322 TEXT-BOOK OF PHYSIOLOGY. 



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 waste products. The blood at the same time changes 

 to a variable extent in chemic composition; because of the loss of oxygen and 

 the gain of carbon dioxid it also changes in color from red to bluish-red. 



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 physical 

 properties and physiologic functions 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 

 serve to collect the blood from the capillary areas and return it to the right side 

 of the heart. As they emerge from the capillary areas the veins, which in 

 these regions are termed venules, are quite small. By their convergence 

 and union the veins gradually increase in size in passing from the periphery 

 toward the heart. Their walls at the same time correspondingly increase 

 in thickness. The veins from the lower extremities, the trunk, and abdom- 

 inal 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 media, and the tunica adventitia. The media, 

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

 the artery, but a larger amount 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 the heart, especially after a temporary obstruction. 



Veins are distinguished by the presence of valves throughout their course. 

 These are arranged in pairs and formed by a reduplication of the internal 

 coat, strengthened by fibrous tissue. They are always directed toward the 

 heart and in close relation to the walls of the veins, so long as the blood is 

 flowing forward. An obstruction to the flow causes the valves to turn 

 backward until they meet in the middle line, when they act as a barrier 

 to regurgitation. Under these circumstances the elastic tissue permits the 

 veins to distend and accommodate the blood. With the removal of the 

 obstruction the recoil of the elastic tissue, and perhaps the contraction of the 

 muscle-tissue, forces the blood quickly onward. 



HYDRODYNAMIC CONSIDERATIONS. 



The blood flows through the arteries, capillaries and veins in accordance 

 with definite laws. During its transit certain phenomena are presented by 



