Excerpt: 'You: Being Beautiful' on the Biology of Sex

In the book he co-wrote with Michael Roizen, Dr. Mehmet Oz takes a look at beauty in all its aspects.

"You: Being Beautiful" is an "owners manual" to discovering inner and outer beauty and lends advice for work, love and life.

In the excerpt below, Oz takes a look at the biology of sex and the scientific side of what gets men and women all hot and bothered.

Read an excerpt of the book below and then click here to read more from the "GMA" Library.

The Biology of Sex

Humans are the most sexual species around. How do we know? (The answer is not from National Geographic specials.) One example: Women are sexually active for almost their entire lives and throughout all times of their menstrual cycle— meaning that they can choose to have sex even during times when they are physiologically unable to produce offspring. That means that sex must have some higher purpose and function than simply reproduction. Another: Sex drive does not need to decrease with age, meaning that we strongly desire the physical connection even after we're unable to bear children.

What's that higher purpose? For one, sex can serve as that nirvana moment between couples—a time when you feel complete happiness and intimacy, a time when you express your love to your mate. In other words, sex is designed to make you feel good. Real, real good. How good? For starters, consider that:

Men who have sex three times a week can decrease their risk of heart attack and stroke by 50 percent.

Women who enjoy sex tend to live longer than those who don't.

Great sex makes your body feel and be the equivalent of two to eight years younger—same for men who have 150 to 350 orgasms a year, compared to the average of once per week.

Having orgasms seems also to help decrease general pain.

Increasing sex from once a month to once a week, according to researchers, is the happiness equivalent of an additional $50,000 in income for the typical American.

It's also interesting to look at the gender-based evolutionary functions of sex.

Thousands of years ago, the woman felt that it was her job to grow the species and raise the children, so she needed someone who could protect the family. Her body responded better to intimacy (she provided that intimacy so that men could help her reach orgasm). A man had different intentions.

When he saw a bunch of marauders marching through camp, he would get aroused by the threat to his family and mate—a signal that his sperm needed to beat out other men's sperm.

So a man responds sexually to anxiety, risk, and excitement, in contrast to a woman's desire for intimacy.

That hard-wired difference is one way to explain the different ways that men and women feel aroused—and it's the basis for helping you figure out how to better mesh the sexual preferences and differences in your own relationships so that you don't only go through the motions when it comes to sex but also experience the emotions.

While you may think that the biggest sex organ of all is one that's covered up by the latest style from Jockey or Victoria's Secret, your brain is actually your biggest sex organ. Some researchers have said that sexual thoughts, for example, go through a man's brain once every 52 seconds and through a woman's only once a day. And even conservative researchers say that men have many more sexual thoughts than women do.

Perhaps that's because men have 2.5 times the amount of brain space devoted to sexual drive that women do (or because women have more important things to think about).

Sex, of course, is more than just thinking about it; it's also about craving it. That craving originates in a part of the brain called the insula. Blocking messages to the insula is one of the ways that cigarette cessation techniques work—good news for many, they don't block sexual craving messages; in fact, bupropion, the drug we most often use in our breathe-free program with nicotine, actually increases libido in most people. The insula (remember it from chapter 8?), a primitive area of the brain, is especially active in women who have more frequent orgasms.

Let's now look at the way men and women biologically work when it come to sex:

Related

Excerpt: 'You: Being Beautiful' on Energy

WATCH: Chronic Pain Puts a Strain on Beauty

Excerpt: 'You: Being Beautiful'

WOMEN: During sex, your pupils dilate, nostrils flare, heart rate increases, oxytocin level increases, sweat glands open for cooling, breasts enlarge by 25 percent, and nipples increase in height by half an inch. Infrared cameras also show increased blood flow to the lips, nose, and labia. All of these things happen as the sexual stimuli build up to the almighty orgasm (see Figure 10.3).

A good question to ask right about now: Why do women have orgasms?

Evolutionarily, it was one of the ways that women could tell whether a man would be a good lifelong partner, because it could help women distinguish between a caring, patient male and a selfish or impatient one. Nevertheless, female orgasm can be so subtle that some women don't even know when they've had one.

Here's what happens:

During intercourse, your vaginal walls make fluids that let your partner's penis slide with just the right amount of friction. Together with the sights, sounds, and smells of sex, the stimulation to the clitoris, labia, and breasts all builds up a crescendo of intense physical sensation.

This is about the time when your brain tells your vagina and nearby muscles to contract. Why? To bring his penis in deeper and increase the chance of his sperm hitting its target—the egg.

In the process, some women even ejaculate. During orgasm, the uterus dips in like an anteater and sucks up the semen into the uterus to further increase the chance of fertilization.

The female orgasm also causes hormones to increase contractions in the vagina and uterus and help move semen into the uterus (women who orgasm between 1 minute before and 45 minutes after their partner's ejaculation have a higher tendency to retain sperm compared to those who don't have an orgasm).

The female orgasm, of course, isn't an easy thing to describe.

The brain serves as the main conductor in this symphony, but it might involve many different instruments, sometimes including the area known as the G-spot, which is parallel to a gathering of nerves on the male prostate.

Women usually do not have a single spot like some magic sex-me- here button but rather a region of nerves like those spread over the surface of the male prostate. That's because as a woman's reproductive organs develop in utero, her rudimentary prostate moves away so these nerves end up on the vaginal wall. So if you insert your index finger upward into the vagina and make the "come here" movement, you will touch the G-spot region that exists in some women.

The region is often not that sensitive either, but you never know until you try. The fact that women can be stimulated to orgasm through not only the genitals but also the mouth, nipples, and other parts of the body points to the complexity of the system—and reinforces the fact that the true biology of sex really evolves within the brain. (One theory is that sexual stimuli are carried from the cervix and uterus to the brain through the vagus nerve—one of the nerves stimulated during deep breathing and meditation.)

MEN: If you allow us a few moments to talk about the male anatomy, we think you'll be pretty amazed.

Biologically, men's sexual organs are much different than those of other species. For one, a man's penis doesn't have a bone, unlike those of other species. Why? The bone makes for easy and fast access for males in the animal kingdom (to inseminate their partners quickly); men give up the bone but gain a disproportionately large penis for their body size in return.

The evolutionary implications: One, men use the penis as a tool of attraction, implying that women do place some value in using it as a diagnostic for evaluating potential mates (not so overtly these days). And two, the lack of bone implies that men do equate emotions with sex, since they must be aroused for an erection; instead of easy and fast access, which can be painful to the females, it takes more care to have a sexual relationship between two people.

Another interesting observation: Humans have proportionally smaller testicles than males in other species; that's because other species need to ejaculate more semen to fertilize partners who are in heat prior to other males of the tribe and ensure propagation of their genes.

Human males don't need the size because of the biological drive to be monogamous (at least serially).

Now, it doesn't take a sexologist to know the purpose of the male orgasm: Find the egg, fertilize the egg, begin shopping for Barbies. But what's interesting is that this mad dash to the egg isn't some New York City marathon where all the starter sperm strap on their Nikes with the goal of making it to the finish line. Some of the sperm do that, but others are more like defensive linemen. Their job: to stop other men's sperm from scoring.

Some sperm even have a dual role—blocking other sperm but allowing their own sperm with their genes to penetrate more effectively.

Now, during a man's orgasm, the brain is firing like a lit-up pinball machine, causing contractions in most muscles of the body. The purpose: Like a woman's, these contractions help increase the chance of pregnancy by enabling the penis to penetrate as deeply as possible. The glands that make semen, mostly the prostate, squeeze repeatedly, propelling sperm as deep and as far as possible. The prostate, by the way, is often referred to as the male G-spot, because it's made up of some of the same types of tissues as some of the spots identified around the nerve plexus that is the G-spot in women and can be stimulated in a similar fashion by some adventuresome couples.