The Chakras
According to the Yoga texts, the human being has different levels, or planes of existence. Our physical body is the densest and the grossest part. It is made of the food we are eating and is the grossest manifestation of Prakriti or matter. Within it, or better said, beyond it, there is a subtle body made of energy and represents the pranic, or energetic support of our physical body. It is in this subtle body that dwell the 7 centers - called chakras or wheels - each represented by a flower of lotus.
Six of them are distributed along the spine, the seventh being located at the top of the head. Being part of the subtle body, the chakras actually are formed there where the main NADIS (the life current channels) are crossing each other. Even though they influence and impact our body they are not parts of it and do not have a particular location in our physical body. In the same time they are not just abstract notions; they are energetic centers and their outer functioning can be perceived in our everyday life if one pays a close attention to that aspect.
Hatha Yoga: The chakras
To illustrate this better, I will quote Hans Rieker in his "Commentry of Hatha Yoga Pradipika":
"In times of danger there is usually a convulsive contraction of Muladhara Chakra; in case of acute danger, it intensifies as the often mentionned experience of "seeing the whole life flash through the mind". In sexual excitement, the Svadistana Chakra is noticeable. Best known is the influence of Manipura chakra on crying and laughter, which are related to the region of the diaphragm. One speaks of loving devotion as coming from the heart; it really involves the neighbouring of Anahata chakra. The well knowing choking sensation when a speaker is "blocked" relates to the Vishudda chakra. The index finger on the brow means that the Ajna chakra has spoken and the halo of a saint has its center in Sahasrara chakra.."