. Physiology, experimental and descriptive . ry possi-ble way. It cannot go back, for it fills the pockets of thesemilunar valves, and closes them with a click. A rapidwave is sent forward that gives the pulse, and a slower butstill rapid stream flows along thearteries, through the pulmonaryartery to the lungs, and throughthe aorta and its branches to allthe other parts of the body. The elastic reaction of the ar-teries thus makes steady the flowof blood which is intermittent as itleaves the heart. Action of the Medium-SizedArteries. — Each organ requiresa supply of blood in proportion toits a
. Physiology, experimental and descriptive . ry possi-ble way. It cannot go back, for it fills the pockets of thesemilunar valves, and closes them with a click. A rapidwave is sent forward that gives the pulse, and a slower butstill rapid stream flows along thearteries, through the pulmonaryartery to the lungs, and throughthe aorta and its branches to allthe other parts of the body. The elastic reaction of the ar-teries thus makes steady the flowof blood which is intermittent as itleaves the heart. Action of the Medium-SizedArteries. — Each organ requiresa supply of blood in proportion toits activity. An actively working organ, like the brain, demands much more blood than bonepractically inactive. Further, working tissues, such as thebrain and muscles, need a great deal more blood while theyare at work than when they are resting. An organ needinga constant large supply of blood might secure this by having alarge artery. But how can the supply be regulated so that anorgan may receive, now more, now less, according to its needs?. Isolated Fibers Fibers Joined Fig. 24. Plain (Unstriated) Mus-cular Fibers from the Bladder. ACTION OF SHALL ARTERIES. 77 This is regulated by the medium-sized arteries leading tothe parts. In the walls of the small and medium-sized arte-ries are muscle fibers of a different kind from those of theskeleton. These fibers are spindle-shaped, as shown in Fig. 24,and do not have the cross-markings of the fibers of the skele-tal muscles; they are in consequence called Non-striated, Smooth Connective Tissue Endothelium
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