AP Biology

Answer: D, Gas exchange occurs in the circulatory and pulmonary systems. Gas exchange in humans is carried out by mechanisms in the lungs and heart.

Key Takeaway: Three processes are essential for the transfer of oxygen from the atmosphere into the body and its exchange with carbon dioxide: ventilation, diffusion, and perfusion. Ventilation is simply the act of breathing, and diffusion is the free movement of gasses within the body requiring no energy. Perfusion is the process of pumping blood through the lungs by the circulatory system. These three processes involve the coordinated efforts of the circulatory and pulmonary systems and result in the constant exchange of carbon dioxide and oxygen in the body.