Overview Of The Cardiovascular System

Conceptually, it is useful to subdivide the cardiovascular system into the following four functional components: blood, vessels, heart, and its associated control systems. As shown in Fig. 1, blood circulates within a closed series of vessels consisting of a systemic and a pulmonary circuit that are in series with each other. The blood functions as the carrier for transporting substances to and from the various tissues. Although simple diffusion can be an effective transporter of material, it is practical only over relatively short distances. The circulating blood keeps the internal fluid matrix well stirred such that most of the transport is by convection rather than by diffusion. Once a substance arrives at the tissues, it diffuses across the capillary walls as dictated by the concentration gradients. For example, the blood picks up oxygen in the lungs, nutrients in the intestine, and hormones in the various endocrine organs, where each of these substances is in excess. These substances will then be delivered to tissues where their concentrations are low on a subsequent pass through the circulation. Blood also acts to remove noxious products from the cells. Carbon dioxide produced by metabolizing cells is picked up by the circulating blood and transported to the lungs, where it is removed. Metabolic wastes and excess electrolytes are removed from the blood at the kidneys. The blood even removes excess heat from metabolizing

FIGURE 1 Diagram of the human cardiovascular system illustrating how blood flow is distributed to the various organ systems.

tissues and transports it to the skin where it can be radiated into the environment.

The cardiovascular circuitry depicted in Fig. 1 illustrates that the right heart, lungs, left heart, and systemic vasculature are in series with one another; thus, the volume of blood flowing through any of these four regions per unit of time must be equal. Flow leaving either the right or left heart is cardiac output, whereas flow returning to the heart is venous return. At a steady state, cardiac output and venous return must be equal. In Fig. 1, cardiac output and venous return are illustrated as being 5 L/min. The right heart, pulmonary artery, lungs, and pulmonary veins are collectively referred to as the pulmonary circulation. The left heart, systemic arteries, peripheral capillary beds, and systemic veins make up the systemic circulation.

The study of the physical principles governing the movement of blood within the cardiovascular system is known as hemodynamics. In its simplest form, the cardiovascular system can be considered to be a pump (the heart) in series with a system of tubes through which it pumps blood. In this chapter, we examine the physical principles that dictate the motion of fluids and see how they apply to the cardiovascular system.

HYDROSTATICS: THE PHYSICS OF FLUID AT REST

A liquid within a container exerts a force on the walls of the container that we call pressure and has the units of force per unit area. Gravitational forces have a profound effect on the pressure within a fluid system because fluid at the bottom of a container is compressed by the weight of the fluid above it. The pressure (P) at any point in a container of fluid that is open to the atmosphere depends on the vertical distance (h) between that point and the surface of the fluid, the density of the fluid ( p), and the acceleration due to gravity (g). This relationship is given as:

For the example given in Fig. 2, a tank is filled with water having a density of 1 g/cm3. At a depth of 136 cm, the pressure on a side tube would be 133,280 dynes/cm2 because the acceleration of gravity is 980 cm/sec2. Because the fluid is not moving, the pressure is equal in all directions and a second gauge oriented vertically at the same point records the same pressure. The gauges in

FIGURE 2 In an open system in which there is no flow, the pressure exerted by a column of water 136 cm high is 100 mm Hg. Also, side pressure and end pressure are equal.

the figure show 100 rather than 133,280, because the units dynes/cm2 are seldom used in medicine. Rather, pressure is usually expressed relative to that required to push a column of mercury up a manometer tube. A mercury manometer makes for a simple yet accurate measuring device in the physician's office. A direct conversion is possible because mercury is 13.6 times as dense as water (100 mm x 13.6 = 1360 mm, or 136 cm). Thus, a 100-mm-high column of mercury (10 cm) will exert a pressure of 13.6 x 980 x 10, or 133,280 dynes per cm2. That means that the 136-cm-high column of water produces a pressure equal to a 100-mm-high column of mercury. In this new system, we would express this pressure as simply 100 mm Hg. Any other force that acts on the fluid will either add to or subtract from the pressure caused by the gravitational acceleration. For example, in Fig. 3, a compressional force of 100 mm Hg is applied to the surface of the tank. That pressure will be added to the preexisting pressures throughout the tank so that a pressure of 200 mm Hg will now be seen at the bottom of the tank.

HYDRODYNAMICS: THE PHYSICS OF FLUID IN MOTION

The movement of fluid is referred to as flow. Flow can be viewed in two ways. One way is to consider the displacement of a volume of fluid per unit of time. This is what is conventionally meant by the term flow and the

100 mm Hg

100 mm Hg

FIGURE 3 If an additional compressional force equal to 100 mm Hg is applied to the system described in Fig. 1, both end and side pressure will equal 200 mm Hg.

units of liters/minute or gallons/hour are familiar. Another way to appreciate flow is to consider the linear displacement of the individual particles of the fluid. As the fluid flows, each particle is moving at a finite velocity. Although one does not normally think of fluid movement in terms of velocity, fluid velocity is actually the major determinant of the distribution of forces within a moving fluid.

As a fluid flows through a tube, the mean velocity of flow (v) must be directly proportional to the flow rate (Q) and inversely proportional to the cross-sectional area (A) at that point. Thus:

In Fig. 4, a tube has three different diameters along its length. The flow rate will be the same in all three regions (assuming that fluid is neither created nor destroyed within the tube). If flow is set to 200 mL/ sec, the velocity of flow will be different in each region. The individual fluid particles will be moving through region 1 at an average of 100 cm/sec, through region 2 at 20 cm/sec, and through region 3 at 200 cm/sec. The narrower the tube the faster the individual fluid particles must travel to accommodate the flow. It should be emphasized that these velocities represent an average. As seen below, the velocities of the individual fluid particles are actually heterogeneous, with some traveling faster than this average and others traveling more slowly.

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