Equine Alimentary System - Anatomy & Physiology

Introduction

The horse is a monagastric hindgut fermenter. The horse evolved for grazing and it does so for up to 17 hours a day. A high proportion of the horse's dietary carbohydrate is in the form of starch. A mature horse eats 2-2.5% of it's body weight in dry matter every day, 1.5-1.75% of this should be fibre (hay/haylage). This is to prevent a rapid drop in pH in the large intestine and also to stimulate peristalsis in the gut and prevent build up of gas.

Undigested material spends a long time in the caecum and large intestine being digested by microbial fermentation, mainly cellulose (95% after 65 hours).

In the hindgut of the horse; 75-85% of insoluble carbohydrates is digested, 15-30% of soluble carbohydrates and 30% of protein digestion. A lot of absorption of volatile fatty acids (VFAs) and water occurs in the large intestine which pass readily into the blood. Electrolytes are also absorbed in the large intestine; 95% of sodium and chloride and 75% of potassium and phosphate. To mix the contents of the large intestines, the taenia and circular muscle of the tunica muscularis contract. This also transports the ingesta through the large intestine and brings the products of fermentation in contact with the epithelium.

Small Intestine

The arrangement is similar to that of the dog's small intestine, but the position of the jejunum is influenced by the large ascending colon and caecum. The jejunum is restricted to the left dorsal part of the abdomen. The ileum opens directly into the caecum. The transit time of food in the small intestine is quite rapid. 25% of a liquid marker has reached the caecum within 1.5 hours. Most ingesta reach the large intestine within 3 hours after intake. 70-80% of protein is digested and 70%+ of starch is digested by enzymes.

Caecum

The caecum is the main site of microbial fermentation, followed by the ascending then descending colons. It is located on the right side of the abdomen. It is very large, roughly 1m in length with a 30L capacity. It consists of a base, body and apex (blind ending). The base lies in the right dorsal part of the abdomen, in contact with the abdominal roof. The apex lies on the ventral abdominal wall, and terminates at the level of the xiphoid cartilage. It exists at the junction with the ileum and colon.

The caecocolic orifice is where the caecum opens into the ascending colon. This exists as a transverse slit formed by a constriction of the ascending colon. There is a sphincter at this point which prevents backward flow of ingesta when the colon contracts.

The ileum opens into the caecum at the ileal papilla. This is a small projection into the caecum housing the ileal sphincter and venous plexus that, together, control the ileal orifice.

Taenia are present. Taenia are formed by concentration of the longitudinal muscle layer. Between the taenia are sacculations, or haustra. Haustra appear as folds on the interior surface. There are four taenia over the caecum; dorsal, ventral, lateral and medial. The dorsal taenia provides the attachment site for the ileocaecal fold, which joins the caecum to the ileum.

The lateral taenia provides the attachment site for the caecocolic fold, which joins the caecum to the ascending colon. The ventral taenia is free.

The medial and lateral taenia are where the caecal vessels and lymph nodes are located. Ingesta is regularly transported from the ileum to the caecum, this movement can be heard upon auscultation of the right dorsal quadrant of the caudal abdomen. Ausculatation of this area is carried out in the assesment of colic. In the horse, the caecum is responsible for the digestion of complex carbohydrates such as cellulose.

Colon

Ascending colon

The ascending colon is very large and takes up most of the ventral abdomen. It is the shape of a double "U", where one "U" is on top of the other. There are four limbs that lie parallel to each other, and three flexures that change these direction of the limbs.

The sequence of the limbs and flexures of the ascending colon is as follows; Right Ventral Colon (for those with an RVC bias remember, "the RVC comes first!"), passes out of the caecocolic orifice on the right side of the abdomen and continues cranially to the xiphoid region; Sternal Flexure, passes across the midline from right to left, Left Ventral Colon, runs caudally on the left ventral abdominal floor; Pelvic Flexure, turns dorsally just cranial to the pelvic inlet and then runs cranially to the diaphragm, Left Dorsal Colon, runs cranially, parallel and dorsal to the left ventral colon; Diaphragmatic Flexure, turns caudally at the diaphragm; Right Dorsal Colon, continues caudally on the right. It is the shortest limb of the ascending colon.

The transverse colon continues on from the right dorsal colon as the right dorsal colon turns medially. The right dorsal colon is attached by a mesentery to the dorsal abdominal wall, the base of the caecum, the root of the mesentry and the pancreas. This anatomical arrangement of mesentery allows the left ascending colon to twist and is a common cause of colic (colonic torsion).

The ventral parts of the ascending colon are attached to the dorsal parts by a short mesocolon. The mesocolon houses the blood vessels, nerves and lymphatics. In the ventral colon many important digestive and absorptive functions take place, whilst the dorsal colon is mainly responsible for transportation of ingesta. Taenia are present. Different parts of the colon can be distinguished by the number of taenia present:

The right and left ventral colon and the sternal flexure have four taenia. The left dorsal colon and pelvic flexure have one taenia and the right dorsal colon and diaphragmatic flexure have three taenia.

Transverse Colon

The transverse colon is short. It passes from across the midline from right to left. It passes cranial to the root of the mesentery The transverse colon has two taenia. It turns caudally to become the descending colon at the level of the left kidney.

Descending Colon

The descending colon is between 2-4m long. It is suspended by a long mesentery; mesocolon descendens. The descending colon has two taenia. Between the two taenia are distinct sacculations that house the faecal balls.

Microbial Environment

Microbes convert carbohydrates to volatile fatty acids (VFAs). The horse receives 75% of it's energy requirements from VFAs. The large intestine is buffered by the secretion of large amounts of bicarbonate from the pancreas and the ileum. Glands in the wall of the large intestine may also produce bicarbonate. The microbial population exists in the caecum and ventral colon.

It is mixed; there are both bacteria and protozoa. Microbes are anaerobic. The microbial population is dependent on diet and frequency of feeding, as different microbes are suited to digesting different things. The number of microbes can change 100 fold in a 24 hour period. VFAs produced are absorbed across the intestinal wall. Urea from the blood is transported to the intestinal lumen to be used by microbes, which also use nitrogen from the diet. Environmental factors of the caecum and ventral colon can influence fermentation of microbial population.

Environmental factors include: Frequent intake of food, constant temperature, constant mixing, removal of the products of fermentation by absorption and peristalsis and the stable osmotic environment i.e. normal intake of water.

VFA's produced include Acetate, Propionate and Butyrate. Factors that promote VFA production include an optimum pH of 6.5, an anaerobic environment and gut motility.

Rectal Palpation

Rectal palpation is a useful technique and is often used to assess colic. Structures that can be palpated per rectum include; faecal balls in the descending colon, the bladder, the reproductive organs in the mare, the base of the caecum, the root of the mesentery, the left kidney, +/- the nephrosplenic ligament, the left dorsal colon and the pelvic flexure of the ascending colon. NB: This is a common site of impaction.

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