The respiratory system collaborates with the circulatory system to supply oxygen and eliminate metabolic waste products. It also aids in the regulation of blood pH. Respiration is the process through which oxygen and carbon dioxide are exchanged between the environment and the cells of the body. The two main organs involved in respiration are the lungs and the heart. Lungs: The lungs consist of a network of airways that lead into alveoli where gas exchange takes place. Gas exchange is the process by which gases are transferred across a barrier, in this case, the thin membrane called the alveolar wall. Oxygen enters the body through the nose or mouth and flows into the airways where it is absorbed by small sacs called alveoli. These sacs are where gas exchange occurs between blood and air. When you breathe in, oxygen from the air enters these sacs where it is taken up by hemoglobin in your red blood cells. This binds to oxygen, making it transportable by blood throughout the body. Carbon dioxide, on the other hand, is released from cells that use oxygen to produce energy. This binding reaction requires a lot of energy, so after work is done within the cell, the blood carries it back to the lungs where more oxygen can be taken in and more carbon dioxide can be released. This cycle continues every time you breathe in and out.