The CO2 ruminal buffering system

The ruminal buffering system is dominated by dissolved carbon dioxide and bicarbonate. It works by combining hydronium and bicarbonate extruded from the cells in exchange for ruminal short chain fatty acids (energy) and sodium. This reaction is catalysed by the enzyme carbon anhydrase present in the rumen fluid. The product is dissolved carbon dioxide, a molecule which is a combination of liquid water and CO2 gas. it follows that CO2 gas can escape the fluid into the gas cap leaving water behind. The CO2 gas will escape the rumen through eructation, buffering the rumen fluid.

During CO2 holdup the buffer system is disrupted and CO2 not longer effervescent freely from the fluid. CO2 holdup is a normal feature of the rumen environment, during brief periods after feeding, which helps ruminal bacteria to growth and digest more efficently nutrients. All ruminal bacteria are carbophilic.

During ruminal acidosis however, this transient effect is extended for long post prandial periods and so much so that dCO2 concentrations can reach critical levels.

As an example, in humans, the average outdoor air level is commonly measured at 400 ppm (parts per million). Indoor spaces with good air exchange typically have levels ranging from 400 to 1,000 ppm. However, when the concentration reaches the range of 1,000 to 2,000 ppm, it is often considered as an indicator of poor air quality. At these levels, individuals may experience discomfort such as headaches, sleepiness, and a sense of stagnant, stale, and stuffy air. If the concentration surpasses 2,000 ppm and reaches the range of 2,000 to 5,000 ppm, the negative effects become more pronounced.

Ruminal dissolved carbon dioxide during acidosis can reach more than 5,000 ppm. Humans under those conditions will be dead in couple of minutes. The abnormal “retention” of dissolved CO2 in the rumen for extended postprandial periods explain the typical clinical signs of subacute ruminal acidosis in cattle or SARA.

This figure illustrate the normal function of the buffer systems catalysed by ruminal carbon anhydrase (CA), the protons and bicarbonate are combined to produce dissolved carbon dioxide. in turn the CO2 gas leaves the fluid releasing water and buffering the rumen fluid. The disruption of this system leads to retention of dCO2 which will produce changes that can lead to many nutritional diseases, we will review them in the next pages.