Sexual arousal

Sexual arousal describes the physiological and psychological responses in preparation for sexual intercourse or when exposed to sexual stimuli. A number of physiological responses occur in the body and mind as preparation for sexual intercourse, and continue during intercourse. [|Male arousal] leads to erection, cremaster reflex and pre-ejaculate; in [|female arousal], the body's response is engorged sexual tissues, changes to the cervix, and vaginal lubrication.
Sexual arousal has several stages and may not lead to any actual sexual activity after mental stimuli have brought about accompanying physiological changes. Physical stimuli, such as touch, and the internal fluctuation of hormones can influence sexual arousal. Given sufficient and appropriate sexual stimulation in the right context, sexual arousal intensifies to a high level and can precipitate an orgasm as part of the climax. Mental or physical stimulation may also be pursued for their own sake, even in the absence of orgasm.

Erotic stimuli

Depending on the situation, a person can be sexually aroused by a variety of factors, both physical and mental. A person may be sexually aroused by another person or by particular aspects of that person, or by a non-human object or scenario. The physical stimulation of an erogenous zone or acts of foreplay can result in arousal, especially if it is accompanied with the anticipation of imminent sexual activity. Sexual arousal may be assisted by a romantic setting, music or other soothing situation. Sexual arousal can come from porn or other sexual material. The potential stimuli for sexual arousal vary from person to person, and from one time to another, as does the level of arousal.
Stimuli can be classified according to the sense involved: somatosensory, visual, and olfactory. Auditory stimuli are also possible, though they are generally considered secondary in role to the other three. Erotic stimuli which can result in sexual arousal can include conversation, reading, films or images, or a smell or setting, any of which can generate erotic thoughts and memories in a person. Given the right context, these may lead to the person desiring physical contact, including kissing, cuddling, and petting of an erogenous zone. This may in turn make the person desire direct sexual stimulation of the breasts, nipples, buttocks or genitals, and further sexual activity.
Erotic stimuli may originate from a source unrelated to the object of subsequent sexual interest. For example, many people may find nudity, erotica or pornography sexually arousing. This may generate a general sexual interest that is satisfied by sexual activity. When sexual arousal is achieved by or dependent on the use of objects, it is referred to as sexual fetishism, or in some instances a paraphilia.
There is a common belief that women need more time to achieve arousal. However, recent scientific research has shown that there is no considerable difference for the time men and women require to become fully aroused. Scientists from McGill University Health Centre in Montreal used the method of thermal imaging to record baseline temperature change in genital area to define the time necessary for sexual arousal. Researchers studied the time required for an individual to reach the peak of sexual arousal while watching sexually explicit movies or pictures and came to the conclusion that on average women and men took almost the same time for sexual arousal – around 10 minutes. The time needed for foreplay is strongly individual and varies from one occasion to the next depending on circumstances.
Unlike many animals, humans do not have a mating season, and both sexes are capable of sexual arousal throughout the year.

Disorders

When a person fails to be aroused in a situation that would normally produce arousal and the lack of arousal is persistent, it may indicate a sexual arousal disorder or hypoactive sexual desire disorder. There are many reasons why a person fails to be aroused, including a mental disorder such as depression, drug use, or a medical or physical condition. There may also be a lack of sexual desire generally or for the current partner. A person may always have had no or low sexual desire or the lack of desire may have started later in life.
Contrastingly, a person may be hypersexual, either having high sexual desire in relation to culture or expected development, or suffering from a persistent genital arousal disorder, causing spontaneous, persistent, and uncontrollable arousal, and the physiological changes associated with arousal.

Physiological and psychological responses

Physiological responses

Sexual arousal causes various physical responses, most significantly in the sex organs. Sexual arousal for a man is usually indicated by the swelling and erection of the penis when blood fills the corpus cavernosum. This is usually the most prominent and reliable sign of sexual arousal in males. In a woman, sexual arousal leads to increased blood flow to the clitoris and the rest of the vulva, as well as vaginal transudation - the seeping of moisture through the vaginal walls, which serves as lubrication. In both sexes, pupil dilation is an involuntary physiological response to sexual arousal. However, the degree of pupil dilation varies with individuals, as does the degree of maximal pupil dilation.

In males:

In females:

Male sexual arousal. On the left, the male genitalia are in regular, flaccid state. On the right, the male is sexually aroused, the penis is erect and the scrotum is tense.

Erection of nipples
Penile tumescence and erection
The veins in the penis may become more prominent
Tightening and/or retraction of the foreskin often exposing the glans penis
Emission of pre-ejaculatory fluid
Swelling of the testicles
Ascension of the testicles
Tensing and thickening of the scrotum
Pupil dilation

Female sexual arousal. On the left, the female genitalia are in regular state. On the right, the female is sexually aroused, the vulva is wet and its labia are slightly engorged.

Erection of nipples
Vaginal lubrication
Vasocongestion of the vaginal walls
Tumescence and erection of the clitoral glans and labia
Elevation of the cervix and uterus
Tenting, i.e. expansion of the inner two-thirds of the vagina
Change in shape, color and size of the labia majora and labia minora
Pupil dilation
Widening of the areola

Male

It is normal to correlate the erection of the penis with male sexual arousal. Physical or psychological stimulation, or both, leads to vasodilation and the increased blood flow engorges the three spongy areas that run along the length of the penis. The penis grows enlarged and firm, the skin of the scrotum is pulled tighter, and the testicles are pulled up against the body. However, the relationship between erection and arousal is not one-to-one. After their mid-forties, some men report that they do not always have an erection when they are sexually aroused. Equally, a male erection can occur during sleep without conscious sexual arousal or due to mechanical stimulation alone. A young man—or one with a strong libido—may experience enough sexual arousal for an erection to result from a passing thought, or just the sight of a passerby. Once erect, his penis may gain enough stimulation from contact with the inside of his clothing to maintain and encourage it for some time.
As sexual arousal and stimulation continues, it is likely that the glans or head of the erect penis will swell wider and, as the genitals become further engorged with blood, their color deepens. As the testicles continue to rise, a feeling of warmth may develop around them and the perineum. With further sexual stimulation, their heart rate increases, blood pressure rises and breathing becomes quicker. The increase in blood flow in the genital and other regions may lead to a sex flush in some men.
As sexual stimulation continues, orgasm begins, when the muscles of the pelvic floor, the vasa deferentia, the seminal vesicles and the prostate gland itself may begin to contract in a way that forces sperm and semen into the urethra inside the penis. Once this has started, it is likely that the man will continue to ejaculate and orgasm fully, with or without further stimulation.
Equally, if sexual stimulation stops before orgasm, the physical effects of the stimulation, including the vasocongestion, will subside in a short time. Repeated or prolonged stimulation without orgasm and ejaculation can lead to discomfort in the testes.
After ejaculation, men usually experience a refractory period characterized by loss of their erection, a subsidence in any sex flush, less interest in sexual activity, and a feeling of relaxation that can be attributed to the neurohormones oxytocin and prolactin. The intensity and duration of the refractory period can be very short in a highly aroused young man in a highly arousing situation, perhaps without even a noticeable loss of erection. It can be as long as a few hours or days in middle-aged and older men.

Female

The beginnings of sexual arousal in a woman's body is usually marked by vaginal lubrication, swelling and engorgement of the vulva, and internal lengthening and enlargement of the vagina. There have been studies to find the degree of correlation between these physiological responses and the woman's subjective sensation of being sexually aroused: the findings usually are that in some cases there is a high correlation, while in others, it is surprisingly low.
Further stimulation can lead to additional vaginal wetness and further engorgement and swelling of the clitoris and the labia, along with increased redness or darkening of the skin in these areas as blood flow increases. Further changes to the internal organs also occur including to the internal shape of the vagina and to the position of the uterus within the pelvis. Other changes include an increase in heart rate as well as in blood pressure, feeling hot and flushed and perhaps experiencing tremors. A sex flush may extend over the chest and upper body.
If sexual stimulation continues, then sexual arousal may peak into orgasm. After orgasm, some women do not want any further stimulation and the sexual arousal quickly dissipates. Suggestions have been published for continuing the sexual excitement and moving from one orgasm into further stimulation and maintaining or regaining a state of sexual arousal that can lead to second and subsequent orgasms. Some women have experienced such multiple orgasms quite spontaneously.
While young women may become sexually aroused quite easily, and reach orgasm relatively quickly with the right stimulation in the right circumstances, there are physical and psychological changes to women's sexual arousal and responses as they age. Older women produce less vaginal lubrication and studies have investigated changes to degrees of satisfaction, frequency of sexual activity, to desire, sexual thoughts and fantasies, sexual arousal, beliefs about and attitudes to sex, pain, and the ability to reach orgasm in women in their 40s and after menopause. Other factors have also been studied including socio-demographic variables, health, psychological variables, partner variables such as their partner's health or sexual problems, and lifestyle variables. It appears that these other factors often have a greater impact on women's sexual functioning than their menopausal status. It is therefore seen as important always to understand the "context of women's lives" when studying their sexuality.
Reduced estrogen levels may be associated with increased vaginal dryness and less clitoral erection when aroused, but are not directly related to other aspects of sexual interest or arousal. In older women, decreased pelvic muscle tone may mean that it takes longer for arousal to lead to orgasm, may diminish the intensity of orgasms, and then cause more rapid resolution. The uterus typically contracts during orgasm and, with advancing age, those contractions may actually become painful.