THE CIRCULATORY SYSTEM: THE COMMON CARRIER FOR THE BODY 87 



heavier and more muscular than the walls of the veins. In both, a smooth, 

 thin lining of epithelial cells (endothelium) is surrounded by several 

 layers of smooth muscle cells and connective tissue. The walls of the 

 capillaries lack the smooth muscle and connective tissue layers and con- 

 sist of a single layer of thin endothelial cells. 



The great arteries that leave the heart, the aorta and pulmonary artery, 

 branch into successively smaller ones, so that all parts of the body are 

 supplied with an abundance of small arteries. These continue to branch 

 until they pass into the enormous network of capillaries. The latter, after 

 a short course through the tissues, unite to form small veinlets. These, in 

 turn, unite to form larger and larger veins and finally pour their blood 

 into the great veins that lead back to the heart — the pulmonary veins 

 from the kings, entering the left auricle, and the two venae cavae, entering 

 the right auricle. The veins of the lower part of the body empty into the 

 inferior vena cava; those of the head, neck, and arms into the superior 

 vena cava. 



The capillaries have the task of bringing the blood into the immediate 

 vicinity of the cells, and it is essential that no cell be far removed from 

 a capillary. For this condition to exist it is necessary that the capillaries 

 be exceedingly numerous. It has been calculated that the capillaries in 

 the human body, if placed end to end, would form a tube 62,000 miles 

 long. Since each capillary is only about a millimeter in length, the number 

 of capillaries is beyond comprehension. To illustrate the closeness of the 

 spacing of cells and capillaries, it may be noted that there are about 2,500 

 capillaries in a cross section of 1 sq. mm. of skeletal muscle, or about 

 1,562,500 per square inch. The total surface of the 62,000-mile capillary 

 tube mentioned above is about 67,000 square feet, or 1J^ acres; and it 

 has been calculated that 1 cc. of blood is exposed to a capillary surface of 

 7,300 sq. mm., or 8 square feet. The cross-sectional area of the total 

 capillary network is estimated as 400 to 1,000 times that of the aorta, and 

 considering the relatively small amount of blood in the entire vascular 

 system, it is evident that all these capillaries cannot be filled at the same 

 time. As is mentioned below in connection with the lymphatic system, 

 only a part of the capillaries of a resting organ contain blood at any one 

 time, but the entire capillary system of the organ may become filled dur- 

 ing periods of activity. 



FUNCTIONAL DIVISIONS OF THE CIRCULATORY SYSTEM 



The Pulmonary Circulation. In man, as in all the higher vertebrates, 

 the circulatory system is double. The right auricle and ventricle receive 

 only the dark venous blood that has returned to the heart from all the 

 tissues of the body except the lungs. When the heart contracts, the blood 

 from the right ventricle is forced into the pulmonary artery, which leads 



