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The Systemic Circulation

blood, capillaries, veins, arteries, left and body

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THE SYSTEMIC CIRCULATION.

The circulation of the blood throughout the whole body is similar to the circulation through the lungs already described (p. 302). The former is called the systemic or great circulation, the latter the pulmonary or lesser circulation.

The blood which fills the left ventricle is, by contraction, forced into the aorta, which is al ready filled with blood, so that to make room for the extra quantity the vessel must distend, and the blood already in it must be forced on wards. As this is happening many times in a minute the blood in the aorta, is continually being forced into its branches. Now, as we have seen, it gives off branches to every part of the body, to head and neck, to the upper limbs, to the chest and the organs in it, to the organs contained within the belly, and to the lower limbs. So that, to every region of the body, from the crown of the head to the sole of the foot, blood is being carried by arteries. As these arteries penetrate into the various regions and organs of the body they give off branches, which are continually becoming smaller, till they become microscopic in size and are found penetrating every part. The smallest arteries end in thin-walled capillaries, which form such a close net-work that a pin cannot be passed into a tissue without opening into some of them. The arteries, even the smallest of them, are simply tubes for conducting the blood to its destination; but the capillaries are something more, for their walls are so thin that, though continuous, they permit fluid portions of the blood to ooze through to bathe the tissues that surround them. The blood in the capillaries is, therefore, in communication with the tissues surrounding them, and exchanges material with them even as it flows through them. At length the capillaries begin to join together to form larger and larger vessels, and so gradually veins are formed, at first microscopic in size, but gradually the blood passes into larger veins formed by the junction of smaller ones and by the addition of other coats, till the large veins of the particular limb or organ are reached.

These join the veins coming from other limbs and organs, until the two large venous trunks are formed, the superior and inferior velum cavoc, the one corning from the upper and the other from the lower parts of the body, which carry the blood to the right auricle. From the right auricle it passes to the right ventricle, then through the lungs by the pulmonary artery and its branches and back to the heart, but to the left side, by the pulmonary veins, as already described. By passing into the left ventricle the blood has completed its circuit of the body and lungs.

Thus blood which has issued from the left ventricle passes through two sets of capillaries before it returns to the left ventricle—the ca pillaries of the tissue which it is sent to nourish, and the capillaries of the lung. The blood which is sent to the stomach, bowels, spleen, and pan creas has, however, an unusually long circuit. After passing through the arteries of these organs, traversing their capillaries, and enter ing their veins, the blood reaches the portal vein, formed, as we have seen (p. 200), by the junction of the veins of the organs mentioned. The blood then passes through the capillaries of the portal vein in the liver and enters the hepatic vein, and so on to the inferior versa cava. Thus blood sent to the stomach and bowels, spleen and pancreas, traverses three sets of capillaries, those of the particular organ, stomach or spleen, &c., those of the portal system in the liver, and those of the lung. This is the longest route a portion of blood may take from the moment it, leaves the left ventricle to the moment it returns to it. The shortest possible route is through the. substance of the heart itself. A portion of blood entering the aorta, and imme diately passing off by the coronary arteries (p. 302), will merely traverse the capillaries in the substance of the heart and return to the right auricle by the coronary vein, completing its circuit by passing through the lungs.

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