The following paragraph written by a Malaysian student studying in America aptly captures the concept of regio-specific heterogeneity, the topic of the present chapter:

America is not homogeneous, neither is its heterogeneity immediately apparent. Rather, as so many of my American friends have been so keen to enlighten me, American people differ from state to state. The mainstream American culture of fast-food chains, shopping malls and pop music is there for the world to see. But it is the only uniting factor across their vast homeland. A person from Iowa is vastly different from a person from California. Even in Virginia alone, those who lived in the northern part of the state near Washington, DC are totally different from those who grew up in central Virginia where Sweet Briar is located. Taken as a whole, America is bursting with ethnic and cultural diversity, but in reality, it is small pockets of homogeneity that combine to form a picture of unparalleled heterogeneity.1

The vascular endothelium lines the blood vessels of the body, and in humans, the estimated number of individual cells constituting this lining is somewhere on the order of 1-6 x 1013. Like Americans, the mainstream "endothelial culture" of blood, tissue, and nutrients is there for all to note. These uniting factors do not, however, reveal the vast differences of specific circulations [1, 2]. Several vascular human diseases are exquisitely restricted to specific types of vessels. Vasculitis often shows a marked predilection for specific arteries, veins, or capillaries; tumor cells may metastasize to selective vascular beds, and atherosclerosis is generally restricted to the larger arteries.

Although scientists initially thought the endothelial cell layer was essentially an inert barrier between the blood and

1 Thoughtfully penned by a Citizen of the World;

the tissue, this extensive "organ" is now known to carry out a diverse array of specialized functions which can vary markedly from one organ to another. Today the endothelium is viewed as a dynamic, mutable, heterogeneous, distributed organ with essential secretory, synthetic, metabolic, and immunologic functions. The picture of "unparalleled heterogeneity" is revealed as follows.

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Essentials of Human Physiology

Essentials of Human Physiology

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.

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