3.2 Digestive System

Overview

The digestive system, also known as the gastrointestinal (GI) system or alimentary system, is a long, muscular tube that begins at the mouth and ends at the anus. It also has some accessory organs. The functions of the digestive system include the intake, transport, and digestion of food, along with the absorption of nutrients and the removal of physical waste.

Animals can be classified into different groups depending on the type of food they eat. The three groups are herbivores, carnivores, and omnivores.

• Herbivores are animals whose diet is plant-based. Some examples of herbivores include horses, cows, and some bird species.
• Carnivores are animals that eat meat (that is, other animals); these animals include cats, snakes, and sharks.
• Omnivores are animals that eat both plant- and animal-derived foods. Some examples of omnivores are humans, dogs, bears, and chickens. 

Path of Digestion

^{CrashCourse. (2015, September 7). The digestive system, part 1: Crash Course anatomy & physiology #33 [Video]. YouTube. https://www.youtube.com/watch?v=yIoTRGfcMqM&list=PL8dPuuaLjXtOAKed_MxxWBNaPno5h3Zs8&index=34}

Structures of the Digestive System

Mouth (oral/buccal cavity): Begins the process of both mechanical and chemical digestion via mastication, which starts the chemical breaking down of carbohydrates. The mouth includes the lips, cheeks, tongue, gingivae, and teeth.

Lips: Form the opening to the mouth.  The lips are very tactile, allowing animals to pick up and hold food in their mouth; for example, horses’ lips are sensitive and very mobile for grazing and drinking.  

Cheeks: Composed of muscle and fatty tissue; help keep the food in the mouth.

Tongue: Composed of muscle tissue; keeps the food between the teeth during chewing and aids in swallowing. The tongue is also very important for many species for tasting, prehension, licking, vocalizing, and grooming; these functions are more important in some species than others.  

The dorsal surface of the tongue has small structures or “bumps” called papillae, which contain the taste buds. A cat uses their papillae to hold saliva to clean their fur, and for lapping up water. 

Gingiva (sing.)/Gingivae (pl.): Also called the gums; consist of mucous membranes that internally line the mouth and surround the teeth.

Teeth: Primarily responsible for grinding and breaking down food into small enough pieces to swallow and digest. The number of teeth varies among species.

• Primary or deciduous teeth are the first set of teeth that develop in most species, more commonly known as the “baby teeth.” These teeth usually fall out or resorb into the body at maturity as the permanent “adult” teeth erupt. Retained deciduous teeth are teeth that have not fallen out or been resorbed. In cats and dogs, they are usually extracted at the time of spay or neuter.
• Secondary or permanent teeth are the “adult” teeth.

The teeth are arranged in upper (maxilla) and lower (mandible) arcades. There is a dental formula for each species representing the typical distribution of teeth. The four main types of teeth are as follows:

1. Incisors: Front teeth used for shearing or cutting
2. Canines (“fangs”): Used for tearing and defense
3. Premolars: Cheek teeth used for grinding and chewing food
4. Molars: Located caudal to the premolars and used for grinding and chewing food

 

Pharynx: Also called the throat; a cavity that is part of both the respiratory and the digestive systems. When food enters the pharynx, involuntary muscle contractions close off the airways to prevent food from entering the lungs. When an animal is not eating, the airways are open for air to enter from the mouth and nose. 

Esophagus: A muscular tube that leads from the pharynx to the stomach. When the esophagus reaches the stomach, a ring-like muscle called a sphincter opens and closes to allow the passage of food into the stomach.

Stomach: A sac-like organ that secretes enzymes for chemical digestion. It also has the function of storing and mixing food. Animals with a single stomach, such as humans, dogs, cats, horses, and pigs, are known as monogastric (“one stomach”). Ruminants, such as cows, goats, sheep, camels and giraffes, have multi-chambered stomachs. The most common type of ruminant stomach has four compartments, each with their own function: the reticulum, the rumen, the omasum, and the abomasum. After swallowing, ruminants regurgitate their food to re-chew it. The video below shows a giraffe regurgitating food. Watch as the food is swallowed (moving caudally down the neck), and then regurgitated (moving cranially up the neck).

^{Fick, J. (April 14, 2013). Giraffe eating cud [Video]. YouTube.  https://www.youtube.com/watch?v=c2dzm9u7KTc&ab_channel=Jon%C3%A9Fick}

Small intestine: A long, muscular tube that connects the stomach to the large intestine. Further nutrient digestion occurs here, and it is also where most of the nutrient absorption into the bloodstream takes place in the body. It is divided into three parts: the duodenum, jejunum, and ileum (listed from the most proximal to the most distal).

Large intestine: The distal continuation of the small intestine consisting of the cecum, colon, rectum, and anus. The large intestine’s primary functions are the absorption of water and salts, fermentation, and the formation and excretion of feces. The large intestine can vary between species. The terminal end of the large intestine is the rectum, which is connected to the outside world by the anus.  

(Sturdy & Erickson, 2022)

Common Pathological Conditions of the Digestive System

Common Procedures

Fecal Testing Example

Fecal examinations are a common procedure done in most clinics. Fecal tests are used to detect bacteria, viruses, parasites and abnormal material found in the stool. In Figure 3.3, you can see an intestinal parasite egg found in a fecal floatation. More information can be found in 5.4 Laboratory Terminology.

 

Ultrasound: An imaging technique to visualize of internal body structures that uses high-frequency sound waves to produce an image.

References

Sturdy, L., & Erickson, S. (2022). The language of medical terminology. Open Education Alberta. https://pressbooks.openeducationalberta.ca/medicalterminology/, licensed under CC BY-NC-SA

Images
(images are listed in order of appearance)

Digestive system by Clker-Free-Vector-Images, Pixabay licence

Canine Teeth by Kelly Robertson, NorQuest College. Used with permission.

Ascaris suum egg by Strongyle, CC BY-SA 3.0