Measurements made after injection of flow tracers (in particular microspheres) whose distribution to the various organs mirrors organ blood flows indicate that in humans the BM blood flow is approximately 10 mL/min/100 g tissue; the hematopoietic BM receives therefore approximately 2 to 4 percent of the cardiac output. Similar values have been found in animals.
Various factors regulate the BM blood flow: cytokines conveyed by the bloodstream such as erythropoietin (Epo) or secreted locally such as G-CSF, endothelium-derived vasoactive substances such as nitric oxide (NO), neurotrans-mitters, and neuropeptides released by efferent nerve terminals or by vascular or hematopoietic cells.
BM blood flow increased threefold in rats with 50 percent blood loss and in rats supplemented with high doses of Epo. The administration of G-CSF to rats also caused a twofold increase in the BM blood flow. Following injection of either cytokine, cardiac output, mean arterial blood pressure, and ventilation rate remained unchanged, which excludes a generalized systemic effect of the cytokines on the vasculature. The increase in blood flow after Epo was found not only in BM, but also in bone. The increase in blood flow after G-CSF was restricted to the BM; blood flows in nonhematopoietic organs (muscle, small bowel, skin, kidneys), in spleen, and even in the bone itself were unchanged. Epo and G-CSF appear therefore to cause different perfusion patterns, with different effects on the blood supplies of the nutrient artery versus the muscular arteries, as well as on venous collection. Inhibition of NO formation in BM greatly decreased the increase in blood flow observed after bleeding or administration of G-CSF, indicating that the cytokines may induce the synthesis and/or release of the vasodilatory substance by ECs. NO may then affect the tone of nearby VSMCs.
Nerve fibers are observed in BM of humans or rodents, most conspicuously in association with the vasculature. In the mouse, approximately 92 percent of the efferent nerve terminals with many synaptic vesicles have been found nearby arteries and arterioles (in contact with VSMCs or periarterial adventitial cells), while only 3 percent were in contact with sinuses and 5 percent within the hematopoietic parenchyma. In humans, approximately 75 percent of the arteries and arterioles and 6 percent of the capillaries are associated to nerve fibers. In spite of so widespread a BM innervation, whether there is neuronal control of BM blood flow remains controversial; noticeably, the increase in BM blood flow induced by bleeding or administration of G-CSF has not been modified by denervation.
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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.