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Veins & Their functions
• For years, the veins were considered to be
nothing more than passageways for flow of
blood to the heart.
• It has become apparent that they perform other
special functions that are necessary for
circulation of blood.
• They are capable of constricting and enlarging
and thereby storing
either small or large
quantities of blood and making this blood
available when it is required by the remainder of
the circulation.
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Veins & their functions
The peripheral veins can also propel
blood forward by means of a so-called
venous pump.
 They help to regulate cardiac output,
an exceedingly important function
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Veins & Their Functions
 THE VEINS convey the blood from the capillaries of the
different parts of the body to the heart. They consist of two
distinct sets of vessels, the pulmonary and systemic.
 The Pulmonary Veins, unlike other veins, contain arterial
blood, which they return from the lungs to the left atrium of
the heart.
 The Systemic Veins return the venous blood from the body
generally to the right atrium of the heart
 The Portal Vein, an appendage (accessary) to the systemic
venous system, is confined to the abdominal cavity, and
returns the venous blood from the spleen and the viscera of
digestion to the liver. This vessel ramifies in the substance of
the liver and there breaks up into a minute network of
capillary-like vessels, from which the blood is conveyed by
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the hepatic veins to the inferior vena cava.
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The Portal Circulation
 The liver is unusual in that it has a double blood supply;
the right and left hepatic arteries carry oxygenated
blood to the liver, and the portal vein carries venous
blood from the GI tract to the liver.
 The venous blood from the GI tract drains into the
superior and inferior mesenteric veins; these two
vessels are then joined by the splenic vein just
posterior to the neck of the pancreas to form the
portal vein. This then splits to form the right and left
branches, each supplying about half of the liver.
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 On entering the liver, the blood drains into the hepatic
sinusoids, where it is screened by specialized macrophages
(Kupffer cells) to remove any pathogens that manage to
get past the GI defenses. The plasma is filtered through the
endothelial lining of the sinusoids and bathes
the hepatocytes; these cells contain vast numbers of
enzymes capable of braking down and metabolizing most of
what has been absorbed.
 The portal venous blood contains all of the products of
digestion absorbed from the GI tract, so all useful and nonuseful products are processed in the liver before being
either released back into the hepatic veins which join the
inferior vena cava just inferior to the diaphragm, or stored in
the liver for later use.
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• The veins commence by minute plexuses which receive the
blood from the capillaries. The branches arising from these
plexuses unite together into trunks, and these, in their passage
toward the heart, constantly increase in size as they receive
tributaries, or join other veins.
• The veins are larger and altogether more numerous than the
arteries; hence, the entire capacity of the venous system is much
greater than that of the arterial; the capacity of the pulmonary
veins, however, only slightly exceeds that of the pulmonary
arteries.
• The veins are cylindrical like the arteries; their walls, however,
are thin and they collapse when the vessels are empty, and the
uniformity of their surfaces is interrupted at intervals by
slight constrictions, which indicate the existence of valves in
their interior.
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Veins & Their Functions……contd
 They communicate very freely with one another, especially in
certain regions of the body; and these communications exist
between the larger trunks as well as between the smaller
branches.
 Thus, between the venous sinuses of the cranium, and between
the veins of the neck, where obstruction would be attended
with imminent danger to the cerebral venous system, large and
frequent anastomoses are found.
 The same free communication exists between the veins
throughout the whole extent of the vertebral canal, and between
the veins composing the various venous plexuses in the
abdomen and pelvis, e. g., the spermatic, uterine, vesicles, and
pudendal.
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Veins and their functions……contd
Unlike arteries veins are not supported
by the heart. An independent pumping
system is therefore required to carry
the blood – against gravity -back to the
heart.
The heart serves as a pump for the
transport of blood in the arteries and
transports in the case of a human adult
7,000 litres of blood per day in
the circulatory system.
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‘calf muscle pump’.
 This volume of blood must also be carried back to
the heart by the veins and on account of man's
upright posture an additional pump mechanism is
needed to carry blood against gravity towards the
heart.
 This requires a complicated mechanism: The
‘muscle-vein pump’ of the leg muscles, also called
the ‘calf muscle pump’. It fulfils the most
important function in transporting back blood, as
the veins in the leg cover the longest distance to
the heart.
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‘systemic venous channels ’.
 The systemic venous channels are subdivided into three
sets, viz.,
 superficial
 deep veins
 venous sinuses.
 The Superficial Veins (cutaneous veins) are found
between the layers of the superficial fascia
immediately beneath the skin; they return the
blood from these structures, and communicate
with the deep veins by perforating the deep
fascia.
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Deep Veins
 The Deep Veins accompany the arteries, and are usually
enclosed in the same sheaths with those vessels.
 With the smaller arteries—as the radial, unlar, brachial,
tibial, peroneal—they exist generally in pairs, one lying on
each side of the vessel, and are called venæ comitantes.
 The larger arteries—such as the axillary, subclavian,
popliteal, and femoral—have usually only one
accompanying vein.
 In certain organs of the body, however, the deep veins do
not accompany the arteries; for instance, the veins in the
skull and vertebral canal, the hepatic veins in the liver,
and the larger veins returning blood from the bones
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Venous Sinuses
 Venous Sinuses are found only in the
interior of the skull, and consist of canals
formed by a separation of the two layers of
the dura mater; their outer coat consists of
fibrous tissue, their inner of an endothelial
layer continuous with the lining membrane
of the veins.
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How the muscle-vein pump functions:
 The leg’s movement activates the calf muscles. The muscle
belly thickens and presses together the deep veins lying
between the muscles.
 The veins narrow, blood has less space to diffuse and flows
faster against gravity in the direction of the heart.
 So-called venous valves support the blood in flowing back
from the legs.
 Valves located at opposite sites act as non-return valves. If
the blood flows towards the heart, the valves lie back
against the vascular wall and allow the blood to flow
unhindered in the direction of the heart.
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Muscle Pump
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Muscle Pump
However, if the blood flows back, the
venous valves shut, obstruct the
passage and prevent the blood from
flowing “back”.
Should the muscles relax, this results in
suction which refills the empty veins
with venous blood.
The number of venous valves in the
different veins varies from 2 to 20 valves
per vein.

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Muscle Pump
 In the limbs, the veins are surrounded by
skeletal muscles, and contraction of these
muscles during activity compresses the veins.
Pulsations of nearby arteries may also compress
veins.
 Since the venous valves prevent reverse flow,
the blood moves toward the heart.
 During quiet standing, when the full effect of
gravity is manifest, venous pressure at the ankle
is 85–90 mm Hg
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Muscle Pump
 Pooling of blood in the leg veins reduces
venous return, with the result that cardiac
output is reduced, sometimes to the point
where fainting occurs.
 Rhythmic contractions of the leg muscles
while the person is standing serve to lower
the venous pressure in the legs to less than
30 mm Hg by propelling blood toward the
heart.
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Varicose Veins
 This heartward movement of the blood is
decreased in patients with VARICOSE VEINS
because their valves are incompetent.
 These patients may develop stasis and ankle
edema.
 Even when the valves are incompetent, muscle
contractions continue to produce a basic
heartward movement of the blood because the
resistance of the larger veins in the direction of
the heart is less than the resistance of the small
vessels away from the heart.
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Venous Pressure in the Head
 In the upright position, the venous pressure in
the parts of the body above the heart is
decreased by the force of gravity.
 The neck veins collapse above the point where
the venous pressure is close to zero, and the
pressure all along the collapsed segments is
close to zero rather than subatmospheric.
However, the dural sinuses have rigid walls and
cannot collapse.
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Venous Pressure in the Head
 The pressure in them in the standing or sitting
position is therefore subatmospheric.
 The magnitude of the negative pressure is
proportionate to the vertical distance above the
top of the collapsed neck veins, and in the superior
sagittal sinus may be as much as –10 mm Hg.
 This fact must be kept in mind by neurosurgeons.
Neurosurgical
procedures
are
sometimes
performed with the patient seated. If one of the
sinuses is opened during such a procedure, it sucks
air, causing a i r e m b o l i s m
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Venous Pressure & Flow
 The pressure in the venules is 12–18 mm Hg.
 It falls steadily in the larger veins to about 5.5 mm
Hg in the great veins outside the thorax.
 The pressure in the great veins at their entrance
into the right atrium (central venous pressure)
averages 4.6 mm Hg but fluctuates with respiration
and heart action.
 Peripheral venous pressure, like arterial pressure, is
affected by gravity. It is increased by 0.77 mm Hg
for each centimeter below the right atrium and
decreased by a like amount for each centimeter
above the right atrium the pressure is measured .
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Effects of gravity on arterial and venous pressure. The scale on the right indicates the increment (or decrement) in
mean pressure in a large artery at each level. The mean pressure in all large arteries is approximately 100 mm Hg
when they are at the level of the left ventricle. The scale on the left indicates the increment in venous pressure at
each level due to gravity. The manometers on the left of the figure indicate the height to which a column of blood in a
tube would rise if connected to an ankle vein (A), the femoral vein (B), or the right atrium (C), with the subject in the
standing position. The approximate pressures in these locations in the recumbent position—ie, when the ankle,
thigh, and right atrium are at the same level—are A, 10 mm Hg; B, 7.5 mm Hg; and C, 4.6 mm Hg.
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Velocity of blood in veins
When blood flows from the venules
to the large veins, its average
velocity increases as the total crosssectional area of the vessels
decreases.
 In the great veins, the velocity of
blood is about one fourth that in
the aorta, averaging about 10 cm/s.
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Venous Pressures—Right Atrial Pressure
(Central Venous Pressure) & Peripheral Venous Pressures
 Blood from all the systemic veins flows into the
right atrium of the heart; therefore, the pressure in
the right atrium is called the central venous
pressure.
 Right atrial pressure is regulated by a balance
between
 (1) the ability of the heart to pump blood out of the right
atrium and ventricle into the lungs and
 (2)the tendency for blood to flow from the peripheral veins
into the right atrium.
• If the right heart is pumping strongly, the right atrial pressure
decreases.
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Venous Pressures—Right Atrial Pressure
 Conversely ,weakness of the heart elevates the right atrial
pressure.
 Any effect that causes rapid inflow of blood into the right
atrium from the peripheral veins elevates the right atrial
pressure.
 Some of the factors that can increase this venous return
(and thereby increase the right atrial pressure) are:
 (1) increased blood volume
 (2) increased large vessel tone throughout the body with
resultant increased peripheral venous pressures
 (3) dilatation of the arterioles, which decreases the
peripheral resistance and allows rapid flow of blood
from the arteries into the veins.
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Venous Pressure ….contd
 The same factors that regulate right
atrial pressure also enter into the
regulation of cardiac output because
the amount of blood pumped by the
heart depends on both the ability of
the heart to pump and the tendency
for blood to flow into the heart from
the peripheral vessels.
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Measuring Venous Pressure
 Central venous pressure can be measured directly
by inserting a catheter into the thoracic great veins.
 Peripheral venous pressure correlates well with
central venous pressure in most conditions.
 To measure peripheral venous pressure, a needle
attached to a manometer containing sterile saline is
inserted into an arm vein. The peripheral vein
should be at the level of the right atrium (a point 10
cm or half the chest diameter from the back in the
supine position).
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Measuring Venous Pressure
 The values obtained in millimeters of saline can be
converted into millimeters of mercury (mm Hg) by
dividing by 13.6 (the density of mercury).
 The amount by which peripheral venous pressure
exceeds central venous pressure increases with the
distance from the heart along the veins.
 The mean pressure in the antecubital vein is normally
7.1 mm Hg, compared with a mean pressure of 4.6 mm
Hg in the central veins.
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Measuring Venous Pressure ….contd
 A fairly accurate estimate of central venous
pressure can be made without any equipment
by simply noting the height to which the
external jugular veins are distended when the
subject lies with the head slightly above the
heart. The vertical distance between the right
atrium and the place the vein collapses (the
place where the pressure in it is zero) is the
venous pressure in mm of blood.
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Measuring Venous Pressure ….contd
 Central venous pressure is decreased during
negative pressure breathing and shock.
 It is increased by positive pressure breathing,
straining, expansion of the blood volume, and heart
failure.
 In advanced congestive heart failure or obstruction
of the superior vena cava, the pressure in the
antecubital vein may reach values of 20 mm Hg or
more.
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Thoracic Pump
 During inspiration, the intrapleural pressure falls
from –2.5 to –6 mm Hg. This negative pressure is
transmitted to the great veins and, to a lesser
extent, the atria, so that central venous pressure
fluctuates from about 6 mm Hg during expiration
to approximately 2 mm Hg during quiet inspiration.
 The drop in venous pressure during inspiration aids
venous return. When the diaphragm descends
during inspiration, intraabdominal pressure rises,
and this also squeezes blood toward the heart
because backflow into the leg veins is prevented by
the venous valves.
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Effects of Heartbeat
 The variations in atrial pressure are transmitted
to the great veins, producing the a, c, and v
waves of the venous pressure-pulse curve
 Atrial pressure drops sharply during the ejection
phase of ventricular systole because the
atrioventricular valves are pulled downward,
increasing the capacity of the atria. This action
sucks blood into the atria from the great veins.
The sucking of the blood into the atria during
systole contributes appreciably to the venous
return, especially at rapid heart rates.
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Effects of Heartbeat
Close to the heart, venous flow becomes
pulsatile. When the heart rate is slow, two
periods of peak flow are detectable, one
during ventricular systole, due to pulling down
of the atrioventricular valves, and one in early
diastole, during the period of rapid ventricular
filling.
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Venous Pressure….contd
 The normal right atrial pressure is about 0 mm
Hg, which is equal to the atmospheric pressure
around the body.
 It can increase to 20 to 30 mm Hg under very
abnormal conditions, such as:
 (1) serious heart failure or
 (2) after massive transfusion of blood, which greatly
increases the total blood volume and causes
excessive quantities of blood to attempt to flow into
the heart from the peripheral vessels.
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Venous Pressure…..contd
 The lower limit to the right atrial pressure is usually
about -3 to -5 mm Hg below atmospheric pressure.
 This is also the pressure in the chest cavity that
surrounds the heart.
 The right atrial pressure approaches these low
values when the heart pumps with exceptional
vigour or when blood flow into the heart from the
peripheral vessels is greatly depressed, such as
after severe haemorrhage.
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