Physiology and Hygiene for Secondary Schools by Francis M. Walters;A.M.
page 56 of 527 (10%)
page 56 of 527 (10%)
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body. The contraction of a muscle will, for example, close the small veins
in its vicinity and diminish the capacity of the larger ones. The natural tendency of such pressure is to empty the veins in two directionsâone in the same direction as the regular movement of the blood, but the other in the opposite direction. The valves by closing cause the contracting muscle to push the blood in one direction onlyâtoward the heart. The valves in the veins are, therefore, an economical device for _enabling variable pressure_ in different parts of the body _to assist in the circulation_. Veins like the inferior vena cava and the veins of the brain, which are not compressed by movements of the body, do not have valves. *Purposes of the Muscular Coat.*âThe muscular coat, which is thicker in the arteries than in the veins and is more marked in small arteries than in large ones, serves two important purposes. In the first place it, together with the elastic tissue, keeps the capacity of the blood vessels _equal to the volume of the blood_. Since the blood vessels are capable of holding more blood than may be present at a given time in the body, there is a liability of empty spaces occurring in these tubes. Such spaces would seriously interfere with the circulation, since the heart pressure could not then reach all parts of the blood stream. This is prevented by the contracted state, or "tone," of the blood vessels, due to the muscular coat. In the second place, the muscular coat serves the purpose of _regulating_ the amount of blood which any given organ or part of the body receives. This it does by varying the caliber of the arteries going to the organ in question. To increase the blood supply, the muscular coat relaxes. The arteries are then dilated by the blood pressure from within so as to let through a larger quantity of blood. To diminish the supply, the muscle contracts, making the caliber of the arteries less, so that less blood can |
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