(American Scientist, Volume 85, p234 )  Because of its cross-sectional area, the carotid-artery sinus tends to fall between two regimes of fluid flow. Even in healthy individuals, a standing vortex, or recirculation zone, that could play a

role in the initiation of atherosclerotic disease, may form in the sinus.This zone shows up clearly in the image above. In the work of Mineo Motomiya and Takeshi Karino the filming of tracer particles reveals streamlines (blue) that diverge at the bifurcation and form spiral secondary flows. ( Adapted from Motomiya and Karino 1984.)

( A FIST SIZED PUMP, THE BODY p86 )

The endless circling of the blood depends upon one tireless muscle: the heart (above). Though it is only about the size of the fist and weighs a half to three quarters of a pound, the heart does enough work each day to lift the body a mile straight up. In this frontal view of the organ, an injection pigment gives color to the web of veins, arteries and capillaries that serve the heart.

(Figure 2.2:1, Blood flow through the heart.

The Arrows show the direction of blood flow. The symbols are SVC, superior vena cava; IVC,inferior vena cava; RA, right atrium; RV, right ventricle; PA pulmonary artery; LV,left ventricle. The valves are T, tricuspid, P, pulmonary, AO aortic, M ,mitral. From Folkow and Neil (1971) Circulation, Oxford Univ. Press, New York, p153.