When William Harvey boldly challenged centuries of medical orthodoxy in 1628 by proclaiming that blood circulated throughout the body, he still had trouble explaining one critical concept: how arteries actually connected to veins. For with no way to magnify the vessels, all the great experimenter could conjure up was the old idea that there must be some sort of vascular pores arterial blood seeped through . The Italian anatomist, Marcello Malpighi, was therefore quite surprised just 30 years later when he trained the lens of his new-found microscope onto the fine vascular architecture of a frog's lung and found not pores, as he had expected, but rather an entirely new class of veritable microvessels—which became known as "capillaries"—as the definitive connection shuttling blood between arteries and veins .
For more than 300 years, scientists have understood capillaries to be just as Malpighi described them that day in Bologna—as a third class of blood vessels distinct in form and function from their arterial and venous counterparts. However, recent research on a novel class of cell-signaling molecules known as ephrins, some of which reliably mark either arteries or veins on a molecular level, suggests that this traditional view of capillaries may no longer be sufficient, as capillary vessels seem to exhibit clear arterial-venous identities of their own. These identities are defined by the differential expression of members of the Ephrin-B family down to the level of the smallest microvessels, demonstrating that the microvasculature, far from just being a homogenous collection of nonarterial, nonvenous capillaries linking arterioles to venules, consists rather of two discrete arterial and venous continuums: the arterial microcirculation and the venous microcirculation. These new observations of arterial and venous microcirculatory identities likely reflect previously uncharacterized genetic, morphologic, and functional differences within the microvasculature, and should provide a novel conceptual framework for the study of vascular biology and the treatment of angiogenesis-dependent disease.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.