VPB 121: Veterinary Physiology - II (2+1)

LARGE INTESTINE

Large intestine of herbivores

• Large intestine is of great importance in herbivores. In simple stomached animals, the enormous caecum and colon are involved in the microbial fermentation hence they are called as hind gut digesters. The absorption of the fermented end products of cellulose and digestible products synthesised by bacteria occur in large intestine of simple stomached herbivores.

Structure

• Caecum and colon of simple-stomached herbivores are very capacious and sacculated, whereas in ruminants, they are simple and non-sacculated. Glands and villi are present in large intestine of herbivores. The glands secretes mainly mucous but has no enzymatic digestive function. In carnivores c olon is short and non-sacculated, caecum is poorly developed, intestinal glands are present throughout large intestine but villi are absent. The glands of large intestine secretes only mucous not the enzymes.

Bacterial action

• No bacterial fermentation occurs in small intestine of carnivores. However, in large intestine, putrifactive (proteolytic) and cellulolytic type of bacteria act on the ingesta which have escaped digestion in small intestine. The products of bacterial action in large intestine are indole, skatole, paracresol, VFA, NH₃ and gases CO₂, CH₄, H₂S and B complex vitamins. These products may be absorbed into the portal blood and carried to liver.

Digestive process

• In monogastric animals, hind-gut maintains conditions favourable for microbial fermentation, which includes substrate availability, control of pH and osmolality, anaerobiosis, retention of substrate and removal of end products of fermentation. There is extensive urea recycling in caecum and colon supplies N₂ for microbial growth.
• Bacterial action in large intestine is mainly on structural and non-structural carbohydrates and proteins. The digestion of cellulose in large intestine yields VFA, CO₂ and CH₄. Ammonia is produced from proteins and urea is absorbed from large intestine of horse.
• In ruminants, enzyme digestion occurs after microbial fermentation and the host animal also utilises the microbial cell bodies. In simple stomached herbivores, microbial fermentation follows after enzymatic digestion and only fermentation products and not the microbial cell bodies are available for absorption by the host. The VFAs absorbed from the large intestine contributes to the energy needs of the host animal.
• Volatile fatty acids acetic, propionic and butyric acids are found in the large intestine of ruminants and horse. Appreciable quantities are also produced in pigs, rabbit and fowl. Very small amounts are found in large intestine of cat and dog. Bacterial synthesis of B complex vitamins occurs in caecum and colon.
• The gases found in large intestine are CO₂, CH₄ and very small amounts of H₂S, O₂ and considerable quantities of N₂ which are expelled in part through anus and in part absorbed into the blood and eliminated through lungs.

Functions of large intestine

• The digestive processes are practically complete in the small intestine. Materials escape absorption in small intestine are gradually propelled into the large intestine through the ileocaecal valve. Large intestine is involved in the functions of reabsorption of water that has been poured out by digestive glands. It also acts as reservoir for waste material that constitutes the faeces, which are expelled at intervals by the act of defecation.