The small intestine of an average sized horse (450 kg) is about 20 metres long. It is anatomically divided into 3 segments:
Duodenum - the first 1-1.5 meters. The bile duct and pancreatic duct empty their contents into the duodenum at the junction with the jejunum.
Jejunum - the middle part of the intestine (> 80% of the length). The wall of the jejunum has an abundance of digestive glands that secrete enzymes and buffers into the lumen. The jejunum has a large absorptive surface, and is the major site for the absorption of nutrients including vitamins and minerals.
Ileum - the final 1-2 meters of the small intestine. The ileum's structure and function is much like that of the jejunum.
The horse has no gall bladder. The bile, which is produced by the liver, is therefore secreted continuously into the small intestine. Bile solids (phospholipids and bile salts) help in the emulsification of fat, and are a pre-requisite for lipid digestion. The pancreas secretes a bicarbonate-rich fluid which neutralises the acidic contents of the stomach digesta. Gastric contents are neutralised within a few metres of their emergence into the small intestine. The pancreas also secretes enzymes that hydrolyse proteins (trypsin), fats (lipase) and starch (amylase). Enzymes secreted by the glands of the intestinal wall continue to digest end products from pancreatic enzyme reactions into small components that can be absorbed by the cells of the intestinal epithelium.
The transport velocity through the small intestine is approximately 20 cm per minute, thus giving the small intestine about 1.5 hours to digest the feed and extract nutrients. In the last part of the ileum the digesta accumulates before it is squirted under pressure into the large intestine at the junction between the colon and the caecum. Emptying of the ileum appears to take place 3-6 times per hour, and delivers 0.2 to 1.5 litres of intestinal contents to the hind-gut at a time.
The digestive secretions increase the water content of the digesta until it has the consistency of a thick soup that can be propelled through the long and curving intestine. At the terminal ileum the water content is about 96% (4% dry matter) on a roughage diet, and in the range 90% to 96% on diets with a greater proportion of grains and commercial concentrates.