Stomach

The stomach is a muscular, hollow organ in the gastrointestinal tract of many animals, including several invertebrates. The stomach has a dilated structure and functions as a vital digestive organ. In the digestive system the stomach is involved in the second phase of digestion, following chewing. It performs a chemical breakdown due to enzymes and hydrochloric acid.

The anatomy of the stomach in animals varies significantly among species. In dogs and cats, the stomach is a single compartment organ divided into four anatomical regions: the fundus, corpus, antrum, and pyloric sphincter. The fundus is the upper part of the stomach, primarily involved in storing ingested food and gases. The corpus, or body, is the central region where most of the digestive processes occur. The antrum is the lower part of the stomach that grinds food and mixes it with gastric juices. The pyloric sphincter controls the passage of partially digested food (chyme) from the stomach into the duodenum where peristalsis takes over to move this through the rest of the intestines.[1-2]

Histologically, the stomach is lined with a simple columnar epithelium and contains various types of glands. The fundic glands, located in the fundus and body, consist of parietal cells, chief cells, and mucous neck cells. Parietal cells secrete hydrochloric acid, while chief cells produce pepsinogen, an inactive precursor of the enzyme pepsin. Mucous neck cells secrete mucus to protect the stomach lining from acidic gastric juices.[3-4]

In ruminants like alpacas, the stomach is divided into multiple compartments: C1, C2, and C3. C1 and C2 are involved in fermentation and microbial digestion, while C3 functions similarly to the abomasum in other ruminants, performing enzymatic digestion.[5]