Introduction.. Definitions..Basic Anatomy...Stomach...Small Intestine...Large Intestine...Liver and.Pancreas ...Vomit Centers...

Diseases: Primarily GI...Non-GI

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Introduction:

The causes of gastrointestinal motility problems (i.e. vomiting, regurgitation, diarrhea, constipation, ileus) in dogs and in cats are numerous. Sometimes, problems result directly from an insult to the digestive system producing maldigestion and malabsorption that results also in abormal motility; sometimes the problem is a mechanical obstruction or just a paralysis of the built-in motility apparatus; however, in many instances, gastrointestinal disturbances occur in response to non-gastrointestinal diseases, including metabolic, infectious, parasitic, neurological, neoplastic or other perturbations that only, secondarily, affect the processes promoting aberrant or normal gastrointestinal motility. In order to appreciate the possible outcomes from any direct or indirect perturbation to the motility and functionality of the digestive systems in dogs and cats, it is helpful if the viewer has at least a basic understanding and appreciation of the anatomy, gross and cellular physiology of the entire digestive system and then of the components of other systems insofar as these secondary systems affect gastrointestinal motility. This page provides an overview of these basics, then moves on to discuss selected (the more common) abnormalities that are the most likely reasons your pet vomits and/or has diarrhea. A larger list of potential causes of vomiting or potential causes of diarrhea are available; however, as stated, only the most common causes will be addressed here.

Definitions

Some of the terms used in veterinary (and human) medicine which describe anatomical structures and the symptoms of gastrointestinal disorders are defined and explained here; these will on occasion be part of this discussion of vomiting and diarrhea.

Anatomy of the Gastrointestinal Tract

Gross anatomy

The digestive system begins with the oral cavity, and ends with the rectum. In between there is the esophagus, stomach, small intestine (duodenum, jejunem and ileum), cecum and large intestine (colon). This discussion will bypass describing the oral and esophageal portions of the digestive system other to acknowledge that they exist and they are also subject to afflictions, though inherently different than the subject of this page; the focus here is instead on those areas involved in the creation and/or expression of vomit and diarrhea.

Microscopic anatomy

General

The gastrointestinal tract from stomach to colon is essentially, a tubular structure composed of layers. A diagrammatic representation of these layers is shown here.

While different portions of the tract are composed of the same fundamental layer structure, some modification and specialization of form and function are seen as well.

Stomach

General Information:

A diagramatic representation of the stomach is shown here. The stomach represents a continuation of the esophageal portion of the digestive tract. It is the first portion encountered in the abdominal cavity, and, in comparison to the rest of the gastrointestinal tract, is capable of great distension to accommodate ingesta. Grossly, it is slightly curved...the larger "outer" portion...the greater curvature...resides primarily on the left side of the abdomen in cats or on both sides of the abdomen in dogs, and is in close proximity to the spleen and transverse portion of the colon; the "inner" lesser curvature, in contrast, faces the liver, which lies cranially (towards the head). The stomach is relatively tapered at the proximal (beginning....where it joins the esophagus) and distal (furthest away...where it joins the small intestine) ends.

The stomach is also composed of layers, and the appearance and functionality of these layers varies somewhat, depending upon the anatomical portion of the stomach and the specialized function required. In general, the mucosa layer is glandular...mucus-secreting, acid-secreting, hormone-secreting and enzyme-secreting glands are present in variable numbers. In the relaxed state, the mucosa layer assumes a distinct and prominent longitudinally folded appearance, an arrangement of the mucosa called rugae. The extra mucosal tissue present in rugae will accommodate and permits extensive distension of the stomach with ingesta. Different anatomical regions of the stomach contain a preponderance of one or more of these glandular types in the mucosal layer. The submucosa and muscularis layers are especially developed in the stomach, to accommodate the prolonged churning and mixing of contents (see below).

Anatomy of the Stomach:

Functionality of the Stomach:

Small Intestine

General Information:

The small intestine, also known as the "small bowel", is comprised of three segments: beginning at the entrance from the stomach (pyloric sphincter), they are, sequentially, the duodenum, the jejunem and the ileum. These portions of the intestinal tract, collectively the small intestine, are concerned with the digestion and absorption of nutrients initially delivered from the stomach, which are mixed with enzymes and bile from the pancreas and liver, respectively. The entire small intestine is suspended by a thin membrane within the abdomen, the mesentery, within which are contained major blood vessels and lymph vessels which supply the intestine. In addition to digestive enzymes, there is a large population of bacteria which normally reside within the small intestine. These function to facilitate nutrient breakdown and supply certain vitamins and some amino acids. While within the intestine, these bacteria function in a positive way for the host animal. However, any breakdown in the integrity of the intestine which allows translocation of bacteria or bacterial toxins to the general circulation can result in bacterial colonization and severe infection of any of the host animal's body systems, and even death!

A diagrammatic representation of the small intestine layer composition is shown here. Please note that the basic layer structure described above is present here as well, but now, the inner most portion of the mucosa, the epithelial portion, has assumed a specialized, folded configuration ("villi"). Additionally, the lamina propria sublayer of the mucosa contains extensive aggregations of lymphoid tissue, called Peyer's Patches (PP in pictures, below). The exact purpose or function of these areas is not certain.

Anatomy of the Small Intestine

The small intestine mucosa layer is folded into "villi" (singular is "villus")--fingerlike projections-- to maximize the digestive-absorptive surface area of this portion of the GI tract.  This configuration of the mucosa is shown here:

The layers underlying the mucosa transport absorbed nutrients; the muscle layers are concerned with the orderly transport of undigested waste towards the large intestine for further processing. Organized movement through the intestine (peristalsis) is mediated via the muscularis layers and nerve complexes (myenteric plexus) strategically located within and between these. The neurologic input governing intestinal motility (stimulating intestinal muscular contractions) is via the parasympathetic and sympathetic autonomic nervous systems and localized circuitry responsive to stretch and chemical mediators, independent of the central nervous system.There is a more or less continuous synthesis and turnover of villi cells, with newer cells emerging in the crypt portion of a villus and migrating towards the tip, replacing the older tip cells, which are shed.

Large Intestine

General Information

The cecum marks the transition between the large and small intestine. In dogs and cats, this area is vestigial...it is for all intents and purpose, the beginning of the large intestine...the colon. The bulk of the large intestine functions to extract water and electrolytes, store and periodically eliminate nutritional waste. There are many bacterial here as well, and these further break down waste material and supply some vitamins which are also absorbed

Anatomy of the Large Intestine

The colon in domestic animals is named according to its relationship to its normal direction of motility. The part beginning with cecum, on the right side of the abdomen, is the ascending portion (because movement of contents is "upwards"...towards the head); next comes the transverse portion, named because is now lies perpendicular to the axial skeleton, and crosses from the right side to the left side of the abdomen. The remaining portion, the descending colon, so named because motility is now directed toward the rear of the animal (a direct shot towards the anus), lies on the left side of the abdomen, passes through the pelvic canal, and terminates at the rectum. Contents in the rectum are eliminated periodically through the anus.

Grossly and histologically, the large intestine is, in many ways, similar in appearance to the small intestine. This similarity ...and the differences...are illustrated in the following visual representation.

Liver and Pancreas

General Information

The liver and pancreas each have many functions. This discussion concerns only their respective roles in gastrointestinal function and dysfunction.

Liver

The liver resides in the left and right abdominal cavity, against the diaphragm, cranially (towards the head) and abuts the stomach and portions of the duodenum caudally (towards the rear). The relationship of liver to the components of the gastrointestinal tract is shown here (the gall bladder...see below...is hidden):

It is composed of various lobes...sections...and also, between two of the lobes, there is a sac-like structure, the gall bladder, which stores bile synthesized by liver cells ("hepatocytes").  A conduit from the gall bladder, the common bile duct, delivers bile directly to the duodenum when the gall bladder contracts, in response to a meal.

Blood flow from the gastrointestinal system passes through the portal system...a portion of the circulatory system in which venous blood is shunted to the liver before mixing with venous blood from the general circulation. The liver then removes and/or detoxifies noxious or otherwise harmful chemicals, bacterial toxins, etc.

With respect to the gastrointestinal system, the liver supplies bile and bile acids (stored in the gall bladder) via a duct (the common bile duct) into the duodenum in response to feeding and of emptying of stomach contents into the small intestine. The bile acids serve to emulsify fats into discreet packets which are further processed by the action of other enzymes, ("lipases") derived locally or from secretions by the pancreas and transported via blood or lymphatic systems for further processing by the liver or muscle tissue and for storage at various locations. (More about fat metabolism can be obtained elsewhere.)

The liver also functions to synthesize proteins, coagulation factors, carbohydrates, and certain vitamins. There are other substances made by the liver which help to maintain the integrity of the vascular supply to the gastrointestinal system. Liver disease (including disease of the biliary system) or failure can directly or indirectly lead to aberrant gastointestinal function, including, but not limited to vomiting and diarrhea.

Pancreas

The lobes of the pancreas are in intimate proximity to the duodenum and to the stomach, approximating a V-shape as shown here:

Stomach, duodenum and pancreas..

The pancreas is both an endocrine and exocrine organ, (endocrine because it synthesizes and secretes insulin into the bloodstream where it acts on various tissues for absorption of glucose; and exocrine, in close proximity to the duodenum, where, via the pancreatic duct to the duodenum, it secretes digestive enzymes involved, primarily in the breakdown of protein and fat). In cats, but not dogs, the pancreatic and common bile duct share a common point of entry into the duodenum. Failure due to inflammation ("pancreatitis") with or without aberrant release of digestive enzymes into the abdominal cavity or an intrinsic deficiency of digestive enzymes ("exocrine pancreatic insufficiency"), a congenital or acquired disorder can result in vomiting, diarrhea or both.

Vomit Centers of the Brain

The control of vomiting (aka "emesis") is frequently mediated via certain neurological centers of the brain. There is a primary vomit center, and a chemoreceptor trigger zone (crtz) also known as the area postrema. The process is mediated by releases of specific neurotransmitters in response to appropriate noxious stimuli in the body that bind to receptors on the crtz or on the vomit center, itself. The chemoreceptor trigger zone which is not protected by the blood-brain-barrier, detects circulating noxious chemicals and receives neurological input from the GI tract (when affected by appropriate stimuli) and elsewhere; the crtz then signals the vomit center to initiate the vomit reflex. Some stimuli directly affect the vomit center. For example, stretch receptors on a distended or irritated small intestine or colon transmit signals to the vomit center directly, initiating the vomit reflex. Below is a rather schematic diagram showing the relevant areas, the various receptors and the approximate sequence of events.

Selective Diseases of the GI Tract Causing Vomiting and/or Diarrhea

In general, anything which affects the integrity of villus production/turnover will cause malabsorption problems and potentially, result in diarrhea, vomiting or both. Additionally, infiltrative diseases, affecting the normal thickness of any of the intestinal layers can likewise affect absorption and have similar consequences.

Diseases of the Stomach

Parasites

Gastric dilatation (+ or - volvulus)

Obstruction

Gastritis

Ulcer

Hypertrophic gastropathy

Motility disorders

Diseases of the Small Intestine

Parvovirus enteritis

Hemorrhagic Gastroenteritis Syndrome

Small Intestinal Bacterial Overgrowth

Dietary Indiscretion

Parasites

Allergic Gastroenteritis

Diseases of the Large Intestine

Irritable Bowel Syndrome

Parasites (Whipworms , Hookworms and Giardia)

Fiber-Responsive

Diet-Responsive (Highly digestible or Hypoallergenic)

Constipation

Clostridium or Campylobacter enterotoxicosis

Chronic Idiopathic Colitis

Parvovirus colitis

Selective Diseases External to the GI Tract Causing Vomiting and/or Diarrhea

Diseases of the Pancreas

Diseases of the Liver

Diseases of the Central Nervous System

Cardiac Diseases

Endocrine Gland Diseases

Diseases of the Abdominal Cavity

Infectious Diseases

Miscellaneous Neoplasias

Drugs and Poisons

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