Physiology of microcirculation,
venous and lymphatic vessels
Microcirculation
● The microcirculation is the blood flow
through blood vessels smaller than 100 µm
(i.e. arterioles, capillaries, and venules).
● Function:
1. Transport of cells, oxygen and
other substances to/from the tissues
2. Regulation of body temperature
Microcirculation consists of 3
components:
1. Haemomycrocyrculation (arterioles,
precapillares, capillares, postcapillares venules,
venules, arterioles-venules anastomosis)
 2. Substance’ transport to intercticium, where
some hydrostatic and oncotic pressure creates
 3. Limphatic vessels – their walls more thin than
in arteriales and don’t contain basal membrane.
Intercellular cracks – they are the main way of
penetration of tissue fluid into the lumen of
lymphatic vessels
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Arterioles
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An arteriole is a small
diameter (<20 μm, up to 5-9
μm) blood vessel that extends
and branches out from an
artery and leads to capillaries.
Arterioles have thin muscular
walls (usually only one to two
layers of smooth muscle) and
are the primary site of
vascular resistance.
In a healthy vascular system
the endothelium, inner lining
of arterioles and other blood
vessels, is smooth and
relaxed. This healthy
condition is promoted by the
ample production of nitric
oxide in the endothelium.
Arteriola
Metharteriola
Shunt
Arterial
capillares
Precapillary
sphincter
Venous
capillares
Postcapillary
sphincter
Venula
Total peripheral resistance

Total peripheral resistance refers to the cumulative
resistance of the thousands of arterioles with precapillares in
the body.
 It is approximately equal to the resistance of the arterioles,
since the arterioles are the chief resistance vessels in the
body.
 Total Peripheral Resistance =
Mean
Pressure // Cardiac
CardiacOutput.
Output.
MeanArterial
Arterial Pressure

The total peripheral resistance of healthy lung arterioles is
typically about 0.15 to 0.20 that of the body, so pulmonary
artery mean blood pressures are typically about 0.15 to 0.20
of aortic mean blood pressures.
Capillary
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Capillaries, are the smallest of a body's blood vessels,
measuring 5-10 μm.
They connect arteries and veins, and most closely interact
with tissues.
Capillaries have walls composed of a single layer of cells,
the endothelium.
This layer is so thin that molecules such as oxygen, water
and lipids can pass through them by diffusion and enter the
tissues.
Waste products such as carbon dioxide and urea can
diffuse back into the blood to be carried away for removal
from the body.
Capillary permeability can be increased by the release of
certain cytokines.
The Organization of a Capillary Bed
Figure 21.5a, b
Types of Capillaries

There are three structural types of
capillaries:
1.Continuous
2.Fenestrated
3.Sinusoids
Type of Capillaries: Continuous

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Continuous capillaries most
abundant in the skin & muscles
– Least permeable, lack pores
– Endothelial cells provide an
uninterrupted lining
– Adjacent cells are connected with
tight junctions
– Intercellular clefts allow the
passage of fluids
Continuous capillaries of the brain
– Have tight junctions completely
around the endothelium
– Constitute the blood-brain barrier
Type of Capillaries: Fenestrated

Fenestrated Capillaries
– They have pores
(fenestrations)
– Found wherever active
capillary absorption or
filtrate occurs
 Example: intestinal villi,
ciliary process of eye,
endocrine glands,
glomeruli of kidney
Characterized by:
– Greater permeability than
continuous capillaries
Type of Capillaries: Sinusoidal

Sinusoidal capillaries
are modified, very
permeable (leaky)
capillaries.
 They have a large
lumens with large
fenestrae.
– Found only in the
liver, bone marrow,
lymphoid tissue and
in some endocrine
organs.
TYPES OF CAPILLARY

1. Somatic.
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2. Visceral

3. Sinusoidal
Capillary Beds: Microcirculation
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Vascular shunts – metarteriole
– thoroughfare channel
True capillaries – 10 to 100 per capillary bed
– branch off the metarteriole
Precapillary sphincter
– Cuff of smooth muscle that surrounds each true capillary
– Regulates blood flow into the capillary
Blood flow regulated by vasomotor nerves & chemical conditions
Oxygen, CO2,
small solutes,
nutrients move
across capillaries
primarily through
diffusion.
Concentration
gradient
Electrochemical
Hydrostatic
and Osmotic
pressure
Fig 14.9
14-22
Morpho-functional properties of venous system
 Veins
are the vessels, which are carry out
blood from organs, tissues to heart in
right atrium. Only pulmonary vein carry
out blood from lungs in left atrium. There
are superficial (skin) and deep veins.
They are very stretching and have a low
elasticity. Valves are present in veins.
Plexus venosus are storage of blood.
Blood moving in veins under gravity.
Mechanism of regulation

Difference of pressure in venous system is a
cause of blood moving. From the place of high
pressure blood moving to the place of low
pressure.
 Negative pressure in chest is a cause of blood
moving.
 Contraction of skeletal muscles, diaphragm
pump, peristaltic movement of veins walls are the
causes of moving.
Blood flow in veins
 Blood
flows through the blood vessels,
including the veins, primarily, because of the
pumping action of the heart, although venous
flow is aided by the heartbeat, the increase in
the negative intrathoracic pressure during
each inspiration, and contractions of skeletal
muscles that compress the veins (muscle
pump).
Venous pressure
 Venous
pressure is pressure of blood, which
are circulated in veins.
 Venous pressure in healthy person is from 50
to 100 mm H2O.
 Increase of venous pressure in physiological
condition may be in the action of physical
activity. Determine of venous pressure is
called phlebotonometry and give for doctors
information about activity of right atrium.
Methods for assessing blood flow in
the veins
Magnetic
resonance
venography of
normal sinuses of
the brain
(posterior-lateral
projection).
Phlebography

а – atria wave –contraction of
right atrium
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с – passing of carotid artery
pulse on vein
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х – systole of ventricles
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v – ventricular – filling of
atrium by blood

y – passing of blood in right
atria
Lymphatic system
 The
lymphatic system is a complex network
of lymphoid organs, lymph nodes, lymph
ducts, and lymph vessels that produce and
transport lymph fluid from tissues to the
circulatory system.
 The
lymphatic system is a major component of
the immune system.
Morpho-functional properties of lymphatic system
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Lymph system has capillaries, vessels, where present
valves, lymphatic nodes. In lymphatic nodes are
lymphopoiesis, depo of lymph, their function is barrierfilter. Lymph flow in vein system through the chest
lymph ductus.
Functions of lymph:
1. support of constant level of volume and components
of tissue fluid;
2. transport of nutritive substances from digestive tract in
venous system;
3. barrier-filter function.
4. take place in immunology reactions.
Lympathatic system

The lymphatic system has three primary functions.
First of all, it returns excess interstitial fluid to the
blood.
 The second function of the lymphatic system is the
absorption of fats and fat-soluble vitamins from
the digestive system and the subsequent transport
of these substances to the venous circulation.
 The third and probably most well known function
of the lymphatic system is defense against
invading microorganisms and disease.
Lymphatic capillaries
 Lymphatic
capillaries begin as one side
closed capacities, which are drained by
smallest lymphatic vessels.
 Pressure of lymph inside the capillary
is lower than in intracellular space,
which helps to lymph flow. Capillary
wall has basal membrane and one layer
of endotheliocytes.
Lymph
-The lymphatic capillaries are
responsible for returning
interstitial fluid and proteins to
the vascular compartment.
-Lymph capillaries merge into
large thoracic duct which
empties into the large veins.
-Lymph vessels have smooth
muscle for movement and
surrounding skeletal muscle
contractions and contain open
ends.
Lymph
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Lymph originates as blood plasma that leaks from the
capillaries of the circulatory system, becoming interstitial
fluid, and filling the space between individual cells of
tissue.
Plasma is forced out of the capillaries by hydrostatic
pressure, and as it mixes with the interstitial fluid, the
volume of fluid accumulates slowly.
Most of the fluid is returned to the capillaries by osmosis
(about 90% of the former plasma).
The excess interstitial fluid is collected by the lymphatic
system by diffusion into lymph capillaries, and is processed
by lymph nodes prior to being returned to the circulatory
system.
Once within the lymphatic system the fluid is called lymph,
and has almost the same composition as the original
interstitial fluid.
Lymphatic circulation

The lymphatic system acts as a secondary circulatory system, except
that it collaborates with white blood cells in lymph nodes to protect
the body from being infected by cancer cells, fungi, viruses or
bacteria.
 Unlike the circulatory system, the lymphatic system is not closed
and has no central pump; the lymph moves slowly and under low
pressure due to peristalsis, the operation of semilunar valves in the
lymph veins, and the milking action of skeletal muscles.
 Like veins, lymph vessels have one-way, semilunar valves and
depend mainly on the movement of skeletal muscles to squeeze fluid
through them.
 Rhythmic contraction of the vessel walls may also help draw fluid into
the lymphatic capillaries.
 This fluid is then transported to progressively larger lymphatic vessels
culminating in the right lymphatic duct (for lymph from the right
upper body) and the thoracic duct (for the rest of the body); these
ducts drain into the circulatory system at the right and left subclavian
veins.
Function of Limphatic node
1. Lymph nodes play role of
filtration barrier. This function
possible due to the presence of
macrophages and net of
reticular fibers in the lumen of
the sinuses.
2. Lymph nodes are organs of
lymphopoiesis (B - and Tlymphocytes)
3. Lymph nodes – they are
deposit of lymph.
The main drains of the
lymphatic system in which
lymph flows in to the venous
system is the Lymphatic
thoracic duct and cervical
lymphatic duct, which collects
lymph from the head and
surrounding areas.
Ultrasound examination of lymph
nodes
Production of lymph
 Fluid
efflux normally exceeds influx
across the capillary walls, but the extra
fluid enters the lymph and drains through
them back into the blood. This keeps the
interstitial fluid pressure from rising and
promotes the turnover of tissue fluid.
 The normal 24-hour lymph flow is 2-4 L.
Mechanism of lymph flow

Lymph flow is due to movements of skeletal
muscle, the negative intrathoracic pressure during
inspiration, the suction effect of high velocity
flow of blood in the veins in which the lymphatic
vessels terminate, and rhythmic contractions of
the walls of the large lymph ducts.
 Since lymph vessels have valves that prevent
backflow, skeletal muscle contractions push the
lymph toward the heart.
 Pulsations of arteries near lymphatic vessels may
have a similar effect.
Thymus

The thymus is an organ located in the upper
anterior portion of the chest cavity.
 The thymus plays an important role in the
development of the immune system in early life,
and its cells form a part of the body's normal
immune system.
 It is most active before puberty, after which it
shrinks in size and activity in most individuals and
is replaced with fat.
 Function: Production (maturation) of T cells.
Spleen
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The spleen is located in the upper left part of the
abdomen, behind the stomach and just below the
diaphragm.
The spleen is the largest collection of lymphoid tissue in
the body.
It is regarded as one of the centres of activity of the
reticuloendothelial system.
Its absence leads to a predisposition to certain infections.
Function:
–
–
–
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Blood reservoir
Destruction of old red blood cells
Immune functions
Blood cells production in embryogenesis
Tonsils
 Tonsils
are clusters of lymphatic tissue just
under the mucous membranes that line the
nose, mouth, and throat (pharynx).
 There are three groups of tonsils.
 The pharyngeal tonsils are located near the
opening of the nasal cavity into the pharynx.
When these tonsils become enlarged they
may interfere with breathing and are called
adenoids.
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