Hypertension is the medical term for persistent and sustained high blood pressure. It is one of the most (51) forms of heart disease. An understanding of blood pressure in (52) , healthy individuals is necessary in order to understand hypertension. Blood pressure is simply the force (53) against the walls of the body's arteries (血管) as blood flows through. The force, produced primarily by the pumping (54) of the heart, is essential for the (55) of the blood and its life-supporting nutrients(营养) to all parts of the body. Every time the heart (56) , called a systole, blood pressure increases. When the heart relaxes (57) beats(diastole), the pressure decreases. The normal systolic pressure of a person at vest is between 100 to 140, and the normal diastole is 60 to 90. Blood-pressure readings are expressed by both (58) , with the systolic over the diastolic: for example, 140/90. As these ranges suggest, there is a wide (59) of blood pressure for healthy people. Also, blood pressure (60) for healthy people during different times of the day and under different. (61) . It is lower when an individual is asleep than when he is excited or exerting himself physically. Therefore, a single blood-pressure reading above 140/90 does not (62) abnormality. When the pressure is continuously (63) , a person is considered to be suffering from hypertension. Hypertension is a stealthy(隐秘的) problem: a physician can discover it easily enough, but not a patient. (64) elevations, for example, may produce headaches, dizziness, fatigue, and weakness. But the patient is not likely to (65) these symptoms as having any other source than the increased elevation. (51)