Rumen - Anatomy & Physiology

Rumen Topography (Sheep) - Copyright RVC 2008

Introduction

The rumen is the first chamber of the ruminant stomach. It is the largest chamber and has regular contractions to move food around for digestion, eliminate gases through eructation and send food particles back to the mouth for remastication.

The rumen breaks down food particles through mechanical digestion and fermentation with the help of symbiotic microbes. Volatile fatty acids are the main product of ruminant digestion.

Structure

Rumen Anatomy (Sheep) - Copyright RVC 2008

Grooves correspond with thickened smooth muscle pillars on the inside of the rumen
- Ruminal pillars divide the dorsal and ventral ruminal sacs
- Coronary pillars divide the caudal blind sacs
- Cranial pillar divides the dorsal and cranial sacs

Covered by greater omentum

38-40°C

Anaerobic

pH 6.7

Buffered

Large holding capacity

Water intake lowers the ruminal temperature so bacteria are tolerant to temperature changes towards the lower end of the scale

Objects are often lodged in the rumino-reticular fold. When the rumen contracts, the object can be pushed through the reticulum wall into the pericardium and heart.

Laterally compressed

Extends from the cardia at the level of the 8th rib to the pelvic inlet

Serosa covers the entire rumen except dorsally where the rumen attaches to the abdominal roof allowing more freedom for ruminal movement and expansion

Ruminal contractions can be felt for in the left paralumbar fossa

1-2 contractions should be felt per minute

Opening at the cardia into both the rumen and reticulum is called the reticuluar groove (see oesophageal groove)

Function

Waste removal

Simpler products of digestion are assilimated directly, others continue down the digestive tract for further digestion

Mixes food

Moves food forwards through the stomach chambers

Sensors in the rumen can determine the courseness of the food. Course, tough feed needs stronger and more frequent ruminal contractions.

Vagus nerve (CN X) is needed for control of stomach movements

Reflex control through sensory receptors in the medulla

See rumination

See eructation

Ruminoreticular contraction

Diagram of the contractions of the ruminoreticulum - Copyright RVC 2008

Primary mixes food
- Mixing cycle of ruminoreticulum
- 2 contractions of the reticulum (2nd most powerful) which continues over the rumen
- Ingesta flows from the reticulum to cranial ruminal sac to reticulum (or ventral sac)
- Every 60 seconds

Secondary lets gas out
- See eructation
- Ingesta flows from ventral blind sac to dorsal blind sac to dorsal sac (eructation) to ventral sac

Vasculature

Cranial mesenteric artery

Celiac artery

Right and left ruminal arteries

Innervation

Dorsal vagus (CN X) (most important)

Ventral vagus (CN X)

Lymphatics

Caudal mediastinal lymph node enlargement put pressure on the dorsal vagus effecting ruminal contractions

Numerous small lymph nodes are scattered in the ruminal grooves

The lymph drains to larger atrial nodes between the cardia and omasum, then to the cistera chyli

Histology

Rumen Microbes

Have a variety of microbes that can utilise many substrates

Dominance of different bacterial species depends on pH. Ergo, microbial populations are not constant

Microbes digest cellulose and hemi-cellulose

Microbes provide a source of all amino acids

Microbes synthesise vitamins (especially the B vitamins)

Rumen Microbial Population

Bacteria
- Over 2000 species
- 99.5% obligate anaerobes

Protozoa
- Large
- Unicellular organisms
- Prey on bacteria
- Numbers affected by diet

Fungi
- Digest fibre
- Numbers present usually low

Common Rumen Microbes

Species	Type	pH
Ruminococcus flavefauens	Fibre	6.15
Fibrobacter succinogens	Fibre	6
Megashpaera eisdeni	Lactate user	4.9
Streptococcus bovis	Lactate producer	4.55