Blood Pressure

Measurement of Blood Pressure

Blood pressure can be measured using an instrument called a sphygmomanometer.

A column of mercury is linked to an inflatable cuff which is wound around the upper arm. A stethoscope is then used to listen to the sounds of the blood in the brachial artery, at the bend of the elbow.

The sounds start at the systolic pressure:
(heart contraction => higher pressure)

and finish at the diastolic pressure:
(heart relaxation => lower pressure).

Hence blood pressure is expressed as :

"height of column of Hg at systolic pressure "
"height of column of Hg at diastolic pressure".

Normal Blood Pressure is about

mm Hg.


High Blood pressure is also known as 'hypertension'.

Low blood pressure is also known as 'hypotension'.

The following table summaries key causes, effects, and symptoms of 'high' and 'low' blood pressure:


Causes of Condition

Effects / Symptoms

High Blood Pressure

  • May be of unknown cause
    (essential hypertension, or hyperpiesia)
  • May result from kidney disease, including narrowing of the renal artery (renal hypertension)
  • Or endocrine diseases (such as Cushing's disease or phaeochromocytoma)
  • Or disease of the arteries (such as contraction of the aorta) - which is known as secondary, or symptomatic hypertension.

More general contributory factors are :

  • Stress, Obesity, Age, Social Class, Smoking, Lack of exercise, Poor diet.
  • Damage to arteries & veins.
  • Holes get blocked up by cholesterol.

Hypertension is symptom-less until the symptoms of its complications develop.

These include :

  • Atherosclerosis
  • Heart failure
  • Cerebral haemorrage
  • Kidney failure.

Low Blood Pressure

Can occur following:

  • Excessive fluid loss (e.g. through diarrhoea, burns or vomiting)
  • Severe blood loss (haemorrage) from any cause.

Other causes may include:

  • Myocardinal infarction
  • Pulmonary embolism
  • Severe infections
  • Allergic reactions
  • Arrhythmias
  • Acute abdominal conditions (e.g. pancreatitits)
  • Addisons disease
  • Drugs (e.g. an overdose of the drugs used to treat hypertension).

Temporary Hypotension:

  • Simple faint (syncope)
  • Light-headed
  • Sweats
  • Impaired conciousness

Severe Hypotension:

  • Peripheral circulatory failure (cardiogenic shock)
  • Unrecordable blood pressure
  • Weak pulses
  • Suppression of urine production

See also the structure and functions of blood, blood vessels, blood clotting, the structure of the heart, the functions of the heart, and systemic circulation.

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