Avian Digestion in small and large intestine

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Avian Digestion in small and large intestine

Small intestine

Small intestine begins at the exit from the gizzard and ends at the junction of the small intestine, caeca and colon.

It is relatively long and has a constant diameter. Of the three parts of the small intestine duodenum, jejunum and ileum, only the duodenum can be easily distinguished in the fowl.

There is no clear demarcation between the jejunum and ileum.

Much of the digestion of the food and all of the absorption of the nutrients takes place in the small intestine

  • Structures of small intestine
    1. Outer serous membrane
    2. A layer of longitudinal muscle
    3. A layer of thick circular muscle
    4. Structures located between the two muscle layers are-
      • Blood vessels
      • Lymph vessels
      • A network of nerve fibers.
    5. An ill-defined sub-mucosa
    6. Mucous membrane consisting of-
      • Thick muscularized mucosae of longitudinal and circular muscle.
      • Corium – many glands, lymphoid tissue, muscle fibers and a variety of free cells.
      • Inner epithelial surface. 
Duodenum

The duodenum starts at the gizzard and forms an elongated loop about 20 centimeters long. The pancreas lies between the arms of the loop and being attached to each arm of the duodenum actually holds the two arms together.

Lymphoid tissue in the duodenum is very plentiful and is usually located in the corium.

Bile ducts from the gall bladder attached to the liver and two to three pancreatic ducts enter the small intestine by a common papilla at the caudal end (closest to the rear) of the duodenum.

Pancreas is located closely associated with the duodenum being attached to each side of the duodenal loop and lying between the two arms.

Villi

It is a long flattened, fingerlike projections extending into the lumen  of the intestine like flexible fingers.

 A single layer of columnar epithelium together with goblet cells covers the lining. The goblet cells secrete mucous.

A lacteal (lymph vessels), capillaries, bundles of plain muscle fibres, nerves and other tissues and cells occupy the core of the villus. The villi have the function of providing a vastly increased surface area for the more efficient absorption of the nutrients.

The villi are very actively involved in the absorption process.

The efficiency of the absorption is influenced by the surface area available for the nutrients to move through.  

 Jejunum and the ileum

The jejunum and the ileum, together about 120 cm long, commence at the caudal end of the duodenum where the bile and the pancreatic duct papilla are located and terminates at the ileo-caecal-colic junction. It is a junction of small intestine, the two caeca and the colon. This portion of the small intestine is similar in structure to the duodenum except that- the villi are shorter and there is less lymphoid tissue.

Meckel’s diverticulum is a constant feature about half way along the small intestine appearing as a small projection on the outer surface of the small intestine. This projection represent this point of attachment of yolk sac during the development of the embryo.

Functions of small intestine
  1. Produces number of enzymes involved in the digestive process
  2. Site of final digestion of the food
  3. Site of absorption of the end products of digestion

Large intestine

The large intestine is very short and ends at the cloaca. This section includes paired caeca, colon and rectum and the rectum being the terminal part.

The bursa of fabricius is located immediately above the cloaca of young birds, but disappears when the birds have reached approximately one year old.

Caeca

Paired caeca or blind pouches are about 16-18 centimeters long in the adult.

  • Structure of the caeca
    1. Outer serous membrane
    2. Outer longitudinal muscle
    3. Circular muscle
    4. Inner longitudinal muscle forming the muscularis mucosae of the mucous membrane
Caecal function
  1. Microbial digestion of cellulose.
  2. Reflux of urine into the ceca exposes the cecal microflora to urea and uric acid, which are then degraded; the nitrogen is not recycled for use by the host.
  3. Microbial synthesis of B compex vitamins 

Cloaca

Cloaca is a tubular cavity opening to the exterior of the body provides a common passage to the digestive and urogenital tracts.

  • Cloaca divides into three chambers as follows:
    1. The copradaeum – a continuation of the colon-rectum
    2. The urodaeum – middle part into which the ureters and genital ducts open
    3. The proctodaeum – opens to the exterior of the vent. Birds less than one year old have a dorsal opening leading into the blind, rounded sac – the bursa of fabricius.
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