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Key Stage 5 biology I Lesson Plan 8 – Heart and circulation Resource Sheet 8.3
Circulation
An average adult contains about 5 litres of blood which the heart pumps through
a network of blood vessels.
Blood is pushed out of the heart into the arteries. These divide up inside the
tissues of the body to form the much smaller capillaries. Unlike the arteries,
which are able to expand and contract because of their muscular walls, the
capillaries have very thin walls and a fixed diameter. It is through the capillary
walls that the exchange of materials between the blood and cells takes place.
From the capillaries the blood is collected by veins, which return it to the heart.
A plan of the main parts of this network is shown in diagram 1, along with
illustrations showing the differences between the main types of blood vessel.
head
pulmonary
artery
carotid arteries
lungs
pulmonary vein
venae cavae
(great veins)
heart
RV
pocket valves
open
dorsal aorta
LA
RA
LV
liver
pocket valves
closed
gut
hepatic portal vein
elastic fibres and
smooth muscle
rest of body
endothelium
thin endothelium
(one cell thick)
lumen
collagen fibres
vein
collagen
fibres
lumen
capillary
Diagram 1. A plan of the circulatory system
artery
elastic fibres
and
smooth muscle
Key Stage 5 biology I Lesson Plan 8 – Heart and circulation Resource Sheet 8.3
Arteries
Blood does not flow steadily through the arteries. As blood is only expelled from
the heart as it contracts (systole) the blood in the arteries moves in pulses. The
elastic, muscular walls of the arteries help to even out the blood flow so that
by the time it reaches the capillaries it is flowing smoothly. In terms of structure
the arteries have thick walls and a relatively narrow space through which blood
flows. This space is called the lumen.
Arterioles
Arterioles are small diameter blood vessels that branch out from arteries and lead
to the capillaries. They have thinner walls than arteries but are still muscular and
elastic.
Capillaries
The capillaries are very fine blood vessels, often only just wide enough for red
blood cells to move down in single file. Their walls consist of a single layer of
flattened cells called pavement endothelium. These thin walls optimise the
diffusion of dissolved substances into or out of the blood in the capillaries.
Blood flow through different networks of capillaries is controlled by sphincter
muscles at the point where the arterioles branch to form the capillaries. Some
networks will also have a blood vessel which allows blood to completely bypass
the capillary network and travel straight to the venule collecting blood from the
capillary network. These bypass vessels are called arteriovenous shunt vessels.
Arteriole
Artery
Capillaries
Venule
Tissue cells
Vein
Diagram 2. The relationship between arteries, arterioles, capillaries, venules and veins
Key Stage 5 biology I Lesson Plan 8 – Heart and circulation Venules
Venules are small blood vessels that lead from the capillary networks back to
the veins. Venule walls comprise a thin endothelium, a middle layer of muscular
tissue and an outer layer of collagen fibres.
Veins
Veins have thinner walls and larger lumen than arteries. Vein walls comprise a
lining of endothelial cells, a middle layer of muscular tissue and an outer layer
of collagen. The middle layer of muscular tissue is much thinner than that of an
artery. Veins also differ from arteries in having valves which prevent the backflow
of blood. These valves are necessary as the blood pressure in veins is much lower
than in arteries and blood in veins is also mostly having to flow against the force
of gravity.
Blood Pressure
Blood pressure generally refers to arterial pressure and is the force exerted by
circulating blood on the walls of blood vessels. It is considered to be one of the
principal vital signs.
Arterial pressure is usually measured with a sphygmomanometer, which uses the
height of a column of mercury to measure the circulating pressure. Even though
these mercury based devices are increasingly obsolete in the face of modern
electronic devices, blood pressure values are still recorded in millimetres of
mercury (mmHg).
Blood pressure values are given as two numbers. These refer to systolic pressure
and diastolic pressure. An example might be written as 120/80 mmHg, and spoken
as “one twenty over eighty”.
An individual’s blood pressure may vary for a wide range of reasons including in
response to stress, nutritional factors, drugs, or disease. In the UK blood pressure
is judged to be normal if it falls into the following bands:
• Systolic: 110-140 mmHg
• Diastolic: 70-90 mmHg
Hypertension refers to arterial pressure being significantly above these limits
and hypotension refers to it being significantly below these limits. Along with
body temperature, blood pressure is the most commonly measured physiological
parameter
Resource Sheet 8.3