394 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the capillaries to the heart. It is apparent, therefore, that there is no direct 

 connection between the pressure and the speed of the blood at a given point, 

 inasmuch as they change together along the arteries and change inversely 

 along the veins. How varied the combinations may be of pressure and speed 

 will be seen in studying the regulation of the circulation. 



In the great veins, as in the arteries, the speed is very high compared with 

 the capillaries. In the capillaries the speed of the blood is least, while in 

 the tubes which supply and which drain them the speed is great. The physi- 

 ological value of these facts is clear. It has already been pointed out that the 

 blood moves slowly through the short and narrow tubes, where its exchanges 

 with tissue and with air are effected, and swiftly through the long tubes of 

 communication. What are the physical conditions which underlie these 

 physiological facts? 



The speed of the blood varies inversely as the collective sectional 

 area of its path. If the circulation in an animal continue uniform for a time 

 during several breaths and heart-beats it is evident that the forces con- 

 cerned must be so balanced that, during that time, equal quantities of blood 

 will have entered and left the heart, the arteries, the capillaries, and the veins, 

 respectively. If the arteries, for instance, lose more blood than the heart 

 transmits to them, this blood must accumulate in the veins till the arteries 

 become drained and the supply to the capillaries fails. The very maintenance 

 of a circulation, then, implies that equal quantities of blood must pass any 

 two points of the collective blood-path in equal times, except when a general 

 readjustment of the rate of flow may lead to a temporary disturbance of it. 

 It will be seen at once that this principle is consistent with the widest differ- 

 ences of rate between individual arteries of the same importance, or between 

 individual veins or capillaries. If in one artery the flow be increased by one- 

 half, and in another be diminished by one -half, the total flow in the two 

 arteries collectively will be the same as before. 



If the principle just stated be considered in connection with the anatomy 

 of the blood-path, the differences of speed in the arterial, capillary, and venous 

 systems will at once be understood. The wider arteries and veins are few. 

 Dissection shows that when an artery or vein divides, the calibre, and, with 

 the calibre, the "sectional area" of the branches taken together, is commonly 

 larger than that of the parent trunk. In general it is a law of the arterial 

 and venous anatomy that the collective sectional area of the vessels of either 

 system increases from the heart to the capillaries. The smaller the individual 

 vessels are, the wider is the blood-path which they make up collectively. 

 Widest of all is the blood-path where the individual vessels are smallest that 

 is, in the capillary system. The collective sectional area of the capillaries is 

 several hundred times that of the root of the aorta. The collective sectional 

 area of the veins which enter the right auricle is greater, perhaps twice as 

 great, as that of the root of the aorta. The venous system, regarded as a 

 single tube, is of much greater calibre than the arterial. It is perhaps better 

 to make these general statements than to compare the different figures given 



