This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The reason of the slower flow of the capillaries is that their united area is considerably greater than that of the arteries supplying them, so that the same quantity of blood flowing through them in a given time has a wider channel to flow in and therefore moves more slowly. The area of the veins is smaller than that of all the capillaries, but greater than that of the arteries, and so the rate of movement in them is intermediate.
We may picture to ourselves the vascular system as a double cone, widening from the ventricles to the capillaries, and narrowing from the latter to the auricles. Just as water forced in at a narrow end of this would flow quickest there, slowest at the widest part, and quicker again where it passed out the other narrow end, so the blood flows quick in the aorta and hollow veins,* and slow in the capillaries, which, though thousands of times smaller than the great arteries and veins, are millions of times more numerous. The channel through which the blood flows in them is, therefore, when they are all taken together, very much greater than that to which it is confined in the large arterial and venous trunks.
How may the arteries be recognized? In what are the smallest arteries seen to end? DO the capillaries vary much in size? What is the direction of flow in the veins? How does its rate differ from that in the arteries? From that in the capillaries?
Why does blood flow slowest through the capillaries? Why in the veins quicker than in the capillaries, but slower than in the arteries?
How may we picture the vascular system? Illustrate. How do capillaries differ in size from the large arteries?
*A good illustration taken from physical geography is afforded by the Lake of Geneva, in Switzerland. This is supplied at one end by a river which derives its water from the melting glaciers of some of the Alps. From its other end the water is carried off by the river Rhone. In the comparatively narrow inflowing and outflowing rivers the current is rapid; in the wide bed of the lake it is much slower.