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Blood Pressure
The pressure in blood vessels decreases as the distance from the heart
increases. The pressure starts high in the arteries leaving the heart, as
they are most affected by contraction of the ventricles. The left ventricle
with thicker muscle can contract with greater force than the thinner
muscle of the right ventricle. Consequently, the pressure is higher in the
aorta than in the pulmonary artery.
Diagram below shows blood pressure in blood vessels from the left and
right sides of the heart:
The effects of the ventricle contracting and relaxing, can be clearly seen
producing peaks and troughs in the pressure in the arteries. The pressure
remains high during diastole, due to the springing back of the elastic wall
of the arteries, in the elastic recoil mechanism. Moving away from the
heart, the pressure falls due to increasing distance from the heart and
increased friction. The pressure falls dramatically in the capillaries due
both to the above and to loss of fluid through the capillaries.
The main features of blood vessels are as follows:
-In arteries, blood flows in spurts or pulses because the contraction of the
ventricles is not continuous.
-A high-pressure pulse is caused by contraction of the ventricles and
blood is forced into the arteries.
-Lower pressure between pulses is due to the ventricles relaxing. The fall
in pressure is reduced by the elastic recoil mechanism.
-The greatest drop in blood pressure takes place in the arterioles because
of increased friction combined with increasing distance from the heart.
-Elastic tissue allows expansion of the arteries without bursting,
maintains high pressure and smooths out the flow.
-Smooth muscle allows vasoconstriction and vasodilation which can be
used to regulate the distribution of blood in temperature regulation and to
divert more blood to the muscles during exercise (by reducing blood to
the gut, for example).
-Capillaries are thin and permeable, allowing exchange of materials
between the cells and the blood.
-Veins contain a lot of blood in their large lumens and there is very low
pressure.
-Blood is pushed along in veins by contraction of the large muscles they
pass through, and semilunar valves keep the blood flowing in one
direction.
Blood pressure is a measure of the amount and the force of blood against
the walls of blood vessels. The pressure in the arteries needs to be at a
certain level to help to force some of the contents of the blood into the
tissue fluid. A weakened heart may produce a low blood pressure and
affect the composition of tissue fluid and the amount of substances
delivered to the cells of the body. High blood pressure is often produced
as a result of the build up of fat deposits in some of the arteries.
Therefore monitoring of blood pressure can give an indication of the
health of the circulatory system.
The pumping of the heart, combined with the narrowness of the smaller
blood vessels, produces a considerable pressure in the ARTERIES. This
is what is meant by BLOOD PRESSURE.
Blood only flows along the blood vessels because it is UNDER
PRESSURE.
Blood pressure is measured in millimetres of mercury (mmHg). Two
readings are taken:
(1) the pressure of the blood when the heart CONTRACTS
(SYSTOLIC BLOOD PRESSURE) and
(2) the pressure of the blood when the heart RELAXES
(DIASTOLIC BLOOD PRESSURE).
The normal systolic pressure of a young adult is about 120 mmHg;
normal diastolic pressure is about 75 mmHg.
Constant high blood pressure is HARMFUL. It makes the heart work
harder. Eventually the overworked heart may fail altogether. High blood
pressure can also damage the kidneys and eyes and increase the risk of an
artery tearing open. A stroke can occur when one of the arteries that
supplies blood to the brain ruptures.
Hypotension:
The technical term for low BP with dizziness and fainting episodes.
Many healthy people have a lower BP than average for people of their
age and have no symptoms. Others may have some underlying disorder
that causes hypotension. The most common type is postural hypotension
caused by suddenly sitting or standing up.
Hypertension:
Characterised by abnormally high BP at rest. The World Health
Organisation (WHO) defines hypertension as being consistently above
165/95 mm Hg, but many people are classed as mildly hypertensive if
their BP is over 140-160/90-95 mm Hg.
Summary
Changes in blood pressure
-Blood pressure (BP) is the force exerted by the blood on the walls of the
blood vessels.
-It is generated by the walls of the left ventricle during contraction.
-BP is measured in mmHg.
-At first sight, BP recordings look like a fraction (e.g. 120/80 mmHg, the
so-called average BP for a young adult). However, this is only a way of
displaying an upper systolic reading when the ventricles are contracting
and a lower diastolic reading when they are relaxed.
-The highest BP is found in large arteries close to the heart, such as the
aorta, the carotid arteries and the arteries in the arms.
-There is a gradual drop in BP as the blood is forced through the medium
and small arteries and the arterioles.
-The veins have little BP and blood has to be assisted back to the right
atrium by skeletal muscle pressure in the limbs and the presence of valves
to prevent backflow.
-Graph shows how BP varies in the circulation: