Final answer:
RBCs move fastest in the aorta at 30 cm/sec and velocity decreases in arterioles and capillaries for exchange, then increases in venules and veins returning blood to the heart.
Step-by-step explanation:
Red Blood Cells (RBCs) move the fastest in the aorta and have a high concentration in both arterioles and venules. The speed of RBCs is greatest in the aorta where blood travels at 30 cm/sec, but it decreases significantly in the arterioles and capillaries to about 0.026 cm/sec to facilitate gas and nutrient exchange.
Then, as blood flows from the capillaries to venules and veins, its velocity increases again, though not as much as in the aorta, aiding the return of blood to the heart.