Rumination - the process of regurgitating fermented plant material for further chewing and to stimulate digestion. The action commonly referred to as "chewing the cud".

Almost all herbivorous mammals have evolved to form symbiotic relationships with microbes that can ferment the vegetation they eat by synthesizing enzymes that can break down the cellulose and lignin of a plant's cell walls. The herbivores provide a home and food for these microbes and in return are able to absorb the fatty acids and sugars that are a byproduct of their fermentation. Ruminants do this better than most herbivores.

Ruminants are special because they have a stomach composed of four connected chambers. The first chamber, called the rumen, is where they house these microbes that anaerobically break down the food they eat, essentially a large fermentation vat. To aid in the fermentation process the ruminant will occasionally regurgitate plant matter from the stomach part to further break it down with vigorous chewing. From the rumen the food passes through two other stomach chambers that filter small food particles through to the fourth stomach chamber, the "true" stomach, where the food is enzymatically digested.

Using this method, the okapi receives the maximum amount of nutrition from its food source and is able to eat quickly and digest slowly, limiting the amount of time it is vulnerable predators.

The nervous system is one of the most important aspects of a mammal's physiology, it is the system that connects all others. The okapi's nervous system allows it to not only sense environmental changes through the use of its eyes, ears, and sense of smell, but also rapidly process the information received from these sensory organs to determine whether action is necessary or not.

The autonomic nervous system is especially important to animals that may end up as prey to another animal. This system has 2 parts, the sympathetic and parasympathetic, and is one that controls the bodily functions not under conscious control, such as digestion or heart rate.

The sympathetic nervous system is what enables the "fight or flight" response in many animals. To ungulates like the okapi this system is of paramount importance because it is what allows not only the rapid increase in heart and respiratory rates but also greater blood flow to the limbs and other muscles that allow the okapi to escape danger from an equally fast predator.

The parasympathetic system is what enables the body to return to a state of rest or calm. This system stimulates digestion and preserves the bodies regular heart rate and rate of breathing. It maintains the normal functions of a body at a rest and and keeps things in order until they change.

The cardiovascular system in a ruminant such as the okapi is very similar to that of a human. It involves a closed circulatory system, meaning the blood remains within a network of veins as it travels throughout the body providing oxygen. That is the main function of this type of system, to circulate oxygen from the lungs to every part of the body that requires it and to carry deoxygenated blood full of carbon dioxide back to the lungs to be expelled and enable the process to repeat.

The heart is the center of a system like this one, the contracting and relaxing of the heart muscles keep blood moving from the center of the body to the limbs and other extremities, supplying the tissues and organs there with oxygen that those individual cells need to function properly.

This system is closely connected to the one above, the nervous system, because it is the nervous system in conjunction with the endocrine system that can stimulate the heart rate, causing it to raise or lower. Raising the heart rate allows for more oxygenated blood to reach further parts of the body faster than usual and this act can be crucial when it comes to running away from a predator or avoiding other danger.