The Arterial System

The Hydraulic Filter Converts Pulsatile Flow to Steady Flow

The principal functions of the systemic and pulmonary arterial systems are to distribute blood to the capillary beds throughout the body. The arterioles, which are the terminal components of the arterial system, regulate the distribution of flow to the various capillary beds. In the region between the heart and the arterioles, the aorta and pulmonary artery, and their major branches constitute a system of conduits of considerable volume and distensibility. This system of elastic conduits and high-resistance terminals constitutes a hydraulic filter that is analogous to the resistance-capacitance filters of electrical circuits.

Hydraulic filtering converts the intermittent output of the heart to a steady flow through the capillaries. This important function of the large elastic arteries has been likened to the Windkessels of antique fire engines. The Windkessel in such a fire engine contains a large volume of trapped air. The compressibility of the air trapped in the Windkessel converts the intermittent inflow of water to a steady outflow of water at the nozzle of the fire hose.

The analogous function of the large elastic arteries is illustrated in Fig. 7.1 . The heart is an intermittent pump. The cardiac stroke volume is discharged into the arterial system during systole. The duration of the discharge usually occupies about one third of the cardiac cycle. As shown in Fig. 4.13 , most of the stroke volume is pumped during the rapid ejection phase. This phase constitutes about half of systole. Part of the energy of cardiac contraction is dissipated as forward capillary flow during systole. The remaining energy in the distensible arteries is stored as potential energy (see Fig. 7.1A–B ). During diastole, the elastic recoil of the arterial walls converts this potential energy into capillary blood flow. If the arterial walls had been rigid, capillary flow would have ceased during diastole.

Hydraulic filtering minimizes the cardiac workload. More work is required to pump a given flow intermittently than steadily; the steadier the flow, the less is the excess work. A simple example illustrates this point.

Consider first that a fluid flows at the steady rate of 100 mL/s through a hydraulic system that has a resistance of 1 mm Hg/mL/s. This combination of flow and resistance would result in a constant pressure of 100 mm Hg, as shown in Fig. 7.2A . Neglecting any inertial effect, hydraulic work, W, may be defined as

that is, each small increment of volume, dV, pumped is multiplied by the pressure, P, that exists at that time. The products are integrated over the time interval, t ₂ – t ₁ , to yield the total work. When flow is steady,

In the example in Fig. 7.2A , the work done in pumping the fluid for 1 s would be 10,000 mm Hg/mL (or 1.33 × 10 ⁷ dyn-cm). Next, consider an intermittent pump that generates a constant flow of fluid for 0.5 s and then pumps nothing during the next 0.5 s. Hence flow is generated at the rate of 200 mL/s for 0.5 s, as shown in Fig. 7.2B and C . In panel B , the conduit is rigid, and the fluid is incompressible. However, the system has the same resistance to flow as in panel A. During the pumping phase of the cycle (systole), the flow of 200 mL/s through a resistance of 1 mm Hg/mL/s would produce a pressure of 200 mm Hg. During the filling phase (diastole) of the pump, the pressure in this rigid system would be 0 mm Hg. The work done during systole would be 20,000 mm Hg/mL. This value is twice that required in the example shown in Fig. 7.2A .

If the system were very distensible, hydraulic filtering would be very effective and the pressure would remain virtually constant throughout the entire cycle (see Fig. 7.2C ). Of the 100 mL of fluid pumped during the 0.5 s of systole, only 50 mL would be emitted through the high-resistance outflow end of the system during systole. The remaining 50 mL would be stored by the distensible conduit during systole, and it would flow out during diastole. Hence the pressure would be virtually constant at 100 mm Hg throughout the cycle. The fluid pumped during systole would be ejected at only half the pressure that prevailed in Fig. 7.2B . Therefore the work would be only half as great. If filtering were nearly perfect, as in Fig. 7.2C , the work would be identical to that for steady flow (see Fig. 7.2A ).

Naturally, the filtering accomplished by the systemic and pulmonic arterial systems is intermediate between the examples in Fig. 7.2B and C . The additional work imposed by the intermittent pumping, in excess of that for steady flow, is about 35% for the right ventricle and about 10% for the left ventricle. These fractions change, however, with variations in heart rate, peripheral resistance, and arterial distensibility.

The greater cardiac energy requirement imposed by a rigid arterial system is illustrated in Fig. 7.3 . In a group of anesthetized dogs, the cardiac output pumped by the left ventricle was allowed to flow either through the natural route (the aorta) or through a stiff plastic tube to the peripheral arteries. The total peripheral resistance (TPR) values were virtually identical, regardless of which pathway was selected. The data (see Fig. 7.3 ) from a representative animal show that, for any given stroke volume, the myocardial oxygen consumption ( $\text{M}\dot{\text{V}}{\text{o}}_{\text{2}}$ ) was substantially greater when the blood was diverted through the plastic tubing than when it flowed through the aorta. This increase in ${\text{M}\dot{\text{V}}}_{{\text{o}}_2}$ indicates that the left ventricle had to expend more energy to pump blood through a less compliant conduit than through a more compliant conduit.

Arterial Elasticity Compensates for the Intermittent Flow Delivered by the Heart

The elastic properties of the arterial wall are determined by the composition and mechanical properties of the vessel. Two important constituents of the arterial wall are elastic fibers, composed of elastin and microfibrils, and collagen. Elastin is elaborated by endothelial cells and is found in the tunica intima, whereas collagen is derived from myofibroblasts in the tunica adventitia.

The elastic properties of the arterial wall may be appreciated by considering first the static pressure-volume relationship for the aorta. To derive the curves shown in Fig. 7.4 , aortas were obtained at autopsy from individuals in different age groups. All branches of each aorta were ligated and successive volumes of liquid were injected into this closed elastic system. After each increment of volume, the internal pressure was measured. In Fig. 7.4 , the curve that relates pressure to volume in the youngest age group (curve a) is sigmoidal. Although the curve is nearly linear, the slope decreases at the upper and lower ends. At any point on the curve, the slope (dV/dP) represents the aortic compliance. Thus in young individuals, the aortic compliance is least at very high and low pressures and greatest at intermediate pressures. This sequence of compliance changes resembles the familiar compliance changes encountered in inflating a balloon. The greatest difficulty in introducing air into the balloon is experienced at the beginning of inflation and again at near-maximal volume, just before the balloon ruptures. At intermediate volumes, the balloon is relatively easy to inflate; that is, it is more compliant.

The previously described effects of the subject’s age on the elastic characteristics of the arterial system were derived from aortas removed at autopsy (see Fig. 7.4 ). Such age-related changes have been confirmed in living subjects by ultrasound imaging techniques. These studies disclosed that the increase in the diameter of the aorta produced by each cardiac contraction is much less in elderly persons than in young persons ( Fig. 7.5 ). The effects of aging on the elastic modulus of the aorta in healthy subjects are shown in Fig. 7.6 . The elastic modulus, E _p , is defined as:

where ΔP is the aortic pulse pressure, ( Fig. 7.7) , D is the mean aortic diameter during the cardiac cycle, and Δ D is the maximal change in aortic diameter during the cardiac cycle.

The fractional change in diameter (ΔD/D) of the aorta during the cardiac cycle reflects its change in volume (ΔV) as the left ventricle ejects its stroke volume into the aorta each systole. Thus E _p is inversely related to compliance, which is the ratio of ΔV to ΔP. Consequently, the increase in elastic modulus with aging (see Fig. 7.6 ) and the decrease in compliance with aging (see Fig. 7.4 ) both reflect the stiffening (arteriosclerosis) of the arterial walls as individuals age.

The Arterial Blood Pressure is Determined by Physical and Physiological Factors

The determinants of the pressure within the arterial system of intact subjects cannot be evaluated precisely. Nevertheless, arterial blood pressure is routinely measured in patients, and it provides a useful clue to cardiovascular status. We therefore take a simplified approach to explain the principal determinants of arterial blood pressure. To accomplish this, the determinants of the mean arterial pressure , defined in the next section, are analyzed first. Systolic and diastolic arterial pressures are then considered as the upper and lower limits of the periodic oscillations around this mean pressure.

The determinants of the arterial blood pressure may be subdivided arbitrarily into physical and physiological factors ( Fig. 7.8 ). The arterial system is assumed to be a static, elastic system. The only two physical factors are considered to be the blood volume within the arterial system and the elastic characteristics ( compliance ) of the system. The following physiological factors will be considered: (1) the cardiac output, which equals heart rate × stroke volume ; and (2) the peripheral resistance . Such physiological factors operate through one or both of the physical factors.