THE BLOOD- VASCULAR SYSTEM. 



blood-vascular system comprises the heart and blood-vessels with their 

 J_ contained fluid, the blood. The composition of the blood and the minute 

 anatomy of the blood-vessels will be considered in the section on Histology. 



The Heart is the central organ of the entire 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 very minute vessels, 

 called arterioles, which in their turn open into a close-meshed network of micro- 

 scopic 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 

 again 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 a septum into two halves, right and left, each 

 half being further constricted into two cavities, the upper of the two being termed 

 the auricle and the lower the ventricle. The heart therefore consists of four 

 chambers or cavities, two forming the right half, the right auricle and right 

 ventricle, and two the left half, the left auricle and left ventricle. The right 

 half of the heart contains venous or impure blood ; the left, arterial or pure blood. 

 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 capil- 

 laries 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, and in doing so becomes changed 

 from arterial or pure blood into venous or impure blood, which is collected by the 

 veins and through them returned to the right auricle of the heart. From this 

 cavity the impure blood passes into the right ventricle, from which it is 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 auricle by the pulmonary 

 veins. From this cavity it passes into that of 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 intes- 

 tine is not returned directly from these organs to the heart, but is collected into 

 a large vein, termed the portal vein, by which it is carried to the liver. In the 

 liver this vein divides, after the manner of an artery, and ultimately ends in 

 capillary vessels, from which the rootlets of a series of veins, called the hepatic 

 reins, arise ; these carry the blood into the inferior vena cava, which conveys it 

 to the right auricle. 



From this it will be seen that the blood contained in the portal vein passes 

 through two sets of capillary vessels : (1) those in the spleen, pancreas, stomach, 

 etc., and (2) those in the liver. 



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