Abomasum - Anatomy & Physiology

Overview

The abomasum is the fourth chamber in the ruminant. It functions similarily to the carnivore stomach as it is glandular and functions to digest 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.

Displacement of the abomasum to the left or to the right is a common disorder affecting dairy cows due to high concentrate feed.

Structure

Abomasum Anatomy (Sheep) - Copyright RVC 2008

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). There is some control at the pyloric sphincter. The abomasum is large in newborn animals. The proximal ends of the abomasal folds forms a plug preventing reflux into the omasum. It has thin walls and a serousa covering.

Function

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.

Vasculature

The vasculature of the abomasum includes the cranial mesenteric artery, the celiac artery and the left gastric and left gasrtoepiploic arteries.

Innervation

The innervation of the abomasum includes the dorsal vagus nerve (CN X) and the ventral vagus nerve (CN X) (most important).

Lymphatics

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.

Histology

Abomasum Histology (Sheep) - Copyright RVC 2008

• Simple columnar epithelium

• 3 layers of tunica muscularis- inner oblique, middle circular and outer longitudinal

• Lamina muscularis thicker (3 separate layers)

• Gastric glands in lamina propria of mucosal layer in pyloric region (lighter part)

• Heavily coated by mucous for protection

• Submucosa contains loose connective tissue, many blood vessels and unilocular adipocytes

• Deep gastric pits

• Coiled glands in lamina propria opening into gastric pits

• Pink inner mucosa

• Rugae are present in the pyloric region

• Torus (large swelling) is present at the pyloric passage to narrow the lumen

• Dark mucosa of fundus and body contain peptic glands

Species Differences

Small Ruminants

The abomasum can contact the liver. The abomasum is proportionately larger than in cattle.

Links

Test yourself with the Abomasum Flashcards

Click here for Abomasum Histology

Click here for Rumen - Anatomy & Physiology

Click here for Reticulum - Anatomy & Physiology

Click here for Omasum - Anatomy & Physiology

Video links:

1.Pot 52 Lateral view of the Abdomen of a young Ruminant

2.Pot 175 Sections of the Ruminant Stomach

3.Pot 47 Ovine Omasum and Abomasum

4.Left sided topography of the Ovine Abdomen and Thorax

5.Right sided topography of the Ovine Abdomen

6.Structure of the ruminant forestomachs