In all higher organisms the cardiovascular system represents an elaborated transport network, which is essential for the maintenance of vital functions by supplying oxygen and nutrients to tissue and removing by-products of metabolism. In order to adapt to the varying demands of the tissues, the circulatory system has evolved central and local control mechanisms that act in concert to maintain an adequate blood flow. At a given blood pressure, the blood flow to each organ is determined by the peripheral vascular resistance of this organ, which is adjusted by a variety of local mechanisms affecting the tone of the smooth muscle cells in the so-called resistance vessels, that is, small terminal arteries and large and small arterioles. Endothelial cells, situated at the interface between blood and the vessel wall, play a crucial role in controlling vascular tone and homeostasis, particularly in determining the expression of pro- and antiinflammatory genes. Many of these effects are mediated by the generation and release of endothelium-derived auta-coids, which are generally short-lived and locally acting vasoactive compounds. In vivo, endothelial cells are constantly subjected to mechanical stimulation by the viscous drag generated at the luminal surface of endothelial cells by the streaming blood (wall shear stress) and the cyclic strain of the vascular wall, which results from the pulsatile changes in blood pressure. Such stimuli not only determine acute autacoid production but can also affect the expression of autacoid-producing enzymes.
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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.