120 READINGS IN BIOLOGICAL SCIENCE 



SO much to the thinner walls of the veins as to the low pressure of the blood 

 within. If we measure the average pressure of the blood in successive re- 

 gions of the vascular tree, we find that there is a continuous decrease from 

 the heart through the arterial and capillary and venous regions. In the 

 aorta close to the heart the average pressure is always highest. It is lower 

 in the arterioles and capillaries and still lower in the veins. The lowest 

 pressure is in the veins closest to the right auricle. At this point the pres- 

 sure is about at atmospheric pressure or (in mammals) even lower. The 

 rate at which blood escapes from a hole in a vessel demonstrates these 

 differences very well; it spurts rapidly from an artery and flows much 

 more slowly from capillaries or veins. Even large vessels in the neck near 

 the heart may bleed very little through a small hole. 



The heart is capable of adjusting its output to the rate of activity of 

 the body. When more blood is required, mechanisms are automatically 

 set into operation which increase the rate and strength of the heart beat 

 and hasten the rate of the circulation. But note that this mechanism alone 

 would increase the rate of blood flow to all the organs simultaneously. 

 It allows for no differentially greater flow to one organ or system than to 

 another. If the heart pumps more blood, all organs share in the general 

 increased blood flow. Not only can the output of the heart change, but 

 also there are mechanisms which change the distribution of the blood 

 to the various organs differentially, in accordance with their varying rates 

 of metabolic activity. 



Disorders of the blood vessels are fairly common and their effects are 

 to be understood on the basis of the disarrangements of blood-vessel physi- 

 ology which they entail. Some blood-vessel defects have already been re- 

 ferred to in connection with intra-vascular clotting of blood. Plugging 

 of an infected blood vessel with a thrombus often serves the useful end 

 of decreasing the bursting of the damaged vessel, but it also may produce 

 serious damage or death. Plugging of vessels to parts of the brain or to 

 the heart may be suddenly fatal. 



The effects of occlusion depend, first, on the importance of the organ 

 whose vessel is occluded and, second, on whether or not other vessels also 

 carry blood to that organ, as is generally the case. 



Allied in their effects to those of complete obliteration of vessels are 

 conditions which abnormally narrow the caliber of vessels. This may be 

 in the nature of a more or less localized blood-vessel spasm, or a thicken- 

 ing and hardening of the walls of the arterioles and arteries. Again, the 

 effects depend upon what blood vessels are involved. Hardening (and nar- 

 rowing^) of the arteries of the brain may so interfere with proper nutri- 

 tion of that organ as to cause serious mental derangements. Involvement 

 of the kidney vessels may so damage those organs as to make impossible 

 the proper elimination of wastes. 



Hardening of the arteries is essentially a disease of the old. It seems in 



