Blood pressure, or more accurately, arterial blood pressure is, literally, the pressure within the arteries produced by the beating of the heart and maintained by the elasticity of the arterial walls and the aortic valve.

There are two parts to the blood pressure, which are the systolic and diastolic blood pressures. The systolic blood pressure is the direct pressure created by the beating of the heart which is transmitted throughout the body by the fluid that is the blood. The diastolic blood pressure is the pressure that the arterial blood manages to fall to before the next heartbeat. Blood pressure is thus often quoted as x/y mmHg where x is the systolic blood pressure and y is the diastolic pressure and the unit of measurement is the height of milimeters of mercury.

The importance of blood pressure

Blood pressure is a function both of cardiac output and of peripheral vascular resistance. There is a good analogy with electrical circuits - if you think of the heart as providing the potential difference and the arterial system as providing the resistance to flow, the blood pressure is the current in the circuit. No current flow equals the lightbulb going out.

It is the blood pressure that maintains tissue perfusion. A minimum blood pressure is needed to keep the vital organs well perfused and viable. In fact, the body seems designed around maintaining blood pressure to vital organs.

If someone is running away from a would be attacker, his/her adrenal glands would be pouring out adrenaline - this has the effect of causing vasoconstriction in such temporarily unnecessary parts of your body as the gastrointestinal tract (no point digesting food if survival is at stake), vasodilatation in the muscles (better get moving) and causing the heart to beat like crazy (keep the blood flowing).

In a person who is suffering from severe blood loss, the body's response is to shut down peripheral circulation (cold and clammy hands), conserving the remaining blood to perfuse the brain, heart and kidneys while the heart beats faster to keep the blood pressure up.

How to measure blood pressure

Blood pressure is measured using a sphygmomanometer which is basically a cuff that goes around the arm which is attached to a pressure reading (which is traditionally a actual reservoir of mercury and a vertical tube in which the mercury can rise but can be a dial or digital nowadays) and which can be pumped up. Choose a cuff size that: i) fits on the patient's upper arm and ii) the length of which is at least as long as the width of the patient's arm.

The proper way to take a resting blood pressure is by having the patient either sitting down or lying in bed. Place the cuff around the patient's upper arm above the elbow - the patient should place his/her arm so that the area cuffed is approximately at the same level as his/her heart. Then, with a hand palpating the patient's radial pulse on the same side as the cuff, commence inflating the cuff, while keeping an eye on the pressure reading. When the sensation of the patient's pulse just ends as the pressure rises is approximately the systolic blood pressure. Confirm by pumping the pressure slightly higher and letting the pressure fall, feeling carefully for the return of the pulse. With this number in mind, release the pressure and place a stethoscope in the patient's cubital fossa (the inside of their elbow). Now commence inflating the cuff again to just above the previously obtained systolic blood pressure by palpation. Release the pressure slowly and listen carefully for when there is a beating sound. The pressure at the first sound heard is the systolic blood pressure by auscultation. Continue allowing the pressure to subside slowly. Listen carefully and note when the pressure sounds become weaker and then disappear altogether. The pressure at which the sounds is just barely audible before disappearing is the diastolic blood pressure.

The sounds heard while measuring blood pressure are called Korotkoff sounds.

Hypertension is defined as a blood pressure at rest that is consistently above 140mmHg systolic and/or above 90mmHg diastolic. The reason that hypertension is treated is that it is associated with an increased risk of cardiac disease and stroke (amongst other things). Particularly high blood pressures which are difficult to control with drugs is malignant hypertension, for which further investigations must be performed as there is some underlying pathology.

Hypotension, on the other hand, is when the blood pressure falls sufficiently for arterial perfusion to the brain and organs to suffer. It is a little more difficult to define exactly what is hypotension as what may be low for some people may well be normal for others - for instance, a blood pressure of 90/60mmHg may be symptomatic for me but is entirely normal for someone I know. Most people, however, will be symptomatic if their systolic blood pressure falls below 80mmHg. Almost everyone will be very sick if their systolic blood pressure falls below 60mmHg.

A patient who has the symptoms of shock with a very low blood pressure is a condition that is a medical emergency. Urgent medical attention is necessary.

Exhaustive knowledge on blood pressure would take more space than is available in this node.