A human releases the co2 because it is the waste material of the organs in the body. the plants will get the co2 or carbondioxide because it is necessary in photosyntheisis. during photosynthesis, oxygen or o2 will be the waste product thus it must excrete from the plant. Now the body will get the o2 through breathing because it serves as food for the organs.
The answer you should have generated is about the human cardiorespiratory system.
Lets start with the lungs.
First,the blood reaches the lungs, from there,you inhale and ingest oxygen.The oxygen travels to the lung's alveoli and exchange gases with the blood.The oxygen is added to your blood and carbon dioxide is taken out for you to exhale.Again,blood travels around the body until it reaches to the lungs and the process is repeated.