Oxygen from the atmosphere enters the bloodstream through a process called gas exchange in the lungs. Step 1: Inhalation When you inhale, air from the atmosphere travels down your trachea, through the bronchi and bronchioles, and finally reaches tiny air sacs in your lungs called alveoli. Step 2: Diffusion at the Alveoli The walls of the alveoli are extremely thin, only one cell thick, and are surrounded by a dense network of equally thin blood vessels called capillaries. Oxygen in the inhaled air has a higher partial pressure in the alveoli than in the deoxygenated blood flowing through the capillaries. Step 3: Movement into Blood Due to this difference in partial pressure, oxygen diffuses across the thin alveolar and capillary walls, moving from the alveoli into the blood plasma and then into the red blood cells. Step 4: Binding to Hemoglobin Inside the red blood cells, oxygen quickly binds to a protein called hemoglobin, forming oxyhemoglobin. This allows the blood to transport a large amount of oxygen efficiently to the body's tissues.