The circulatory and respiratory systems begin to change. The growth spurt usually triggers the feet and legs to grow, followed by the body's trunk and ending in the shoulders and chest. Bones become harder and more dense, while muscle and fat contribute to an adolescent increase in weight. Sexual development involves enlargement, maturity and the reproduction of the primary sex organs. For boys, the testes begin to produce part of the fluid for sperm cells and the prostate gland to produce semen.
Males are able to ejaculate the semen, which contains the sperm.
For girls, the ovaries release the mature ova into the fallopian tubes and menstruation occurs. The Teen Health Project at the Dalhousie Medical School in Nova Scotia, Canada examines all aspects of adolescent development including growth and human sexuality. According to the project, girls experience puberty about 2 years earlier than most boys. The first signs of puberty in girls are usually the development of breast buds or pubic hair, appearing in most girls between the ages of 8-10 years. Then breast enlargement, pubic hair growth, a height spurt and the onset of menstruation all occur some time in the next 4-10 years.