Blood Pressure is a measurement of the pressure of the blood flowing against the walls of the arteries. Arterial blood pressure is constantly changing during the course of the cardiac cycle. The highest pressure in the cycle is called the systolic blood pressure, the lowest is the diastolic blood pressure ; the both readings are necessary to enable you to evaluate the status of your blood pressure. Many factors such as physical activity, anxiety, or the time of day, can influence your blood pressure. Blood pressure is typically low in the morning but high from afternoon to evening. It is lower in the summer but higher in the winter.