THE CIRCULATION OF THE BLOOD 33 i 



rial. Though extremely short, the capillaries divide and subdivide a number 

 of times, forming meshes or networks, the closeness and general arrange- 

 ment of which vary in different localities. 



As the endothelial cells are living structures and characterized by irrita- 

 bility, contractility and tonicity, it may be assumed that the capillary wall as 

 a whole is characterized by the same properties. Upon the possession of 

 these properties the functions of the capillary depend. 



The function of the capillary wall is to permit of a passage of the nutritive 

 materials of the blood into the surrounding tissue spaces and of waste 

 products from the tissue spaces into the blood. The structure of the capil- 

 lary wall is well adapted for this purpose. Composed as it is of but a single 

 layer of endothelial cells, the thickness of which defies accurate measurement, 

 it readily permits, under certain conditions, of the necessary exchange of 

 materials between the blood and the tissues. The forces which are con- 

 cerned in the passage of materials across the capillary wall are embraced 

 under the terms diffusion, osmosis, and filtration. As a result of the inter- 

 change of materials the tissues are provided with nutritive materials and re- 

 lieved of the presence of the products of metabolism. As the blood loses 

 oxygen and gains carbon dioxid, it changes in color from a scarlet red to a 

 bluish red. In consequence of the exchange of materials, the blood under- 

 goes a change in composition, the extent and character of which varies in 

 accordance with the activities and character of the organ traversed by it. 



In order that the nutritive materials may pass through the capillary 

 wall in amounts sufficient to maintain the necessary supply of lymph in the 

 lymph or tissue spaces, it is essential that the blood shall flow into and out 

 of the capillary vessels constantly and equably, in volumes varying with the 

 activities of the tissues, under a given pressure and with a definite velocity. 

 These conditions are made possible by the cooperation of the physiologic 

 properties and physiologic actions of the heart and vascular apparatus, the 

 nature of which will be explained in subsequent pages. 



The Structure, Properties, and Functions of the Veins. The veins 

 arise from the distal side of the capillary vessels. As they emerge they are 

 quite small and designated venules. By their convergence and union the 

 the veins gradually increase in size in passing from the periphery toward 

 the heart. Their walls at the same time correspondingly increase in thick- 

 ness. The veins from the lower extremities, the trunk, and abdominal 

 organs finally terminate in the inferior vena cava. The veins from the head 

 and upper extremities terminate in the superior vena cava. -Both venae 

 cavae empty into the right auricle. 



A typical vein consists of the same three coats as the artery: viz., the 

 tunica intima, the tunica meditkantii the tunica adventitia. The media, 

 however, does not possess as much of either the elastic or muscle tissue as 

 the artery, but a larger am&unt of the fibrous tissue. Hence they readily 

 collapse when empty. In Virtue of their structure the veins also possess 

 both elasticity and contractility, ; though in'a far less degree than the arteries. 

 These properties come into play and are of value in furthering the movement 

 of the blood toward thtHieart, especially after a temporary obstruction. 



Certain veins, especially the large veins of the extremities, are character- 

 ized by the presence of valves throughout their course. These are arranged 

 in pairs and formed by a reduplication of the internal coat, strengthened 



