Chapter 25

Upon completion of this chapter, the reader should be able to:

25.1 State the structures that make up the digestive system
25.2 Describe the function of the digestive system
25.3 Discuss dentition in various animal species
25.4 Differentiate between the monogastric, ruminant, nonruminant, and avian digestive systems
25.5 Describe common disorders associated with the digestive system

The digestive system, also known as the gastrointestinal (GI) system, is responsible for the digestion of food.
Definition of Digestion: The process of breaking down food particles into nutrients that the body uses to sustain life.
Nutrients are converted into a form usable by cells and transported through the digestive system.
The complexity of nutrient absorption varies across species.
Major structures of the digestive system include:
- Mouth (with or without teeth)
- Stomach
- Intestines

The digestive system, commonly referred to as the alimentary canal, utilizes teeth for processing food.
Types of Teeth:
- Carnivores: Have adaptations for a meat-based diet, including long canine teeth (fangs) for tearing food (e.g., dogs and cats).
- Herbivores: Their teeth are structured for grinding plant material (e.g., cows, sheep, goats, horses).
- Omnivores: Their teeth adapt to both plant and meat diets (e.g., pigs and humans).
Tooth Structure:
- Enamel: Hardest body substance, covers the crown of the tooth, protecting it.
- Crown: Visible part above the gum line.
- Dentin: Underneath enamel, similar to bone.
- Root: Below gum line, holds the tooth in place.
- Pulp Cavity: Contains nerves, arteries, and veins.
Types of Teeth According to Lifespan:
- Deciduous Teeth (Baby Teeth): Curved shape, shed when maturity is reached.
- Permanent Teeth (Adult Set): Straighter shape appears as replacements.
Dentition Arrangement:
- Specific terms describe the arrangement (dental formula) and positions of teeth in the upper (numerator) and lower (denominator) jaws.
- The dental formula varies by species but is consistent among domestic animals.

Mouth Components:
- Lips
- Tongue
- Hard and soft palate
- Salivary glands
- Teeth
Saliva Production:
- Four salivary glands produce saliva for food breakdown.
- Saliva contains the enzyme amylase which begins digestion.
Tongue Functions:
- Holds and mixes food with saliva, contains taste buds (e.g., papillae).
Esophagus Function:
- Tube that conveys food to the stomach, regulated by sphincter muscles that control entry and exit.
Mucosa Function:
- Lining tissue that facilitates food movement through the GI system.

A single, simple stomach divided into three parts:
1. Cardia: Entrance filtering food to the stomach.
2. Body: Expands as food enters.
3. Pylorus: Exit passage of the stomach.
Stomach Characteristics:
- Lined with rugae when empty, sphincter muscles regulate food movement.
- Bile synthesis by the liver aids digestion and is produced for use in the small intestine or stored in the gall bladder.
Small Intestine Sections:
1. Duodenum: First section, associated with the pancreas.
2. Jejunum: Middle section.
3. Ileum: Final section.
Cecum Role:
- Aids in digestion, marking the transition to the large intestine.
Large Intestine Structure:
- Comprised of ascending, transverse, descending colon, and rectum leading to anus.

A single large stomach comprising multiple compartments:
- Rumen: Primary storage and fermentation area, about 80% of the stomach.
- Reticulum: Acts as a filter and traps indigestible materials (5% of stomach).
- Omasum: Absorbs water and nutrients (8% of stomach).
- Abomasum: True stomach with digestive acids and enzymes (10% of stomach).
Ruminating Process:
- Involves regurgitation of cud for further breakdown and requires large amounts of bacteria for digestion of fibrous plant materials.

Found in horses, rodents, rabbits; resembles monogastric but has a large, well-developed cecum.
Plant fibers are digested through fermentation in the cecum.

Characterized by structures not present in other animals, adaptations for grinding and digesting hard food particles.
Beak: Used instead of teeth, with salivation occurring to aid in food swallowing.
Crop: Holds and softens food before further digestion.
Gizzard: Muscular organ that grinds hard food particles.
Cloaca: Common end point for excretion of feces and urates.

Process: Bringing food back up from stomach; occurs only in monogastric animals.
Can be preceded by nausea and requires withholding food and water to allow GI rest.
Restart feeding with bland, low-fat food post-vomiting.

Characterized by soft, watery waste; can cause dehydration.
Animals with diarrhea should be examined for cause, and signs include frequency and volume of stool.

Determined by skin turgor, hydration status through gum moisture; severe cases may need IV or SQ fluids.
Fluid Replacement Calculation:
$ext{Fluid volume (ml)} = ext{Percentage of dehydration} imes ext{Body weight (kg)} imes 1000$

Signs include stomach pain and straining to produce bowel movements.
Causes include poor diet and inadequate hydration; management may involve enemas or dietary changes.

Common in horses causing severe stomach pain and potentially fatal complications.
Signs include lying down, rolling, and lack of bowel movements. Immediate veterinary intervention is necessary.

Affects dogs and ruminants; involves excessive gas causing abdominal distension.
In serious cases can lead to gastric dilatation volvulus (GDV) in dogs, requiring urgent medical treatment.

Occurs when non-digestible items block the GI tract; diagnosed via radiographs.
Surgery may be necessary to remove the obstruction and deal with necrotic tissue.

The digestive system consists of various specialized systems reflecting different dietary needs in animal species.
Common conditions related to digestive health include vomiting, diarrhea, dehydration, constipation, colic, bloat, and obstructions, many of which can involve emergencies requiring prompt medical attention.