The Four Compartments of the Stomach

 
 


Drawing of ruminant stomach. 
Adapted from Ferguson, 1985.

 THE RETICULUM
Food passes from the rumen to the reticulum, which has a honeycombed lining with ridges and deep pits that increase its surface area (Kent, 1996).  This is the compartment where a cow forms boluses for regurgitation and rumination (Ritter).

THE OMASUM
The omasum acts as a temporary holding site for ingesta moving from the reticulum to the abomasum (Kent, 1996).  It removes water and electrolytes from the feed particles and further reduces the size of these particles (Motility).  The omasum functions like a filter that only lets finely processed contents to pass (Ritter).

 
THE RUMEN
The rumen is the largest of the four compartments of the stomach.  Its contents account for abount 15% of the total body weight of a cow (Ferguson, 1985). Nitrogen constantly gets recycled to the rumen from the bloodstream.  This allows the cattle to survive with very little nitrogen in their diets (Owens and Zinn, 1988).  The rumen houses the microorganisms that perform many of the digestive processes.  The feed particles may remain in the rumen for as many as five days while these microbes work (Ferguson, 1985).  It maintains an ecosystem with conditions favorable for microbial life (Yokoyama and Johnson, 1988).  These conditions include:
  • High concentration of ammonia (Allison, 1965)
  • Low concentration of amino acids (Allison, 1965)
  • Low oxygen content which favors microbes that grow only in absence of oxygen (Yokoyama and Johnson, 1988)
  • pH 6-7
  • Temperature 38-42°C
New microorganisms are always entering the rumen with food, water, and soil.  However, they do not survive because they cannot compete with the microbes specifically adapted to the rumen's conditions (Yokoyama and Johnson, 1985).

 
THE ABOMASUM
The abomasum is the final of the four compartmetns in the ruminant stomach.  It is the only part that has secretory tissue (Merchen, 1988).  The secretions have HCL and pepsin (Ritter) which kill the microorganisms that aided digestion in the rumen (Ferguson, 1985).