The internal lining of the stomach is divided into two halves. The upper part of the stomach is nearest to where food enters from the esophagus. It consists of non-glandular (squamous) epithelial tissue that does not have much secretory function in relation to digestion. This portion of the stomach has limited protective mechanisms and can become exposed to acid when there is limited forage intake or during exercise.
The lower half of the stomach is lined by glandular epithelium. This is where gastric acid is secreted. Therefore, the horse also secretes mucus and other protective factors in this area to protect against the acid it is producing. The two sections are divided by a demarcation known as the margo plicatus.
The long and complicated nature of the horses' digestive tract can predispose them to many problems.
• Colic: the term "colic" refers to abdominal pain rather than a specific disease. Conditions that commonly cause colic include gas, impactions with feed or sand, abnormal motility, and a myriad of other causes
• Gastric ulcers (equine gastric ulcer syndrome or EGUS)
• Infectious diseases caused by bacteria, viruses, parasites, and other organisms
• Other non-infectious diseases caused by overeating, eating poor-quality food, chemicals, obstruction, inflammation and cancer