The complexity begins with your anatomy. While you probably know that generally, your journey to orgasm starts with vaginal or clitoral stimulation, you might not realize that there's still debate among researchers about the exact anatomy of the clitoris. The most visible part of this intriguing organ is the small bundle of extra-sensitive nerve endings that sits right underneath where the two inner labia meet up top. From there, the clitoris actually extends internally in two shafts that sit along either side of the vagina. Experts may still be mapping the clitoris in full, but pretty much everyone understands the sexual purpose: pleasure.
To understand your orgasm, you should also know that the vaginal canal is lined with the soft tissue of the mucous membrane covering layers of stretchy muscle. (This canal leads to the cervix, a narrow passageway that sits in front of the uterus. This is the long journey upon which sperm must embark in order to fertilize an egg. Some research suggests that the female orgasm may help improve your chances of getting pregnant by improving "sperm retention," but you have to time it right.)
During arousal, you'll notice your heart rate increase, your skin may begin to feel (and look) flushed, and your genitals will swell with blood. But you're also building up a lot of muscle tension throughout your body.
At the same time, your brain is working up quite a potent cocktail of chemicals. That includes the neurotransmitter dopamine, which is commonly associated with pretty much anything that feels good. But during an orgasm, you're also getting a huge release of oxytocin, which can promote feelings of closeness and empathy (among many other things).
Also, according to a small 2006 study published in the European Journal of Neuroscience, there's some evidence that, in women, the brain's hippocampus and amygdala show decreased activity during sex — but before climaxing. Both of those areas are associated with emotional regulation, especially fear and anxiety. And, during orgasm, activity decreased in other brain areas, too. That suggests that feeling safe and relaxed may be especially important for women to reach an orgasm.