The biological problem has to do with the flow of blood through blood vessels, like capillaries, veins and arteries. The blood flow is greatest along the central axis of the tube and decreases as the distance r from the axis increases until v (the velocity of the blood) becomes zero at the wall. The central axis is the middle of the tube (as if a straight line had been drawn down the center of the tube.) The blood flow decreases as it moves farther from the central axis toward the walls of the tube, because of the friction of the walls of the tube. The letter r stands for the distance from the central axis to the wall of the blood vessel.
