504 



ORGAN SYSTEMS OF MAN 



artery of a horse and noted that the blood 

 rose to a distance of over 9 feet. He also 

 observed that the blood rose and fell gently 

 in the tube with each heart beat. He there- 

 fore concluded that the pressure in the 

 artery was sufficient to maintain a column 

 of blood 9 feet high, and that the variation 

 in height was due to the contracting ventri- 

 cle. 



Blood pressure is measured even today in 

 a similar manner, although with more re- 

 fined instruments. For experimental pur- 

 poses, where blood pressure recordings are 

 desired over some period of time, the blood 

 from an artery is allowed to flow into a 

 tube fitted to a recording device (Fig. 

 19-12). Continuous recordings can be made 

 in this manner. This would be a rather for- 

 midable way to observe the blood pressure 

 of a patient in the physician's office and 

 there would be few who would submit to 

 such treatment. The years following Hales' 

 discovery led to much experimentation, and 

 before the turn of the nineteenth century, 

 other methods had been devised to obtain 

 blood pressure without entering an artery. 



The most successful method is univer- 

 sally employed today. It consists simply of 

 placing a rubber bag that can be inflated 

 around the upper arm where it will squeeze 

 the arm artery until no more blood can be 

 forced through it. The bag is attached to 

 a mercury manometer, an instrument that 

 records the pressure in millimeters of mer- 

 cury. Mercury is used because it is a heavy 

 fluid and changes in pressure can be re- 

 corded with a small instrument. Water 

 could be used but the instrument would 

 be very inconvenient because the tube 

 would have to be several feet long. Once 

 the blood is prevented from going through 

 the arm artery, the pressure in the cuff is 

 slowly released, barely allowing blood to 

 pass through at the highest pressure, called 

 systolic pressure because it is the point of 

 greatest force due to ventricular systole. 

 This can be heard through a stethoscope 

 placed over the artery at the elbow. As the 



pressure is released still further, more and 

 more blood flows through the artery and 

 the sound becomes louder and louder, sud- 

 denly falling off sharply. The pressure read- 

 ing just before this point is reached is re- 

 ferred to as the diastolic pressure, because 

 the blood is moving during the entire car- 

 diac cycle and it therefore records the 

 pressure which is maintained in the arteries 

 when the semilunar valves are closed. In 

 other words, it is the lowest pressure in the 

 arteries or when the heart is at rest. A great 

 deal of important information can be ob- 

 tained by the physician about the condi- 

 tion of the arteries and heart by taking 

 blood pressure readings, and they have be- 

 come a routine part of medical examina- 

 tions. 



THE BLOOD 



It is interesting to recall that not too 

 long ago it was considered sound medical 

 practice to withdraw blood (blood-letting) 

 from the veins during disease, while today 

 such a procedure would be considered 

 "fatal," if not for the patient, certainly for 

 the doctor who attended him! Medical 

 practice today is to conserve the patients' 

 blood, or even add to it by transfusion in 

 certain types of illness and in cases of seri- 

 ous injury. Many thousands of lives were 

 saved during the recent war because stored 

 blood could be given to injured men. 



In spite of earlier blood-letting customs, 

 blood has been held in high regard from 

 ancient times and today still plays a part 

 in rituals of many primitive tribes. Such 

 terms as "blood lines" in breeds of domestic 

 animals or "good or bad blood" or "blue 

 blood" denote the hereditary importance 

 that has been attached to blood. Despite 

 such common beliefs, it is now known defi- 

 nitely that there is no difference of this sort 

 either between the bloods of individuals or 

 of various races of man alive today. Any 

 efforts to perpetuate this fallacy are based 

 on emotion rather than fact. 



