Gas exchange is the process by which living organisms take in oxygen and release carbon dioxide. In humans and many animals, this occurs in the lungs, where oxygen from the air enters the bloodstream, and carbon dioxide, a waste product of metabolism, is expelled. This exchange is essential for cellular respiration, which provides energy for the body. In plants, gas exchange happens through small openings called stomata on leaves. During the day, plants take in carbon dioxide from the air and release oxygen through a process called photosynthesis. At night, the process reverses, and plants primarily take in oxygen and release carbon dioxide as they respire.