Blood Pressure

What is Blood Pressure?

Blood pressure is the force or pressure which the blood exerted on the walls of the blood vessels. It is expressed as force per unit area of the vessel. The systemic arterial blood pressure, usually called simply arterial blood pressure. The arterial blood pressure is the result of the discharge of blood from the left ventricle into the already full aorta.

It helps to drive blood at a high velocity along the arteries in the closed circulatory system. During the peak period of ventricular contraction, pressure in the arteries reaches a maximum and this is called systolic blood pressure (SBP). During the relaxation of the ventricle, blood pressure falls down. It reaches a minimum just before the beginning of the next systole. This minimum is called diastolic blood pressure (DBF). In a cardiac cycle of average duration, the systole lasts for about 0.3 sec and the diastole 0.5 sec. Therefore in a subject, having a cardiac cycle of 0.8 sec duration, there will be SBP and DBF 120 and 80 mm Hg respectively. The difference between the systolic pressure and the diastolic pressure is called pulse pressure.

How to measure blood pressure?

Blood Pressure is determined by sphygmomanometer, an instrument devised by Riva-Rocci, and subsequently improved by Von Recklinghausen. Sphygmomanometer is fundamentally a mercury manometer, but instead of the classical U-tube of the manometer, one of its limbs is long and the other is very short, that acts as a reservoir of mercury. The reservoir, via a rubber tube, is connected with an arm cuff. The cuff, in turn, via rubber tube is connected with a hand pump. Air can be introduced into the cuff by pumping the hand pump, a process called 'inflation'. Air from the cuff can be driven out by unscrewing the pump called deflation.
The physician or medical person measures the blood pressure during two phases in the heart's cycle. They first measures systolic pressure, which is the peak pressure exerted on arterial walls when the left ventricle of the heart contracts in the process of pumping out blood. Then, they checks diastolic pressure, which is the reduced pressure felt just before the next beat, when the heart is relaxed and blood is flowing into it. The two readings are recorded on blood pressure chart as a division, the systolic over the diastolic.

Blood pressure measuring process

At the time of the measurement of Blood Pressure, a cuff is wrapped around the persons upper arm, and then air is pumped into the cuff until its pressure against the arm is sufficient to stop the flow of blood in the main artery. Then, listening with a stethoscope held over the artery just below the cuff, the tester releases air gradually until he hears the pulse resume. At this moment the pressure of the air in the cuff is slightly less than that of the blood in the artery, and the reading that shows up on the gauge is the systolic pressure. The tester then releases still more air, stopping as soon as no more sounds can be heard. The reading on the scale is the diastolic pressure.

Blood Pressure Regulation

On several factors such as heart rate, strength of heart beat, blood output and resistance to blood by the blood vessels, the blood pressure depends. Resistance to blood flow is altered by contraction or relaxation of the smooth muscle in the blood vessel walls, especially those of the arterioles. Increased peripheral resistance causes a rise in blood pressure, whereas a decrease, a fall in blood pressure. All these activities are regulated by the vasomotor centre of the medulla. Pressoreceptors, located on the walls of carotid and aortic sinuses, regulate vasomotor centre activity. Stimulation of parasympathetic fibers in these areas produces vasodilatation, resulting in the reduction in blood pressure. The opposite occurs when blood pressure is low. Many other factors like emotional stress, excitement, pain, irritation, etc. increase sympathetic activity and therefore blood pressure. Some chemical substances such as epinephrine, histamine, and acetylcholine act directly on the smooth muscle of arterioles or on the vasomotor centre, resulting in alterations in blood pressure.

Abnormal Blood Pressure

Continued rise of blood pressure above 140/90 mm of Hg is known as hypertension. Blood pressure below 100/60 mm of Hg in an adult signifies hypotension. Hypertension occurs in about 10 per cent of middle-aged people. Hypertension encourages hardening of the arteries. It can lead to heart failure, coronary artery disease, and strokes. A chronic vasoconstriction of arterioles may also cause hypertension because the narrower the arterioles, the higher is the resistance against the blood flow and, consequently, higher is the blood pressure.
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