Physiology of the Gastrointestinal Tract

Nutrition is the cornerstone of the treatment of digestive diseases. However, considering actual pathophysiological knowledge about gastroenterology, it seems obvious that there is no diet adapted to all kinds of digestive cases. The general objectives of the diet are: stimulating dietary consumption, improving digestion and nutrient absorption, maintaining normal digestive motility and intestinal transit, and decrease inflammation when it exists. In addition, the dietetic strategy must plan to provide the right nutrients to optimize the bacterial flora and to protect the mucosal barrier.

Jürgen ZENTEK
DMV, Prof, specialist degree in animal nutrition, Dipl. ECVCN

Jürgen Zentek graduated from the Faculty of Veterinary Medicine (Tierärzliche Hochschule) in Hanover, Germany in 1985. After employment in a veterinary practice, in 1987 he led a research project at the Department of Animal Nutrition, studying the energy intake and skeletal development in growing Great Danes. He obtained his degree as a specialist in animal nutrition and dietetics in 1993. After a year in Bristol, UK, at the School of Veterinary Science, he took the Chair of Clinical Nutrition at the Veterinary University of Vienna in 2000, becoming the Head of the Institute of Nutrition. Since 2005 he has been a Professor of the University of Berlin. His ongoing research is on clinical dietetics of domesticated animals, the relationship between nutrition, intestinal microflora and immunity of the GI tract.

Valérie FREICHE
DMV, Prof, specialist degree in animal nutrition, Dipl. ECVCN

Valérie Freiche graduated from the National Veterinary School of Alfort in 1988 where she remained as an intern then assistant in the Department of Medicine until 1992. Having developed her own practice in the Paris region Valérie initially worked with dogs and cats before choosing to concentrate on gastroenterology. Between 1992 and 2006, she has been responsible for gastroenterology consultation and gastrointestinal endoscopy at the National Veterinary School of Alfort. She also had the same role in a referral practice, in Paris. Since 2006, she works in a referral practice in Bordeaux, in internal medicine and gastroenterology. Valérie is the President of the Internal Medicine Studies Group (GEMI) of the French Association of Veterinarians for Companion Animals (AFVAC). Valérie regularly participates in conferences and post-university training sessions in gastroenterology.

1. Physiology of the Gastrointestinal Tract

Oral Cavity (Figure 1)

Morphologically and physiologically, domestic cats are highly specialized carnivores, as shown by their dentition, nutritional requirements and sense of taste (Bradshaw, 2006). The tongue is rough and has multiple hooklike appendages. These filiform or fungiform papillae enable the cat to lick up liquids and to scrape flesh off bones (Ojima et al, 1997). There are approximately 250 fungiform papillae on the tongue of an adult cat; they are most numerous on the tip. Their size – and the mean number of taste buds – increases from the tip to the back of the tongue (Robinson & Winkles, 1990). A cat’s sense of taste – except sweetness – is mediated via taste buds mainly located in the tongue. The cat has specific and unique feeding preferences linked to its ability to smell amino acids and peptides (Zaghini & Biagi, 2005). The dentition of cats is typical of carnivores. Cats have 26 milk teeth that are replaced at age five to seven months by 30 permanent teeth. The permanent dentition is made up of 12 incisors, 4 canines, 10 premolars and 4 molars (see Chapter 11).

Figure 1. General digestive tract anatomy in the cat.

Esophagus (Figure 1)

The esophagus is a tube that transports food from the mouth to the stomach. At body weights of 4 - 5 kg, the average length is 22 - 23 cm. The cervical segment of the esophagus accounts for about one third of the whole length and the thoracic segment about two thirds (the abdominal segment is very short in the cat) (Hegner & Vollmerhaus, 1997). Coordinated contraction of the longitudinal and circular esophageal musculature is important for the peristaltic transport of a food bolus through the esophagus (Dodds et al, 1973). Motility is subject to a myogenic control system and additional nerve control mechanisms (Preiksaitis & Diamant, 1999). The esophageal glands produce a mucinous secretion that helps lubricate the food bolus. Although cats are able to swallow large pieces of food or prey, the esophageal passage of capsules or tablets may be prolonged or tablets may become trapped due to their diameter or surface structure (Graham et al, 2000). The possibility of medication-induced esophagitis should be considered when administering ulcerogenic drugs to cats.

Stomach (Figure 1)

The stomach has a comparatively large capacity for prey or food storage. The stomach may be subdivided into several anatomical and functional regions. The cardia is the site of entry, the fundus, body and antrum are the middle parts and the pylorus is the transitional zone to the duodenum. Normally, gastric emptying delivers food to the small intestine at a rate that allows optimal intestinal absorption of nutrients (Wyse et al, 2003). The pylorus is surrounded by muscle tissue and regulates food transport into the duodenum. The pyloric muscle prevents the reflux of duodenal contents and bile into the stomach lumen.

Endocrine G-cells are scattered diffusely in the basal part of the mucosa and produce gastrin, a major stimulus of gastric secretory response to meal intake (Cerny et al, 1991). In the stomach, hydrochloric acid secretion by the oxytic cells and pepsin, secreted as pepsinogen by the chief cells, initiate protein digestion.

Lipase activity occurs in the surface mucous cells in newborn cats after ingestion of milk (Knospe & Plendl, 1997). Lipase is localized as pepsin in the chief cells but is also present in pepsin-free cells, the mucus surface cells of the fundus and the antrum (Descroix-Vagne et al, 1993).

Gastric motility and emptying is subject to various regulatory mechanisms, including reflectory, neural and endocrine factors. Diet composition may affect gastric emptying, with fat and large particle size having a delaying effect (Strombeck, & Guilford 1996a; Hall & Washabau, 1999). The stomach can retain ingesta for up to 15 hours before it passes to the intestine (Brugère, 1996). The gastric transit time, determined from the first exit of barium-impregnated polyethylene spheres (BIPS) from the stomach had a median of 6 h (range 3 to 8) in sedated and a median of 2.5 (range 2 to 6) in unsedated cats. The median of 50% gastric emptying time was 6.4 h (range 2.5 to 10.9), and complete gastric emptying was seen after 12 h with a range 6 to 27 h. The orocecal transit time of BIPS was 6.5 h and the 50% orocecal transit time was 8.8 h (range 4.6 to 12.8) (Sparkes et al, 1997).

Small Intestine (Figure 1)

The duodenum, jejunum and ileum are the three histologically defined parts of the small intestine. Bile and the pancreatic secretions enter into the duodenum via the common bile duct and are necessary for the solubilization of fat and the enzymatic digestion of the intestinal content.

The small intestinal mucosa has a specific structure with crypts and microvilli covered by a single epithelial layer. The crypts are the location for cell proliferation. The absorptive enterocytes bear a high density of microvilli, which increases the surface area substantially. The paracellular space is closed by different proteins with specific functions that prevent uncontrolled permeation of bacteria or macromolecules through the intestinal wall. A mucous layer, the glycocalix, consisting of carbohydrates and proteins, covers the brush border. The glycocalix has a high enzymatic activity for the breakdown of macromolecules to absorbable units and provides a specific microenvironment for bacteria associated with the gut wall.

Besides its absorptive capacity, the small intestine has a considerable secretory capacity via the crypts and the goblet cells. Endocrine cells contribute to the regulation of the digestive processes. The duodenal glands are located caudally to the pylorus and produce mucous secretion with neutral, sulphated and carboxylated acid mucosaccharides (Takehana & Abe, 1983). The compounds in food that have passed through the small intestine undigested or unabsorbed enter the large intestine and are fermented by microbial enzymes. A sphincter terminates the small intestine and prevents reflux of chyme and bacteria.

Large Intestine (Figure 1)

The cecum, colon and rectum are the three parts of the large intestine where undigested organic matter is fermented and fluid, minerals and bacterial metabolites are absorbed. Due to the carnivorous character of the cat, the size of the large intestine is small (Table 1), probably because there was no evolutionary need for a large fermentation chamber (Chivers & Hladik, 1980). The large intestine has no microvilli and its surface morphology differs considerably from the small intestine. The crypts of Lieberkuhn contain absorptive and secretory cells. The large intestine of cats is characterized by a dense microbial community with high metabolic activity.