II – For
a better understanding of blood pressure
Description
of the arteries and the blood pressure
2.1 - Working
of the blood circulation: the vessel and the heart
2.2 - Definition of the blood pressure
Who
is concerned with high blood pressure?
2.3 - Epidemiological
data
2.4 - Influence of sex
2.5 - Race
The risk is raised by the existence of other
diseases
2.6 - Cholesterol
2.7 - Diabetes
2.8 - Tobacco
2.9 - The age
2.2 -
Definition of the blood pressure
The purpose of the blood
pressure is to push blood and the red corpuscles to the tissues of
the organism so that they can perform their function: to oxygenate
organs. At the tissue level, blood pressure is very low and the speed
of the blood has been slowed down so that oxygen can pass to the
organs more efficiently.
The blood pressure is defined
as the product between the multiplication of the resistance of the
vessel in which there is the blood and the cardiac output.
Blood
pressure = Resistance of the vessels x cardiac output
The resistance of the vessel corresponds to the rigidity of the wall of the
artery, which can vary in many circumstances.
The cardiac output corresponds
to the capacity of the heart to pump blood at each contraction (or
systole), thus corresponding to a volume of blood pumped per minute
(approximately 5 litres per minute).
The resistance of the vessel
may rise for instance when the nervous system increases its tonicity
or when age causes a thickening of the wall.
In the same way, when the
heart beats in a more energetic way, the cardiac output raises thus
making the blood pressure increase.
- Fluctuation of the
blood pressure
- According to the localization
The more the blood moves
away from the heart in the arteries the less significant the blood
pressure will be. The reason is that the oxygen carried along by
the blood must have enough time to oxygenate the organs.
In practice, it was noted
that the blood pressure measured at the arm is close to the one measured
in the aorta. But the blood pressure measured at the wrist seems
to be less reliable.
- According to time
The blood pressure fluctuates
in the short, middle or long-term.
In fact, there is a cycle
of blood pressure over the life span. It is higher for older subjects
than for young subjects. Thus, the age is a significant factor of
the blood pressure variation.
But the blood pressure
also varies according to seasons, since it is lower in summer than
in winter.
Variation
of the systolic blood pressure during the year
The blood pressure also
varies over 24 hours; it is higher in working life, i.e. the day,
than in period of sleep. A recording of the blood pressure during
24 hours, with an ambulatory blood pressure monitor, provides this
data.
Lastly,
the blood pressure fluctuates in the short-term according to our
emotions, the respiratory rate and our nervous commands, which automatically
control the blood pressure.
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last modified on : 28 sep 2003 |
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