Anatomy of the Human Body - Book V Angiologyby Henry Gray
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THE VASCULAR system is divided for descriptive purposes into (a) the blood vascular system, which comprises the heart and bloodvessels for the circulation of the blood; and (b) the lymph vascular system, consisting of lymph glands and lymphatic vessels, through which a colorless fluid, the lymph, circulates. It must be noted, however, that the two systems communicate with each other and are intimately associated developmentally.
The heart is the central organ of the blood vascular system, and consists of a hollow muscle; by its contraction the blood is pumped to all parts of the body through a complicated series of tubes, termed arteries. The arteries undergo enormous ramification in their course throughout the body, and end in minute vessels, called arterioles, which in their turn open into a close-meshed network of microscopic vessels, termed capillaries. After the blood has passed through the capillaries it is collected into a series of larger vessels, called veins, by which it is returned to the heart. The passage of the blood through the heart and blood-vessels constitutes what is termed the circulation of the blood, of which the following is an outline.
The human heart is divided by septa into right and left halves, and each half is further divided into two cavities, an upper termed the atrium and a lower the ventricle. The heart therefore consists of four chambers, two, the right atrium and right ventricle, forming the right half, and two, the left atrium and left ventricle the left half. The right half of the heart contains venous or impure blood; the left, arterial or pure blood. The atria are receiving chambers, and the ventricles distributing ones. From the cavity of the left ventricle the pure blood is carried into a large artery, the aorta, through the numerous branches of which it is distributed to all parts of the body, with the exception of the lungs. In its passage through the capillaries of the body the blood gives up to the tissues the materials necessary for their growth and nourishment, and at the same time receives from the tissues the waste products resulting from their metabolism. In doing so it is changed from arterial into venous blood, which is collected by the veins and through them returned to the right atrium of the heart. From this cavity the impure blood passes into the right ventricle, and is thence conveyed through the pulmonary arteries to the lungs. In the capillaries of the lungs it again becomes arterialized, and is then carried to the left atrium by the pulmonary veins. From the left atrium it passes into the left ventricle, from which the cycle once more begins.
The course of the blood from the left ventricle through the body generally to the right side of the heart constitutes the greater or systemic circulation, while its passage from the right ventricle through the lungs to the left side of the heart is termed the lesser or pulmonary circulation.
It is necessary, however, to state that the blood which circulates through the spleen, pancreas, stomach, small intestine, and the greater part of the large intestine is not returned directly from these organs to the heart, but is conveyed by the portal vein to the liver. In the liver this vein divides, like an artery, and ultimately ends in capillary-like vessels (sinusoids), from which the rootlets of a series of veins, called the hepatic veins, arise; these carry the blood into the inferior vena cava, whence it is conveyed to the right atrium. From this it will be seen that the blood contained in the portal vein passes through two sets of vessels: (1) the capillaries in the spleen, pancreas, stomach, etc., and (2) the sinusoids in the liver. The blood in the portal vein carries certain of the products of digestion: the carbohydrates, which are mostly taken up by the liver cells and stored as glycogen, and the protein products which remain in solution and are carried into the general circulation to the various tissues and organs of the body.
Speaking generally, the arteries may be said to contain pure and the veins impure blood. This is true of the systemic, but not of the pulmonary vessels, since it has been seen that the impure blood is conveyed from the heart to the lungs by the pulmonary arteries, and the pure blood returned from the lungs to the heart by the pulmonary veins. Arteries, therefore, must be defined as vessels which convey blood from the heart, and veins as vessels which return blood to the heart.
Structure of Arteries.—The arteries are composed of three coats: an internal or endothelial coat (tunica intima of Kölliker); a middle or muscular coat (tunica media); and an external or connective-tissue coat (tunica adventitia). The two inner coats together are very easily separated from the external, as by the ordinary operation of tying a ligature around an artery.
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