Some awesome information on the digestive tract
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It's a big read but some amazing information if you have the time. :)
Structure & Function Of The Digestive Tract
Essentially the digestive tract is a tube that extends from the mouth to the anus.
The bulk of the tract is located in the abdominal cavity. This can be described as a large ‘box’, the sides of which are the diaphragm in front, the muscles below the spine forming the top, and the muscles of the ‘belly’, the bottom. The entire contents of the digestive tract account for approximately 5% of the body weight of a mature horse.
We will now travel through the digestive tract, looking at each component and its role in the digestive system.
1. Mouth and associated structures
The mouth extends from the lips back to the pharynx. The cheeks form the sides and the hard palate the roof. The tongue lies in the bottom of the mouth and helps to move food back to the pharynx.
Horses drink by using the tongue like a suction pump. The ears of a horse can be observed to go forward with each swallow, then drop back between swallows.
The mucous membrane
The mouth is lined by a mucous membrane which is basically a barrier between the contents of the digestive tract and the rest of the body.
The role of the mouth is to grind up food to a small consistency suitable for swallowing. Feed is grasped with the incisor teeth while the lips pick up loose feed.
The feed is then ground up between the molars, the chewing process cracking the outer shell of grains, reducing the particle size of feeds, and increasing the surface area of food particles. Basically food has to be ground down to less than 1mm in length before swallowing.
An adult horse will make approximately 3000-3500 chewing movements per kilogram of hay, and 800-1200 chews per kg of concentrate.
The role of saliva
Saliva is secreted from the salivary glands and acts as a buffer and a lubricant for swallowing. It does not digest food.
Unlike humans, horses only produce saliva in the presence of food (we only need to think about it!), and they secrete approximately 10 - 12 litres per day.
2. Pharynx
The pharynx is involved with both the digestive and respiratory systems, and is a short, funnel-shaped tube between the mouth and the oesophagus.
It has a muscular activity that forces food into the oesophagus. Once food is inside the oesophagus the soft palate descends to prevent food returning to the mouth.
This same mechanism stops a horse breathing through the mouth. If food or water was to try and get out it would go via the nasal passages.
3. Oesophagus
The oesophagus is a muscular tube approximately 120 - 150 cm long. It extends from the pharynx down the left side of the neck, entering the thorax and passing above the trachea and heart, before penetrating the diaphragm and entering the stomach.
The oesophagus can be seen in the jugular groove on the left side of the horse. When no food is present the oesophagus is completely collapsed.
Progressive waves of constriction push food down the tube in waves in a process called peristalsis, and this progression cannot be reversed.
Thus if a horse is seen to have feed and water coming out through the nostrils one might assume the oesophagus is blocked. This can occur in a condition known as "choke".
4. Stomach
The stomach of the horse is relatively small at 10 - 20 litres, and represents approximately 10% of the total digestive tract. In a 16 hand high horse it is about the size of a football. In comparison, cows have 4 stomachs holding a total of 250 litres!
The stomach is basically held in place by the oesophagus, assisted by pressure from the rest of the digestive tract below.
At the entrance of the oesophagus to the stomach there is a very tight muscular ring called the cardiac sphincter.
This ring is so tight that horses cannot vomit, and in fact it is so effective the stomach will rupture before the sphincter opens. This means the horse must be very careful not to eat anything that might cause a digestive upset.
The stomach is J-shaped, and is divided into four regions. These are the oesophageal region, cardiac region, pyloric region and the fundic region.
Because of this shape, water drunk during or after feed passes over the top of the feed and does not wash the feed contents out. This fact refutes the old wives tale that water must be withheld during and after feeding to stop feed being wasted.
Food only stays about 20 minutes in the stomach, but this organ is rarely empty because of the continuous grazing habit of horses.
Progression of feed through the stomach will only slow down if the animal stops eating. If the horse is fasted, it may take 24 hours for the stomach to clear.
The main role of the stomach is to mix food thoroughly, secrete gastric juices to begin the digestive process, and act as a reservoir to maintain a constant supply to the small intestine.
The lining of the stomach contains a mucous layer that produces digestive juices like hydrochloric acid (to break down solid particles and help kill bacteria eaten in the feed) and pepsin (to digest protein).
Production levels of digestive juices reach about 10 - 30 litres per day and are stimulated by the presence of food (in humans this acid is stimulated by the sight of food). The saliva that comes down from the mouth acts as a buffer to prevent the stomach from getting too acidic.
Food exits into the small intestine at the pylorus.
5. Small Intestine
The small intestine is a long, narrow tube approximately 20 - 25 metres in length and 5 - 10 cm in diameter. It holds about 50 - 70 litres, or about 30% of the contents of the digestive tract.
Food moves through the small intestine at a rapid rate, within a couple of hours, moving at about 30 cm per minute.
The 3 parts of the small intestine
The small intestine is divided into three parts. The first metre is called the duodenum and this is relatively fixed in position. It forms an S-shaped bend which contains the pancreas. The second, much longer part is called the jejunum. It is approximately 20 metres long. The third part is called the ileum. It is between 1 - 1.5 m long.
The second and third parts are highly coiled and twisted. They normally sit in the upper left part of the abdomen (the flank) but are very mobile and will go wherever they are pushed.
The small intestine is suspended in the region by an extensive fan-shaped membrane called the mesentery.
Interaction with the liver and pancreas
Both the liver and pancreas secrete substances into the small intestine. The main function of the small intestine is to use the enzymes supplied by the pancreas to digest soluble carbohydrates, protein and fats contained in feed such as grains and roughage.
Vitamins (A, D, E, K and B) and minerals (calcium and some phosphorous as well as trace minerals) are also digested here.
Nearly 50 - 70% of carbohydrate digestion and absorption and almost all amino acid absorption occur in the small intestine, making it the most important site of digestion in the horse.
A major anatomical feature of the small intestine is the presence of millions of finger-like projections called villi inside the mucous layer. The purpose of these is to greatly increase the surface area of the intestine to assist this absorptive process.
The digested nutrients move across the walls of the small intestine and into the blood stream, where they proceed to the liver.
In the horse, bile is secreted from the liver into the duodenum. Bile is a product of the destruction of red blood cells, and its purpose is to break down fat globules so they can be broken down further by other enzymes.
In humans bile is secreted by the gall bladder, but horses do not have this organ.
6. Large Intestine
The large intestine is divided into the caecum, large and small colon, and rectum. It is relatively larger and more complicated than in other species.
Primarily a fermenting chamber, the rate of passage here is quite slow, taking approximately 50 - 60 hours.
Horses consume large quantities of cellulose in their diets as this is a major structural component of plant species. Normal digestive enzymes (such as those in the small intestine) cannot break down cellulose, and so the large intestine houses a large population of bacteria to break the cellulose down into substances that can be absorbed by the body.
Volatile fatty acids
These substances are called volatile fatty acids (VFA's) and they are absorbed across the mucous membrane into the blood stream where they head to the liver for processing (joining nutrients from the small intestine).
Another major structural component of plant material is lignin, which is found in longer grass or forage. Even the micro-organisms in the large intestine cannot break this down, and so horses grazed on this type of forage tend to have what is called a "grass-gut". Physically this appears as a large rounded belly somewhat like that of a pregnant mare.
There are 3 different types of VFA's - acetic, propionic and butyric. The amount and proportion produced will depend on the composition of the diet.
Starch that is not digested and absorbed in the small intestine also reaches the hindgut and is fermented to volatile fatty acids plus D-lactic acid.
Excessive production of D-lactic acid can cause serious problems and should be avoided.
As well as digesting nutrients, microbes also produce nutrients such as vitamin K, B-complex vitamins, proteins, and fatty acids, and these are absorbed in the large colon.
The large intestine is also the main site of water absorption, and thus the contents of the digestive tract are quite dry once they leave the large intestine.
Caecum - The caecum is a large comma-shaped bag 1.25 m long and holding 25 - 35 litres. It extends from high in the right flank downward and forward to the region of the diaphragm.
The caecum is basically a large fermentation vat, and its contents are always liquid. A useful veterinary procedure called an abdominal tap examines the fluid from the caecum as a diagnostic tool in conditions such as colic.
The entry and exit points of the caecum are located about 5 cm apart at the top of the organ, near the horse's right hip bone.
This odd design can cause problems if the animal eats lots of dry feed without adequate water, or if a rapid change of diet occurs. Both these situations can cause a compaction at the lower end of the caecum, which in turn causes pain.
Rapid changes of diet cause problems because the microbial population is quite specific as to the feedstuffs it can digest.
Digesta reaches the caecum approximately three hours after a meal, and remains in the large intestine for a total of about 36 - 48 hours.
A couple of hours after your horse has eaten, you can place your ear against the right side of the horse and listen to the gut sounds microbial digestion causes.
Large colon - The large colon is about 3 metres long and varies in diameter from 5 cm near the caecum to 50 cm. It is divided into four sections by three sharp bends, or flexures.
The pelvic flexure is the main feature of note as this is a narrow, sharp bend that is a favourite spot for blockages.
The large intestine is held in place by its bulks. A short transverse colon joins the large colon to the small colon.
Small colon - This 3 metre long and very narrow (5 - 10 cm) coiled tube sits in a jumble with the small intestine. Because it is fairly free moving it can be involved in abdominal problems such as a twisted gut.
Rectum - This is the final, short, straight portion of the tract, and it is a storage area for faeces. It is about 30 cm long and lies in the pelvic cavity above the bladder.
The pelvic flexure of the large colon, coils of the small colon, and the uterus can be felt through the rectum walls.
The digestive tract ends at the anus where the mucous membrane once again meets the skin.
Associated Organs
Liver
The liver is squeezed in between the diaphragm and the gastrointestinal tract, and is connected to the duodenum (small intestine) by the hepatic duct. It weighs about 5 kg.
Its main role is to process absorbed nutrients, store energy, secrete bile, and break down exhausted red blood cells. These are extremely important functions and thus the health of the liver is critical.
Inflammation of the liver is called hepatitis.
Pancreas
The pancreas is also attached to the duodenum, and is a soft, flat organ that secretes the enzymes used in the small intestine to digest food, and sodium bicarbonate to neutralize acid (from the stomach).
It also secretes hormones such as insulin, which is used to control the passage of glucose from blood into body cells. The rate of production of pancreatic secretions is constant, but it increases by five times when food is present in the stomach.
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