2l6 



How a Complex Animal Uses Food unit iv 



Fig. 2i8 Taking blood pres- 

 sure. JVhat causes blood 

 pressure? Why should per- 

 sons who have abnory/ially 

 high blood pressure be care- 

 ful not to exercise stretiu- 

 ously? (encyclopaedia bri- 



TANNICA FILMS, INC.) 



These branches subdivide again and again 

 so that small arteries reach all parts of 

 the body. In the figure of the cat's in- 

 testine you see a small artery subdivided 

 into still smaller arteries. 



The walls of an artery are thick com- 

 pared with those of veins and especially 

 compared with the capillary wall. An 

 artery is lined inside with a thin and 

 very smooth membrane {serous mem- 

 brane) which obstructs the flow of 

 blood very little. Outside the inner mem- 

 brane is the elastic tissue. Outside the 

 elastic tissue lie rings of involuntary 

 muscle. See Figure 216. Nerve messages 

 that cause the muscles to contract make 

 the bore of the artery smaller; in other 

 words the artery can carry less blood. 

 On the other hand when these muscles 

 are completely relaxed the artery is a far 

 wider artery. In which condition is the 

 artery leading to your face when you 

 are blushing? In which condition, nor- 

 mally, would the artery to the small in- 

 testine be when digestion and absorption 

 are going on? Since these muscles in the 

 walls of arteries are involuntary, all the 

 changes in the size of arteries go on with- 



out conscious control and often without 

 your knowing it. 



Blood Pressure. When the heart pushes 

 blood into an artery it does so with great 

 force; the blood, in its turn, pushes 

 against the wall of the artery. The pres- 

 sure against the wall of the artery is very 

 great; if the wall were rigid and brittle 

 it might break. An elastic artery wall ex- 

 pands, however, thus reducing the pres- 

 sure on it. When we are young our 

 arteries are very elastic and our blood 

 pressure is said to be low. As a rule after 

 we are about forty years of age our ar- 

 teries slowly become less elastic and our 

 blood pressure grows greater. This is 

 normal; only unusually great increases in 

 blood pressure are dangerous. Physicians 

 measure blood pressure by using a device 

 that stops the flow of blood in an artery 

 by pressing against the artery wall. A 

 mercury gauge measures the pressure it 

 takes to press the walls of the artery to- 

 p-ether so that the flow of blood is 

 stopped; this indicates the pressure of the 

 blood against the artery wall. 



Of course, your blood pressure rises 

 when your heart beats harder. For this 



