Mechanical and Chemical Regulation of Arterial and Venous Specification - Mechanical and Chemical Signaling in Angiogenesis - page 2

1 Introduction
Blood circulates throughout the body. This was first clearly shown by a series of simple, but
ingenious, experiments performed by William Harvey in the early seventeenth century [ 1 ].
This finding has lead to the establishment of the modern concept of ''artery and vein''.
The presence of the vascular system and its relationship to a variety of organs
had already been documented by careful and impressive histological analyses of
developing embryos by Aristotle in 300s BC. He had already speculated inter-
dependence of the vascular system and organs in their establishment. However,
he believed that pulsations in the vascular system were due to respiration, and the
heart simply moved as a result of the same breathing process.
In the Middle Ages and early Renaissance, the concept based on Galenic
physiology that had begun in the second century remained prevalent. According to
this ''ancient'' understanding of the arteries and veins, all veins emanate from the
liver and deliver fluids (blood) that maintain and nourish the body, and all the arteries
from the heart disseminated vitality in the form of spiritus throughout the body.
By the end of fifteenth century, the Italian physician Jacopo Berengario da
Carpi proposed a putative cross-talk between arteries and veins. He stated that
''no artery without its vein to accompany it. Thus the artery may keep the vein
alive, and the vein may give blood to the artery in its needs, the blood by which the
vital spirit is made and the artery itself is nourished.'' This notion was followed by
Leonardo da Vinci who stated: ''All the veins and arteries arise from the heart. The
reason for this is that the maximum thickness found in the veins and the arteries
occurs at the junction which they make with the heart. The more removed they are
from the heart, the thinner they become and divide into smaller branches''.
By early sixteenth century, many anatomists already knew that the walls of the
arteries are thicker than those of veins, but they attributed this difference to the
nature of the substances that passed through each of them. Therefore, arteries and
veins were still treated as two separate systems.
However, in the early seventeenth century, William Harvey unambiguously
showed that the blood is delivered to the organ and then return to the heart, thus
the establishment of the concept that arteries and veins belong to the single
circulatory system [ 1 ]. He defined artery as the ''vessel'' that delivers the blood
from the heart to peripheral organs, and vein as the ''channel'' through which the
blood returns from the periphery to the heart.
2 Mechanical Basis of Arterial and Venous Specification
(Specification by Mechanical Signals)
Due to their locations relative to the heart, arterial and venous lumens are exposed
to differential hemodynamic forces. Two classes of hemodynamic forces are known
to modulate structure and function of blood vessels: shear stress and pressure.