The oxygen enters the bloodstream from the alveoli, which are tiny sacs in the lungs where gas exchange takes place. The transfer of oxygen into the blood is through simple diffusion. While oxygen moves from the capillaries and into body cells, carbon dioxide moves from the cells into the capillaries and starts with high concentration but fades to low concentration.
OXYGEN TRANSPORT IN BLOOD CAPILLARIES. In the capillaries, oxygen is transported within red cells through the solution of hemoglobin, then through the cell membrane and the blood plasma. Since cells and plasma are in motion, both convection and diffusion (free and facilitated) may be important.
