Anatomy, Part 7

The Digestive System

Food provides the energy necessary for life, but when food first enters the body, it is in a form that the cells of the body cannot use. Therefore, the body must break it down into small molecules which can pass through the lining of the bowel, through the blood vessel walls, and, finally, through the walls of the cells to where the energy is needed. The process of breaking down food into fuel that the body can use is known as digestion, and the parts of the body which bring this about are collectively known as the digestive system.

The digestive system is made up of:

• The mouth, pharynx and oesophagus

• The stomach

• The small intestine

• The large intestine

• The rectum and anus

• The liver

• The pancreas

THE MOUTH, PHARYNX AND OESOPHAGUS

The system starts with the lips and muzzle, which select the food to be eaten. The food is then bitten off (in the case of grass and long-stemmed hay) or taken in by the incisor teeth. From here, the tongue pushes the food back to the molar teeth, where it is ground up and mixed with saliva.

Three pairs of glands secrete saliva:

• The parotid glands are found at the bases of the ears.

• The sublingual glands lie under the tongue.

• The mandibular glands are at the back of the mandible on each side.

The saliva warms and lubricates the food. It also contains enzymes that help to break down the starch in the food.

After chewing and mixing the food with saliva, the tongue forms a bolus, or ball, out of the chewed-up food. This bolus is then swallowed and passes through the pharynx and down the oesophagus. The bolus is propelled down the oesophagus by waves of muscular contractions called ‘peristalsis’. The oesophagus runs down the neck, into the chest, between the lungs, through the diaphragm and into the stomach.

THE STOMACH

Food enters the stomach through the cardiac sphincter, a ring-shaped muscle that operates as a one-way valve, preventing food from leaving the stomach and re-entering the oesophagus. This prevents the horse from vomiting.

Inside the stomach, the food is churned around and mixed with gastric juices and hydrochloric acid, which help the digestive process by further breaking down the food. Part of the stomach-lining also contains glands that secrete digestive enzymes:

• Pepsin, which begins to break down proteins.

• Lipase, which begins to break down fats.

• Rennin, which coagulates the milk drunk by foals.

The stomach is quite small and can only hold about 2.27kg of hard food. It is never more than two-thirds full of food; when this level is reached, some of the broken-down food, or chyme, is released through the pyloric sphincter into the small intestine. Therefore, it is important not to overfill the stomach by feeding too much at once, as food can easily be sent through the system too quickly for efficient digestion.

THE SMALL INTESTINE

Most of the digestion and absorption of nutrients occurs in the small intestine. Its average length is 20m, and it can be divided into three parts, the duodenum, the jejunum and the ileum.

The duodenum measures about 1m in length, and it contains bile, which is secreted by the liver. Bile emulsifies fats (gives them a bigger surface area) and neutralises acid from the stomach. Also present in the duodenum are enzymes from the pancreas, which turn carbohydrates into simple sugars such as glucose and fructose and proteins into amino acids.

The jejunum is the largest section of the small intestine. This is where most nutrients, in particular, fats, amino acids, and simple sugars, are absorbed into the bloodstream. From the bloodstream, they will either be used directly by the body or stored in the liver until needed.

The ileum continues the process of digestion; calcium, phosphorus and other minerals are absorbed into the bloodstream here. Finally, the ileum controls the flow of partially digested food (now consisting only of fibre and water) into the caecum of the large intestine.

THE LARGE INTESTINE

The large intestine can be divided into three parts: the caecum, the large colon and the small colon.

The caecum is a large, blind-ending sac. It is over 1m in length and can hold 25 litres or more of liquid. It is the equivalent of the human appendix. The caecum contains many millions of gut flora or beneficial bacteria, whose job is to break down the fibrous material from the diet, which is all that remains after the nutrients have been removed in the small intestine. There are many types of bacteria, each specialising in processing a particular foodstuff, although they can adapt to the kind of food eaten over time. This is why it is important to make any changes in the horse’s diet gradually to allow the flora in the caecum to adjust to the change. The action of the bacteria digesting the fibre in the caecum produces fatty acids and vitamins B and C, which are then absorbed. Water is absorbed throughout much of the system and also from the caecum.

Bacterial action continues inside the large colon but with fewer bacteria involved. The breakdown of food in the large colon can take several days, which explains its large size. The space inside the abdominal cavity is limited, so the bulky colon must be folded to fit into it. These tight bends in the folds can become blocked with food, giving the horse colic. Parts of the large colon can also become twisted around one another, cutting off the blood supply – and again resulting in colic.

The small colon (also known as the descending colon) will absorb water, electrolytes and any remaining nutrients that are left after transit through the caecum and large colon.

THE RECTUM AND ANUS

The remaining fibrous waste is expelled as manure through the rectum and anus.

THE LIVER

The liver is the largest gland in the horse’s body, weighing 5kg. It is located inside the abdominal cavity against the back of the diaphragm. It is also one of the most important organs in the body as many digestive processes occur here; it detoxifies the blood; and it plays a major part in protecting the body from disease and infection.

The liver has a double blood supply; it receives oxygenated blood from the hepatic artery and deoxygenated blood containing nutrients directly from the small intestine via the hepatic portal vein. Blood from both vessels enters the lobes of the liver, where oxygen, most of the nutrients and several toxins are removed by the hepatic cells. From here, the nutrients are either stored in the liver or sent out into the body; the toxins are either neutralised or stored; and the blood is returned to the heart through the main blood vessel, the vena cava.

The major functions of the liver include:

• Processing protein, carbohydrate and fat

• Storing vitamins

• Secreting bile to be used in the small intestine for processing fats and cholesterol

• Removing toxins, drugs and hormones from the body

• Breaking down and removing the remains of dead blood cells

• Formation of blood proteins

• Producing heparin, an anticoagulant

THE PANCREAS

The pancreas is found next to the liver, and its role in digestion is to produce pancreatic juice. This juice is slightly alkaline to work with bile to neutralise the acid from the stomach, giving an ideal environment for the enzymes in the small intestine to work. There are also enzymes present in the pancreatic juice itself, which help process proteins, carbohydrates and fats.

The second function of the pancreas is to produce insulin, which helps stabilise the levels of sugar in the blood.

SUMMARISING DIGESTION

There are essentially five stages involved in processing food by the body for use in the cells. These are:

• Ingestion – taking food into the body by eating.

• Movement of food – the passage of food along the length of the gastrointestinal tract mostly by peristalsis.

• Digestion – the breaking down of food both by mechanical and chemical means.

• Absorption – the transferring of nutrients from the gastrointestinal tract into the blood and lymphatic systems, and from there to the cells where they can be used.

• Defaecation – the removal of any non-digested material from the gastrointestinal tract.

NOTE: The horse’s digestive system evolved to nourish an animal living in the wild. A horse living in the wild would have covered many kilometres every day in search of food. A large proportion of his time would be spent grazing, and his diet would be varied and consist of grasses, herbs and shrubs, which would have provided adequate energy for his needs.

Domestication has changed the horse’s habits, diet and workload dramatically. A horse in a domestic environment may only graze for a few hours a day, and this is usually in a field with a limited variety of plants and grasses. The horse is also now expected to participate in various vigorous activities like dressage and showjumping, that place far greater demands on his energy. On top of this, he is often given two or three large concentrate meals every day to supplement his diet. All of these changes have put a strain on the digestive system of the modern horse.