A horse’s digestive system is a complex and impressive machine. Understanding its anatomy is vital to ensure your horse eats and digests food as efficiently as possible, and therefore remains healthy. This blog is aimed at explaining the basics of the digestive system in horses, so you can understand how it works and what may go wrong.
The stomach is a muscular sac, located on the left side of the abdomen. It is capable of holding up to 22 gallons of food. There are two regions in the stomach: glandular and non-glandular. The glandular region produces hydrochloric acid (HCl) which is secreted to break down protein. Cells in the stomach also produce mucus to protect it from the low pH of the gastric contents, and peptidases to break down proteins. Minimal microbial digestion occurs in the stomach.
Once ingesta leaves the stomach, it passes into the small intestine, which is 21 – 25m long and makes up 75% of the total gastrointestinal tract (GIT) volume. The inner wall of the small intestine is covered in 0.5-1mm long finger-like projections called villi, which increase surface area and facilitate nutrient absorption. The villi contain microbes which help break down complex carbohydrates and produce VFA, methane, carbon dioxide and water. The small intestine absorbs fat, sugar, starch and many vitamins and minerals, such as zinc, iron, calcium and phosphorous.
Ingesta is deposited in the cecum in the form of large balls that can be felt on palpation of the right dorsal quadrant of the caudal abdomen (see figure above). The microbiome in the cecum is highly diverse and diet influences the composition and ratio of the different microbial species. In the hindgut, fermentation produces ammonia, carbon chains and amino acids that bacteria utilize for energy. The large colon is the largest compartment in the equine digestive tract; it can hold up to 90 litres of fibre. During this phase, the fibre is broken down and absorbed by microbial fermentation. In addition, the large colon can absorb a great deal of water to help keep the animal hydrated.
The liver is contained within the rib cage and to the right of the midline. It is less lobated than in other animals and its functions include the metabolism of carbohydrates, lipids, proteins, salts, and hormones. It also produces bile, which is essential for the digestion of fats.
After passing through the pylorus, the digested food enters the duodenum via the torus pyloricus, which is a circular muscle, and the jejunum via the ileocecal valve/cecum. The ileum is connected to the cecum by an azygos vein, which can become distended and cause problems if it backs up into the stomach. The cecum can be removed by performing a procedure called a cecal trocarization.