Porth's Essentials of Pathophysiology, 4e

377

Control of Cardiovascular Function

C h a p t e r 1 7

120

100

80

60

40

20

Pressure (mm Hg)

0

Lt. vent.

Lg. art.

Sm. art.

Arte- rioles

Rt. vent.

Pul. art.

Aorta

Caps.

Veins

64%

FIGURE 17-2. Pressure and volume distribution in the systemic circulation. The graphs show the inverse relation between internal pressure and volume in different portions of the circulatory system. (From Smith JJ, Kampine JP. Circulatory Physiology: The Essentials. 3rd ed. Baltimore, MD: J.B.Lippincott; 1990.)

60

40

20

16%

Total blood

volume (%)

4%

4%

4%

0

In the pulmonary circulation, the blood volume (approximately 450 mL in the adult) can vary from as low as 50% of normal to as high as 200% of normal. An increase in intrathoracic pressure, such as occurs when exhaling against a closed glottis, impedes venous return to the right heart. This can produce a transient shift from the pulmonary to the systemic circulation of as much as 250 mL of blood as blood backs up in the vena cava and large veins. Body position also affects the distribution of blood volume. In the recumbent posi- tion, approximately 25% to 30% of the total blood volume is in the pulmonary circulation. On standing, gravity causes a rapid displacement of this blood to the lower part of the body. Because the volume of the sys- temic circulation is approximately seven times that of the pulmonary circulation, a shift of blood from one system to the other has a much greater effect on the pulmonary than on the systemic circulation. The movement of blood from the arterial to the venous side of the circulation depends on a pressure dif- ference or gradient, moving from an area of higher pres- sure to one of lower pressure. The pressure distribution in different parts of the circulation is almost an inverse of the volume distribution (see Fig. 17-2). The pressure in the arterial side of the circulation, which contains only approximately one sixth of the blood volume, is much higher than the pressure on the venous side of the circulation, which contains approximately two thirds of the blood. This pressure and volume distribution is due in large part to the structure and relative elasticity of the arteries and veins. The pressure difference between the arterial and venous sides of the circulation (approxi- mately 84 mm Hg) provides the driving force for the flow of blood in the systemic circulation. The pulmo- nary circulation has a similar arterial–venous pressure difference, albeit it of a lesser magnitude, that facilitates blood flow.

SUMMARY CONCEPTS

■■ The cardiovascular or circulatory system, which consists of a pump (heart), a series of distributing (arteries and arterioles) and collecting (veins and venules) blood vessels, and an extensive system of exchange vessels (capillaries), functions mainly as a transport system that circulates nutrients and other materials to and removes waste products from tissues throughout the body. ■■ The circulatory system can be divided into two parts: the right heart and pulmonary circulation, which moves blood through the lungs and creates a link with the gas exchange function of the respiratory system; and the left heart and systemic circulation, which moves blood throughout all the other tissues of the body. ■■ Blood flow in the circulatory system depends on a blood volume that is sufficient to fill the blood vessels and a pressure difference across the system that provides the force that is needed to move blood forward. The venous system, which is a low pressure system designed to collect and return blood to the heart, contains about two-thirds of the blood; the arterial system, which is a high pressure distributive system, contains about one-sixth of the blood; and the capillaries, which have the lowest pressure and function as an exchange system for gases, nutrients, and wastes, contain the least amount of blood.