Complete Guide to Digestion in Rabbits: Process, Enzymes, and Absorption Explained

The digestive system of rabbits is designed to handle a herbivorous diet rich in cellulose and other plant materials. It involves both mechanical and chemical processes that take place in various parts of the gastrointestinal tract. From the moment food enters the mouth and undergoes mastication, to its journey through the stomach and small intestine where enzymes work their magic, we'll explore each step of the digestive process.

Enzymes play a critical role in digestion, with different types specialized in breaking down specific nutrients such as carbohydrates, proteins, and fats. We'll delve into the functions of carbohydrases, proteinases, lipases, and nucleases, and how they contribute to nutrient breakdown.

As we progress, you'll discover how absorption takes place in the small intestine, where the digested nutrients are transported to nourish the rabbit's body and support its growth. Additionally, we'll explore fascinating rabbit behavior, such as coprophagy, where they consume soft feces to re-digest and absorb more nutrients.

1. Digestion is defined as the conversion of nondiffusible food elements into diffusible constituents.
2. Digestion is the conversion of large molecules into small ones.
3. In the digestive system the process by which food substances are broken down by mechanical and chemical means.
4. Mechanical digestion comprises mastication or chewing, liquefaction of food by digestive juices, swallowing, and peristalsis.
5. The major utility of breaking up food into small bits during chewing is to increase the surface area of food. It helps in enzymatic action.
6. Chemical digestion includes the enzymatic action of food.
7. All enzymes are chemical proteins.
8. All digestive enzymes are hydrolytic. They catalyze the hydrolysis of nutrients.
9. In hydrolysis of nutrients, a small amount of energy is released as heat.

Digestive enzymes:

1. Digestive glands secrete digestive enzymes. Four main types of digestive enzymes are; Carbohydrases, Proteinases, Lipases, and Nucleases.
2. Carbohydrases include amylase (polysaccharides to disaccharides) and disaccharidases (Maltase, sucrase, and lactase).
3. Proteinases can be endopeptidase and exopeptidase.
4. Lipase (steapsin) acts on ester bonds of fats or triglycerides.
5. Nuclease hydrolyses nucleic acid into nucleotides and finally into nitrogenous bases, pentose sugar, and phosphate groups, e.g., D Nase and R Nase.

Digestive Juices in the Alimentary Canal

Digestive juice - Location

• Saliva → Mouth cavity
• Gastric juice → Stomach
• Bile → Duodenum
• Pancreatic juice → Duodenum
• Intestinal juice → (succus entericus) Small intestine

Digestion in the mouth cavity

1. In rabbits, digestion starts from the mouth. Masticated food in the mouth is mixed with saliva secreted by salivary glands (3 pairs in man, 4 pairs in rabbit)
2. pH of saliva is about 6.8 (slightly acidic, almost neutral)
3. Food mixed with saliva in the buccal cavity is called 'bolus'.
4. Saliva contains a starch-splitting enzyme ptyalin (salivary amylase) acts on cooked starch changing them into sugar maltose, isomaltose, and limiting dextrins.
5. Saliva contains an antibacterial enzyme lysozyme. It dissolves the cell wall of Gram-positive bacteria and kills them.
6. There is no digestion in the esophagus. It conducts the food from the mouth into the stomach.

Digestion in Stomach

• Stomach secretes gastric juice (pH 1-3.5)
• Protein digestion starts in the stomach.
• The stomach is the chief site of protein digestion.
• The columnar epithelium of the stomach forms many gastric pits with gastric glands.
• Gastric glands are located on the mucosa of the stomach.
• Gastric glands are lined with three kinds of secreting cells: Zymogenic (main, peptic, or chief) cells, parietal (oxyntic) cells, and mucous cells.
• The main, peptic or zymogen cells secrete digestive proenzymes namely pepsinogen and prorenin.
• Oxyntic cells secrete HCI.
• Pepsinogen is activated into pepsin by HCI. It is a protein-splitting endopeptidase.
• Protein → Pepsin → Proteoses + Peptones
• Pepsin can digest even collagen of connective tissue fibers, but not the keratin of nails or hair.
• The mucus secreted by the stomach protects its wall from the action of pepsin.
• Rennin is found in calf gastric juice. It is a milk-coagulating proteinase.
• Inactive pro rennin is converted into active rennin again by HCI
• Rennin acts on casein, a protein in milk, changing it into calcium para-caseinate. It is known as the curdling of milk.
• Rennin + Milk protein (Casein) → Ca⁺⁺ → Cal.paracaseinate
• But adult cows or human infants do not secrete rennin, The function of rennin is taken over then by pepsin.
• Another enzyme of the stomach is gastric lipase. It splits the butterfat molecules found in milk.
• Lipase + fats → Fatty acids and glycerol
• The presence of gastric lipase in rabbits is doubtful.
• A carbohydrate-splitting enzyme gastric amylase is present in both rabbits and men in very minute quantities. Their role in digestion is negligible.
• Thus the semi-digested thick pasty solution is formed. This is called chyme. It passes into the duodenum.

Digestion in the Small Intestine

• The food in the small intestine is mixed with three digestive fluids namely bile, pancreatic juice, and intestinal juice.

Bile

• Bile is secreted by the liver and is stored in the gallbladder.
• Bile is alkaline, viscous, yellow to green in color, pH 7.8-8.6. There is no digestive enzyme in bile, its main role in digestion is the emulsification of fat.
• In bile, there are salts like sodium bicarbonate, sodium taurocholate, and sodium glycocholate. Emulsification of fats takes place through these bile salts.
• The action of bile on fat is non-enzymatic.
• The presence of bile in the small intestine is also necessary for the absorption of the fat-soluble vitamins A, D, E, and K.
• If the gallbladder is removed, the digestion of fat is affected. Acidity may continue in the duodenum.
• The bile pigments present in bile juice are biliverdin and bile rubin. They are formed by the breakdown of hemoglobin of worn-out RBC in the liver.

Pancreatic juice

• Pancreatic juice, secreted by exocrine pancreatic acini, is a complete digestive juice.
• Pancreatic juice takes part in the digestion of proteins, carbohydrates, and fats.
• Pancreatic juice is colorless watery fluid, slightly alkaline, pH 7.5 - 8 due to the presence of sodium bicarbonate.
• Pancreatic amylase is a starch-splitting enzyme similar to ptyalin hydrolyzing starch and glycogen to maltose, isomaltose, and Limit dextrins.
• Trypsin and chymotrypsin are proteolytic endopeptidases.
• They are initially inactive trypsinogen and chymotrypsinogen.
• The enterokinase secreted by the intestine stimulates trypsinogen to activate trypsin.
• The trypsin changes chymotrypsinogen into active chymotrypsin.
• Both trypsin and chymotrypsin hydrolyze proteins, proteoses, and peptones to polypeptides.
• Trypsin cannot coagulate milk.
• In predator animals drinking the blood of their prey, trypsin hydrolyses, fibrinogen of blood into fibrin leading to blood coagulation.
• Like pepsin, trypsin cannot hydrolyze keratin.
• Pancreatic juice contains the enzyme lipase.
• Pancreatic lipase hydrolyses fat into fatty acids and glycerol.

Intestinal juice

• Intestinal juice or succus entericus is mainly secreted by the crypts of Lieberkuhn.
• Intestinal juice is a clear yellow fluid with a slightly alkaline nature and pH of 7.6, contains water, mucus, and enzymes that complete the digestion of carbohydrates, proteins, fats, and nucleic acids.
• The important enzymes in intestinal juice are.
• Enterokinase → Erepsin
• Disaccharidases → Intestinal lipase
• Polynucleotidase → Nucleosidase

1. Enterokinase activates trypsinogen to trypsin.
2. Erepsin contains exopeptidase, which changes polypeptides to amino acids.
3. Disaccharidases act as follows:
   1. Sucrose invertase → Glucose + Fructose
   2. Maltose Maltase → Glucose + Glucose
   3. Lactose Lactase → Glucose + Galactose
4. Intestinal lipase act as follows:
   1. Emulsified fat → lipase → Fatty acids + Glycerol
5. Polynucleotidase and nucleosidase act as follows:
   1. Nucleic acid → Potynucleotaidse → Nucleotides
   2. Nucleotides → nucleotidase → Nucleosides + Phosphate
   3. Nucleosides → Nucleosidase → Nitrogenous bases + Pentose sugar
6. Digestion of all major nutrients of food is completed in the small intestine.
7. The end products of carbohydrate digestion are monosaccharides.
8. The end products of protein digestion are amino acids.
9. The end products of fat digestion are fatty acids and glycerol.
10. The end products of nucleic acid digestion are nitrogenous bases, pentose sugars, and phosphoric acid.

Thus the digestion of various food stuffs is completed and as the food passes through the intestine it receives more water and forms a fluid emulsion called chyle. The enzymatic action remains continuous throughout the process and finally, proteins, carbohydrates, and fats are hydrolyzed into amino acids, glucose, fatty acids, and glycerol.

The digestion of cellulose occurs in the caecum which is well-developed in Rabbits, as cellulose decomposing bacteria and protozoans are present in caecum.

Absorption

1. Absorption of digested food mainly occurs in the ileum of the small intestine.
2. Fatty acids and glycerol are absorbed by lacteals in villi. They are taken in lymph vessels.
3. Amino acids and glucose are carried by the hepatic portal vein.
4. Some nutrients such as fructose and maltose are absorbed from the intestine by the simple process of diffusion and finally carried to the liver through the portal circulation.
5. Water is absorbed by osmosis from the intestinal lumen to intestinal cells.

Hormonal control of digestion

1. Gastrin: G-cells of the pyloric gland secrete a hormone called gastrin which stimulates the secretion of gastric juice.
2. Enterogastrone: Enterogastrone produced by the small intestine slows down the secretion of gastric juice.
3. Cholecystokinin: The release of bile into the duodenum is promoted by the hormone CCK, which induces rhythmic contraction of the gallbladder.
4. Secretin: Secreted by duodenum stimulates the pancreas and controls the volume of pancreatic juice including water and electrolytes.
5. Pancreozymin: Secreted by the duodenum, controls the amount of enzymes in pancreatic juice.
6. Enterocrinin: Secreted by the small intestine stimulates the intestine to secrete intestinal juice.

Assimilation

The transport of digested food materials from the blood into tissues takes place where they undergo oxidation releasing energy. This process is called assimilation.

The digested food materials involved in the synthesis of new substances help the growth of the animal.

Egestion

The indigestible food passes through the colon and is pushed outside the rectum in the form of semi-solid feces. In the colon and rectum, the water is reabsorbed.

Coprophagy

The feces passed outside are soft and moist due to the presence of large quantities of plant cell walls, which are digested by enzymes. This fecal matter is eaten up by rabbits and thus passed through the gut once more for digestion and absorption of simplified cellulose. By this habit, rabbit gets the maximum amount of nutrients from the food and it is called coprophagy or pseudo rumination or refection.