What are the four stages involved in the human sexual response?

The first step involves excitement, which occurs during foreplay. In males, the penis fills with blood and an erection occurs. In females, blood flow increases to the breasts, nipples, labia, and clitoris. The vaginal walls seep fluid, which helps lubricate the vagina and makes insertion of the penis much easier. The second step is called the plateau. In men, blood flow increases to the testes, which enlarge, and muscle contraction pulls them closer to the body. In females, the outer third of the vagina swells because blood flow increases to this area. Breathing rate and heart rate continue to increase. Orgasm is the third step of arousal; muscle contractions of the reproductive structures are characteristic of this stage. In males, ejaculation occurs, while in females, rhythmic contractions in the vagina and uterus take place. The last stage is called resolution, which involves the return to normal of the body functions.