KS5 Biology
Lesson Plan 8 – Heart and Circulation
Science at Work in Healthcare
Post – 16 Science Education Pack
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.
Diagram 1. A plan of the circulatory system
KS5 Biology
Lesson Plan 8 – Heart and Circulation
Science at Work in Healthcare
Post – 16 Science Education Pack
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.
Diagram 2. The relationship between arteries, arterioles, capillaries, venules and veins
KS5 Biology
Lesson Plan 8 – Heart and Circulation
Science at Work in Healthcare
Post – 16 Science Education Pack
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.