Digestive System of Frog

Rana tigrina is the most common species of frog. Toads have a poisonous gland in their skin, but it is absent in frogs. Frogs exhibit varying body temperatures, which fluctuate according to their surroundings, making them known as cold-blooded or poikilothermic animals. They possess the ability to alter their skin coloration to blend with their environment, a defense mechanism termed mimicry, aiding in evasion from predators. 

Additionally, during winter, frogs are not commonly observed as they retreat to deep burrows, practicing a form of hibernation known as winter sleep to shield themselves from harsh temperatures. Similarly, in summer, they undergo aestivation, another form of dormancy, to escape extreme heat and preserve energy.

1.0Frog Classification

2.0Digestive System

Food and feeding

• The adult frog is carnivorous, feeding on small insects, worms, snails, and similar prey. In contrast, tadpoles are herbivorous, subsisting on aquatic plants.

Ingestion

• Ingestion is taking food into the alimentary canal through the mouth. The food is captured with the help of a sticky tongue. During ingestion, the mouth opens, the tongue extends to catch the prey, and then quickly retracts into the buccal cavity, then closes the mouth.
• Peristalsis refers to the wave-like muscular movements that propel food through the oesophagus and the rest of the alimentary canal.

Digestion

• Digestion involves the intricate transformation of intricate food substances into easily absorbable and soluble forms through the action of specialized enzymes.

Digestion of food in Bucco-pharynx

• The buccopharyngeal cavity does not produce any enzymes, so no enzymatic digestion occurs in the buccal cavity.

Digestion in Oesophagus

• The mucus lining of the oesophagal wall lubricates and moistens the food. No enzymes are present. The food is moved downward by peristalsis.

Digestion in stomach

• The process of digestion initiates within the stomach, where food undergoes initial breakdown aided by muscular contractions, followed by further enzymatic processing facilitated by gastric juice. 
• When food enters the stomach, it prompts the release of gastrin hormone, which in turn prompts gastric glands to secrete gastric juice. Gastrin also plays a crucial role in regulating the secretion of hydrochloric acid (HCl) from the oxyntic cells.

Function of HCL

• It eradicates harmful bacteria and fungi.
• It creates an acidic environment essential for pepsin activity.
• It activates enzymes by converting inactive pepsinogen into active pepsin.
• Gastric juice contains pepsinogen, which is converted into its active form, pepsin, in an acidic medium. Pepsin then breaks down the protein content of food into soluble peptones and proteases.
• Carbohydrates and fats are not broken down by pepsin.
• The partially digested food transforms into a semi-solid, acidic substance called chyme. It stays in the stomach for approximately 2-3 hours, when the digested nutrients are absorbed. The remaining food then moves into the duodenum.
• Pepsinogen (inactive) + HCL                                   Pepsin (active)
• Pepsin + Proteins                                                       Proteases + Peptones

Digestion in duodenum

• The chyme passes through the pyloric valve into the duodenum. Here, it encounters Bile, pancreatic juice, and intestinal juice, collectively acting upon the food. The presence of acidified chyme triggers the mucosa of the duodenum to release the hormones secretin and cholecystokinin.
• Secretin prompts the pancreas to release pancreatic juice, while cholecystokinin triggers the gallbladder to contract and release bile into the duodenum through the hepatopancreatic duct.

Bile

• Bile, an alkaline fluid produced by the liver, does not chemically act on food but plays a crucial role in neutralizing acidic food. 
• It aids in emulsifying fats, breaking them into fine droplets, and provides the necessary alkaline medium for the enzymes in pancreatic and intestinal juices to function effectively.

Pancreatic juice

• These enzymes, including trypsinogen, amylase, and lipase, function optimally in an alkaline environment, facilitating the breakdown of food through hydrolysis.

Trypsinogen + Enterokinase               Trypsin

Proteases + Trypsin                             Amino Acid

Starch + Amylopsin                              Maltose

Fats + Lipase                                         Fatty acids + Glycerol

Digestion in ileum

• The succus entericus, a secretion of the intestinal glands found in the ileum, comprises a range of enzymes, including enterokinase, erepsin, maltase, lipase, and sucrase.

Proteases + Erypsin                               Amino acid

Maltose + Maltase                                 Glucose

Fat + Lipase                                           Fatty acids + Glycerol

Sucrose + Sucrase                                Simple sugar glucose

3.0Absorption, Assimilation and Egestion

• Nutrients from digested food are primarily absorbed in the duodenum and ileum because of villi and tiny finger-like projections. 
• These villi greatly enhance the surface area for absorption. Amino acids, glucose, and fructose are absorbed at the top and bottom of the villi, while fatty acids and Glycerol are taken up by lymph vessels known as lacteals.
• Once absorbed, these nutrients are transported via the bloodstream to various tissues and cells, which participate in metabolic processes to produce energy.
• Any undigested food, termed faeces, is expelled through the cloacal aperture in a process known as egestion.