Track the route of a RBC from the left brain to the left big toe.

The red blood cell leaves the left ventricle and enters the aorta. This red blood cell is heading for the right toe, so it is shunted to the thoracic aorta and then the abdominal aorta. At the hip level, this little red blood cell enters the right femoral artery and heads down the right leg via the right popliteal artery and then one of the many arteries that bifurcate from the that artery into the foot. The red blood cell enters a smaller arteriole which leads to the right big toe. In the big toe, the arteriole has narrowed to a capillary. In the arterial end of the capillary, the red blood cell gives off it's cargo of 02 by diffusion through the wall of the capillary into the tissue cells of the big toe. At the vein end of the capillary, the blood takes CO2 from the tissues of the toe, again by diffusion. Then our little red blood cell begins its return trip, first entering a venule which widens into a vein in the lower right leg, to the right popliteal vein and then the right femoral vein, which empties into the right inferior vena cava in the lower abdominal area. From there, the red blood cell continues being pushed upward through the inferior vena cava until it merges with the superior vena cava above the right atrium. From there, our red blood cell is pumped into the right ventricle, and then out through one of the pulmonary veins to the lungs, where the veins constrict to venules and then, at last, to a capillary running next to an alveolus in the lungs. Here, the blood surrenders its CO2 into the alveolus and picks up O2 from the alveolus, all by diffusion through the walls of the alveolus and the capillary. Now, our red blood cell is returned to the heart by pulmonary venules becoming one of the pulmonary veins (right or left depending in which lung our red blood cell was sent to), and the pulmonary veins enter the left atrium of the heart, where our red blood cell is sent to the left ventricle and our little red blood cell is ready to be sent where the blood current takes it next.

Journey complete!