It is an important diagnostic index for many reasons. Firstly, any condition that dilates or contracts the blood vessels or affects their elasticity, affects the blood pressure. Secondly, any disease of the heart that interferes with its pumping power, affects the blood pressure. In a healthy animal, the blood pressure normal for its species is maintained within a certain range with great constancy. Thus if blood pressure is abnormally low or high, it usually indicates that greater health problems are present.
Blood pressure is measured at two point, a high point and a low point.
The high point is the point at which the heart contracts to empty its blood into the circulation, called systole. The low point is the point at which the heart relaxes to fill with blood returned by the circulation, called diastole. In humans, blood pressure doesn't vary greatly. Blood pressure increases with age because the arteries loose elasticity. This is called secondary hypertension.