Anatomy of the circulation
• The
blood reaching the skin originates from deep vessels.
• These
then feed interconnecting vessels which supply the vascular plexuses of
fascia, subcutaneous tissue and skin.
Deep vessels
The deep vessels arise from the aorta and divide to form the
main arterial supply
to the head, neck, trunk and limbs.
Interconnecting vessels
The interconnecting system is composed of:
• Fasciocutaneous
(or septocutaneous) perforating vessels
• These
vessels reach the skin by traversing fascial septae.
• They
provide the main arterial supply to the skin in the limbs.
• Musculocutaneous
vessels
• These
vessels reach the skin via direct muscular branches from the deep
system.
• These
branches enter muscle bellies and divide into multiple perforating
branches, which travel up to the skin.
• The
musculocutaneous system provides the main arterial supply to the skin of
the torso.
Vascular plexuses of fascia,
subcutaneous tissue and skin
Vascular plexuses of the fascia, subcutaneous tissue and skin
are divided into six layers.
1 Subfascial
plexus
• A small plexus lying on the undersurface of the fascia.
2 Prefascial
plexus
• A larger plexus particularly prominent on the limbs.
• Predominantly supplied by fasciocutaneous vessels.
3 Subcutaneous
plexus
• Lies at the level of the superficial fascia.
• Mainly supplied by musculocutaneous vessels.
• Predominant on the torso.
4 Subdermal
plexus
• Receives blood from the underlying plexuses.
• The main plexus supplying blood to the skin.
• Represented by dermal bleeding observed in incised skin.
5 Dermal
plexus
• Mainly composed of arterioles.
• Plays an important role in thermoregulation.
6 Subepidermal
plexus
• Contains small vessels without muscle in their walls.
• Has a predominantly nutritive and thermoregulatory function.
Angiosomes
• An angiosome is a composite block of tissue supplied by a
named artery.
• The area of skin supplied by an artery was first studied by
Manchot in 1889.
• His work was expanded by Salmon in the early 1930s, and more
recently by
Taylor and Palmer.
• The anatomical territory of an artery is the area in which the
vessel branches
ramify before anastomosing with adjacent vessels.
• The dynamic territory of an artery is the area into which staining
extends after
intravascular infusion of fluorescein.
• The potential territory of an artery is the area that can be
included in a flap if it is
delayed.
• The vessels that pass between anatomical territories are
called choke vessels.
The transverse rectus abdominis muscle (TRAM) flap illustrates
the angiosome
concept well.
• Zone 1
• This receives musculocutaneous perforators from the deep
inferior epigastric
artery (DIEA) and is therefore in its anatomical territory.
• Zones
2 and 3
• There is some controversy as to which of the following zones
is 2 and which
is 3; the numbers of these zones are interchanged in various
texts.
• The portion of skin lateral to zone 1 is in the anatomical
territory of the
superficial circumflex iliac artery (SCIA). Blood has to travel
through a set of
choke vessels to reach it from the ipsilateral DIEA.
1•
The portion of skin on the other side of the linea alba is in the anatomical
area
of the contralateral DIEA. This area is reliably perfused in a
TRAM flap based on
the contralateral DIEA and is therefore within its dynamic
territory.
• Zone
4
• This lies furthest from the pedicle and is in the anatomical
territory of
the contralateral SCIA.
• Blood passing from the flap pedicle to zone 4 has to cross two
sets of choke vessels.
• This portion of the TRAM flap has the worst blood supply and
for this reason
it is often discarded.
Arterial characteristics
From his detailed anatomical dissections Taylor made the
following observations:
1 Vessels
usually travel with nerves.
2 Vessels
obey the law of equilibriumaif
one is small, its neighbour will tend to be
large.
3 Vessels
travel from fixed to mobile tissue.
4 Vessels
have a fixed destination but a varied origin.
5 Vessel
size and orientation is a product of growth.
The microcirculation
• Terminal
arterioles are present in the reticular dermis and terminate as they enter
the capillary network.
• The
precapillary sphincter is the last part of the arterial tree containing muscle
within its wall. It is under neural control and regulates the
blood flow into the
capillary network.
• Arteriovenous
anastomoses (AVAs) connect the arterioles to the efferent veins.
• Blood
flowing through AVAs bypasses the capillary bed and has a thermoregulatory
rather than nutritive function.
• AVAs
are of two types:
1 Indirect
AVAs are convoluted structures known as glomera and are densely
innervated by autonomic nerves.
2 Direct
AVAs are much less convoluted and have a sparser autonomic supply.
• The
blood supply to the skin far exceeds its nutritive requirementsamuch of it
bypasses the capillary beds via the AVAs and has a primarily
thermoregulatory function.
Control of blood flow
The
muscular tone of vessels is controlled by the following factors.
Pressure
of the blood within vessels (myogenic theory)
•
The myogenic theory was originally described by Bayliss and states that:
•
Increased intraluminal pressure results in constriction of vessels.
•
Decreased intraluminal pressure results in their dilatation.
•
This mechanism helps to keep blood flow constant and is the cause of the
immediate
hyperaemia
observed on release of a tourniquet.
Neural innervation
• Arterioles, AVAs and precapillary sphincters are densely
innervated by sympathetic
fibres.
• Neural control regulates skin blood flow in the following
ways.
• Increased arteriolar tone results in a decrease of cutaneous
blood flow.
• Increased precapillary sphincter tone reduces the blood flow
into the capillary
networks.
• Decreased AVA tone results in an increase in the non-nutritive
blood flow
bypassing the capillary bed.
Humoral factors
• Epinephrine (adrenaline) and norepinephrine (noradrenaline)
cause vasoconstriction
of the vessels.
• Histamine and bradykinin cause vasodilatation.
• Low oxygen saturation, high carbon dioxide saturation and
acidosis also result
in vasodilatation.
Temperature
Increased heat produces cutaneous vasodilatation and increased
flow which predominantly
bypasses the capillary beds via the AVAs.
