Digestive System
Last Updated on Tuesday, 12 July 2011 04:35 Written by Sandesh Monday, 24 May 2010 05:37
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DIGESTIVE SYSTEM OF FROG
The digestive system includes the structures which are concerned with the obtaining of food; its digestion, absorption of digested food and finally the removal of indigestible residue.
o Alimentary canal
o Digestive glands
o Physiology of Digestion
I. Alimentary Canal
Alimentary canal of frog is complete. It is a long coiled tube of varying diameter, extending between the mouth and cloaca. It comprises of following parts:
1) Mouth
2) Buccal cavity
3) Pharynx
4) Oesophagus
5) Stomach
6) Small intestine
7) Large intestine
Cloaca
1) Mouth: The wide gape-like mouth marks the beginning of alimentary canal. It lies between the two jaws, the upper jaw which is immovable and the lower jaw which can be freely moved up and down. Mouth extends from one side of the snout to another.
2) Buccal cavity: the mouth leads into the buccal cavity which is large and shallow. There are several mucous glands inside the buccal cavity for lubricating food. Salivary glands are absent in frog. Buccal cavity comprises teeth, tongue and internal nares.
o Teeth- Teeth are absent in lower jaw where as upper jaw bears numerous, closely set small, pointed and somewhat hook-like teeth. These teeth occur in a row on either side of maxillary and pre-maxillary bone thus they are know as maxillary and pre maxillary teeth. Besides these, two small bones on the roof of buccal cavity, known as vomer, also bear two groups of teeth known as vomerine teeth. The teeth in frog are not meant for chewing; they simply hold the prey and prevent it from slipping out. Important character of frog’s teeth are as follows:
The teeth are all alike in shape and structure= homodont
directly attached to the upper jaw =acrodont
old and damaged teeth are continuously replace= polyphyodont
Each tooth bears two parts, a base and a crown. Crown is the free part made up of hard ivory-like substance the dentine.The tip of the crown is covered by hard, resistant, shining, white material known as enamel. Each tooth contains a central pulp-cavity open at a side. It is filled with highly vascular and nutritive pulp.
o Tongue- A thick, fleshy tongue lies at the floor of buccal cavity. The anterior end of tongue is attached to the inner border of the lower jaw whereas the posterior end is free and bifid. The tongue can be flicked out and retracted to catch the prey. It is provided with large number of tubular lingual glands which also secrets sticky fluid.
o Internal nares- The roof of buccal cavity bears anteriorly, just in front of vomerine teeth, a pair of small openings the internal nares. Nasal cavity opens inot the buccal cavity through it. They aid in respiration.
3) Pharynx:
Posteriorly, buccal cavity leads into pharynx which lies behind the angle of jaws. Buccal cavity and pharynx are sometimes described as bucco-pharyngeal cavity. Various apertures open into pharynx.
o A pair of Eustachian tubes which leads into the cavities of middle ear opens into the pharynx at the sides of its roof.
o A slit-like opening called Glottis which leads into the lungs, opens in the middle of the floor of the pharynx
o In male frogs, opening of Vocal sacs are seen on the floor of pharynx, at the either side near the angle of two jaws.
Thus pharynx leads into many regions namely, the middle ear through Eustachian tube, lungs through glottis, vocal sacs and finally into the Oesophagus.
Histologically, the wall of alimentary canal is made up of serosa, muscularis, submucosa and mucosa.
3) Oesophagus:
Due to the absence of neck the oesophagus is short but wide. It is highly distensible and tubular structure. It directly passes into stomach and thus there is no external line of demarcation between the oesophagus and stomach.
4) Stomach:
The stomach lies behind the oesophagus and is the most enlarged part of the alimentary canal. It is situated on the left side of the body cavity, a little beneath the liver. It is about 4 cm long, broad and slightly curved bag or tube like structure with thick muscular walls. It is wide anteriorly and narrows posteriorly. The wide anterior part is known as cardiac stomach whereas the short narrow posterior part is known as pyloric stomach. The pyloric end of stomach is slightly constricted and opens into the large intestine. The opening is guarded by a circular ring sphincter muscles, called pyloric valve.
The histological structure of oesophagus and stomach is quite similar. The inner surface of the stomach is thrown into several prominent longitudinal folds which allow its distension when the food is received. It has well developed muscularis. The mucous epithelium of stomach contains multicellular gastric glands secreting the enzymes known as pepsinogen. The oxyntic glands secrete hydrochloric acid.
Stomach serves for storage and digestion of food.
5) Small intestine: It is long coiled and narrow tube, about 30cm in length supported by mesentery. It consists of two parts, duodenum and ileum.
i. Duodenum: Duodenum which is the first part of the small intestine is a narrow tube about 3 to 5 cm in length. It runs ahead parallel to stomach forming a U. It receives a common hepato-pancreatic duct from liver and pancreas bringing bile and pancreatic juice.
The histological structure of duodenum is similar to that of oesophagus and stomach except that the musoca is thick forming transverse folds. It is irregular and branched like fingers called villi.
ii. Ileum: It is the longest part of the alimentary canal and makes several loops before joining into the large intestine. Histologically, ileum is similar to duodenum. It helps in digestion of food and absorption of digested food.
6) Large intestine or rectum:
It is a short; wide tube about 4 cm long running straight behind to open into cloaca. The inner lining of rectum forms numerous longitudinal folds. Its function is the reduction of water and preparation and storage of feaces.
7) Cloaca:
Cloaca is the last part of alimentary canal. It opens into the exterior through cloacal aperture. It is the common passage for the removal of undigested food from the alimentary canal but also for the removal of urine and reproductive cells, such as ova and sperms.
II. Digestive Glands
Besides gastric glands and intestinal glands, two large glands are also associated with the alimentary canal of frog. They are liver and pancreas.
1) Liver: Liver is the largest gland found in the body of vertebrates. It is reddish brown; multi-lobed gland situated close to heart and lungs. Liver of frog consists of 3 lobes- right, left and median lobe. Between right and left lobes of liver, there is large, spherical, greenish and thin-walled sac, the gall-bladder which serves as storage of bile secreted by liver cells. Bile flows into the gall bladder from liver, by a number of hepatic ducts. The duct of the gall bladder is known as cystic duct. The cystic duct from gall bladder and hepatic duct from liver joins to form common bile duct. It runs through pancreas and joins pancreatic duct to form a hepato-pancreatic duct which finally opens into the duodenum. The function of bile juice is to emulsify fat.
Histologically, liver is much branched tubular gland. The secretory unit of liver is called lobule. Between the lobules are branches of hepatic artery, hepatic portal vein and bile duct. Each lobule is made up of very large number of hepatic cells, which are polygonal in shape. Each cell has large, prominent nuclei. The hepatic cells are arranged in columns between bile capillaries which unite to form bile ducts.
Liver receives blood from two sources:
Hepatic artery- carries oxygenated blood
Hepatic portal vein- carries blood containing absorbed food material from gut
Function of Liver: [Important for exam- 3 marks question]
a. The cells of liver secret bile. Bile is a greenish alkaline fluid stored in the gall bladder. It doesn’t contain digestive juice but contains bile salts such as sodium bicarbonate, sodium glycocholate and sodium taurocholate. The presence of these salts in bile neutralizes acidity of food in intestine and prepares it for the activity of enzymes of pancreas which can only act on alkaline medium.
b. The liver plays a major role in maintaining blood concentrations of glucose. When in excess, glucose is converted into insoluble carbohydrate and glycogen which are stored in liver cells. This process is called glycogenesis. On the other hand, when the concentration of glucose falls in blood, liver reconverts glycogen into glucose. This process is called glyocogenolysis. In this way liver helps to maintain body’s homeostasis.
c. The liver maintains protein concentration of blood. The excess amino acids are converted into ammonia by liver. They combine with Co2 and change into urea. The so produced urea is excreted into the urine by the kidneys.
d. The liver stores important substances such as glucose (in the form of glycogen). It also stores fat-soluble vitamins (vitamins A, D, E and K), vitamin B 12 , and minerals such as copper and iron.
e. The liver removes harmful substances (such as ammonia and toxins) from the blood and then breaks them down or transforms them into less harmful compounds
f. It destroys old erythrocytes in adult and forms new ones in embryo.
2) Pancreas: Pancreas is the second largest gland. It is long, flattened, irregularly lobed and somewhat Y-shaped gland of light yellow colour. It lies in mesentery between the duodenum and stomach
Histologically, it is much branched, grape-like gland with many lobules. Each lobule consists of a cluster of secretory cells which encloses a very narrow central space.
Function of pancreas:
Pancreas functions both endocrine as well as exocrine. The endocrine part is formed by islets of Langerhans which produces hormone insulin. Insulin regulates the blood sugar level. The exocrine part of pancreas secrets pancreatic juice.The pancreatic juice is alkaline mainly due to the presence of sodium bicarbonate. It contains three types of digestive enzymes, namely, amylopsin, trypsin and steapsin or lipase. The ducts from pancreas open into common bile duct to form hepato-pancreatic duct which opens into duodenum. The secretion of pancreas is thus poured into duodenum through hepato-pancreatic duct.
III. Physiology of Digestion
Frog feed chiefly on insects therefore it can be considered insectivorous. The prey is captured by rapid flicking of tongue and swallowed whole, without mastication, passing down the oesophagus into stomach. The process of digestion can be divided into three parts: Buccal digestion, Gastric digestion and intestinal digestion.
3) Buccal Digestion:
No physical or chemical changes occur to the captured prey in buccal cavity due to the absence of digestive glands. The salivary glands are absent in the buccal cavity. The lining of bucco-pharyngeal cavity secrets mucus, which lubricates the food and makes it easy to swallow.
From buccal cavity the prey is pushed into the oesophagus. Here the prey undergoes physical change due to constant peristaltic movement of oesophageal wall. No digestion occurs in oesophagus.
2) Gastric Digestion:
The peristaltic movement of oesophagus pushes the food forward into stomach. Stomach performs three main functions: storage, mechanical mixing and chemical modification.
4) Storage: The chief advantage of storage is in making continuous feeding unnecessary.
5) Mechanical mixing: The wall of stomach undergoes peristaltic movements (i.e. contraction and relaxation), which allows food to go down, break into pieces. Besides, it also aid the food to mix thoroughly with the gastric juice secreted by gastric glands situated in the inner lining of stomach.
6) Chemical modification:
Gastric glands of stomach wall secrets the gastric juice containing hydrochloric acid and an inactive pre-enzyme pepsinogen. In the presence of hydrochloric acid, pepsinogen changes into active enzyme pepsin. Pepsin acts upon proteins of food and changes it into proteases and peptones. Acid not only provides acidic medium but also kills bacteria and fungi present in food and makes it soft. Food in stomach, after being acted upon by digestive enzymes is further churned into thick creamy form called chyme. The chyme now passes from stomach into duodenum through pyloric sphincter.
3) Intestinal Digestion:
The food, in form of chyme reaching duodenum is acidic in nature. The acidic chyme stimulates the duodenum to produce two basic types of intestinal hormones, namely:
i) secretin- stimulates pancreas to produce pancreatic hormone
ii) cholecystokinin- activates gall-bladder to release bile juice.
Bile and pancreatic juices are poured side by side into duodenum by hepato-pancreatic duct.
Bile is a greenish alkaline fluid secreted by liver. It doesn’t contain digestive juice but contains bile salts such as sodium bicarbonate, sodium glycocholate and sodium taurocholate. The presence of these salts in bile neutralizes acidity of chyme and prepares it for the activity of enzymes of pancreas which can only act on alkaline medium.
Pancreatic juice is the watery alkaline fluid from pancreas containing various types of enzymes all of which act best in alkaline medium. The three enzymes secreted by pancrease which plays important role in digestion are:
i. trypsinogen
ii. amylopsin
iii. lipase(steapsin)
All the three above mentioned enzymes remain in inactive state until they are activated by Enterokinase. Enterokinase is an enzyme secreted by intestinal juice, succus entericus present in the intestinal mucosa. It brings about following reactions:
• Inactive Tyrpsinogen is converted into active Trypsin by the action of Enterokinase. Trypsin acts on protein and converts it into Peptones, Proteoses and Polypeptide into amino acids.
• Amylopsin or amylase reduces starches into maltose.
• Lipase coverts emulsified fats into fatty acid and Glycerol.
Besides enterokinase, succus intericus contains several enzymes that act upon all kinds of food stuffs.
i) Peptidase-changes polypeptides into amino acids
ii) Maltase- changes maltose into glucose
iii) Sucrase- changes sucrose into glucose and fructose
iv) Lipase- splits fats into fatty acids and glucose
4) Egestion, Absorption and assimilation: The undigested food residue passes out through the cloacal aperture as feaces. This process is called egestion.
After digestion has been completed, the absorption of digested food occurs through the walls of small intestine. Absorption is the process of taking the digestive food by the process of osmosis into blood. The site for absorption is the small intestine.
Absorbed food stuffs may be used for two basic purposes of nutrition:
i) liberation of energy during respiration
ii) assimilation as part intimate structure of animal
