Oxygen is transported throughout the body via hemoglobin in the red blood cells.
Oxygen transport and exchange provide the necessary oxygen to the body. This process is called respiration. Air is breathed in through the mouth and nose and after various processes it enters the blood through diffusion. The oxygen enters the red blood cells and in exchange carbon dioxide is taken out.
Oxygen levels in the blood are directly related to the alveoli. The alveoli increase the surface area of the lungs. For example, if a person has pulmonary disease then it means they have a decreased surface area for oxygen exchange. This in turn would mean lower blood oxygen levels.