Human Physiology - William F. Ganong 2002

Blood Circulation
Hemodynamics and Lymph Dynamics
Venous Circulation

Blood moves through Blood Vessels, including Veins, primarily due to the pumping action of The Heart. However, venous blood flow is also assisted by heart contractions, the decrease in negative intrathoracic pressure during each inspiration, and the contractions of skeletal Muscles that compress the veins (the Muscle pump).

Venous Pressure and Flow

Pressure in the venules is 12-18 mmHg. As the caliber of the veins increases, the pressure drops, reaching 5.5 mmHg in the large veins outside the thoracic cavity. Pressure in the great veins where they enter the right atrium (central venous pressure) averages 4.6 mmHg, though it fluctuates depending on the respiratory cycle and Cardiac Activity.

Peripheral venous pressure, like arterial pressure, is subject to gravitational forces. It increases by 0.77 mmHg for every centimeter below the right atrium and decreases by approximately the same amount for every centimeter above it (see Fig. 30-17).

Blood flows from venules into large veins, where its velocity increases due to a reduction in the total cross-sectional area of the vessels. In the large veins, the velocity is four times lower than in the aorta, amounting to about 10 cm/s.

Thoracic Pump

During inspiration, intrathoracic pressure drops from -2.5 to -6 mmHg. This negative pressure is transmitted to the large veins and, to a lesser extent, to the atria. Consequently, central venous pressure fluctuates from about 6 mmHg during expiration to approximately 2 mmHg during quiet inspiration. The drop in venous pressure during inspiration facilitates venous return. During inspiration, the Diaphragm descends and intra-abdominal pressure rises, which also AIDS blood flow toward the heart, while venous Valves prevent backflow into the VEINS OF THE lower extremities.

Effect of Cardiac Contraction

Pressure Changes in the atria are transmitted to the large veins, producing the a, c, and v waves of the venous pulse curve (see Chapter 29). Atrial pressure drops sharply during the ejection phase of ventricular systole because the atrioventricular valves are pulled downward, which increases atrial capacity. As a result, blood is "suctioned" from the large veins into the atria. This process promotes venous return, especially at high heart rates.

As blood approaches the heart, venous flow becomes pulsatile. At low heart rates, two periods of maximum flow are recorded: one during ventricular systole due to the downward Displacement of the atrioventricular valves, and another at the onset of diastole during the period of rapid ventricular filling (see Fig. 30-15).

Muscle Pump

In the limbs, veins are surrounded by skeletal muscles; the contraction of these muscles during physical activity compresses the veins. Pulsation of nearby Arteries can also compress veins. Because venous valves prevent backflow, blood is forced to move toward the heart. When a person is standing, the effects of gravity are fully operative, and venous pressure in the calf is 85-90 mmHg (see Fig. 30-17). Blood pooling in the lower extremities causes a decrease in venous return and Cardiac Output, which can sometimes lead to fainting. Rhythmic contractions of the lower limb muscles while standing help propel blood toward The Heart and reduce venous pressure in the lower extremities to 30 mmHg or less. This upward movement of blood is impaired in patients with varicose veins because their valves become incompetent. However, even with incompetent valves, muscle contractions still facilitate blood movement toward the heart. This occurs because the resistance of large veins located closer to the heart is lower than that of smaller vessels situated farther away.

Venous Pressure in the HEAD

In the upright position, venous pressure in body regions located above the heart is reduced by the force of gravity. The neck veins collapse above the level where venous pressure approaches zero. Pressure throughout the collapsed segments remains near zero, but does not drop below atmospheric pressure. The Dural Venous Sinuses have rigid walls and cannot collapse, so the pressure within them during sitting or standing is subatmospheric. The magnitude of this negative pressure is directly proportional to the vertical distance above the apex of the collapsed neck veins and reaches 10 mmHg in the superior sagittal sinus. Neurosurgical Procedures are sometimes performed in the sitting position. If one of the sinuses is opened during such a Procedure, air may be drawn into it, leading to an air embolism.

Air Embolism

Because air, unlike liquid, is compressible, its presence in the Circulation can have adverse consequences. Forward blood flow relies on the incompressible nature of blood. A large volume of air filling the heart causes a cessation of circulation and leads to sudden death. This happens because during ventricular contraction, the air is compressed rather than being pushed into the arteries. A small amount of air escaping the heart enters the vessels, where air bubbles obstruct small blood vessels, causing a sharp increase in flow resistance that slows or halts circulation. Obstruction of small cerebral vessels leads to severe and even fatal neurological disorders. Hyperbaric Oxygenation (see Chapter 37) can be therapeutic in these cases because pressure reduces the size of gas emboli. The volume of air required to cause a fatal air embolism in laboratory animals depends, among other factors, on The rate of its entry into the veins. In some cases, even 100 mL causes no adverse effects, whereas at other times as little as 5 mL can prove fatal.

Measurement of Venous Pressure

Central venous pressure can be measured by placing a catheter into the great veins of the thoracic cavity. Peripheral venous pressure generally correlates with central venous pressure.

To measure peripheral venous pressure, a needle connected to a manometer containing a sterile isotonic solution is inserted into an arm vein. The peripheral vein must be at the level of the right atrium (10 cm, or half the chest diameter from the back in the supine position). Values obtained in millimeters of isotonic solution can be converted to millimeters of mercury 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 to the veins. The normal mean pressure in the antecubital vein is 7.1 mmHg, and the mean pressure in the central veins is 4.6 mmHg.

Central venous pressure can be measured very accurately without any equipment by determining the degree of distension of the external jugular veins in the supine position with the head slightly elevated above the heart. The vertical distance between the right atrium and the point of venous collapse (where pressure equals zero) represents the venous pressure expressed in millimeters of blood.

Central venous pressure is decreased by negative intrathoracic pressure and in Shock. It is increased by positive intrathoracic pressure, exercise, an expanded circulating blood volume, and Heart Failure. In severe heart failure or SUPERIOR VENA CAVA obstruction, pressure in the antecubital vein can reach 20 mmHg or higher.