Understanding Blood Pressure
Blood pressure is a vital indicator defined as the force that the flowing blood exerts on blood vessel walls. Generally, arterial pressure, referring to the pressure within larger arteries that carry blood away from the heart, is termed blood pressure. The most commonly used device for measuring arterial pressure is the sphygmomanometer, which originally measured blood pressure by the height of a mercury column. Even today, blood pressure readings are expressed in millimeters of mercury (mmHg), although mercury is no longer used in aneroid and electronic devices.
Diastolic arterial pressure is the lowest pressure during the resting part of the cardiac cycle, while systolic arterial pressure is the peak pressure occurring at the start of the cardiac cycle when the ventricles contract. Mean arterial pressure is the average pressure over the cardiac cycle, with pulse pressure representing the difference between the maximum and minimum pressures.
A typical resting, healthy adult's blood pressure is around 115 mmHg systolic and 75 mmHg diastolic (written as 115/75 mmHg and pronounced "one fifteen over seventy-five"). Arterial blood pressure measurements can exhibit significant variations. Blood pressure is dynamic, with natural fluctuations from one heartbeat to another and throughout the day (following a circadian rhythm). It also changes in response to stress, dietary factors, medications, or medical conditions. Hypertension refers to abnormally high arterial pressure, as opposed to hypotension, which indicates abnormally low pressure. Alongside body temperature, blood pressure measurements are among the most commonly assessed physiological parameters.
High Blood Pressure (Hypertension)
Arterial hypertension, in itself, is not typically an acute issue. However, due to its long-term indirect effects and its role as an indicator of other problems, it is a significant concern for diagnosing physicians.
Elevated pressures place mechanical stress on the arterial walls, leading to increased cardiac workload and the development of atheroma (abnormal tissue growth) in the artery walls. Higher pressure contributes to more stress and atheroma progression, which, in turn, results in the weakening, thickening, and enlargement of the heart muscle over time.
Chronic hypertension is the primary cause of chronic renal failure and a risk factor for heart attacks, strokes, heart failure, and arterial aneurysms. A mild increase in arterial pressure can also reduce one's lifespan. With severely high pressures, mean arterial pressures 50% or more above average, a person's life expectancy is limited to just a few years unless properly treated.
In the past, most attention was given to diastolic pressure, but it is now recognized that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are risk factors. In some cases, reducing excessive diastolic pressure can actually increase risk, likely due to the increased difference between systolic and diastolic pressures (refer to the article on pulse pressure).
Low Blood Pressure (Hypotension)
Low blood pressure, or hypotension, can be confused with hypertension due to similar pronunciation. Hypotension is a medical concern only when it leads to symptoms such as dizziness, fainting, or, in severe cases, shock.
When arterial pressure and blood flow fall below a certain threshold, perfusion to the brain is significantly reduced, resulting in lightheadedness, dizziness, weakness, or fainting. Nevertheless, individuals with consistently low arterial pressures who function well tend to have lower rates of cardiovascular disease events compared to those with normal arterial pressures.
Sometimes, arterial pressure drops significantly when a person stands up from a sitting position, known as orthostatic hypotension (postural hypotension). Gravity reduces the rate of blood return from the veins below the heart to the heart, decreasing stroke volume and cardiac output.
Healthy individuals typically experience rapid vein constriction below the heart and an increased heart rate to counteract the effects of gravity. This is an involuntary process regulated by the autonomic nervous system. It usually takes a few seconds to fully adjust, and inadequate or slow compensations may result in reduced blood flow to the brain, leading to dizziness and potential loss of consciousness. Increased G-forces, as experienced by acrobatic jet pilots during maneuvers, significantly exacerbate this effect. Reorienting the body perpendicular to gravity effectively eliminates the problem.
Other causes of low arterial pressure include:
- Hemorrhage - blood loss
- Toxins, including toxic doses of blood pressure medications
- Hormonal abnormalities, such as Addison's disease