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

March 9, 2009 — -- 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 isnot from National Geographic specials.) One example: Women are sexually activefor 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 physiologicallyunable to produce offspring. That means that sex must have somehigher purpose and function than simply reproduction. Another: Sex drive doesnot need to decrease with age, meaning that we strongly desire the physical connectioneven after we'reunable to bear children.

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

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

Thousands of years ago, the woman felt that it was her job to grow the speciesand raise the children, so she needed someone who could protect the family. Herbody 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 ofmarauders marching through camp, he would get aroused by the threat to hisfamily and mate—a signal that his sperm needed to beat out other men'ssperm.

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

That hard-wireddifference is one way to explain the different ways that menand women feel aroused—and it'sthe basis for helping you figure out how tobetter mesh the sexual preferencesand differences in your own relationshipsso that you don'tonly gothrough the motions when it comesto sex but also experience the emotions.

While you may think that thebiggest sex organ of all is one that'scovered up by the latest style fromJockey or Victoria's Secret, yourbrain is actually your biggest sexorgan. Some researchers have saidthat sexual thoughts, for example,go through a man'sbrain once every52 seconds and through a woman'sonly once a day. And even conservative researcherssay that men have many more sexual thoughts than women do.

Perhapsthat'sbecause men have 2.5 times the amount of brain space devoted to sexualdrive that women do (or because women have more important things to thinkabout).

Sex, of course, is more than just thinking about it; it'salso about craving it.That craving originates in a part of the brain called the insula. Blocking messagesto the insula is one of the ways that cigarette cessation techniques work—goodnews for many, they don'tblock sexual craving messages; in fact, bupropion, thedrug we most often use in our breathe-free program with nicotine, actually increases libido in most people. The insula (rememberit from chapter 8?), a primitive area ofthe brain, is especially active in women who havemore frequent orgasms.

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

WOMEN: During sex, your pupils dilate, nostrilsflare, heart rate increases, oxytocin level increases,sweat glands open for cooling, breastsenlarge by 25 percent, and nipples increase inheight by half an inch. Infrared cameras alsoshow increased blood flow to the lips, nose, andlabia. All of these things happen as the sexualstimuli build up to the almighty orgasm (see Figure10.3).

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

Evolutionarily, it was one of the ways that women couldtell whether a man would be a good lifelong partner, because it could help womendistinguish between a caring, patient male and a selfish or impatient one. Nevertheless,female orgasm can be so subtle that some women don'teven know whenthey'vehad one.

Here'swhat happens:

During intercourse, your vaginal walls make fluids that let your partner'spenis 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 allbuilds up a crescendo of intense physical sensation.

This is about the time whenyour brain tells your vagina and nearby muscles to contract. Why? To bring hispenis 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 inlike an anteater and sucks up the semen into the uterus to further increase thechance of fertilization.

The female orgasm also causes hormones to increase contractionsin the vagina and uterus and help move semen into the uterus (women who orgasm between 1 minute before and 45 minutes after their partner'sejaculationhave a higher tendency to retain sperm compared to those who don'thavean orgasm).

The female orgasm, of course, isn'tan 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 maleprostate.

Women usually do not have a single spot likesome magic sex-me-herebutton but rather a region of nerves likethose spread over the surface of the male prostate. That'sbecauseas a woman'sreproductive organs develop inutero, her rudimentary prostate moves away so thesenerves end up on the vaginal wall. So if you insertyour index finger upward into the vagina and makethe "come here" movement, you will touch theG-spot region that exists in some women.

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

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

Biologically, men's sexual organs are much different than those ofother 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 disproportionatelylarge 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 inusing it as a diagnostic for evaluating potential mates (not so overtly these days). Andtwo, the lack of bone implies that men do equate emotions with sex, since they mustbe aroused for an erection; instead of easy and fast access, which can be painful tothe females, it takes more care to have a sexual relationship between two people.

Another interesting observation: Humanshave proportionally smaller testicles than malesin other species; that's because other speciesneed to ejaculate more semen to fertilize partnerswho are in heat prior to other males of the tribeand ensure propagation of their genes.

Humanmales don't need the size because of the biologicaldrive to be monogamous (at least serially).

Now, it doesn'ttake a sexologist to know thepurpose of the male orgasm: Find the egg, fertilizethe egg, begin shopping for Barbies. Butwhat'sinteresting is that this mad dash to the eggisn'tsome New York City marathon where all thestarter 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 likedefensive linemen. Their job: to stop other men'ssperm from scoring.

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

Now, during a man'sorgasm, the brain is firing like a lit-uppinball 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 topenetrate 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'smade up of someof the same types of tissues as some of the spots identified around the nerveplexus that is the G-spot in women and can be stimulated in a similar fashion bysome adventuresome couples.