Arteriolar tone refers to the degree of contraction of smooth muscle cells that wrap around these microvessels. Smooth muscle contractile activity (i.e., arteriolar tone) determines the diameter of the arteriolar lumen, which, in turn, sets the hydraulic resistance of these microvessels to blood flow. Thus, arteriolar tone importantly contributes to regulation of systemic vascular resistance and blood pressure; the distribution of blood flow to and within the body's tissues and organs; and regulation of solute and water exchange in downstream capillaries and venules. Arteriolar smooth muscle cells integrate a myriad of excitatory and inhibitory stimuli from blood pressure, circulating hormones, neurotransmitters, local metabolites, and endothelium-derived factors to determine arteriolar tone, and hence arteriolar diameter and resistance. Ion channels, proteins that form ion-selective, water-filled pores in the plasma membrane of cells, in both smooth muscle and endothelial cells, play a central role in determining and regulating arteriolar tone. Ion channels provide the major source of intracellular Ca2+ that activates contractile proteins in smooth muscle and stimulates the synthesis and release of vasoactive substances from endothelial cells. In addition, ion channels significantly contribute to the determination and regulation of membrane potential. Membrane potential, in turn, modulates Ca2+ influx through ion channels, the release of Ca2+ from intracellular stores, and cell Ca2+ sensitivity. Ion channel-mediated changes in membrane potential also are transmitted along microvessels to modulate arteriolar tone at distant sites. Thus, ion channels participate in all aspects of regulation of arteriolar tone.
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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.