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;
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 |
