Most animals use sex steroids in differentiation of male and female reproductive structures and oxytocin for the majority of the acute, sudden events that occur at these structures. Oxytocin and vasopressin family members are known from 4 invertebrate phyla and all groups of vertebrates (Youson, 1999). Oxytocin and vasopressin are short peptides composed of nine amino acids (nanopeptides) which are made in the paraventricular and supraoptic nuclei of the hypothalamus. Vasopressin is also made in several other regions of the brain (Debiec, 2007).Oxytocin functions in a paracrine hormone in mammals, being produced in the hypothalamus, pituitutary, mammary gland, ovary, uterus, testis, and prostate. The processes regulated by oxytocin tend to utilize positive feedback mechanisms (such as in birth, milk ejection mediated by neurons followed by recovery, and in the deterioration of the corpus luteum (Ivell, 1999; Caldwell, 2002).

Oxytocin is a factor in social bonding, love, and the intensity of orgasm. (Bianchi-Demicheli, 2007). Levels of oxytocin increase when human couples describe romantic thoughts, such as recollections of their first date (Debiec, 2007). Maternal and romantic love both involve regions of the brain rich in oxytocin and vasopressin receptors (Lehto, 2007). Oxytocin delivered through a nasal spray (which is known to cross the blood brain barrier) increases trust in humans, in part by reducing the activity of the amygdala where fear is processed (Debiec, 2007).

Oxytocin injections in animals increase sexual activity (Salonia, 2005). Patients who have taken synthetic forms of OT to alleviate problems in breast feeding have observed increased libido and lubrication as side effects (Salonia, 2005). Stimulation of the nipple, vagina, and cervix can result in the release of oxytocin and prolactin (Komisaruk, 1988). Some evidence suggests that sex steroids bound to sex hormone binding globulin interact with oxytocin receptors (SHBG is produced in the brain in the same areas where OT receptors are expressed) (Caldwell, 2002).

Blood oxytocin levels increase during arousal and peak during orgasm in both women and men. During the menstrual cycle, oxytocin levels are highest in the follicular and ovulatory stages and significantly decreased during the luteal phase. Increased rates of orgasm have been recorded in the part of the menstrual cycle just prior to ovulation. OT levels correlate with levels vaginal lubrication. Although women taking oral contraceptives do not undergo this cycling of oxytocin, its levels still increase during sexual activity and are correlated with vaginal lubrication (Salonia, 2005; Bianchi-Demicheli, 2007).

Vasopressin increases sex drive (Brizendine, 2006).

There are similarities in the orgasms of men and women. In a study in which men and women were asked to provide written descriptions of their experience of orgasm, medical students, psychologists, and OB-GYN doctors (male and female) could not distinguish between descriptions written by men or women (Bianchi-Demicheli, 2007). Both men and women can orgasm during sleep (Levin, 2007).

Different researchers have classified different stages of sexual pleasure and orgasm. Alternate enumeration of the stages of sexual experience include a three step model of desire, arousal, and orgasm and a 4 step model of excitement, plateau, orgasm, resolution. Many now join the plateau phase with the excitation phase and add two stages of desire, one spontaneous and a second resulting from arousal (Levin, 2007). Orgasm effects a variety of body regions such as the heart (tachycardia--up to 180 beats per minute), blood vessels (initial increase then decrease in blood pressure and the "sex flush"), lungs (hyperventilation), sweat glands, muscles (spasms, pelvic motions, shuddering), and vocalizations (Mah, 2001). During a woman's orgasm, the circumvaginal musculature contracts involuntarily but rhythmically 3-15 times (at about 0.8 second intervals) accompanied by contractions of uterine and anal muscles. Orgasms may last 3 seconds to about 2 minutes. Other physiological changes such as also occur. Consciousness may fade somewhat (Bianchi-Demicheli, 2007).

In men, but not women, there is a post-ejaculatory refractory time (PERT) during which time a second erection and ejaculation are impossible. Multiple orgasms are possible in young males but the incidence decreases rapidly with age (Mah, 2001; Levin, 2007).

Twin studies suggest there is a heritable basis to orgasm although other factors such as her partner, education level, sexual experience, body-image, mental state, and culture can also affect orgasm. For example, a woman might easily reach orgasm with one partner but not achieve orgasm with a second (Bianchi-Demicheli, 2007).In women, sexual function can be affected by sociocultural and economic factors (such as sex education, access to health care, stress associated with cultural norms, perceived body image in light of cultural expectations, and fatigue due to work obligations), issues relating to her sexual partner (including poor communication, reaction to betrayal, imbalance of power in the relationship, fear of abuse, differences in sexual drive, health problems in her partner), psychological factors (aversion or mistrust of sexual pleasure, inhibition because of fear of consequences), and medical factors (health disorders, side effects of medication, pregnancy, STDs) (Ramage, 2007).
Kegel's exercises involve contractions of the muscles of the pelvic floor, as if a woman were ending urination, which can improve muscle tone and increase orgasm (Ramage, 2007).

Although women with spinal injury can achieve orgasm, spinal damage often results in orgasm deficits, indicating a role for the spinal cord. The spinal cord receives input from the pudendal and pelvic nerves and communicates with the periaqueductal gray matter of the midbrain. The medial preoptic area, paraventricular nucleus, (Bianchi-Demicheli, 2007).