There are two types of digestion, physical and microbial. Physical digestion happens during things such as chewing, it is the mechanical break down of food. Prehension is the receiving of food into the mouth. This happens via the lips, tongue and teeth. Mastication is chewing, this is a strong reflex in horses as they chew grass thoroughly. They do not regurgitate grass back up to chew as cows and sheep do. Grass has a high silica content which is difficult to chew.
When the food has been chewed into balls it is called a bolus. Saliva is important for sticking it together, it also makes the food the correct pH for the enzymes and microbes to work. The gap between the front and back teeth is called the diastema, this is where the bit lies. This has happened due to lengthening of the face during evolution. It also helps to separate chewed food from unchewed food.
Degluttination is swallowing, this is a reflex action. There are voluntary and involuntary actions involved. The epiglottis blocks the trachea (the wind pipe), the larynx moves forwards and upwards to let digesta past and the tongue acts a plunger. In the involuntary actions, the soft palate elevtates, there is relaxation of the cardiac sphincter and peristalsis which squeezes the food along. Below is a video from YouTube showing how this works.
The stomach can be divided into four regions. Mainly protein digestion happens here, there is also a bit of storage and it regulates the flow of digesta. The oesophageal region of the stomach is non glandular, there is a small amount of fermentation of carbohydrates here. The cardiac region secretes mucus, this is good for protecting the stomach lining and stops acid damaging the epithelial cells. The fundic and pyloric regions have gastric pits which make the gastric juices. They have G cells which secrete a hormone called gastrin. The fundic region is glandular and secretes mucus. It is stimulated when the stomach stretches with food inside it. Gastrin is the produced and triggers other reactions. The pyloric sphincter controls the rate digesta can leave the stomach. This is why over feeding can cause colic, as the food will build up in the stomach.
The small intestines is the place where the majority of mammalian digestion occurs (this is the digestion not carried out by microbes). There is secretion of mucus, enzymes, alkalis, hormones and bile etc. here which help to neutralise the acid that has escaped from the stomach. A lot of mixing also happens here. The majority of carbohydrates and fats are digested in the small intestine. Problems can occur is the carbohydrates reach the caecum. The rate of movement is slowest in the ileum (the last section) as this is the carbohydrates last chance to be absorbed before the caecum. They are absorbed through the villi and microvilli which are finger-like projections that increase the surface area.
The caecum has an anaerobic environment because air is squeezed out as food falls into the bottom of the sack. The caecum contains friendly bacteria and protozoa, they are anaerobic and produce amino acids. It has a pH of 5, this is critical for the micro organisms to live as below this it will be too acidic and they will die. When they die, toxins are released which can get into the blood stream and cause laminitis.
In the large intestine it is a major site of water re absorption.
The horse's digestive system is quite complicated but an understanding of it will help with choosing the correct diet for your horse.