(I've always been fascinated by all things equine, including their anatomy and how their various body systems work. As a project to learn more about these amazing animals, I've decided to do a series of posts on the equine digestive system. As these come along, they'll be added to the sidebar. Please keep in mind that I am neither a vet nor an expert on these topics - if you have corrections and additions or resources to direct us to, please put that in the comments. Also, you may find it helpful to visit this article, which has excellent pictures of the digestive system and the location of the various organs.)
Once food has passed down the stomach and through the cardiac valve, it enters the stomach, which has a capacity of 8-15 quarts. The stomach is located fairly far back in the ribcage, fairly high up and towards the left side. In the horse, the stomach is a transit point - most of the contents leave after a very short time - 15 minutes or so - after minimal digestion by hydrochloric acid, muscular and enzymic action. Some particulate matter may linger longer, but once again this portion of the digestive system works best with continuous ingestion of forage, rather than occasional ingestion of larger pelleted meals. This is due both to the rapid emptying of the stomach and also the fact that the horse's stomach essentially produces acid on a continuous basis. With a continuous incoming supply of forage to digest, together with the buffering effect of the saliva produced by chewing the forage and which is swallowed with the food, the horse's stomach works without problems.
One of the reasons horses are prone to gastric ulcers is the way we feed them. As soon as a horse does not have access to forage on a continuous basis, and particularly if the horse's diet has a high proportion of pelleted feeds to forage, the stomach cannot function in the most effective way. Since the horse is not chewing forage continuously, saliva isn't produced continuously in volume, and cannot buffer the continuously-produced acid in the stomach. And the rapid emptying of the stomach after food enters means that the stomach lining is more vulnerable to the acid.
When I was first learning about gastric ulcers in horses, I was surprised to find out how common they are. To treat them, essentially we provide artificial buffering to the horse's stomach acid (or in the case of severe ulcers, block the production of acid for a time to allow healing). Horses with ulcers can exhibit a variety of behavior changes, including pain and misbehavior during grooming, tacking or riding. It is sad to think that many horses are punished for behaviors that arise due to treatable pain.
When the food exits the stomach, it passes through the pyloric valve into the small intestine. If there is some blockage of flow through the rest of the digestive system, food can back up into the stomach (gastric reflux), resulting in distention and severe pain. As I understand it, this is one thing vets look for when they pass a nasogastric tube into the stomach during treatment of colic - if the contents of the stomach are under pressure due to gastric reflux, this will be apparent when the tube enters the stomach, and removing some of the contents of the stomach may relieve the pain. Gastric reflux is also a sign of blockage further on in the digestive system.
Stay tuned for the small intestine . . .