Most pet parents don’t give much thought to their dogs’ stomachs — unless there’s tummy trouble. How the stomach works isn’t a particularly glamorous or trendy topic — unless you’re into anatomy, physiology or nutrition. But understanding the stomach’s role in digestion can put you in a better position to recognize when your dog needs to be seen by a veterinarian and to make decisions that help improve pet health.
Temporary storage and much more
Your dog’s stomach is a large, muscular, sac-like organ located between the esophagus (aka throat) and small intestine. It assists in the early stages of digestion through three important functions:
- Storage: The stomach is a short-term food storage site, allowing a meal to be eaten quickly and digested over an extended period of time.
- Mixer/grinder: The stomach acts as a blender in which food is ground into tiny pieces as it’s mixed with enzymes, hydrochloric acid and other substances to create a liquefied food called chyme.
- Controller: The stomach controls the rate of release of chyme into the small intestine.
The inner surface of the stomach sits in large folds that enable the stomach to expand substantially when a meal is eaten. It’s also lined with cells and glands that produce four different substances important to digestion or to normal stomach functions:
Mucus
The most numerous cells lining the inside of the stomach are mucous cells. These cells produce an alkaline, bicarbonate-rich mucus that coats, lubricates and protects the stomach lining. The mucus coating must be produced continuously for the stomach to be protected continually. If there’s an inadequate amount of any mucus ingredient, the stage is set for damage to occur that may ultimately result in stomach ulcers.
Acid
Some cells secrete hydrochloric acid, which is responsible for creating an extremely acidic environment — about pH 2 (pH 7 is neutral), although this can vary from one dog to the next. Such a low pH can remove paint from many surfaces! In the stomach, hydrochloric acid helps convert pepsinogen to pepsin, the enzyme responsible for breaking down proteins. It also disables bacteria and other microorganisms that may have been eaten during the meal.
Proteases (enzymes that break down proteins)
At least eight variations of pepsinogen are secreted primarily by cells of the stomach known as chief cells. These enzyme precursors are converted into pepsins after interacting with hydrochloric acid. Pepsins work only in the stomach to start the breakdown of proteins in food. Enzymes in the small intestine finish the digestion of proteins into amino acids so they can be absorbed.
Hormones
The primary hormone produced by stomach cells is gastrin, which controls acid secretion and stomach contractions. Gastrin stimulates the release of hydrochloric acid that, in turn, interacts with pepsinogen to activate pepsin and start the protein digesting process.
A very effective food grinder and mixer
While most of the digestive system has two thick layers of smooth muscle within its walls, the stomach has an extra layer of muscle that enables complex grinding movements. Low-frequency, sustained muscle contractions in the upper stomach create pressure inside the stomach that helps stomach emptying.
In the lower stomach, wave-like contractions (aka peristalsis) occur. The muscle contractions become stronger as they move toward the sphincter between the stomach and small intestine. As the contractions reach the end of the stomach, muscles contract, pushing food back and up into the stomach. These movements make the stomach an effective food grinder and mixer, turning your dog’s food into a thick liquid called chyme.
On to the next stage
As food becomes chyme in the stomach, it is slowly released into the small intestine for further digestion and absorption of nutrients. The amount of time it takes for food to be processed in the stomach varies with the meal’s ingredients and several other factors. What animal scientists have learned is that foods don’t move uniformly through the digestive system and they don’t leave segments of the digestive tract in the same order as they arrive.
What the stomach doesn’t do
Some pet owners mistakenly assume nutrients are absorbed from the stomach. In reality, the stomach absorbs very few substances. Small amounts of water may be absorbed across the stomach wall, as are small amounts of some fat-soluble medications, such as aspirin and nonsteroidal anti-inflammatory drugs, and alcohol.
A well-functioning digestive system — including a properly working stomach — is essential to your dog’s health. It’s what enables your pet to use the nutrients in food for energy and to build and repair tissues. If you have any questions or concerns about your dog’s digestive system, be sure to talk with your veterinarian.
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