When doctors talk about
blood pressure, what they mean is the pressure within the large blood
vessels as your heart forces blood to circulate around your body. On
the whole, the lower your blood pressure, the better you are in the long
term (except in some very rare con-ditions in which excessively low blood
pressure is part of a disease).
Blood pressure and your circulation
Blood picks up oxygen in the lungs from the air that
we breathe in. This oxygenated blood enters the heart and is then pumped
out to all parts of the body in blood vessels called arteries. Larger
blood vessels branch into smaller and smaller ones and then to microscopic
arterioles, which eventually form tiny networks of blood vessels known
as capillaries. This network of larger arteries, medium-sized arterioles
and tiny capillaries allows blood to reach every cell of the body and
deposit its oxygen, which is used by cells to make the vital energy they
need to survive. Once the blood has deposited its oxygen in the cells,
the deoxygenated blood returns to the heart in veins, to be pumped back
up to the lungs to pick up more oxygen.
During each heartbeat, the heart muscle contracts to
push blood around the body. The pressure produced by the heart is highest
when it contracts, and this is known as the systolic (higher value) pressure.
Then the heart muscle relaxes before its next contraction, and the pressure
is at its lowest, which is known as the diastolic (lower value) pressure.
Both systolic and diastolic pressures are measured when you have your
blood pressure checked.
The dividing line between a normal and an abnormal
blood pressure is not easy to define. Perhaps the best definition is
that level of blood pressure above which treatment has been shown to
be worth while.
Diagram showing
the heart and circulation with veins (blue) draining the blood
back to the heart where it is pumped to the lungs and back to
the rest of the body through the arteries (red). Inset: diagram
of the capillary network in tissues such as skin or muscle, with
oxygen and other nutrients passing through the capillary walls
to the cells. |
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The pumping
cycle of the heart. |
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KEY
POINTS
- High blood pressure is caused by a narrowing of
the microscopic arterioles in all tissues
- Systolic pressure is the pressure in the larger
vessels when the heart contracts
- Diastolic pressure is the pressure when the heart
relaxes between beats
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