SECTION III THE CARDIOVASCULAR SYSTEM Atheroscleroais A 43-year-old mn and his father,age 80.both had heart rates of 60 beats/nin, cardiac outputs of 4.8 L/ain.stroke volumes of 80 ml.and total peripheral resistances of 20 mm H/L/min.Also,the tine distributions of ventricular systole and diastole were similar in hoth individuals:systole occupied about 0.3 s.and diastole occupied about 0.7 s.The principal disparity in the physical characteristics of their cardiovascular systems was that the son had a very compliant arterial systen.whereas the father's arterial systea was very noncompliant.The son's arterial compliance (blood volune accommodated per unit change of pressure) was estimated to be about 3 ml/mm Ha.whereas that of the father was only about 0.3 al/m Hg. 1.What effect did the differences in arterfal compliance have on the nean arterial pressures of these two individuals? 2.What effect did the differences in arterial compliance have on the systolic and diastolic arterial pressures of these two individuals? 3.What effect did the differences in arterial compliance have on the cardiac work of these two individuals? ANSVER 1.The pean arterial pressures were the same in both individuals.despite the differences in arterial compliance.Because both people had cardiac outputs (Qh) of 4.8 L/min,the flow (peripheral runoff,Qr)from arteries to veins through the systemic resistance vessels also had to be 4.8 L/min under steady state comditions. To force the flow of 4.8 L/min through a total peripheral resistance (Rt)of 20 mm Hg/L/min,the mean arteriovenous pressure difference (Pa Pv)over the entire time
SECTION III THE CARDIOVASCULAR SYSTEM Atherosclerosis A 43-year-old man and his father, age 80, both had heart rates of 60 beats/min, cardiac outputs of 4.8 L/min, stroke volumes of 80 ml, and total peripheral resistances of 20 mm Hg/L/min. Also, the time distributions of ventricular systole and diastole were similar in both individuals; systole occupied about 0.3 s, and diastole occupied about 0.7 s. The principal disparity in the physical characteristics of their cardiovascular systems was that the son had a very compliant arterial system, whereas the father's arterial system was very noncompliant. The son's arterial compliance (blood volume accommodated per unit change of pressure) was estimated to be about 3 ml/mm Hg, whereas that of the father was only about 0.3 ml/mm Hg. 1. What effect did the differences in arterial compliance have on the mean arterial pressures of these two individuals? 2. What effect did the differences in arterial compliance have on the systolic and diastolic arterial pressures of these two individuals? 3. What effect did the differences in arterial compliance have on the cardiac work of these two individuals? ANSWER 1. The mean arterial pressures were the same in both individuals, despite the differences in arterial compliance. Because both people had cardiac outputs (Qh) of 4.8 L/min, the flow (peripheral runoff, Qr) from arteries to veins through the systemic resistance vessels also had to be 4.8 L/min under steady state conditions. To force the flow of 4.8 L/min through a total peripheral resistance (Rt) of 20 mm Hg/L/min, the mean arteriovenous pressure difference (Pa – Pv) over the entire time
of a cardiac cycle would have to be Rt x Qr (or 20 x 4.8),which equals 96 m Hg: this computation is based on the definition of total peripheral resistance (Rt). nanely that Rt (Pa Pv)/Qr Because Py is usually slightly greater than 0.Pa will be only slightly less than 96 mm Hg.This reans arterial pressure will be independent of the arterial compliance. 2.The aorta was normally compliant in the younger man.This person's left ventricle ejected a stroke volume of 80 ml in 0.3 sec.Because his aorta was so distensible,most of the ejected blood could be stored in the aorta during the ejection period itself,and only a relatively small fraction of the stroke volune would pass froa the arteries through the small resistance vessels and into the veins during systole (which usually occupfes only a small fraction of the cardiac cycle). The remainder of the stored blood that had been ejected from the left ventricle traverses the resistance vessels throughout diastole.If the maximm increment in blood volue (AVa)in the aorta during the ejection phase of systole was about 60 ml,and if the arterial compliance (Ca)of the grandson were 3 nl/m Hg.then the change in arterial pressure (APa)during ejection would be AVaa/Ca (or 60/3),which equals 20 mm Hg.Thus,at the very beginning of the period of ventricular ejection, Pa would be at its minimum value (defined as the diastolic arterial pressure),and as the blood accunulates in the arterial systea during ejection.Pa rises to its maxim value (defined as the systolic arterial pressure)at some time during eject ion.The diastolic pressure (Pd)would be below the mean arterial pressure and the systolic pressure (Ps)would be above the mean pressure.However,the difference between Ps and Pd.namely the pulse pressure,would be determined by the mximum increment in arterial volume (AVa)during ventricular ejection and by the arterial compliance(Ca）. Similarly,the father's arterial pulse pressure is determined by the sane factors.In this old person,however,the arterial system was mot very compliant. Therefore,very little of the stroke volune,ejected during systole,could he stored in the arteries and thereby provide a relatively steady blood flow throughout
of a cardiac cycle would have to be Rt x Qr (or 20 x 4.8), which equals 96 mm Hg; this computation is based on the definition of total peripheral resistance (Rt), namely that Rt = (Pa – Pv)/Qr Because Pv is usually slightly greater than 0, Pa will be only slightly less than 96 mm Hg. This means arterial pressure will be independent of the arterial compliance. 2. The aorta was normally compliant in the younger man. This person's left ventricle ejected a stroke volume of 80 ml in 0.3 sec. Because his aorta was so distensible, most of the ejected blood could be stored in the aorta during the ejection period itself, and only a relatively small fraction of the stroke volume would pass from the arteries through the small resistance vessels and into the veins during systole (which usually occupies only a small fraction of the cardiac cycle). The remainder of the stored blood that had been ejected from the left ventricle traverses the resistance vessels throughout diastole. If the maximum increment in blood volume (∆Va) in the aorta during the ejection phase of systole was about 60 ml, and if the arterial compliance (Ca) of the grandson were 3 ml/mm Hg, then the change in arterial pressure (∆Pa) during ejection would be ∆Vaa/Ca (or 60/3), which equals 20 mm Hg. Thus, at the very beginning of the period of ventricular ejection, Pa would be at its minimum value (defined as the diastolic arterial pressure), and as the blood accumulates in the arterial system during ejection, Pa rises to its maximum value (defined as the systolic arterial pressure) at some time during ejection. The diastolic pressure (Pd) would be below the mean arterial pressure and the systolic pressure (Ps) would be above the mean pressure. However, the difference between Ps and Pd, namely the pulse pressure, would be determined by the maximum increment in arterial volume (∆Va) during ventricular ejection and by the arterial compliance (Ca). Similarly, the father's arterial pulse pressure is determined by the same factors. In this old person, however, the arterial system was not very compliant. Therefore, very little of the stroke volume, ejected during systole, could be stored in the arteries and thereby provide a relatively steady blood flow throughout
diastole.Instead,almost the entire stroke volume had to be forced through the microcirculation during the brief period of ejection (about 0.3 sec in this exarple). To achieve a flo of alnost 80 nl per 0.3 sec (or 240 nl/sec over this brief period) through a peripheral resistance of 20 m Hg/L/min,the left ventricle would be required to develop a systolic pressure of almost 290 mm Hg.Furthermore,oece ejection ceases,a very small translocation of blood from arteries to veins early in diastole results in a precipitous drop in pressure because the arteries are so rigid.Hence,the diastolic arterial pressure is very low,and the arterial pulse pressure is extrenely large.Ordinarily,of course,such a high systolic arterial pressure is dangerous,and could not be sustained for very long.Yarious changes in hemodynamics (in heart rate,stroke volue,duration of systole,total peripheral resistance,and blood volune)and in the structural characteristics of the heart and arterial system take place during aging.and they enable the heart to adapt to the less coapliant arterial systen 3.The amount of work done by the hearts of those two individuals to eject the sane stroke volume (80 ml)differs markedly.The work done per beat can be estinated by multiplying the stroke volume by the nean pressure that exists during ejection. For the younger man.the stroke work is approximately 80 ml x 95 m Hg or 7.6 L.m Hg.For the father,on the other hand.the stroke work would be 80 ml x 290 mm Hg. or 23.2 L..mg.Therefore this example shows that considerable more work mast be done by the heart to force a given flow through the sane total peripheral resistance when the systeaic arteries are alnost noncompliant than when they are very compllant
diastole. Instead, almost the entire stroke volume had to be forced through the microcirculation during the brief period of ejection (about 0.3 sec in this example). To achieve a flow of almost 80 ml per 0.3 sec (or 240 ml/sec over this brief period) through a peripheral resistance of 20 mm Hg/L/min, the left ventricle would be required to develop a systolic pressure of almost 290 mm Hg. Furthermore, once ejection ceases, a very small translocation of blood from arteries to veins early in diastole results in a precipitous drop in pressure because the arteries are so rigid. Hence, the diastolic arterial pressure is very low, and the arterial pulse pressure is extremely large. Ordinarily, of course, such a high systolic arterial pressure is dangerous, and could not be sustained for very long. Various changes in hemodynamics (in heart rate, stroke volume, duration of systole, total peripheral resistance, and blood volume) and in the structural characteristics of the heart and arterial system take place during aging, and they enable the heart to adapt to the less compliant arterial system. 3. The amount of work done by the hearts of those two individuals to eject the same stroke volume (80 ml) differs markedly. The work done per beat can be estimated by multiplying the stroke volume by the mean pressure that exists during ejection. For the younger man, the stroke work is approximately 80 ml x 95 mm Hg or 7.6 L · mm Hg. For the father, on the other hand, the stroke work would be 80 ml x 290 mm Hg, or 23.2 L · mm Hg. Therefore this example shows that considerable more work must be done by the heart to force a given flow through the same total peripheral resistance when the systemic arteries are almost noncompliant than when they are very compliant