The sound of quickened breath, the feel of your heart pounding, and every inch of your skin crying out for a caress... now you're thinking about sex. Two people entwining in an act of passionate consummation is more than just exciting; it's also an amazing feat of biological engineering. The changes your body undergoes during a sexual act are astounding, complex, and profound. Are you breathless with ... anticipation?

The human body is designed to enjoy sex. Whether or not humans evolved this way to increase pair bonding, using sex as a lure to keep men engaged in childrearing, or to improve the quality of the gene pool may never be known. What is clear, though, is that our bodies have developed an enjoyment of an exquisite range of sensations that encourage us to explore the infinite varieties of the sexual act.

No matter in what ways we choose to enjoy sex, our bodies are well equipped for a wide range of sexual experience. The male penis, which is actually longer than most people think since it extends well into the body, is an impressive structure with an extremely sensitive head and a highly developed network of erectile tissue. It is connected to the testicles inside the scrotum, which provide both sexual motivation and material by the production of testosterone and sperm. The testicles hang outside the body because sperm can be adversely affected by body temperature. Located not too far away is the prostate gland, which provides some of the fluid to nourish the sperm and lubricate the mechanisms of intercourse. The prostate gland is not only a functional workhorse, but it is also filled with nerves that, when stimulated through the anus, can be a source of intense sexual pleasure.

A woman's sexual organs are equally complex, and normally hidden inside the folds of the labia majora and minora, the outer and inner lips. As she becomes sexually excited, however, the lips swell and separate to reveal the erect clitoris and the entrance to the vagina. The clitoris is very similar in structure to a small penis, with erectile tissue and a dense concentration of nerve fibers at the head. The vagina is the entrance to the uterus, where during intercourse sperm may be deposited. Even though women have no prostate gland (the Skene's glands are the homologous female structure), they have sensitive nerves in the anus, which can lead to pleasurable feelings during stimulation. Sex has not always been a subject for science. For most of western history, this act was thought to be too private to be studied and examined objectively. Although there were many scientists who researched sex in a qualitative manner, asking questions about people's experiences and their observations, two scientists named William Masters and Virginia Johnson were some of the first to quantitatively examine sexual physiology. Following the lead of Alfred Kinsey's behavioral studies, they developed experiments and instruments that could actually measure the biological changes during sex, and in doing so were the first to assess the nuts and bolts of physical pleasure. It is for this reason that their four-stage model of sexual desire is so widely accepted: they measured and quantified the sexual response, and used scientific reasoning to break it down into the components of excitement, plateau, orgasm, and resolution.

Excitement. It's the first phase of sexual response, where interest turns into arousal and the body begins to respond to stimulus and attraction. As it begins, everything inside you changes. Blood begins to flow to sensitive areas, muscles tighten in anticipation, and your heart begins to race. Although it results in many different sensations, excitement is basically the sum of only two different biological processes. The first is vasocongestion, the dilation of vessels so that blood can flow into the areas where it's most needed. Vasocongestion in the corpora cavernosa in the male and female erectile tissue, and the corpus spongiosum in the male leads to engorgement of the external genitalia. Vaginal lubrication is also the result of vasocongestion. As blood enters the vaginal walls, an increase in fluids seeping through makes a woman wet.

Muscle contraction is the second process that contributes heavily to the biology of sexual excitement. In both men and women, the nipples become erect and more sensitive. In women, the inner two thirds of the vagina expand away from each other like an inflated balloon, providing more room for penetration. Some studies have suggested that the cervix and uterus pull up to allow for easier passage of sperm. In men, the scrotal sac tenses and is pulled closer to the body, while the testicles are tugged in by the shortening of the spermatic cords. Excitement is the body's way of drawing in to prepare for release.

Plateau is the slow uphill climb of the sexual cycle, when the initial sensations of excitement expand luxuriously in preparation for the explosion of orgasm. It is an extended time of building pressure, increasing tension, and heavy breathing. During the plateau phase vasocongestion is at its peak. The penis becomes fully erect, the glans swells, and pre-ejaculatory fluid, secreted by the Cowper's glands, appears at the tip. In women, a structure known as the orgasmic platform begins to grow. The outer third of the vagina tightens, increasing the sensation of penetration, and the clitoris actually draws back into the body as tension continues to increase in the journey toward orgasm.

Plateau is where we spend most of our time during the sexual journey. Every sensation is enhanced in a body that is prepared to make the most out of each moment of stimulation. Traveling toward release can be the most pleasurable part of the sexual experience, and it can be drawn out into hours of enjoyment.

A great deal of the sexual experience actually takes place within the brain. Functional brain imaging has shown that widespread areas are active during sexual response, not just the small regions that were previously known to be involved. These areas are normally activated by genital stimulation, but in some individuals they can be stimulated by thought alone. Some women can experience arousal to orgasm without a single touch. While the genital sensory regions of the brain in the thalamus and cortex are not activated, the orgasm-related brain regions are. This suggests that human cognitive processes can bypass sensory genital pathways and still generate orgasm just by thinking or even during sleep. This may seem surprising, but remember, men have been talking about their nocturnal emissions for hundreds of years; only recently have scientists been able to scan women's brains for evidence of similar activity.

Orgasm is, for many people, the goal of the sexual experience. It is what the journey is leading toward, the most exciting moment of a wonderful trip, and the pinnacle of the sexual encounter. It's also the shortest phase of the sexual cycle, with rhythmic muscle contraction as its defining biological characteristic. At the moment of orgasm, muscles all over the body contract, although people tend to pay most attention to the spasms in very specific areas…

In some ways the male and female bodies almost seem to be designed to work for the pleasure and satisfaction of their sexual partner. For example, semen is a potent neutral buffer, which is necessary to prevent sperm from dying in the acidic vaginal environment, but it is also an effective lubricant. The vaginal opening tightens involuntarily as intercourse progresses, which enhances pleasure for both partners, as well as encouraging male ejaculation. Men's and women's bodies are therefore not only optimized for individuality, but for interaction.

It's My Pleasure

Orgasm may, in many ways, be a function of the brain and nervous system. The sexual organs contain a dense concentration of nerves, which contribute extensively to the sensations of passionate pleasure. In particular, the glans of the penis and its female counterpart, the head of the clitoris, are some of the most densely innervated parts of the human body. The interior of the vagina is sensitive as well, although much less so than the clitoris, or even the back of the hand. The most concentrated areas of vaginal sensation are at the top of the entrance, near the clitoris. The inner vagina is much less sensitive to touch, and many women don't enjoy contact with their cervix during intercourse. However, some research suggests that more than 25 percent of women may find cervical stimulation pleasurable.

In men, orgasm and ejaculation are closely, but not exclusively, linked. Male ejaculation actually occurs in two phases. The first phase takes place when the prostate, seminal vesicles, and vas deferens contract and force the ejaculate into a bulb at the base of the urethra. The second phase occurs when the urethral bulb and penis contract rhythmically to expel the ejaculate out the tip of the penis. Nonetheless, it is possible for some men to experience orgasm without ejaculation, and these men may be more likely to have multiple orgasms.

In women, the most profound change during the plateau phase is the formation of the orgasmic platform. In contrast to the earlier ballooning of the inner vagina, the orgasmic platform is actually formed by a swelling and tightening of the outer vagina. It is the orgasmic platform that has the most noticeable rhythmic contractions. A woman may experience three contractions, or a dozen, and the muscles of the anus and uterus may contract as well. Women are far more likely to be multi-orgasmic than men, and some women are actually capable of ejaculation during orgasm—but the precise nature of what and how they're ejaculating remains a subject of scientific debate.

Fluid Dynamics

An important function of vaginal lubrication is to enhance the experience of sex. The vagina contains no secretory organs. How, then, does the vagina become lubricated? As the blood vessels in the vaginal walls dilate and increase blood flow, fluid seeps through into the vagina in a process known as transudation. Basically, increased blood flow causes the vessels to sweat.

Does female ejaculation exist? Many people think so. The pornography industry certainly does, and markets numerous videos to instruct women in the technique. The scientific community, on the other hand, is less certain. Major gynecologic journals scoff at the evidence, while sexual health textbooks accept it as fact.

Male ejaculate is a complex fluid designed to nourish and safeguard sperm during their journey through the female reproductive tract. The first component of the secretion is mucin, a potent basic buffer made by the Cowper's glands. This buffer is necessary because from the moment sperm enter the vagina, they are in danger. The normally acidic vaginal pH is fatal to them, and they can survive only because the buffering capacity of semen temporarily raises the pH to neutral so that sperm can make their way into the relatively less hostile environment of the uterus. Prostaglandin, one of the seminal components made in the prostate, actually causes uterine contraction, which may aid in the uptake of sperm. The prostate also contributes other supportive compounds, including immunoglobulins. Finally, the seminal vesicles make their contribution to the mix by producing the majority of the volume of the ejaculate. One of the most important components of their secretion is fructose-a sweet-tasting natural sugar. The reproductive voyage requires a great deal of energy, and this special sugar not only nourishes the sperm on their exhausting journey, but may also contribute to semen's unique taste.

The path that sperm travel during ejaculation is a long and winding road. Sperm take approximately three months to mature. However, despite traveling through the epididymis, they are still not ready to do their job. Before ejaculation, they move through the vas deferens, where they are mixed with the other components of semen in preparation for being expelled through the urethra. Actual ejaculation requires the coordination of a series of valves, which first allow the semen into the ejaculatory ducts—the emission phase—and then expel it. Even after ejaculation, the sperm still aren't prepared for fertilization. The process of sperm activation actually continues up until the moment they reach the egg.

Resolution is the end of the sexual journey, where the body returns to its unaroused state. The heartbeat starts to slow, breathing deepens, and muscles begin to relax. The first thing that women experience is the release of the pressure in their breasts and the return of their clitoris to its resting position. Then the orgasmic platform also relaxes and begins to shrink, as the vagina and uterus go back to their resting shapes.

Resolution takes, on average, between 15 and 20 minutes, but it can take longer in women who have not had an orgasm, because the explosive release of orgasm helps allow the physical tension to subside. For women who experience multiple orgasms, however, even the slightest bit of stimulation can move them back into active arousal, repeating the sexual cycle until they are exhausted and replete. Each orgasm makes it easier to experience the next as the body remains at an elevated level of sexual response.

For men, resolution is a two-stage process. During the first, although the stiffness of the penis releases as the corpora cavernosa empty, the penis maintains its aroused size. The erection doesn't fully subside until the second, much slower stage, when the corpus spongiosum and glans empty. During resolution, most men enter a refractory period where they cannot orgasm again. This period can range from several minutes to more than a day, but there are some men who do not experience a refractory period at all. For these men, loss of erection does not automatically follow orgasm, and they are capable of multiple orgasms within a short period of time. In multiorgasmic men, ejaculation and orgasm may be independent, or the quantity of ejaculate may decrease with each successive orgasm.

Sometimes the phrase "sexual decision making" seems like an oxymoron, because many people do not act rationally about sex. They let their desires take over the thinking portion of their brain, and then blame their mistakes on factors they claim are out of their control. Can your body set off cascades of hormones that make your libido surge, that make you want sex to the point of distraction, and that cause your genitals to ache with the desire for touch?

The feedback system involved in sexual excitement has many unique characteristics that reveal the importance of reproduction in human lives. Even the connections formed by sexually sensitive neurons of the brain are special. These neurons are linked by a unique blood vessel portal system so that they can communicate directly with the specialized cells of the pituitary gland and induce them to begin a hormone cascade.

Sexual arousal is both conscious and unconscious. Women have been shown to be able to think themselves to orgasm, but the actual process of arousal is controlled by unconscious cascades. Specifically, the body's drugs of choice when it comes to sexual arousal are androgens such as testosterone. Androgens are produced not only in the ovaries, but in the adrenal cortex, and they are what revs up a woman's libido (a Latin word meaning desire or lust). They do this in a number of ways: by increasing genital skin sensitivity, communicating to nerves in the spinal column and brain, converting to estrogen to modulate the vaginal environment, and regulating neurotransmitter release to affect sensation and mood.

In men, libido is separated hormonally from the ability to ejaculate and orgasm. This became clear hundreds of years ago when it was found that some eunuchs could still become sexually aroused, even though their lack of testicles made ejaculation impossible. Testicular testosterone is clearly extremely important for male sexual arousal, so how could they function without it? The answer is that androgens are also produced elsewhere in the body. The liver, the skin, and the adrenal glands all make androgens that are weaker than testosterone, but which still can have a sexual effect. While testosterone is the most effective stimulant of both sex drive and ability in men, it is important to consider that sexual activity in men may also be independent of hormones. Testosterone replacement therapy is an effective way to enhance the sexual lives of men who have decreased testosterone production due to aging or other health factors.

During sex, the brain shifts its attention to the sexual act...which seems much more interesting than worrying about cause and effect, actions and consequences. During the sexual act, people are too involved in their activity to be concerned about things that would bother them in the more sober aspects of their daily lives. Worrying about where someone's mouth has been doesn't seem terribly important when that mouth is on you.

Although it's hard not to laugh at the Emo Phillips line, "I used to think that the brain was the most important organ in my body, but then I realized who was telling me this.” The brain actually is the most important organ when it comes to sex. Not only is it the home of thought and fantasy, but it is also where all the signals from the rest of the body are processed and perceived as pleasure or pain. It affects the production of sex hormones, determines who and what you find attractive, and helps you sense arousal in others. Its complexity is what takes sex from reproduction to remarkable.

A hand may stroke your calf, or lips may touch your neck, but all touch is perceived in the same place—in the brain. Whether your skin is stimulated by a breath or a finger, nerves in that location fire, signals are transmitted through the cranial nerves to your brain, and your brain recognizes that you have been touched. During sexual arousal, the brain becomes even more responsive to touch and other sensations, answering each stimulus by increasing blood flow to the most sensitive areas, and setting up a feedback spiral that allows sexual arousal to ascend to its peak.

Touch sensors, types of sensory nerves, are specialized to sense pressure, deep and light touch, vibration, temperature, or stretch. Concentrations of each type of sensory nerve vary in different areas of the skin. Any or all of these sensations can occur during a sexual interlude. An ice cube trailed across the skin, the weight of a lover's arm—the body receives every type of sensation through its own unique receptor, which then sends chemical and electrical signals to relate its encounter to the brain. The lips, in particular, contain a high density of nerves, and a large portion of the sensory apparatus of the brain is devoted to analyzing their tactile experiences. This is one of many reasons why the mouth plays such an important role in the sexual experience.

Food both sustains and satiates. Its purpose is to nourish, but the textures and tastes upon the tongue also can stimulate feelings of pleasure. Like smells, tastes stimulate the brain's pleasure centers (the limbic system), and emotional reactions can become associated with certain textures or flavors. Kissing provides your brain with sensations of both touch and taste, and it can be an extremely erotic activity. In many cultures, the image of a kiss is synonymous with not just passion, but with true romance and love. It's not a universal behavior, however: some cultures actually consider kissing a disgusting practice—unhygienic, repulsive, even suggestive of cannibalism!

For most people, sight is a very important component of sexual arousal. The things that "turn them on" may vary, but there's almost always some type of visual that can spark a sexual response. It's not just a stereotype, however, that men tend to be far more interested in sexual imagery than women; it's a scientific fact. Even though looking at a suggestive picture activates the same pathways in men's and women's brains, men tend to have higher levels of electrical response even when women report being more physically aroused. This suggests that something that occurs during development affects how men and women respond to visual stimuli. While they may both find an image, sexually arousing, they respond in biologically different ways.

What, and how, images are processed into arousing stimuli remains poorly understood. Light is received by the rods and cones of the retina and transmitted to the visual cortex for interpretation, but how these images stimulate sexual thought is still a matter of debate. The visuals that people find sexually stimulating vary between sexes, cultures, and even age groups, and seem to be learned from the environment rather than being instinctive. In men, there seems to be a direct link between seeing a visually arousing image and activation of the testosterone-responsive hypothalamus, but in women the connection between sight and arousal is less well understood. But it's clear that visual stimulation can be erotic to both sexes, even if no one can explain how.

Why does the sound of a quickened breath or a throaty whisper send shivers down a person's spine? No one knows, but it's very certain that something stimulating is going on. Even more interestingly, several studies have found that the way people process sound varies not only by sex but also by sexual orientation, suggesting another link between sensuality and aural pleasure.

Smell is one of the most potent senses for stimulating the human brain. Odors are profound triggers of emotional and sense memory, and they have both conscious and subliminal effects on the mind. Our emotional response to odors is far stronger than to their related visual or auditory stimuli.

Some of the most interesting research on smell has involved pheromones and how they affect human behavior. Pheromones are chemical compounds detected by the nasal sensory neurons within the nose, although pheromones are usually odorless. They are defined by the effect they have on social and sexual behavior. Pheromones have been shown to affect the timing and synchronization of the menstrual cycle in groups of women, mother-infant bonding, and gender-based sexual repulsion and attractiveness. Interestingly, research suggests that the brains of homosexual men respond to male pheromones in a manner similar to that of heterosexual women, but not to female pheromones the way that heterosexual men do.

In contrast to pheromones, odors are processed by the olfactory neurons and the olfactory cortex. They are then interpreted both consciously through the neocortex, and emotionally through the limbic system. It is through the limbic system that odors stimulate memory, attraction, and gut reaction, because of previous associations between that odor and emotional states or activities. Pheromones, and other sexually attractant odors, appear to work at least in part by stimulating the hypothalamus which, with its intimate connection to hormonal control, is a major sexual component of the brain.

The odors of sex are particular stimulants of memory and arousal. Studies have shown that a woman's body odor is most attractive to men when she's in the fertile stage of her menstrual cycle. Evolution has distilled a stimulus that encourages sex when it will be most productive. There is also a reason why women so enjoy snuggling into the center of their partners' chest. One of the apocrine glands is located there, and the scent produced by these glands is regulated by the sexual hormones. In other words, the pheromones created there at any given moment are stimulated by, and in turn enhance, the current mood. It is a comfortable place to nuzzle your lover and inhale their personal sexual scent.

All the senses are involved in sexual desire, and sexual desire is born from both biological and cultural stimulation. What we are attracted to, what turns us on, the things that bring us passion are affected as much by personal history as by biology. Erogenous zones, for instance, are defined as those places on the body that, when touched, give rise to sexual desire. You may think that those locations are mapped and certain, but a person from Asia will give you very different descriptions of their erogenous zones than a friend from North America. We are conditioned by our environment to believe that certain stimuli and behaviors are sexual, and research has shown that individuals can be trained to be aroused by smells, sensations, and images that have no direct relationship to their feelings about sex.