Abomasum - Anatomy & Physiology

ALIMENTARY SYSTEM

Introduction

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

• Lies upon the abdominal floor

• Cranial part split into pylorus and body

• Caudal part

• Covered by lesser omentum

• 15-20 folds inside

• Torus at pyloric exit

• Outflow fairly constant

• Motility at pylorus (peristalsis)

• Some control at pyloric sphincter

• Large in newborn

• Proximal ends of the abomasal folds forms a plug preventing reflux into the omasum

• Thin walls

• Serousa covering

Function

• Chemical breakdown of food

• Secretes hydrochloric acid and pepsinogen

• Some intrinsic motility

• Impaired motility can cause distension

• Movements are slow
  • Contraction first of proximal part
  • More forceful contraction at pyloric part

Vasculature

• Cranial mesenteric artery

• Celiac artery

• Left gastric and left gasrtoepiploic arteries

Innervation

• Dorsal vagus nerve (CN X)

• Ventral vagus nerve (CN X) (most important)

Lymphatics

• Single lymph nodules are present at the junction between epithelium and 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

• Abomasum can contact liver

• Abomasum is proportionately larger than in cattle

Test yourself with the stomach of ruminants flashcards

The Abomasum Flashcards

Links

The Rumen - Anatomy & Physiology

The Reticulum - Anatomy & Physiology

The Omasum - Anatomy & Physiology

Video

Pot 52 Lateral view of the Abdomen of a young Ruminant

Pot 175 Sections of the Ruminant Stomach

Pot 47 Ovine Omasum and Abomasum

Left sided topography of the Ovine Abdomen and Thorax

Right sided topography of the Ovine Abdomen

Structure of the ruminant forestomachs