Why do women's breasts become erect when they are aroused?

During the sexual excitement phase of the sexual response cycle, the first evidence of increased breast response to sexual tension is the erectile response of the nipple, which is the result of involuntary contraction of the rich smooth muscle fibers in the nipple after sexual stimulation. The two nipples often do not respond in sync; one may be fully erect and swollen while the other lags behind. Inverted nipples may bulge out of their resting state, appearing to be in a semi-erect position. If the inversion is irreversible, no signs of nipple reaction will be seen.

A full erection response can increase the length of the nipple compared to before stimulation.

Generally, it increases by 0.5 to 1.0 cm, and the reaction can also increase the diameter of the nipple base by 0.25 to 0.5 cm. Women with large, protruding nipples usually have a smaller tendency to swell and become erect than those with normal-sized nipples. It is also difficult for nipples to respond strongly to sexual stimulation if they are very small, but very small nipples are rare.

The second physiological change during the excitement phase is the increased demarcation and dilation of the breast venous tree pattern.

If the breast has sufficient volume, there will be congestion of the underlying superficial veins, but this will probably not become more pronounced until late in the excitement. Larger breasts often show a more pronounced dilation of the venous tree pattern. The congestion of the mammary venous tree does not usually reach the areola area as it expands toward the center.

The actual volume of the breast will increase significantly, which is the result of deep venous congestion in the breast.

When a woman is in a sexual response state and an erection occurs, the congestion in the lower part of the pendulous breast is more easily observed. If the woman takes a supine position, the overall increase in breast volume will be more obvious. In the late stage of sexual excitement, obvious areola congestion can be seen.

The degree and timing of breast response during sexual excitement vary greatly and often vary from person to person and from time to time. During the plateau phase, the areola adjacent to the erect nipple will also swell, often giving the illusion that the erect nipple has partially subsided. Not until the areola swelling subsides during the resolution period can the nipples that resolved later but remain erect be seen again.