Cardiovascular System - Integration of Cardiac Output and Venous Return

Question 1

stroke volume (SV)

Answer 1

• volume of blood ejected by the ventricle on each heart beat
• typical value 70mL

Question 2

ejection fraction (EF)

Answer 2

• fraction of the end-diastolic volume ejected in each stroke volume
• measure of ventricular efficiency (indicator of contractibility)
• typical value of 55%

Question 3

cardiac ouptut

Answer 3

• volume of blood ejected by the ventricle per unit time

- typical 5L/min

Question 4

main determinants of stroke volume (3)

Answer 4

1. preload
2. contractility
3. afterload

Question 5

preload

Answer 5

• pressure load prior to the contraction of the heart

- determinant of stroke volume

Question 6

contractility

Answer 6

• heart’s ability to produce face at any given stretch

- determinant of stroke volume

Question 7

afterload

Answer 7

pressure against which the heart must work to eject blood during systole

Question 8

(1) systolic and (2) diastolic left ventricular pressure-volume curves

Answer 8

1. systolic curve shows active pressure as a function of end-diastolic volume
2. diastolic curve shows passive pressure as a function of end-diastolic volume
   basis for the Frank-Starling relationship in the heart

Question 9

Otto Frank (experiment and findings)

Answer 9

• german physiologist
• ligated the aorta of frogs heart (isovolumetric contractions)
• measured the pressure generated when the heart was stretched by increasing the diastolic volume
• found that isovolumetric pressure increased with stretch

Question 10

length-tension relationship in cardiac muscle

Answer 10

relationship between the pressure developed during systole and the volume present in the ventricle just prior to systole

Question 11

Ernest Starling (experiment and findings)

Answer 11

• British physiologist
• isolated heart and lung of a dog
• central venous pressure and arterial pressure measured by mercury manometers
• found central venous pressure = pressure at entrance to right atrium

Question 12

Frank-Starling Law of Heart

Answer 12

• ^ central venous pressure, ^ the output of both ventricles
• ^ right atrial pressure, ^ stroke volume of both ventricles
• volume heart ejects in systole = volume it receives in venous return
• stroke volume and cardiac output correlate directly with the end-diastolic volume (which correlates with venous return)
• cardiac output matches the venous return

Question 13

PV loop of the cardiac cycle

Answer 13

area of the PV loop is the PV work done by the heart

see graph

Question 14

effect of preload and afterload on PV loop

Answer 14

• if preload is increased while after load is held constant, SV increases!
• if afterload is increased while preload held constant, decreases SV

Question 15

effect of sympathetic stimulation on PV loop

Answer 15

• increases rate and cardiac contractility, increases the SV

- shifts isovolumetric systolic curve up and to the left

Question 16

effect of sympathetic stimulation on ventricular function curve

Answer 16

• positive inotropic agents shift the cardiac function curve up and to the left
• positive inotropic agents increase the strength of cardiac contraction

Question 17

indicator dilution method for estimating cardiac output

Answer 17

t1: indicator injected rapidly at some time
t2: indicator in bolus in central veins
t3: indicator moves forward into systemic circulation (bolus in heart and lungs)
t4: bolus in systemic arteries

• arterial blood samples are obtained rapidly over 30s
• C(t) depends on rate of injection, area under curve doesn’t
  (area under curve is complicated by recirculation of indicator through coronary circulation)

Question 18

cardiac function curve

Answer 18

• plots the cardiac output against the right arterial pressure
• right atrial pressure is the preload of the heart that determines its degree of stertch and its force of contraction
• as CO increases, EDV increases because of the length-tension relationship in the ventricles
• when EDV becomes very high the curve starts to bend because the ventricles reach a limit (unable to keep up with the venous return)

Question 19

vascular function curve

Answer 19

• plots the venous return (CO) against the right atrial pressure
• inverse relationship between venous return and right atrial pressure is explained by the pressure gradient between the veins and the right atrium

Question 20

cardiac and vascular function curves

Answer 20

operating point of the the cardiovascular system matches cardiac function to vascular function

Question 21

effect of changes in blood volume on the steady-state operating point

Answer 21

• hemorrhage causes loss of blood from the circulation, which reduces the stressed volume and lowers the operating point
• transfusion increases cardiac output and raises operating point

Question 22

effects of changing cardiac contractility on the steady-state operating point

Answer 22

diseased heart decreases the cardiac output and the operating point

Question 23

steady-state operating point during exercise (4)

Answer 23

strenuous exercise:

1. increased heart rate
2. increased cardiac contractility
3. vasoconstriction (some vascular beds)
4. venoconstriction