Step-by-step explanation:
Oxygen is carried from the lungs to the tissues via the circulatory system. After we inhale, oxygen enters the lungs and diffuses across the thin walls of the air sacs (alveoli) and into the bloodstream. This process is facilitated by the high concentration gradient of oxygen between the air in the lungs and the blood in the capillaries surrounding the alveoli.
Once the oxygen is in the bloodstream, it binds to hemoglobin molecules in red blood cells, which act as a carrier. Hemoglobin is a protein that has four heme groups, each of which can bind to an oxygen molecule. In the lungs, each hemoglobin molecule can become fully saturated with oxygen (four oxygen molecules bound), giving the blood a bright red color.
The oxygen-rich blood then travels from the lungs to the heart, which pumps it out to the rest of the body via the arteries. As the blood flows through the capillaries in the body's tissues, the oxygen is released from the hemoglobin and diffuses into the cells, where it is used in cellular respiration to produce energy. The waste product, carbon dioxide, is then picked up by the blood and transported back to the lungs to be exhaled.