There are four pivotal stages in an orgasm: excitement, plateau, orgasm and resolution.
If you're properly aroused with your partner, the brain sends messages which increases blood flow to your genitals, and your heart rate increases. So if things are getting steamy and you feel a a constricting of your chest- bravo! You are a great sexual match. However, the opposite effect can also happen. If you feel less than starry-eyed over your embrace- chances are your hormones don't rage for your partner and you will have to concentrate much more to achieve a proper orgasm.
Another caveat, is that men and women experience orgasms differently. If you think about anatomy back when we were first created, 10k years ago- we had to get the deed done quickly lest be devoured by toothy elephants and giant coyotes. Men had to fertilize the egg quickly. We aren't natural predators and were design to escape predators. For this reason, females are meant to protect their babies with their lives, rather than sit on a nest (okay our team is now drawing pictures of lady nests and it's kind of awesome).
Anyways, for men, the orgasm tends to last between three and 10 seconds, followed by a "refractory period" anything from a minutes to hours - which is why men can't achieve another orgasm immediately after they've experienced one.
Women on the other hand, tend to experience longer bouts of pleasure - around 20 seconds - and they have no refractory period, which gives the opportunity for multiple orgasms.
Men and women also feel pleasure in different areas: for men, the anal sphincter, the prostate gland and the penis muscles all contract.
Women on the other hand, have "rhythmic contractions" of the uterus, vagina, anus and pelvic muscles.
Thirty regions of the brain remain active, and are, at first, flooded with dopamine which makes you crave the feeling of orgasm again, followed by Oxytocin which increases feelings of love and bonding between you and your sexual partner.
The lateral orbital frontal cortex however shuts down, which controls self-evaluation, control and reason.
There you have it. In order to complicate sex even more, we've broken it down to a scary level. Cheers!