Preksha Meditation & Human Health: 1.8 Blood Pressure

Blood pressure is the force the blood exerts against the walls of the blood vessels. Filtration in capillaries depends upon blood pressure; filtration brings nutrients to tissues, is the first step in the formation of urine. Blood pressure is one of the "vital signs" often measured, and indeed a normal blood pressure is essential to life. The pumping of the ventricles creates blood pressure, which is measured in mmHg (millimetres of mercury). When a systemic blood pressure reading is taken, two numbers are obtained: systolic and diastolic, as in 110/70 mmHg. Systolic pressure is always the higher of the two and represents the blood pressure when the left ventricle is contracting. The lower number is the diastolic pressure, when the left ventricle is relaxed and does not exert force. Diastolic pressure is maintained by the arteries and arterioles. Systemic blood pressure is highest in the aorta, which receives all of the blood pumped by the left ventricle. As blood travels farther away from the heart, blood pressure decreases. The brachial artery is most often used to take a blood pressure reading; here a normal systolic range is 90 to 120 mmHg, and a normal diastolic range is 60 to 80 mmHg. In the arterioles, blood pressure decreases further, and systolic and diastolic pressures merge into one pressure. At the arterial end of capillary networks, blood pressure is about 30 to 35 mmHg, decreasing to 12 to 15 mmHg at the venous end of capillaries. This is high enough to permit filtration but low enough to prevent rupture of the capillaries. As blood flows through veins, the pressure decreases further, and in the cava veins, blood pressure approaches zero as blood enters the right atrium. The upper limit of the normal blood pressure range is now 120/80 mmHg. The levels of 125 to 139/85 to 89 mmHg, once considered high-normal, are now called "prehypertension," that is, with the potential to become even higher (Scanlon, 2007). A systemic blood pressure consistently higher than the normal range is called hypertension. A lower than normal blood pressure is called hypotension.