Arousal
Arousal involves the same chemical events and neural pathways in men and women. Carressing genital skin makes electrical impulses in surface nerves that provoke other nerve cells to convert the amino acid Arginine to a short lived, active molecule, Nitric Oxide or 'NO'. The conversion requires an enzyme that includes Niacin as a component.

NO dilates small arteries, causing a sudden increase in blood flow, just like opening up a valve on a hydrant. Trapping blood in the penis results in erection. In women, there is elongation of the vagina, swelling of the labia, and vaginal fluid production.

Erotic stimuli and sensual thoughts make electronic taffic in spinal conduits to pelvic nerves, which are arranged as two thick trunks that go from deep in the pelvis to combine along the underside of the penis and in the clitoris. Superficial sensory nerves join these nerve trunks and their rich blood supply. When these nerve trunks are injured, nerve cells that make NO are depleted or destroyed, and arousal cannot occur.

NO molecules are used up very quickly and have to be replenished almost continuously. Drugs like Cialis and Viagra extend the life of NO which is similar to making more of the material.

Vaginal fluid production requires Estrogen and cell surface estrogen receptors. Estrogen is a 'steroid' hormone, made from cholesterol. Vaginal fluid production declines when estrogen is diminished, as with menopause and low dose birth control pill use.