Description

Laura D. Rosenthal DNP, ACNP, FAANP

Hemodynamics is the study of the movement of blood throughout the circulatory system, along with the regulatory mechanisms and driving forces involved. Concepts introduced here reappear throughout the chapters on cardiovascular drugs, so we urge you to review these now. Because this is a pharmacology text, and not a physiology text, discussion is limited to hemodynamic factors that have particular relevance to the actions of drugs.

Overview of the Circulatory System

The circulatory system has two primary functions: (1) delivery of oxygen, nutrients, hormones, electrolytes, and other essentials to cells and (2) removal of carbon dioxide and metabolic wastes from cells. In addition, the system helps fight infection.

The circulatory system has two major divisions: the pulmonary circulation and the systemic circulation. The pulmonary circulation delivers blood to the lungs. The systemic circulation delivers blood to all other organs and tissues. The systemic circulation is also known as the greater circulation or peripheral circulation.

Components of the Circulatory System

The circulatory system is composed of the heart and blood vessels. The heart is the pump that moves blood through the arterial tree. The blood vessels have several functions:

• • Arteries transport blood under high pressure to tissues.
• • Arterioles are control valves that regulate local blood flow.
• • Capillaries are the sites for exchange of fluid, oxygen, carbon dioxide, nutrients, hormones, and wastes.
• • Venules collect blood from the capillaries.
• • Veins transport blood back to the heart. In addition, veins serve as a major reservoir for blood.

Arteries and veins differ with respect to distensibility (elasticity). Arteries are very muscular and hence do not readily stretch. As a result, large increases in arterial pressure (AP) cause only small increases in arterial diameter. Veins are much less muscular and hence are 6 to 10 times more distensible. As a result, small increases in venous pressure cause large increases in vessel diameter, which produce a large increase in venous volume.

Distribution of Blood

The adult circulatory system contains about 5 L of blood, which is distributed throughout the system. As indicated in Fig. 34.1, 9% is in the pulmonary circulation, 7% is in the heart, and 84% is in the systemic circulation. Within the systemic circulation, however, distribution is uneven: most (64%) of the blood is in veins, venules, and venous sinuses; the remaining 20% is in arteries (13%) and arterioles or capillaries (7%). The large volume of blood in the venous system serves as a reservoir.

FIGURE 34.1 Distribution of blood in the circulatory system. A large percentage of the blood resides in the venous system.

What Makes Blood Flow?

Blood moves within vessels because the force that drives flow is greater than the resistance to flow. As shown in Fig. 34.2, the force that drives blood flow is the pressure gradient between two points in a vessel. Blood will flow from the point where pressure is higher toward the point where pressure is lower. Resistance to flow is determined by the diameter and length of the vessel and by blood viscosity. From a pharmacologic viewpoint, the most important determinant of resistance is vessel diameter: the larger the vessel, the smaller the resistance. Accordingly, when vessels dilate, resistance declines, causing blood flow to increase—and when vessels constrict, resistance rises, causing blood flow to decline. To maintain adequate flow when resistance rises, blood pressure must rise as well.