. Adventures with animals and plants. Biology. 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 capil
. Adventures with animals and plants. Biology. 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 blo
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