Abomasum - Anatomy & Physiology
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The abomasum is the fourth chamber in the ruminant. It functions similarly to the carnivore stomach as it is glandular and digests food chemically, rather than mechanically or by fermentation like the other 3 chambers of the ruminant stomach.
The abomasum differs in its position within the abdomen, depending on fullness of the other chambers of the stomach, intrinsic abomasonal activity, contractions of the rumen and reticulum (to which it is attached) and by age and pregnancy status.
Displacement of the abomasum to the left or to the right is a common disorder affecting dairy cows due to high concentrate feed.
The abomasum lies upon the abdominal floor. The cranial part is split into the pylorus and body. There is also a caudal part. It is covered by the lesser omentum. It has 15-20 folds inside. The torus is at the pyloric exit. The outflow is fairly constant. There is motility at the pylorus (peristalsis) and some control at the pyloric sphincter. The abomasum is large in newborn animals. The proximal ends of the abomasal folds form a plug preventing reflux into the omasum. It has thin walls and a serosa covering.
The function of the abomasum is the chemical breakdown of food. It secretes hydrochloric acid and pepsinogen. It has some intrinsic motility. Impaired motility can cause distension. The movements are slow, contractions occur first in the proximal part and are more forceful at the pyloric part.
The vasculature of the abomasum includes the cranial mesenteric artery, the celiac artery and the left gastric and left gastroepiploic arteries.
Single lymph nodules are present at the junction between the epithelium and the lamina propria. Numerous small lymph nodes are scattered in the abomasal curvatures. The lymph drains to larger atrial nodes between the cardia and omasum, then to the hepatic lymph nodes.
The abomasum has a simple columnar epithelium. There are 3 layers of tunica muscularis - inner oblique, middle circular and outer longitudinal. The lamina muscularis is thicker and has 3 separate layers.
Gastric glands are present in the lamina propria of the mucosal layer in the pyloric region (lighter part). The abomasum is heavily coated by mucous for protection. The submucosa contains loose connective tissue, many blood vessels and unilocular adipocytes. The coiled glands in the lamina propria open into deep gastric pits. The inner mucosa is pink. Rugae are present in the pyloric region and a torus (large swelling) is present at the pyloric passage to narrow the lumen. The dark mucosa of the fundus and body contains peptic glands.
The abomasum can contact the liver. The abomasum is proportionately larger than in cattle.
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