The most literal translation of chi would be motive force or life force. It is the force that makes everything happen and it is the basic thing that forms the foundation of all other things. Like gravity, magnetism and other forces, it is not directly observable and the pathways that it travels along are not actual anatomical structures. Because it is a force, it ricochets and bounces around inside your body in trajectories that zig-zag all over. These trajectories are called channels or meridians and the chi circulates in them in a very regular pattern. It starts in your torso, goes out to your hands, returns to the torso, goes out to your feet, and then returns to the torso again. Each time it returns to the torso it passes through a different internal organ so, in the course of one complete circuit, your chi covers every part of your body and passes through every one of your internal organs.
There is no equivalent to chi in western medicine. Though it travels in the same general direction as the blood and nerve impulses, it is not the same. Chi is more like the push behind the nerve impulses that makes them go or the force that makes your heart pump your blood along. The closest western equivalent to chi would be like the concept of energy as it is portrayed in physics. For example, in physics everything is a different manifestation of energy and in Chinese medicine everything is a different manifestation of chi.
There are two basic forms of chi in the human body: substance, such as organs and tissues, and function, which includes transforming, transporting, holding, raising, protecting and warming. In its transformative function chi helps our bodies turn the raw materials of air and food into fuel. The transporting function helps our bodies get substances from one place to another, moving food along the digestive tract or urine from the kidneys to the bladder. Chi's holding function is what helps us keep things in place, like how the uterus holds a developing baby. Similar to this function, chi also has a raising ability, lifting our organs up against gravity, preventing them from prolapsing. Chi even guards us against the pathogens that cause disease, protecting us when we are exposed to infectious organisms and extremes in weather, and warms our bodies, making all physiological processes possible.
In Chinese medicine our physical body is completely intertwined with our emotional being. Because of this, emotions can directly cause physical diseases and internal imbalances of our chi. All emotions are valid given the circumstance and degree. However, when they are too extreme or when we get stuck in them for too long they can have negative effects. Chi should be abundant and flow smoothly and regularly, nourishing each and every part of the body. Different emotions disrupt this flow in different ways.