126 READINGS IN BIOLOGICAL SCIENCE 



family. These bacteria are much more likely to gain a foothold if rheu- 

 matic heart disease or a congenital defect or some other abnormal condi- 

 tion already exists. The rare acute form is caused by any one of several 

 different kinds of bacteria which may enter the blood stream and attack 

 the heart in the course of an illness elsewhere in the body — for example, 

 pneumonia or meningitis. In the more com.mon subacute form, Strep- 

 tococcus viridans (the green streptococcus) is usually responsible. This 

 germ hides and multiplies in blood-clot nests in the endocardium. With 

 the sulfa drugs and penicillin at the physician's disposal, the outlook for 

 the control of bacterial endocarditis is much more hopeful than it was a 

 very few years ago. 



The Middle-Aged Heart 



HEART DISEASE ASSOCIATED WITH HIGH BLOOD PRESSURE 



High blood pressure, or hypertension, is the most common cause of 

 heart disease in middle age. What hypertension is and why it causes heart 

 disease are known. But what causes hypertension itself is still a puzzling 

 question. Many cases of hypertension are associated with disease of the 

 kidneys (renal hypertension) or with a disease or functional disturbance 

 of the nervous system or endocrine glands. The majority of cases of hy- 

 pertension, however, are labeled "Cause Unknown." 



High blood pressure which develops without any discoverable cause 

 is called essential hypertension. It seems to run in some families, many 

 members of which through several generations have had essential hyper- 

 tension or troubles associated with it. Also it appears to be most common 

 among people who are overweight. 



What ''Blood Pressure'^ Is. Everyone has blood pressure. It is simply 

 the pressure of the blood against the walls of the arteries which are always 

 completely filled with blood. Everyone's blood pressure goes up and down. 

 It is highest during systole, the period when the heart pumps a fresh load 

 of blood into the elastic-walled arteries which stretch to accommodate it, 

 and lowest during diastole, the period when the heart pauses between 

 beats to fill with blood. High blood pressure is commonly taken to mean 

 high systolic pressure. However, the diastolic pressure is fundamentally 

 the more important of the two, because it represents the basic pressure 

 exerted on the arterial walls independently of the additional pressure due 

 to the contraction of the heart. The physician also attaches great impor- 

 tance to the relationship between the systolic and diastolic pressures. The 

 difference between the two is called the pulse pressure. 



The second factor which makes everyone's blood pressure normally an 

 up-and-down affair is the way the arterioles behave during emotional 

 stress. These tiny blood vessels are the smallest branches of the arteries. 

 They are controlled by nerves which automatically make them constrict 



