Female sexual response is characterized by four phases: excitement, plateau, orgasm, and resolution. It involves the participation of the local peripheral and central nervous system, somatic nervous system, and vascular system. The excitement phase is characterized by an increase in heart rate and faster breathing, along with rising blood pressure. The skin may flush, although this does not always happen and is less likely when ambient temperatures are low. Stimulated nerve endings in the skin of the external genitalia send impulses through the sensory nerves of the abdomen to the sacral spinal cord. This sets off autonomic reflexes (parasympathetic and sympathetic) that control increased blood flow to the area and initial glandular lubrication. The clitoris, labia, and vagina swell as a result of this increase in blood flow to these organs. Through the sacral parasympathetic neurons, vaginal lubrication begins, the inner two-thirds of the vagina expands, the breasts become fuller, the uterus elevates, and muscle tension begins to increase.
During the plateau phase, heart rate, blood pressure, and breathing rate continue to increase. Muscles may become tense and involuntary vocalizations may occur. The clitoris becomes very sensitive and withdraws slightly beneath the clitoral hood (skin that surrounds and protects the clitoris). Blood flow continues to the vaginal walls, giving them a dark red or purple hue. Sensory stimulation travels up the spinal cord to the brain, where it enters the limbic system, the part of the brain where pleasure is perceived.
Orgasm may or may not occur and varies widely among women and among different sexual encounters. The distinguishing characteristics of orgasm are involuntary, repeated, quick contractions of the pelvic muscles, uterus, and vagina. During orgasm, the heart rate, blood pressure, and breathing rate are at their peak. An overall sense of sexual pleasure is common, and muscles all over the body may tense. Heart rate reaches its maximum for the sexual response during orgasm.
Resolution follows orgasm or may follow the plateau phase if orgasm does not occur. Blood pressure, heart rate, and breathing rate decrease and return to normal. A sense of fatigue is common, and women may or may not experience a refractory period in which orgasm cannot be achieved again.