Venous System

Introduction

The venous system serves as the major blood reservoir of the body, accommodating large volumes of blood with minimal changes in pressure. Venous return is influenced by vessel compliance, sympathetic activity, body posture, gravity, and mechanisms that facilitate blood flow back to the heart.

Functional Aspects

Components

• The venous system begins at the postcapillary venules and includes venules, small veins, large veins, and the superior and inferior venae cavae.
• The venae cavae return systemic venous blood to the right atrium.

Wall of Veins

• Veins possess the same three layers as arteries: tunica intima, tunica media, and tunica adventitia.
• However, venous walls are thinner and contain less smooth muscle and elastic tissue than arterial walls.

Functions of Veins

• The primary function of veins is to return deoxygenated blood from tissues to the heart for subsequent oxygenation in the lungs.
• Veins are highly compliant capacitance vessels capable of accommodating large volumes of blood with only small increases in pressure.
• Approximately 60–70% of the total blood volume is normally contained within the venous circulation, making veins the major blood reservoir of the body.

Pressure Volume Relationship of Veins

• Veins have thin walls and contain relatively little smooth muscle, making them highly compliant, distensible, and collapsible.
• Because veins are normally partially collapsed, they can accommodate substantial increases in blood volume with minimal rise in venous pressure.
• Consequently, the initial portion of the venous pressure-volume relationship is relatively flat.
• Once veins become fully distended and their lumen assumes a circular shape, further increases in volume produce a steep rise in pressure.
• In contrast, arteries exhibit a more linear pressure-volume relationship because of their thicker and less compliant walls.

Innervation

• Veins receive sympathetic innervation, although the density of innervation is lower than that of arteries.
• Despite fewer nerve fibers, veins respond markedly to sympathetic stimulation because of their high compliance and efficient smooth muscle activation.
• Even mild sympathetic activation produces significant venoconstriction.
• Venoconstriction reduces venous capacitance, mobilizes blood from the venous reservoir, and increases venous return to the heart, thereby supporting cardiac output during exercise, hemorrhage, and stress.

Venous Return

• Venous return is the volume of blood returning to the right atrium per unit time.
• Under steady-state conditions, venous return is equal to cardiac output and is a major determinant of ventricular filling.
• Changes in venous return alter end-diastolic volume (preload) and thereby influence cardiac output through the Frank–Starling mechanism.
• Venous return is affected by total blood volume, sympathetic activity, venous tone, skeletal muscle pumping, respiratory movements, and intra-abdominal pressure changes.

Venous Valves

• Veins, particularly those of the lower limbs, contain venous valves that permit unidirectional blood flow toward the heart.
• During contraction of surrounding skeletal muscles, veins are compressed and blood is propelled centrally, a mechanism known as the skeletal muscle pump.
• Venous valves prevent retrograde flow during muscle relaxation, thereby enhancing venous return and supporting cardiac output.

Venous Pressures

Central Venous Pressure

• Central venous pressure (CVP) is the pressure within the thoracic great veins near their entry into the right atrium.
• CVP reflects right atrial pressure and is an important indicator of venous return and right ventricular function.
• Normal CVP ranges from approximately 0 to 5 millimeters of mercury.
• During vigorous cardiac contraction or increased heart rate, right atrial pressure may transiently become slightly subatmospheric.
• CVP is measured directly by introducing a catheter into a central vein, usually the internal jugular or subclavian vein.
• Assessment of jugular venous pressure provides a useful bedside estimate of right atrial pressure and right heart function.

Peripheral Venous Pressure

• Venous pressure is highest in venules, averaging about 10 to 15 millimeters of mercury.
• As blood flows toward larger veins and the right atrium, venous pressure progressively decreases.
• In the large systemic veins outside the thorax, pressure is usually around 5 millimeters of mercury.

Effect of Gravity on Venous Pressure

• Body posture significantly influences venous pressure because of gravitational effects.
• In the standing position, venous pressure decreases by approximately 0.77 millimeters of mercury for every centimeter above the level of the right atrium.
• Consequently, veins in the head and neck tend to collapse because their transmural pressure becomes very low.
• Collapsed veins have an effective intraluminal pressure close to zero rather than a sustained negative pressure.
• In contrast, venous pressure increases by approximately 0.77 millimeters of mercury for every centimeter below the right atrium.
• Therefore, venous pressure in the feet during quiet standing may reach 80 to 90 millimeters of mercury.
• Unlike superficial veins, the rigid-walled intracranial dural venous sinuses remain patent and may exhibit subatmospheric pressure in the upright posture.