Animal intestine is presented daily with large amounts of ingested fluid and endogenous secretions. The endogenous secretions are particularly voluminous in herbivorous animals. The volume of the secretory fluid in omnivores and carnivores, although generally not as large as that in herbivores, is still substantial and can be affected greatly by the type of diet.
Little water moves across the rumen by the way of mucosa. Sodium is actively absorbed from the rumen, whereas chloride is passively absorbed. Potassium is actively secreted into rumen. The rumen wall is relatively impermeable to calcium, magnesium, and phosphate. In nonruminants, only small amounts of water enter the stomach through gastric mucosa. The absorption sites in the small intestine differ depending on the species. Most of this absorption occurs in the small intestine in a strict carnivore. In the horse and pig, the absorption of water and ions is shifted from the small to large intestine as microbial digestion develops. Nevertheless, the small intestine still absorbs large amounts of water and salt, and the majority of minerals and fat- and water-soluble vitamins. Water-soluble vitamins are absorbed rapidly.
Absorption of fat-soluble vitamins, however, is affected by fat absorption. Most vitamins are absorbed in the upper small intestine, but vitamin B12 is absorbed in the ileum.
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