The Blood Pressure reading is not exactly a fraction. It is more accurately looked at as a ratio. But there isn't really a lot of significance to that either.
The top number is the systolic pressure. Systolic pressure is the pressure the blood exerts on artery walls during each ventricular contraction.
The bottom number is the diastolic pressure. Diastolic pressure is the pressure the blood exerts on artery walls between ventricular contractions. Since the blood is between pulses, it has less pressure, hence the number is lower.
The force of the blood against the walls of the blood vessels is known as blood pressure. The arterial blood pressure undergoes a rhythmic change reaching a maximum during ventricular systole when blood is pumped into the arteries and decreasing during ventricular diastole. The former is called systolic pressure and the latter, the diastolic pressure. The normal systolic pressure in man and in most other mammals ranges from about 120 to 140mm of mercury, whereas the diastolic pressure ranges from about 80 to 90mm of mercury. However, blood pressure is affected considerably by numerous factors including age, sex, heredity, and other physical and emotional states.