ANSWER:The respiratory and the cardiovascular (organ) system allows for oxygen transportation to cells in the body.
EXPLANATION: Air (oxygen) is breathed in through the nose - it passes through the pharynx and larynx to trachea that branches off into the primary bronchi which branches off further into tertiary bronchi which makes up the lungs. At the ends of the bronchi are sacs called areoli and there are tiny capillary blood vessels - this is where gas exchange happens - oxygen passes through areoli to blood in capiliaries where blood becomes oxygen rich. The blood is returned to the heart through atrium into left ventricle where it is pumped out via the aorta. The aorta also branches off into three arteries: brachiochephalic artery, left common carotid artery and the left subclavian artery - which supplies nutrient rich blood to right and left upper limbs and the brain. So the arteries takes oxygen rich blood away from the heart and delivers it to the organs and cells of the body. The veins deliver oxygen poor blood back to the heart through the vena cava and superior vena cava to the right atrium and ventricle of the heart where it is pumped into the pulmonary arteries and delivered to the lungs where gas exchange happens again and the cycle continues. It would be worth noting that it happens simultaneously rather than one at a time as described above.
Other than breathing, how does oxygen enter the body?