CR1.2 Cardiac cycle at rest and exercise

Question 1

Describe the propogation of cardiac conduction.

Answer 1

The sinoatrial (SA) node generates an action potential that spreads to the atrioventricular node (AV). The signal then propogates down the bundle of His through the left and right branch into the Purkinje fibers.

N.B. Action potential speeds are different across different segments of the heart. See CR1.5

Question 2

What are the differences between contraction of the left and right ventricles.

Answer 2

Right ventricular contraction; the free wall is thinner and shortens while the septum is thicker and moves towards the free wall.

Left ventricular contraction; the chamber constricts and shortens.

Question 3

List three elements of end distolic volume (EDV).

Answer 3

1. Venous return (aka. preload)
2. Filling time (time in diastole)
3. Compliance of ventricles (ability of the wall to stretch)

Question 4

Explain hypertrophic cardiomyopathy (HCM).

Answer 4

Increased collagen within the myocardium (fibrosis) reduced compliance and end diastolic volume. This results in a reduction of stroke volume.

Question 5

Define Starling’s Law of the Heart and explain the mechanisms of Starling’s Law of the Heart (length-dependent contractility).

Answer 5

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1. Increased Actin-myosin cross-bridge formation
2. Increased calcium sensitivity of the myofilaments (predominately through troponin C)
3. Increased tension on stretch-activated calcium channels (L-type) leading to increased calcium entry

Question 6

List two mechanisms that control inotropy (length-independent activation).

Answer 6

​Contractility depends on available calcium:

1. Sympathetic nervous system
2. Endocrine input (noradrenaline and adrenaline)

Question 7

Explain in detail the compensation of cardiac output during exercise.

Answer 7

During exercise cardiac output can increase ~5 times to that at rest. This occurs through an increase in heart rate (HR) and stroke volume (SV). SV is increased through an increase in EDV and preload due to increased venous return. Sympathetic nervous system activation results in increased inotropy (and a reduction in ESV) and an increased HR. Increased stroke volume results in increased afterload. This is offset by muscle arteriole vasodilation resulting in a decrease in TPR (e.g. vasodilation of skeletal and cardiac muscle blood vessels due to ß₂-adrenergic signalling and local factors). The increase in CO is greater than the decreased in TPR resulting in an overall increase in MAP.

N.B.

CO = HR x SV

MAP = CO x TPR

Question 8

Fill in the blanks.

Answer 8

Question 9

Coronary blood constricted during ___________.

Coronary blood flow occurs during ________ .

Answer 9

Coronary blood constricted during systole.

Coronary blood flow occurs during diastole.

Question 10

Define and list two determinants of end systolic volume (ESV).

Answer 10

End Systolic Volume (ESV): volume in ventricle(s) at end of systole (ejection).

1. Inotropy (length-independent activation)
2. Afterload

These are interdepdendent.

Question 11

Explain how exercise might affect a normal pressure-volume loop.

Answer 11

During exercise the sympathetic nervous system (SNS) causes;

1. An increase in heart rate (HR) at the SA node
2. Increase in inotropy –> more forceful contractions
3. Increse in preload –> venous return