Respiration is the transport of oxygen and carbon dioxide from the external environment (i.e., the air, or if you are a fish the water) into where it is utilized within cells for energy production (via oxidative phosphorylation). In the human body this process occurs through three major organ systems:
- Respiratory system — which transports oxygen from the air, into the lungs and then into the blood via the alveoli (air sacs). The reverse of this process also occurs in the lung, namely that the waste product carbon dioxide is taken from the blood into the alveoli of the lungs, returned to its natural gaseous state and expelled via the contraction of the diaphragm.
- Cardiovascular system — which transports blood around the body by means of the blood vessels and the pump at the centre of it: the heart.
- Haematologic system — which is what produces blood in the first place to serve (amongst other purposes) to ferry oxygen-rich haemoglobin throughout the body.