When arteries are normally compliant, blood flows through the capillaries throughout the cardiac cycle. However, when the arteries become rigid, blood flow only occurs during systole and ceases during diastole. The physical factors that affect arterial blood flow include the fluid volume within the arterial system and the compliance of the system. Meanwhile, the physiological factors are cardiac output, which is calculated by multiplying heart rate by stroke volume, and peripheral resistance.

To estimate arterial blood pressure (Pa) from an arterial blood pressure tracing, one can divide the area under the pressure curve by the time interval involved. Alternatively, Pa can be approximated using the measured values of systolic pressure (Ps) and diastolic pressure (Pd) with the following formula:

Pa ≈ Ps + (Pd - Ps) / 3

Pa depends on mean blood volume in the arterial system and arterial compliance. Arterial volume (Va) is influenced by the rate of inflow (Qh) into the arteries from the heart and the rate of outflow (Qr) from the arteries through the resistance vessels. These relationships can be expressed mathematically as:

dVa/dt = Qh - Qr

When Qh exceeds Qr, arterial volume increases, leading to a rise in pressure. Conversely, when Qr exceeds Qh, arterial volume decreases, causing a decrease in pressure. Arterial pulse pressure, which is the difference between systolic pressure and diastolic pressure, is primarily determined by stroke volume and arterial compliance.

During the rapid ejection phase of systole, the volume of blood entering the arterial system exceeds the volume exiting through the arterioles. As a result, arterial pressure and volume reach their peak during this phase. Throughout the rest of the cardiac cycle (ventricular diastole), arterial blood volume decreases, leading to a minimum pressure known as diastolic pressure.

The pressure-volume relationship of the aorta varies with age, and arterial compliance decreases as individuals get older. The two physical determinants of pulse pressure are arterial compliance (Ca) and the change in arterial volume. The two physiological determinants of mean arterial pressure (P) are cardiac output and total peripheral resistance. Arterial compliance is defined as the ratio of blood volume to mean blood pressure.
•Fig. 17.11 Whenarteriesarenormallycompliant(A andB),bloodflowsthroughthecapillariesthroughoutthecardiaccycle.Whenthearteriesarerigid,bloodflowsthroughthecapillariesduringsystole (C),butflowceasesduringdiastole(D).
physical factors, or fluid mechanical characteristics, are fluid volume (i.e., blood volume) within the arterial system and the static elastic characteristics (compliance) of the system. The physiological factors are cardiac output (which equals heart rate × stroke volume) and peripheral resistance.
To estimate Pa from an arterial blood pressure tracing, the area under the pressure curve is divided by the time interval involved (see
Fig. 17.13 ). Alternatively, Pa can be approximated from the measured values of systolic pressure (Ps) and diastolic pressure (Pd) by means of the following formula:
Equation 17.14
Consider that Pa depends on only two physical factors: mean blood volume in the arterial system and arterial compliance (
Fig. 17.14 ). Arterial volume (Va), in turn, depends on the rate of inflow, (Qh) into the arteries from the heart (cardiac output) and on the rate of outflow (Qr) from the arteries through the resistance vessels (peripheral runoff). These relationships are expressed mathematically as
Equation 17.15 where dVa/dt is the change in arterial blood volume per unit of time. If Qh exceeds Qr, arterial volume increases, the arterial walls are stretched further, and pressure rises. The converse happens when Qr exceeds Qh. When Qh equals Qr, Pa remains constant. Thus increases in cardiac output raise Pa, as do increases in peripheral resistance. Conversely, decreases in cardiac output or peripheral resistance decrease Pa.
Arterial pulse pressure is systolic pressure minus diastolic pressure. It is principally a function of just one physiological factor, stroke volume, which determines the change in arterial blood volume (a physical factor) during ventricular systole. This physical factor, in addition to a second physical factor (arterial compliance), determines the arterial pulse pressure (see
Fig. 17.14
As described previously, Pa depends on cardiac output and peripheral resistance. During the rapid ejection phase of systole, the volume of blood introduced into the arterial system exceeds the volume that exits the system through the arterioles. Arterial pressure and volume therefore peak; the peak arterial pressure is systolic pressure. During the remainder of the cardiac cycle (i.e., ventricular diastole), exceeds cardiac ejection. The resultant decrement in arterial blood volume thus causes pressure to fall to a minimum, which is diastolic pressure.
Fig. 17.15 illustrates the effect cardiac ejection is zero, and peripheral runoff now greatly of stroke volume on pulse pressure when arterial compliance is constant.
•Fig. 17.12 Pressure-volumerelationshipsofaortasobtainedat autopsyfromhumansindifferentagegroups(denotedbythenumbers•Fig. 17.13 Arterial Systolic, Diastolic, Pulse, and Mean Pres-attherightendofeachofthecurves).Notethatcompliance(ΔV/ΔP)sure. Meanarterialpressure(P)representstheareaunderthearterialdecreaseswithage.(RedrawnfromHallockP,BensonIC.J Clin Invest. pressurecurve(dark red) dividedbythedurationofthecardiaccycle1937;16:595.)(t2− t1).
•Fig. 17.14 Thetwophysicaldeterminantsofpulsepressurearearterialcompliance(Ca)andthechangeinarterialvolume.Thetwophysiologicaldeterminantsofmeanarterialpressure(P)arecardiacoutputandtotalperipheralresistance.
Arterial compliance (Ca), the ratio of blood volume to mean blood pressure (see