Cobra already discussed this. The lower abdomen is composed of the water element, which is absolutely necessary to function normally and be alive. Water cleans and produces movement of the bowel, absorption, and contraction.
Without normal levels of water, the bowel becomes dry and unable to move and receive stimulus, absorb and protect the body from bacteria.
The fire aspect comes from the sexual energy which penetrates the water by rising, that's why the corruption of this is in the bible as the "seas being cut in half to pass by".
Also in a normal energy circulation, the solar and sacral chakra behave like a battery, connecting the two with specific techniques produces real electricity.
If the Sacral were to be fire,the lower female aspect of the body would be missing, thus impossible. Knowing anatomy, most Serotonine is produced by the Solar and Sacral chakra, but this specific Serotonine never reaches the brain. Serotonin is the key element from which bioelectricity runs in our bodies, and it is of the water element, by being the key to conduct the electricity (fire) in the system.
If the Serotonin would reach the brain, it would fry it, as the capacity of the brain of normal humans is nonexistent to sustain that high level of function and "amperage". Rising the sexual energy actually prepares and does this, in a voluntary and controlled way. In any aspect at any point, the body is required to clean and refresh, and this is done in the sacral chakra, where the cycle of birth and death is produced.
Yogic Breathing actually focuses our energy in the pointed focus of the lower belly, which stimulates the metabolism and creates a stimulus for our bodies to grow the material aspect of our bioelectricity. At one point of balance, activating the sacral chakra activates all of the chemicals in which our bioelectricity will have the ability to influence matter on a direct basis.
Expanding these I truly think that rising the "serotonin" to the brain and connecting this pathway is closely related to the steps of MO.