Lecture 3 – Control of Cardiac output

Question 1

Blood flow (Cardiac Output) formula

Answer 1

Blood flow (CO) = Blood pressure / Total peripheral resistance

Question 2

Preload

Answer 2

• Preload - Stretching of heart at rest, increases stroke volume, due to Starling’s law. Stretching of left ventricle on filling.
• Afterload - Opposes ejection, reduces stroke volume, due to Laplace’s law. Resistance to ejection.

Question 3

Energy of contraction (4)

Answer 3

Amount of work required to generate SV.
Depends on starlings law and contractility.
Carries out 2 functions:
Contracts until chamber pressure > aortic pressure - ISOVOLUMETRIC CONTRACTION.
Ejection from ventricle.

Question 4

STARLING LAW - PRELOAD (2)

Answer 4

More blood in = more stretching = more blood out = ejection and contraction is stronger.
• ‘Energy of contraction of cardiac muscle is relative to the muscle fibre length at rest’
• ’When the heart muscle is stretched, the muscle contracts harder’
• Greater stretch of ventricle in diastole (blood entering) = greater energy of contraction = greater stroke volume achieved in systole.

Question 5

Molecular basis of starlings law (4)

Answer 5

States that if muscle is stretched before contraction this allows:
less overlapping actin/myosin
less mechanical inference
potential for more cross-bridge formation
increased sensitivity to calcium ions

Question 6

Preload - roles and effects of starlings law (3)

Answer 6

Responsible for fall in CO during a drop in BV or vasodilation (haemorrhage/sepsis).
Restores CO in response to IV fluid transfusions
Fall in CO during orthostasis (standing up for a long time).

Question 7

Afterload - Laplaces law (3)

Answer 7

Opposes ejection of blood from heart.
Determined by wall stress directed through the heart wall, prevents muscle contraction.
T = Wall stress (S) X Wall thickness (w) = Sw
T = Pressure (P) x Radius (r) /2 (directions of curvature).
T = Pr/2
Sw=Pr/2
S=Pr/2w

Question 8

Afterload - wall stress, small radius (4)

Answer 8

Small ventricular radius
Greater wall curvature
More wall stress directed towards centre of chamber
Less afterload
Better ejection

Question 9

Afterload - wall stress, large radius (4)

Answer 9

Large ventricular radius
Smaller wall curvature
Less wall stress directed towards centre of chamber
More afterload
Less ejection

Question 10

Importance of LaPlaces law (3)

Answer 10

Opposes starlings law at rest
Facilitates ejection during contraction
Contributes to a failing heart at rest and during contraction

Question 11

Afterload - arterial blood pressure and stroke volume???

Answer 11

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Question 12

Laplaces law and heart failure (3)

Answer 12

Increased radius - More dilation not enough contraction - MI –> Volume overload.
Increased pressure - less dilation, more contraction, aortic stenosis/hypertension –> Pressure overload.
Ventricular hypertrophy - Greater myocyte size and more sarcomere. Increasing wall thickness and reducing wall stress and afterload, maintains SV and CO.

Question 13

Starlings law and ventricular pressure-volume loop (5) exercise

Answer 13

Exercise more venous return, increase in EDV.
Increase in preload and stretch.
Shorter isovolumetric contraction phase.
Increase in SV- SL.
More blood into = more blood out of heart.

Question 14

Laplaces law and ventricular pressure-volume loop (5) high bp

Answer 14

High bp, increases afterload.
DOESNT ACCEPT STARLINGS LAW AS IT IS CONTRACTION NOT RELAXATION.
Longer isovolumetric contraction phase - increases pressure in the chamber above, to open the valve.
More energy required, reduces SV.
ESV increases.