In reference to the body, a system is a group of organs that work together to perform a function. For example, the respiratory system ensures that oxygen and carbon dioxide are exchanged inside the air sacs of the lungs. This system includes organs such as the nose, trachea, bronchi, bronchial tubes and alveoli. Another system is the reproductive system. Females have the vagina, uterus, oviducts and ovaries, which allow for the production, fertilization and development of a fetus as well as the production of female hormones. Males have a penis, testes, sperm ducts, urethra, prostate gland, seminal vesicles and cowper's gland. All of these allow for the production and storage of sperm, transport of sperm outside the body and the production of male hormones called androgens. The body is organized from smallest to largest in the following way-cells which form tissues, tissues which form organs, organs which form systems and systems which maintain homeostasis in the body when they function optimally.