Alimentary tract
The alimentary tract and the secretary organs associated with it are involved in the process of digestion. The sequence of events includes ingestion of food, digestion of nutrients and their absorption and elimination of undigested waste material. Digestive enzymes act on the food at different sections of the alimentary tract to breakdown the nutrients into simple units to facilitate absorption. The anatomy of the digestive tract varies widely among different species of animals, according to the nature of their diet. In carnivores, the alimentary tract is simple and the digestive process is aided mostly by the enzymes secreted by the digestive system. A single pouch forms the stomach. In omnivores like pigs, the stomach is made up of a single chamber, but the posterior tract is relatively larger to enable microbial fermentation of crude fibre to some extent.
In ruminants like cattle, sheep and goats, which are essentially herbivores, a complex stomach is present. It has four chambers, namely rumen, reticulum, omesum and abomesum. In rumen and reticulum, extensive microbial fermentation occurs to help in crude fibre digestion and protein synthesis. Omesum is essentially engaged in the act of compression and expulsion of digesta into the abomesum, though some absorption takes place. Abomesum represents the true stomach in ruminants. In non-ruminantherbivores like horses, there is a simple stomach, but an enlarged posterior tract where microbial activity takes place extensively. In birds, certain special structures exist. An esophageal diverticulum called crop is present at the lower portion of esophagus. The stomach is divided into two portions, namely, proventriculus or glandular stomach and gizzard or muscular stomach. Caeca exist bilaterally on either side as blind canals in birds. Cloaca forms a common opening for the elimination of faeces and urine together.
Anatomical Considerations
The mouth forms anterior opening of the alimentary canal in all animals. It chiefly consists of lips, gums, teeth and tongue and is supported by cheeks and jaw muscles in its actions. In birds, the teeth are replaced by strong beaks. Salivary glands secrete salivary secretions into the mouth to initiate digestion. The funnel shaped pharynx forms the anterior end of esophagus. By the act of swallowing or deglutition, food is passed on from mouth into the stomach through esophagus. The posterior end of esophagus is connected to the anterior opening of the stomach. Stomach possesses different sections or regions. The esophageal region is non-glandular. Cardiac glandular zone is present at the anterior part of the stomach. Fundic and body regions of the stomach are glandular and form the middle portions of the stomach. Pyloric region represents the posterior portion of the stomach which connects to the small intestine.
Enzymic Digestion
Soon after the food enters the buccal cavity, the digestive process is initiated. Three pairs of salivary glands and numerous small glands lining the buccal mucous membrane discharge their secretions into the mouth. The three salivary glands are parotid, submaxillary and sublingual glands. Their secretions may be serous, mucous or mixed in nature. Saliva keeps the mouth wet and protects the buccal surface. It has a softening, soaking and lubricating effect on food while it is acted upon by the enzyme. It dissolves the water soluble food particles for better digestion. It possesses a buffering action due to the presence of bicarbonate ions. Copious salivation precedes vomition to protect the mouth surface from the ill effects of the strong acid from the stomach. Saliva also contains lysozyme which has a bactericidal effect. Saliva readily evaporates from the buccal surface and is helpful in thermoregulation during hot weather as is evident in panting. Dogs make use of this mechanism to keep them cool. Saliva contains the enzyme, ptyalin or amylase which acts on the starch to yield dextrins, maltose and glucose. It acts in acidic medium and continues its action in the stomach until inactivated by the action of acid and pepsin in the stomach.
Absorption
Fingerlike projections called villi line the mucous membrane of the small intestine. They provide extensive surface area for absorption of the digested nutrients. A lymph capillary called lacteal is present in the core of each villus. The mechanism of absorption includes simple diffusion, uptake along the concentration gradient with the help of specific ions like calcium or by engulfing or pinocytosis. Bile plays an important role in the digestion and absorption of dietary lipids. Bile salts help in the emulsification of dietary triglycerides to make them accessible to pancreatic lipase for digestion.
Digestion in Birds
The overall digestive process in birds is similar to that of mammals. However, certain anatomical features distinctly characterize the alimentary canal of birds. The mouth which is made of beaks is devoid of teeth. The nasal cavity communicates with the mouth through the upper beak. A diverticulum called crop serves as a storage organ where salivary amylase activity continues for some time. Pepsin is secreted in the proventriculus, the glandular stomach. Gizzard or muscular stomach performs the physical function of grinding the feed material. In the small intestine, duodenum is well defined, with no clear cut parts of jejunum and ileum. The birds possess a pair of caeca where some microbial activity takes place. Extensive reabsorption of water takes place in the large intestine of birds and faeces is eliminated through cloaca along with urine since urethra opens into the rectum near the cloacal opening.
Digestion in Ruminants
The digestive processes in the ruminants are substantially carried out by the microbes which inhabit the rumino-reticular vat. The rumen provides abundant space and conditions necessary for the growth and development of rumen microbes. The feed material is retained in the rumen for a considerable period of time for the microbes to act on them. Substantial quantities of end products of microbial digestion are absorbed across the rumen wall and gases such as carbon dioxide and methane are eliminated by belching. The dry matter content of the material in the rumen is between 10 and 15% and the temperature around 38 to 420C. The pH ranges from 6 to 7 and is buffered by large amounts of saliva which contain bicarbonates and phosphates. The rumen conditions are essentially anaerobic.
Role of Protozoa
Protozoa are established in the young ruminant only due to contact with the adult ruminant. They are not carried by air. The pH of the developing rumen also influences their establishment. Usually below a pH of 5.5 little or no protozoa survive. The rumenprotozoa engulf rumen bacteria and particulate proteins from the protein entering the rumen. The rumen protozoa are generally classified into holotrichs which utilize simple sugars for their energy needs and oligotrichs which make use of polysaccharides. Holotrichs utilize ammonia while oligotrichs use proteins and peptides. Protozoan metabolism results in volatile fatty acids, lactic acid, carbon dioxide, hydrogen and proteolytic products. The protozoa account for about 20% of the protein needs of the host animal and substantial amounts of volatile fatty acids. The role of protozoa in the rumen is of particular significance in low protein diets where the fauna help in better nitrogen retention and less wastage. Protozoan protein is better utilized than the bacterial protein in the small intestine.
Non-ruminant Herbivores
In non-ruminant herbivores like horses, microbial activity takes place mostly in the large intestine. In quantitative terms, the extent of microbial digestion is less than that of rumen digestion. In horses and rabbits and to a limited extent in pigs, this microbial digestion is of practical importance. The end products of large intestinal digestion, namely volatile fatty acids are absorbed across the caecal wall. Colon helps in the re-absorption of water and electrolytes. Protein synthesis by microbes in rabbits is of significance in rabbits which are coprophagic in nature. Microbes also synthesize B-complex vitamins which are absorbed across the caecal and colon walls into the blood stream.