Blood pressure is the
pressure exerted by blood on the walls of the vessels through which it
circulates in the body. The heart pumps
oxygenated blood around the body, via lungs to be re-oxygenated.
There are three main
factors that determine blood pressure:
- the amount of blood in the circulatory system
- the cardiac output, that is the amount of blood pumped by the heart per
minute
- the level of resistance of blood vessels to blood flow.
Our autonomic nervous system (ANS) regulates pumping and so
the rate at which blood is pumped automatically responds to daily activities,
increasing to cope with exercise or stress and decreasing at rest. The heart responds to a drop in blood pressure
by pumping blood at a faster rate.
The circulatory system
can hold large amounts of blood and the capacity of blood vessels - such as
arteries and veins - is determined by their tone or tension. High tone increases resistance so that less
blood can squeeze through, while a lower tone allows the vessels to relax and
so more blood can flow. The distribution
of blood throughout the body alters to respond to activities and so allows more
to the areas that need it. For example
when we exercise the tone of vessels around the working muscles is reduced to
allow more blood to enter and release oxygen to the muscles. Similarly the digestive system vessels relax
when a meal is eaten so that more blood can flow to help with digestion.
Blood pressure is usually measured at the upper
arm using the brachial artery and is generally given as a reading of two
figures e.g. 120/80, the first representing the systolic pressure and the second the diastolic pressure. Blood pressure will vary throughout the course
of the day and the normal range varies from one person to another.
