Blood Vessels
Human Anatomy
Chapter 19
• The blood vessels of the body
form a closed circulatory system.
Blood is pumped from the heart
to the body and returned to the
heart. Blood vessels have the
ability to constrict or dilate and
increase or decrease pressure.
Blood vessels may divide and
grow as the body requires.
Arteries carry blood away from
the heart and veins bring blood
towards the heart.
• I. Structure of blood vessels
walls
• Blood vessels have three layers
or tunics in a circular fashion that
create a cavity called the lumen:
• a. Tunica Intima- contains
endothelium (simple squamous
epithelium). In vessels with a
small diameter the endothelium
is surrounded by a thin layer of
loose connective tissue called
subendothelial layer.
•b. Tunica Media-
this layer is the
thickest layer
composed of
smooth muscle
arranged in
circular sheets and
connective tissue.
The sympathetic
NS controls the
smooth muscle
activity.
Vasoconstriction
occurs when the
smooth muscle
contracts, this
decreases the flow
of blood.
Vasodilation
occurs when the
smooth muscle
relaxes, increases
the diameter and
thus the blood
flow.
c. Tunica externa-
Outer most layer,
composed of
connective tissue. It
is designed to protect,
stretches and anchors
the blood vessels.
d. Lumen- cavity in
the middle of the
blood vessel that is
filled with blood. The
diameter of the lumen
changes with
vasoconstriction or
vasodilation. It can
also be influenced by
plaque build up.
Artery & vein cross section
Artery Wall- notice thick muscular layer.
Vein Wall Capillary walls
• II. Types of blood vessels
• All three vessel types are found in systemic and pulmonary
circulation.
• A. arteries- The closer the artery is to the heart the larger its
diameter and the thicker its walls. As the artery branches it
becomes smaller in diameter and thinner until it becomes a capillary.
• Because arteries have a thick muscle layer they have the ability to
change blood pressure.
• 1. Elastic arteries- (2.5-1 cm) largest, closer to the heart, have thick
sheets of elastin in the tunica media, also called conducting arteries,
able to withstand large fluctuations in blood pressure.
• 2. Muscular arteries- (1cm-0.33 mm) distal to elastic arteries, they
reach the organs, their tunica media is thicker in diameter than the
lumen. Also the tunica media is lined with layer of connective tissue
of both sides (internal elastic lamina and external elastic lamina).
The muscular layer constricts the vessel as needed by each
individual organ.
• 3. Arterioles- (0.3mm-10 mm) They have the smallest diameter, the
tunicas are considerably thinner but are still innervated to induce
vasoconstriction or vasodilation. They connect to capillaries.
Cross-section of elastic artery
• B. Capillaries- these
are the smallest
blood vessels (8-
10mm) forcing RBC
to pass by in one
after the other in line.
They are thin enough
to allow for chemical
exchange, only
composed of a single
layer or endothelial
cells lined by basal
lamina. This is also
the site through which
WBC exit the blood
stream.
•1. Capillary beds- this is a
network or capillaries that is
connected to a terminal
arteriole and postcapillary
venule. The terminal arteriole
branches into the metarteriole
(intermediate between arteriole
and true capillary) that further
branches into the capillary bed.
Throughout the metarteriole
there are precapillary
sphincters (smooth muscle)
that control the flow of blood
into the true capillaries. The
capillaries connect into the
thoroughfare channel
(intermediate between
capillaries and venule) that
connects the venule.
• 2. Capillary permeability- small molecules enter and leave the
capillaries through intercellular clefts. To increase permeability
capillaries containing pores, called fenestrated capillaries, are
located in areas of high exchange. All other capillaries are called
continuous, there are no pores. Permeability happens in different
forms:
• a. direct diffusion- as with oxygen and carbon dioxide
• b. intercellular cleft- as with WBC, most common
• c. cytoplasmic vesicles (caveolae)- as when large molecules like
proteins and carbohydrates are transferred from the intestine into
the blood.
• d. fenestrations- in areas of large exchange as when water exits
blood to make synovial fluid within the synovial joints.
The capillaries that have the lowest permeability are those of the blood
brain barrier. They are only permeable to lipid soluble substances and
components such as oxygen and carbon dioxide.
Continous Capillary Fenestrated Capillary
Sinusoidal Capillary
• 3. Sinusoids- wide leaky fenestrated capillaries in areas of
extensive change and crossing or large materials. These may
be sites in which cells move into the blood like it happens in
bone marrow and spleen.
• C. Veins-blood vessels that carry blood from the capillaries to the heart.
As they move away from the capillaries thay increase in diameter. The
blood pressure in veins is less than in arteries and their walls are also
thinner than arteries.
• 1. Venules- (8-100 mm) they are the smallest and thinnest, specially as
they are closer to the capillaries. As they approach the veins they
become thicker.
• 2. Veins- lumens vary in size but compared to arteries they are larger.
Their tunica externa is thicker than tunica media. The largest veins are
the vena cavae the tunica externa also has bands of smooth muscle.
• 3. Vein valves- these assist in the transport of blood back to the heart.
They prevent back flow. Similar to the valves of the heart, these valve
have cusps formed by the endothelial cells of the tunica intima. In areas
of the body that blood flow is directly against gravity there are more
valves. There are no valves in veins of the thoracic and abdominal
cavities.
• 4. Returning blood to the heart- normal body movements produce
muscle contractions that bring blood back to the heart. Skeletal muscle
contractions force the valves to open and propel blood towards the
heart.
• D. Vascular Anastomoses- areas where several blood vessels unite
or interconnect, they may be arteries or veins. Anastomoses may
provide alternative routes for blood flow. They occur around organs
like the heart, brain, or abdominal organs or around joints. Blood
flow blockage of areas where there are no arterial anastomoses may
result in severe tissue damage.
• E. Vasa vasorum- tiny arteries, capillaries, and veins in larger blood
vessels to provide their own blood supply and nourishment for the
outer half of the wall layers. These tiny blood vessels may arise from
the same blood vessels it nourishes or those near by.
• Part 2: Blood vessels of the body- Focus on knowing the names
of the blood vessel given to you in the practicum list. Also study the
diagrams through out the book so you can visualize the pattern the
blood vessels take throughout the body.
• I. The pulmonary circulation- blood vessels that travel from the heart to
the lungs, within the lungs, and back to the heart. Includes all types of
vessels. Arteries carry deoxygenated blood and veins carry oxygenated
blood.
• II. The systemic circulation-blood vessels that travel from the heart
throughout the body and back to the heart. Arteries carry
oxygenated blood and veins carry deoxygenated blood.
• A. Systemic arteries
• 1. aorta
• 2. arteries of the head and neck
• 3. arteries of the upper limbs
• 4. arteries of the thorax
• 5. arteries of the abdomen
• 6. arteries of the pelvis and lower limbs
• B. Systemic veins
• 1. venae cavae and their major tribuatires
• 2-6 same as those for arteries
• 7. Portal-systemic anastomoses
Blood Vessels entering or leaving the heart
View of iliac and femoral arteries
Vascular system within the liver
• III. Disorders of the blood vessles-
• a. Arthrosclerosis- hardening of artery due to fatty deposits. The
artery loses its flexibility.
• b. Aneurysm- widening or out pocketing or an artery or vein
increasing the changes of the vessel rupturing. It may result from
weaken walls
• c. Deep vein thrombosis of the lower limb- formation of clots in
lower legs. The clot can detach, travel through the body and cause a
embolism or stroke.
• d. Venous disease- inadequate drainage of lower limbs due to
failure of valves, it can possibly causing ulceration.
• e. Microangiopathy of diabetes- common long term diabetes
mellitus complication- patient had thicken but leaky capillaries
slowing tissue fluid flow affecting delicate organs like the kidneys,
and retina, and nerves.
• f. Arteriovenous malformation- congenital condition in which
capillaries do not form and arteries connect directly into veins.
Usually occurs in cerebrum. The vein becomes weaken an
aneurysm can occur.
• Fetal Circulation- study figure 19.26 on page 555