or pulp, as well as odontoblast cells that produce new material for tooth growth. Throughout a person’s life, old and worn out teeth are constantly replaced by new ones that are produced beneath the old ones that are cast out or lost. Polyphyodont teeth are so named because they are replaced several times during the frog’s lifetime.
The buccal cavity has two openings in its roof near the vomerine teeth, the internal or posterior nares connecting with the nasal cavities and allowing respiratory gases to pass to and from the buccal cavity during respiration.
The large, thick, fleshy, and protruding tongue is attached to the inner border of the lower jaw in front and free and notched behind, and it lies on the floor of the buccal cavity. Its upper surface is covered with taste buds in the form of small papillae and mucous glands, the secretions of which make the tongue sticky. No frog digestive system enzymes are produced by taste buds or mucous glands.
To capture and engulf insects, the tongue can be thrown out and quickly retracted. According to Hartog, the tongue is thrown out by the sudden flow of squeezed lymph from one lymph sac to another as a result of muscular contraction, but swallowing is accomplished simply by raising the floor of the buccal cavity in which a flat hyoid cartilage is embedded.
d. Bulgings of Orbits
Two large, oval pale areas, the bulgings of eyeballs, are visible on the roof of the buccal cavity behind the vomerine teeth. During food swallowing, the eyes are depressed into the buccal cavity, pushing the food into the pharynx.
Frog Digestive System
The buccal cavity narrows behind the pharynx, which then opens into the oesophagus via the gullet. The buccal cavity and pharynx are colloquially known as the buccopharyngeal cavity. A wide eustachean opening communicates with the middle ear in the roof of the pharynx on either lateral side.
The glottis, a median slit in the pharynx behind the tongue, guards the lungs’ entrance. It is always open when breathing but closes when food is swallowed. In male frogs, two vocal sac openings form in the angle of the lower jaw on the floor of the pharynx. When croaking, these serve as resonators.
The gullet connects to the oesophagus, a short, broad, and muscular portion of the alimentary canal. This section of the alimentary canal is very short due to the lack of a neck, but it is highly distensible because its inner lining is thrown into a large number of longitudinal folds that allow the oesophagus to expand sufficiently during the passage of ingested food through it to the stomach. It opens into the stomach in such a way that no demarcation line between the oesophagus and the stomach is formed.
It is the most important part of the alimentary canal (frog digestive system), where ingested food is digested with the help of frog digestive system enzymes secreted by the digestive system of frog glands located in its wall. It is a wide, curved tube that connects the oesophagus and the intestine.
A mesentery, the mesogaster, suspends it into the left side of the body cavity, or the coelom. It is divided into two sections: the large, wider anterior cardiac stomach and the smaller, narrower posterior pyloric stomach.
The internal lining of the stomach has numerous longitudinal folds that allow the stomach to expand as needed. It has multicellular gastric glands that secrete pepsinogen enzyme and unicellular oxyntic glands that secrete hydrochloric acid in its mucous epithelium.
The pyloric end of the stomach is constricted, and the sphincter guards the opening into the small intestine (a circular ring-like muscle). It controls the movement of food from the stomach to the intestine.
The stomach leads into the long, tubular and coiled intestine. It is also attached to the dorsal body wall by mesentery. It consists of two parts:
- The small and
- Large intestine.
a. Small Intestine
The small intestine is divided into loops that are held together by a fan-like membrane called the mesentery. The duodenum is the anterior region of the small intestine that curves upwards to form a U with the stomach; the remainder of it is known as the coiled ileum. A common hepatopancreatic duct from the liver and pancreas opens in the duodenum, bringing bile and pancreatic juice. The internal mucous lining forms low transverse folds.
b. Large Intestine
The ileum is a long, narrow, tightly wound tube that connects to the rectum at its lower end (large intestine). The ileum’s internal mucous lining has a number of longitudinal folds. Higher vertebrates lack true villi, glands, and crypts. This section is responsible for food digestion and absorption. Its lower end is connected to the cloaca by a spinctered anus. Its mucosal lining is characterised by low longitudinal folds.
It is a small sac-like structure that receives anus and urinogenital aperture openings. The cloaca opens to the outside via a vent or cloacal spening located at the back of the body.
Physiology of Digestion
The frog is a carnivore that feeds primarily on earthworms, spiders, snails, fish, smaller frogs, and other small insects, which it captures and swallows whole directly into the stomach using its protractible tongue.
Its method of prey capture is remarkable. When feeding, the frog sits in an area where insects congregate. When a prey comes too close, it opens its mouth, flicks out its sticky tongue, and strikes the prey. The prey adheres to the tongue as soon as it comes into contact with it. The tongue is now retracted into the buccal cavity.
The presence of hook-like inwardly directed maxillary and vomerine teeth would prevent the prey from escaping once it was caught in the buccal cavity. It is pushed into the oesophagus from the buccal cavity by the contraction of the pharyngeal wall, and then into the stomach by peristalsis caused by the contraction and dilation of the muscular wall of the oesophagus.
During feeding, whatever food is ingested by the animal that contains complex organic substances that are not immediately useful because they are insoluble and cannot diffuse through the mucous membrane lining of the alimentary canal must be subjected to the physical and chemical changes of digestion in order to be transformed into soluble forms for the immediate use of the animal.
Physical changes are caused by the peristaltic movements of the alimentary canal, whereas chemical changes are caused by organic catalysts known as enzymes, which only hasten chemical reactions without being changed themselves.
They are complex proteins that are produced by the exocrine glands and always act at the optimal body temperature. They can also reverse the reaction, which means that the substances they have altered can be reformed. They are classified according to the type of food on which they act. Thus, proteolytic enzymes digest proteins, diastatic (amylolytic) enzymes digest carbohydrates, and lipolytic enzymes digest fats.
(i) Buccal Digestion
Because the buccal epithelium lacks a frog digestive system gland, the captured prey is not subjected to any physical change (mastication) or chemical action in the buccal cavity.
The prey is pushed directly into the oesophagus from the buccal cavity, where it undergoes physical changes as a result of the constant peristaltic movement of its wall. In addition to mucus, the oesophageal glands secrete an enzyme called pepsin, but no digestion occurs because it does not become active until it reaches the stomach, whereas mucus simply makes the active food inactive and soft, making the passage easier.
(ii) Gastric Digestion
The food reaches the stomach as a result of the oesophageal relaxations and contractions. The stomach is involved in performing three main functions
When food enters the stomach, so-called peristaltic movements allow it to be pushed down, disintegrated into small fragments, and thoroughly mixed with the help of gastric juices secreted by gastric glands found in the stomach’s internal lining. When the glands are activated by the hormone gastrin, which is produced by the stomach wall when food enters the stomach, they secrete their contents.
Gastric juice is secreted by the gastric glands and contains a large amount of water, the inactive pepsinogen enzyme, and free hydrochloric acid. When inactive pepsinogen is mixed with hydrochloric acid, it converts to active pepsin. The acid is useful in preventing bacterial decomposition and dissolving inorganic salts, which softens the food. Pepsin from the stomach and oesophagus degrades food proteins into peptones and proteoses.
After 2-3 hours in the stomach, food is thoroughly churned and mixed by contractions of the muscles of the stomach wall, resulting in a thick creamy acid fluid known as chyme. The muscular contractions of the stomach wall force the chyme to pass into the duodenum in small amounts via the pylorus.
(iii) Intestinal Digestion
Because of the presence of HCl secreted by the oxyntic cells of the stomach’s gastric glands, food enters the duodenum acidic. The acid in the food stimulates the duodenum to produce secretin and cholecystokinin hormones, which then travel through the blood to the pancreas and liver, respectively. Secretin stimulates the pancreas to produce pancreatic juice, whereas cholecystokinin stimulates the gallbladder to produce bile juice.
Through the common hepatopancreatic duct, bile and pancreatic juices are poured side by side into the duodenum. Simultaneously, the intestinal mucosa secretes the intestinal juice known as succus entericus with the help of enterocrinin hormone. Thus, three substances, namely bile, pancreatic juice, and succus entericus, mix with the food in the intestine. All of these have an effect on food and help it to be digested thoroughly.
Absorption is the process by which digested food enters the bloodstream. It primarily occurs in the duodenum and ileum, which are well suited for this due to the development of various folds with villi-like processes that increase the absorptive surface of these two regions of the alimentary canal. Each villus is densely packed with blood capillaries and lymphatic or lacteal vessels. Water, mineral salts, and other nutrients are absorbed directly through the mucosa.
The amino acids, glucose, and fructose diffuse from the mucosa into the intestine’s blood capillaries before reaching the liver via the hepatic portal vein. The liver ensures a steady supply of sugars and amino acids into the bloodstream. Excess sugar is stored as glycogen, whereas excess amino acids cannot be stored in reserve but are converted into urea by liver cells and excreted from the body as urine by the kidneys.
If the blood sugar concentration falls below normal, the reserve glycogen is converted into glucose by the liver cells and released into the bloodstream. The cells obtain the necessary amino acids from the blood in order to synthesise the proteins that form the protoplasm.
The glycerol and fatty acids enter lacteal lymph vessels. Glycerol is easily absorbed because it is soluble in water, whereas fatty acids cannot be absorbed because they are insoluble in water. So, prior to absorption, the fatty acids combine with bile salts to make themselves soluble, allowing them to be absorbed. Glycerol and fatty acids are converted back into fat globules of much smaller molecules after absorption in lacteals. Thus, fats enter the bloodstream as glycerol and fatty acids via lymph vessels.
Different cells pick up the necessary amount of various digested foods from the blood to either build new protoplasm or provide energy. Assimilation is the name given to this phenomeon. Vitamins convert digested food into new protoplasm, and mineral salts form protoplasm components. Various foodstuffs can be converted into other required substances under certain conditions.
The small intestine completes digestion and absorption, while the indigested food enters the rectum via peristalsis for storage and faeces formation. Faeces are formed by old epithelial cells, leucocytes, bile pigments, and a large number of bacteria and are removed on a regular basis through the cloacal opening.