Continuity And Consequences In Nature

(SUPPLY ON DEMAND, THE BODY p48)

The arteries (red ) and veins ( blue ) of muscles doing energy-consuming work, such as javelin throwing, may receive up to nine times more blood than when at rest. Muscles in arterial walls, on signals from the nervous system, can contract so that the flow goes where needed.

 

(American Scientist, Volume 85,p 225)

The carotid-artery bifurcation, like most arterial bifurcations, is a common site of atherosclerotic disease. The common carotid artery divides in the neck into the external and internal carotid arteries. The external carotid supplies the face, scalp, neck and throat tissues with freshly oxygenated blood from the heart, and the internal carotid supplies the anterior portion of the brain. Immediately above the bifurcation, there is a sinus , or swelling in the internal carotid that plays an important role in its hemodynamics. In the color Doppler ultrasound closeup (center) and magnetic-resonance image (right) of a normal carotid in the area of the bifurcation, the area denoted by the blue color and small arrow, respectively, is an area of low-velocity blood flow. The Doppler image was created by ultrasonic waves reflected from the blood; the apparent change in frequency of theses waves can be used to derive the velocity of the fluid from which they were reflected. (From Wolf et al. 1992)

(American Scientist, Volume 85,p 230)  Dominant factor in the initiation of atherosclerotic lesions is thought to be the shear stress that flowing blood imposes on a vessel wall. The shear stress depends on the blood's velocity: The faster the blood is flowing,

the greater the the wall shear stress. At a bifurcation, the velocity profile is skewed toward the inner walls of the bifurcation, which are subjected to higher wall shear stresses. Velocity measurements are graphed against axes representing uniform velocity. These flow patterns were obtained by Mineo Motomiya and Takeshi Karino of the McGill University Medical Clinic by filming the motion of polystyrene microspheres in an isolated (postmortem) human carotid artery rendered transparent by chemical treatment (Adapted from Motomiya and Karino 1984.)