The bacterial digestion produces acids, methane, and CO2 as well as more micro-organisms. The bicarbonate keeps the compartment from becoming too acid and the urea not only helps clear the blood of a waste product, but also acts as a source of nitrogen for the micro-organisms to make amino acids. This mixture may be brought up and chewed thoroughly. It then passes to other compartments and eventually to the abomasum or true stomach that produces pepsin and HCl. The cow garners its sustenance from the micro-organisms.
Rabbits also live on poor quality food.
They have large caecae in which bacteria are allowed to proliferate. They will eat this pellet directly from the anus and thus acquire vitamins and protein. This is called coprophagia . Rats also practice coprophagia and will develop vitamin deficiencies if not allowed access to their feces. Know the stimulus, originating tissue, hormone, effect of the hormone, site of activity of the hormones mentioned.