Functions of various parts of the digestive tract: Small Intestine

The small intestine is a long coiled tube divided into three parts; duodenum, jejunum, and ileum. The duodenum is the upper part of the small intestine which starts from the stomach/abomasum. The bile duct from the liver and the pancreatic duct from the pancreas open into the duodenum. The second, middle part of the small intestine is the jejunum. The jejunum is a coiled tube. Along with the ileum, a straight tube, the jejunum opens into the caecum through the ileo-caecal orifice. The inner lining of the small intestine is folded and has figerlike projections called villi. These structures are richly supplied with blood and lymph for absorption of digested nutrients. The muscular layer of the small intestine causes contraction and expansion movements called peristalsis which mixes and moves the digesta toward the large intestine.
The small intestine is the principal site of absorption of sugars, amino acids, fatty acids, vitamins and minerals. In ruminants, most of the carbohydrates are digested in the rumen; while in non-ruminants, the primary digestion site for carbohydrates is the small intestine. The semi-digested feed flowing from the stomach/abomasum is further digested and absorbed in the small intestine through the action of various enzymes secreted by the intestinal glands and supplied by pancreatic juice. Amylase, maltase, lactase, and sucrase are the major enzymes involved in the digestion of carbohydrates. Trypsin, chymotrypsin, and peptidases take part in the digestion of protein. A summary of various digestive enzymes is given in following table;

Enzyme	Origin	Place of Action	Substrate	Products
Amylase	Salivary Gland Pancreas	Mouth Duodenum	Starch Starch	Maltose Maltose
Maltase	Salivary Glands Intestinal Glands	Mouth Small Intestine	Maltose Maltose	Glucose Glucose
Pepsin	Gastric Mucosa	Abomasum	Protein	Polypeptide
Rennin	Gastric Mucosa	Abomasum	Milk Protein	Coagulates Milk Protein
Lipase	Gastric mucosa Pancreas	Stomach/Abomasum Small Intestine	Lipids Lipids	Faddy Acid Glycerol
Trypsin	Pancreas	Small Intestine	Protein	Peptides/Amino Acids
Peptidases	Intestinal Glands	Small Intestine	Peptides	Amino Acids
Sucrase	Intestinal Glands	Small Intestine	Sucrose	Glucose, Fructose
Lactase	Intestinal Glands	Small Intestine	Lactose	Glucose, Galactose

Functions of various parts of the digestive tract: Stomach - 2

Earlier Post: Functions of various parts of the digestive tract: Stomach
Omasum: The third compartment of the ruminant stomach is known as the omasum. It constitutes about 12.6% of the total volume of the stomach in cattle, buffaloes, sheep and goats. The inner lining of the omasum is raised into longitudinal folds are pillars, which greatly increase the surface area. The principal function of the omasum is to absorb water and reduce size of the feed particles. The contents of the omasum are much drier than those of the rumen and abomasum. Because of the small size of the omasum, its contents are not retained for long and are passed on to the abomasum.
Abomasum: This is the fourth and last chamber of the ruminant stomach. It resembles the simple stomach of the monogastric animals and is the only glandular part of the ruminant stomach. The inner lining of the abomasum has folds which are more prominent in the glandular region. It secretes hydrochloric acid and pepsin. The acidic environment of the abomasum causes destruction of microbial cells which are subsequently digested by the pepsin secreted into the abomasum. The rate of abomasal secretion is influenced by the volume and composition of ingesta entering the abomasum. The digesta leaving the abomasum (in a ruminant) and the simple stomach (in a non-ruminant) is called chyme.
Development of the stomach in young ruminants: In ruminants, the rumen is small and non - functional at birth. The abomasum, on the other hand is comparatively large and well developed. As the animal grows, the rumen gradually develops and attains a size which is 10 times greater than the abomasum in adult animals. Since the rumen does not play any role in digestion during early life, the milk suckled by the calf bypasses the rumen and goes directly in to abomasum through a special mechanism, the oesophageal groove. In the abomasum an enzyme, rennin, is secreted which has a strong coagulating action on milk. This helps in retaining the milk for longer time in abomasum for further digestion. The oesophageal groove is a tube like structure with a slit. Suckling and swallowing of milk and other liquids causes closure of the oesophageal groove, thus directing the liquid food to the abdomasum. Ingestion of solid food causes the oesophageal groove to open, allowing the feed to drop in to the rumen. As the intake of growing calf gradually changes from milk to solid feed, the stomach compartments, particularly the rumen, increase in volume, formation of papillae on the internal surface, and establishment of rumen micro-flora. Early development of the rumen can be encouraged by offering good quality solid feed after two weeks of age. This can also help in early weaning of  young calves.
B. Non - Ruminants (Mono Gastrics): In these animals, the stomach is a "V" - Shaped muscular sac. Muscular contractions of the stomach wall help in breakin gdown the feed particles, and at the same time cause mixing of the stomach contents. About two thirds of the inner surface of the stomach wall has glands which secrete gastric juice containing mainly hydrochloric acid and the enzyme pepsin. The pepsin digests protein into peptides and amino acids and requires acidic medium for its action. Hydrocholoric acid helps in maintaining acidic pH in the stomach. The feed is exposed to gastric enzymes ofr some time, and the semi-digested feed is passed to the small intestine through an opening controlled by sphincter muscles.

Note: Next post on this blog will be comprised of details about "Small Intestine".