What Is Blood Pressure?
Definition: Blood Pressure And Function

Blood Pressure

Contents

• What Is Blood Pressure?
• What Factors Produce And Maintain Blood Pressure?
• How Is It Measured?
• Causes And Effects Of High And Low Blood Pressure

Related Articles

Guide to Hypertension

What Is Blood Pressure?

When your heart beats it pumps blood around your body through a network of veins and arteries. As the blood flows it exerts a force against the walls of those vessels. Blood pressure is the measurement of the strength of this force. Without pressure, blood would not move at all, this makes it an important component of the circulatory system. Blood is always under pressure but the amount of pressure varies in different types of blood vessels: blood pressure in the arteries will be higher than in the veins and capillaries (the vessels furthest from the pumping action of the heart). While most healthy people experience episodes of high blood pressure - it is a normal response to exercise, stress, caffeine or certain medications - levels then return to normal. However, if blood pressure is consistently too high it puts a strain on the blood vessels and heart, which can eventually lead to a heart attack or stroke.

Hypertension is a term to define someone with consistently high blood pressure. The most common cause of hypertension is atherosclerosis - narrowing and hardening of the arteries due to the buildup of fatty cholesterol deposits. The more difficult it is for blood to flow through a vessel, the higher the pressure. If high blood pressure is not treated (by taking medications to reduce it), the blood vessel walls and the heart (because it has to pump harder) become permanently damaged.

What Factors Produce And Maintain Blood Pressure?

• Cardiac output: Determined by the volume of blood pumped out of the heart and the heart rate (how often the heart beats). If cardiac output increases (during exercise for example), blood pressure increases.
• Resistance offered by the arterioles (small blood vessels): vasoconstriction (narrowing of blood vessels caused by contraction of the vessel wall) is controlled by the vasomotor nerves and by adrenaline and noradrenaline. The greater the constriction the higher the blood pressure. Arterioles are constantly widening and squeezing, this helps to keep blood moving.
• Total blood volume: if the amount of circulating blood is reduced, blood pressure is lowered; if there is too much retention of fluid (edema), blood pressure is raised. During pregnancy blood volume increases by nearly 50 percent and the heart rate increases by 10 to 15 beats per second. See, hypertension during pregnancy.
• Viscosity (thickness) of blood: this depends partly on the plasma, especially the amount of plasma proteins and also on the number of erythrocytes (red blood cells). The lower the viscosity, the lower the blood pressure. See, what is blood?
• Elasticity of artery walls: if the arteries harden (atherosclerosis) there is a loss of elasticity and pressure is raised. If the arteries soften, there is lower pressure.

How Is It Measured?

Blood pressure is recorded as two numbers: the systolic pressure (when the heart contracts) over the diastolic pressure (when the heart relaxes/dilates after a beat). The measurement is written one above or before the other. The systolic number on top or first and the diastolic number on the bottom/second. For example, if your blood pressure measurement was 120/80 mmHg (millimeters of mercury), your doctor will say it is "120 over 80."

Normal blood pressure is less than 120 mmHg systolic and less than 80 mmHg diastolic. See blood pressure chart to discover what an unhealthy level is.

Related Articles
Blood pressure diagnosis: Diagnostic tests.
Blood pressure monitors: Useful devices for monitoring your own levels at home.
Why is high blood pressure dangerous?: What it does to your body.

Causes And Effects Of High And Low Blood Pressure

Hypertension (high blood pressure)
Causes of hypertension: stress, medication, kidney disease, narrowing or hardening of the arteries, smoking tobacco, excessive alcohol, unhealthy diet and hereditary factors.
Effects: angina, heart attack, strokes, kidney complaints.

Hypotension (low blood pressure)
Causes: Hypotension can be caused by underactive adrenal glands, hereditary factors; shock may cause short term hypotension.
Effects: dizziness; fainting.

Related Articles on Blood Pressure

For more on the heart and blood, see the following:

• Dangers of stress: What it can do to the body.
• Hypertension prevention: How to prevent high blood pressure.

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