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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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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

Veins & their functions

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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 the hepatic veins to the inferior vena cava.

Veins & Their Functions

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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.

The Portal Circulation

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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 non-useful 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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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.

Veins & Their Functions……contd

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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.

Veins and their functions……contd

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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.

‘calf muscle pump’.

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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.

‘systemic venous channels ’.

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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

 Deep Veins

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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.

Venous Sinuses

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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.

How the muscle-vein pump functions:

16Muscle Pump

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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.

Muscle Pump

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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

Muscle Pump

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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.

Muscle Pump

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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.

Varicose Veins

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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.

Ve n o u s P r e s s u r e i n t h e H e a d

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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 air embolism

Venous Pressure in the Head

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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 .

V e n o u s P r e s s u r e & F l o w

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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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When blood flows from the venules to the large veins, its average velocity increases as the total cross-sectional 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.

Velocity of blood in veins

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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.

Venous Pressures—Right Atrial Pressure (Central Venous Pressure) & Peripheral

Venous Pressures

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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.

Venous Pressures—Right Atrial Pressure

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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.

Venous Pressure ….contd

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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).

Measuring Venous Pressure

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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.

Measuring Venous Pressure

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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.

Measuring Venous Pressure ….contd

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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.

Measuring Venous Pressure ….contd

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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.

Thoracic Pump

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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.

Effects of Heartbeat

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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.

Effects of Heartbeat

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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.

Venous Pressure….contd

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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.

Venous Pressure…..contd

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