Introductory
Note: Knowledge of the structure and function of
blood vessels and other aspects of the heart and vascular
system are parts of training in many therapies, such
as Massage (incl. "Indian Head Massage", "Swedish
Massage", "Acupressure Massage" etc.),
Aromatherapy, Shiatsu, and others. This page is intended
to include the detail required for most Basic / First
Level Courses in these therapies, and some ITEC Diplomas.
The following diagram summarises the sequence
of blood flow through the heart, arteries,
arterioles, capillaries, venules, veins,
then back to the heart:
The following diagram summarises
the structural differences between different
types of blood vessels.
More information about this also follows
in the next section.
2. Structure and
Functions of Blood Vessels
Structure
Functions
Arteries
The walls (outer
structure) of arteries contain
smooth muscle fibre that contract
and relax under the instructions
of the sympathetic nervous system.
Transport
blood away from the heart;
Transport
oxygenated blood only
(except in the case of
the pulmonary artery).
Arterioles
Arterioles are
tiny branches of arteries that
lead to capillaries. These are
also under the control of the
sympathetic nervous system,
and constrict and dialate, to
regulate blood flow.
Transport
blood from arteries to
capillaries;
Arterioles
are the main regulators
of blood flow and pressure.
Capillaries
Capillaries
are tiny (extremely narrow)
blood vessels, of approximately
5-20 micro-metres
(one micro-metre = 0.000001metre)
diameter.
There are networks of capillaries
in most of the organs and tissues
of the body. These capillaries
are supplied with blood by arterioles
and drained by venules. Capillary
walls are only one cell thick
(see diagram), which permits
exchanges of material between
the contents of the capillary
and the surrounding tissue.
Function
is to supply tissues with
components of, and carried
by, the blood, and also
to remove waste from the
surrounding cells ...
as opposed to simply moving
the blood around the body
(in the case of other
blood vessels);
Exchange
of oxygen, carbon dioxide,
water, salts, etc., between
the blood and the surrounding
body tissues.
Venules
Venules are
minute vessels that drain blood
from capillaries and into veins.
Many venules unite to form a
vein.
Drains
blood from capillaries
into veins, for return
to the heart
Veins
The walls (outer
structure) of veins consist
of three layers of tissues that
are thinner and less elastic
than the corresponding layers
of aerteries.
Veins include valves that aid
the return of blood to the heart
by preventing blood from flowing
in the reverse direction.
Transport
blood towards the heart;
Transport
deoxygenated blood only
(except in the case of
the pulmonary vein).
3. Comparison
between Arteries and Veins
Arteries
Veins
Transport
blood away from the
heart;
Transport
blood towards the heart;
Carry Oxygenated
Blood
(except in the case of the
Pulmonary Artery);
Carry De-oxygenated
Blood
(except in the case of the
Pulmonary Vein);
Have relatively
narrow lumens (see
diagram above);
Have relatively
wide lumens (see diagram
above);
Have relatively
more muscle/elastic
tissue;
Have relatively
less muscle/elastic
tissue;
Transports
blood under higher
pressure (than veins);
Transports
blood under lower pressure
(than arteries);
Do not
have valves (except for
the semi-lunar valves of the
pulmonary artery and the aorta).
Have valves
throughout the main veins
of the body. These are to
prevent blood flowing in the
wrong direction, as this could
(in theory) return waste materials
to the tissues.
