The circulatory system
General structure
The circulatory system is a “closed” system consisting of the following
components: heart, arteries, arterioles, capillaries, venules and veins.
• Arteries carry blood away from the heart.
• In most cases they transport oxygenated blood, with two
exceptions: the pulmonary arteries (from the heart to the lungs)
and the umbilical arteries (from the fetus to the maternal placenta).
• Veins, on the other hand, transport blood to the heart and usually carry
oxygen-poor blood, with the exception of the pulmonary veins (from the
lungs to the heart) and the umbilical veins (from the placenta to the
fetus).
• 12% of the blood is present in the heart, 33% in the arterial system (the high-
pressure portion of the circulatory system), 50% in the venous system and 5%
in the capillaries. Nevertheless, all capillaries of an adult human have an
estimated total length of 90,000 km
• In addition, there is the lymphatic system which carries a part of the
interstitial fluid (10%) called lymph back to the heart and the circulation via
a network of thin vessels and lymph nodes.
Distribution of vessels
• The largest vessels are restricted in
number and are in general located
more centrally in the entire body.
• Ramifications (division into
smaller branches) to smaller
vessels and finally to capillarie are
found everywhere, because every
cell is dependent on blood supply.
• The large vessels comprise the
“macrovasculature”, and the
capillaries, arterioles and venules, small arteries and veins with a diameter up
to 0.2 mm form the “microvasculature”.
Function
• Circulating blood delivers nutrients and oxygen to the tissues, and removes
waste products from the tissues.
• The exchange takes place in the capillaries.
• Blood also transports endocrine hormones to the target organs/cells.
• In addition, the blood vessels play an important role in heat exchange.
,General structure of a blood vessel
General structure
The wall of large blood vessels (both arteries (1) and veins (24))
consists of the following three layers.
Tunica intima (2)
• One layer of endothelium (3) on a basal lamina (4),
supported by subendothelial connective tissue (5).
• The basal lamina is normally not visible in sections
(if not specifically stained).
• In arteries, a lamina elastica interna (6) separates the
subendothelial tissue from the next layer. This lamina
usually is visible in stained sections, often as a wavy line.
• The wavy structure of the lamina elastica interna/elastic fibers is considered
a postmortem picture/artifact.
• This structure is present because, postmortem, there is no internal pressure,
allowing the fibers to contract, creating the wavy appearance.
Tunica media (7)
• The tunica media consists mainly of concentric layers of smooth muscle cells
(8), separated by extracellular matrix with elastic and collagenous fibers,
proteoglycans and in some arteries elastic membranes (9).
• In very large vessels, a lamina elastica externa (10) separates the tunica media
from the following layer. This lamina is usually visible in stained sections.
Adventitial layer (11)
• The adventitia mainly consists of connective tissue, with elastic and collagenous
fibers.
• The adventitia blends with the connective tissue of the organ in which this blood
vessel is embedded.
Vas vasorum
• The adventitia of larger vessels contains vasa vasorum (12), small blood vessels
which supply the vessel wall with blood.
• In addition, lymph vessels and vasomotor nerves (13) can be found in the
connective tissue around the blood vessel, which also penetrate the vessel wall.
In histological sections, the smooth muscle cells of the tunica media are usually
contracted, partly due to fixation procedures. Therefore, the lamina elastica
interna often presents as a wavy line.
Arteries
Types of arteries
• Depending on their wall composition, arteries are
classified as elastic(14) or muscular(15) or as arterioles.
• The size of these vessels decreases in this order.
• The ascending aorta and the aortic arch are highly elastic to
accommodate and store the power of blood pushed out during
systole (air-chamber function). However, the last part of the aorta
(the abdominal section before splitting into the common iliac arteries)
is fully muscular.
, Elastic arteries (transport arteries)
• They are the largest vessels, and include the aorta and its larger branches.
• The tunica media (7) contains many concentric elastic membranes (9) with
smooth muscle cells in between.
• They regulate the stiffness of the vessel wall by contracting and thereby
bringing the membranes closer to each other.
• The lamina elastica interna (6) is not always distinguishable from the elastic
membranes, but in the adventitial layer (11), a lamina elastica externa (10)
is present.
• Elastic arteries have an air-chamber function.
2: tunica intima consisting of endothelial cells, subendothelial tissue and the basal
lamina.
Musculous arteries (distribution arteries)
• They have a diameter of many mm.
• They always have a visible lamina elastica interna (6).
• The tunica media (7) is well developed, and may show up to 40 parallel muscle
cell layers. The lamina elastica externa is only found in the larger vessels. The
tunica media regulates the blood flow by contraction.
12: vasa vasorum.
Arterioles
• They have a diameter between 10 and ≈100 µm.
• Large arterioles still show the already mentioned 3-layered structure.
• In smaller arterioles, there may be only one smooth muscle cell (8) layer and
no lamina elastica interna (6). The smallest arterioles continue as capillaries.
3: nucleus of an endothelial cell; 13: vasomotor nerve.
Function of elastic and muscular vessels
• Elastic arteries near the heart, like the aorta,
stretch to accommodate blood from the heart's
forceful pumps and maintain continuous blood
flow during heart relaxation.
• Muscular arteries, which branch from elastic
arteries, have more smooth muscle to actively
regulate blood flow to specific body regions by contracting and relaxing.
Elastic arteries
• Function: To conduct blood directly from the heart and dampen pressure surges.
• Mechanism: Their walls contain a high amount of elastin, a protein that allows
them to stretch when the heart pumps blood out and recoil when
the heart relaxes.
• Result: This action ensures that blood pressure remains relatively constant and
that blood flow continues even between heartbeats.
• Examples: The aorta and its major branches.
Muscular arteries
• Function: To distribute blood to specific body organs and tissues.
• Mechanism: They have a thicker tunica media (middle layer) composed
primarily of smooth muscle, which enables them to actively
constrict or dilate (vasoconstriction and vasodilation).
General structure
The circulatory system is a “closed” system consisting of the following
components: heart, arteries, arterioles, capillaries, venules and veins.
• Arteries carry blood away from the heart.
• In most cases they transport oxygenated blood, with two
exceptions: the pulmonary arteries (from the heart to the lungs)
and the umbilical arteries (from the fetus to the maternal placenta).
• Veins, on the other hand, transport blood to the heart and usually carry
oxygen-poor blood, with the exception of the pulmonary veins (from the
lungs to the heart) and the umbilical veins (from the placenta to the
fetus).
• 12% of the blood is present in the heart, 33% in the arterial system (the high-
pressure portion of the circulatory system), 50% in the venous system and 5%
in the capillaries. Nevertheless, all capillaries of an adult human have an
estimated total length of 90,000 km
• In addition, there is the lymphatic system which carries a part of the
interstitial fluid (10%) called lymph back to the heart and the circulation via
a network of thin vessels and lymph nodes.
Distribution of vessels
• The largest vessels are restricted in
number and are in general located
more centrally in the entire body.
• Ramifications (division into
smaller branches) to smaller
vessels and finally to capillarie are
found everywhere, because every
cell is dependent on blood supply.
• The large vessels comprise the
“macrovasculature”, and the
capillaries, arterioles and venules, small arteries and veins with a diameter up
to 0.2 mm form the “microvasculature”.
Function
• Circulating blood delivers nutrients and oxygen to the tissues, and removes
waste products from the tissues.
• The exchange takes place in the capillaries.
• Blood also transports endocrine hormones to the target organs/cells.
• In addition, the blood vessels play an important role in heat exchange.
,General structure of a blood vessel
General structure
The wall of large blood vessels (both arteries (1) and veins (24))
consists of the following three layers.
Tunica intima (2)
• One layer of endothelium (3) on a basal lamina (4),
supported by subendothelial connective tissue (5).
• The basal lamina is normally not visible in sections
(if not specifically stained).
• In arteries, a lamina elastica interna (6) separates the
subendothelial tissue from the next layer. This lamina
usually is visible in stained sections, often as a wavy line.
• The wavy structure of the lamina elastica interna/elastic fibers is considered
a postmortem picture/artifact.
• This structure is present because, postmortem, there is no internal pressure,
allowing the fibers to contract, creating the wavy appearance.
Tunica media (7)
• The tunica media consists mainly of concentric layers of smooth muscle cells
(8), separated by extracellular matrix with elastic and collagenous fibers,
proteoglycans and in some arteries elastic membranes (9).
• In very large vessels, a lamina elastica externa (10) separates the tunica media
from the following layer. This lamina is usually visible in stained sections.
Adventitial layer (11)
• The adventitia mainly consists of connective tissue, with elastic and collagenous
fibers.
• The adventitia blends with the connective tissue of the organ in which this blood
vessel is embedded.
Vas vasorum
• The adventitia of larger vessels contains vasa vasorum (12), small blood vessels
which supply the vessel wall with blood.
• In addition, lymph vessels and vasomotor nerves (13) can be found in the
connective tissue around the blood vessel, which also penetrate the vessel wall.
In histological sections, the smooth muscle cells of the tunica media are usually
contracted, partly due to fixation procedures. Therefore, the lamina elastica
interna often presents as a wavy line.
Arteries
Types of arteries
• Depending on their wall composition, arteries are
classified as elastic(14) or muscular(15) or as arterioles.
• The size of these vessels decreases in this order.
• The ascending aorta and the aortic arch are highly elastic to
accommodate and store the power of blood pushed out during
systole (air-chamber function). However, the last part of the aorta
(the abdominal section before splitting into the common iliac arteries)
is fully muscular.
, Elastic arteries (transport arteries)
• They are the largest vessels, and include the aorta and its larger branches.
• The tunica media (7) contains many concentric elastic membranes (9) with
smooth muscle cells in between.
• They regulate the stiffness of the vessel wall by contracting and thereby
bringing the membranes closer to each other.
• The lamina elastica interna (6) is not always distinguishable from the elastic
membranes, but in the adventitial layer (11), a lamina elastica externa (10)
is present.
• Elastic arteries have an air-chamber function.
2: tunica intima consisting of endothelial cells, subendothelial tissue and the basal
lamina.
Musculous arteries (distribution arteries)
• They have a diameter of many mm.
• They always have a visible lamina elastica interna (6).
• The tunica media (7) is well developed, and may show up to 40 parallel muscle
cell layers. The lamina elastica externa is only found in the larger vessels. The
tunica media regulates the blood flow by contraction.
12: vasa vasorum.
Arterioles
• They have a diameter between 10 and ≈100 µm.
• Large arterioles still show the already mentioned 3-layered structure.
• In smaller arterioles, there may be only one smooth muscle cell (8) layer and
no lamina elastica interna (6). The smallest arterioles continue as capillaries.
3: nucleus of an endothelial cell; 13: vasomotor nerve.
Function of elastic and muscular vessels
• Elastic arteries near the heart, like the aorta,
stretch to accommodate blood from the heart's
forceful pumps and maintain continuous blood
flow during heart relaxation.
• Muscular arteries, which branch from elastic
arteries, have more smooth muscle to actively
regulate blood flow to specific body regions by contracting and relaxing.
Elastic arteries
• Function: To conduct blood directly from the heart and dampen pressure surges.
• Mechanism: Their walls contain a high amount of elastin, a protein that allows
them to stretch when the heart pumps blood out and recoil when
the heart relaxes.
• Result: This action ensures that blood pressure remains relatively constant and
that blood flow continues even between heartbeats.
• Examples: The aorta and its major branches.
Muscular arteries
• Function: To distribute blood to specific body organs and tissues.
• Mechanism: They have a thicker tunica media (middle layer) composed
primarily of smooth muscle, which enables them to actively
constrict or dilate (vasoconstriction and vasodilation).
