Mechanical Properties of the Heart 1

Question 1

What is needed for contraction of a single ventricular cell?

Answer 1

External calcium

Question 2

What is the approximate size of a ventricular myocyte and T tubules?

Answer 2

100 micrometres

Question 3

Describe the excitation-contraction coupling of the heart including the receptors involved.

Answer 3

Depolarisation causes the opening of L-type calcium channels. This leads to influx of calcium into the myocyte. The calcium then binds to the Ryanodine receptor and leads to the release of calcium from the sarcoplasmic reticulum. SERCA then takes the calcium from the cytoplasm back into the SR. The Na/Ca exchanger pumps out as much calcium as entered the cell in the first place.

Question 4

What is the shape of the force-calcium relationship?

Answer 4

SIGMOIDAL

Question 5

What concentration of calcium is sufficient to generate maximum contraction?

Answer 5

10 micromolar

Question 6

Compare the length-tension relationship in skeletal and cardiac muscle.

Answer 6

Cardiac muscle is much more resistant to stretch. Cardiac muscle exerts a lot more passive force.

Question 7

What are the two types of contraction used by the heart?

Answer 7

Isometric and Isotonic

Question 8

What is Preload?

Answer 8

The weight that stretches the muscle before it is stimulated to contract

Question 9

What is Afterload?

Answer 9

The weight that is not apparent to the muscle in the resting state and is only encountered once the muscle has started to contract

Question 10

What is the effect of increasing preload?

Answer 10

Increasing preload increases the force exerted by the muscle fibres

Question 11

What are the effects of increasing afterload?

Answer 11

Increasing afterload decreases the amount of shortening of muscle fibres and decreases the velocity of shortening of the fibres.

Question 12

What are the in vivo correlates of preload?

Answer 12

End diastolic volume (this is the venous return to the heart that stretches the muscle fibres)

Question 13

What are the in vivo correlates of afterload?

Answer 13

Blood pressure in the vessels leaving the ventricles.

Question 14

What is a simple measure of afterload?

Answer 14

Diastolic arterial blood pressure

Question 15

State Starling’s Law.

Answer 15

Increase in diastolic fibre length increases ventricular contraction.

Question 16

Starling’s law is caused by what two factors?

Answer 16

Changes in the number of myofilament cross-bridges

Changes in the calcium sensitivity of the myofilaments

Question 17

State two possible explanations for increasing calcium sensitivity of the myofilaments.

Answer 17

When the muscles are stretched, the lattice spacing (spacing between filaments) decreases meaning that for the same amount of calcium, more cross bridges can be formed.
When sarcomere length changes, there is a conformational change in troponin C that gives it a higher affinity for calcium.

Question 18

What is Stroke Work?

Answer 18

Work done by the heart to eject blood under pressure into the aorta and pulmonary artery.

Question 19

State the equation for stroke work.

Answer 19

Stroke Work = Stroke Volume x Pressure (at which the blood is ejected)
Stroke volume is greatly affected by preload, afterload and contractility.

Question 20

State the law of Laplace.

Answer 20

When the pressure within a cylinder is kept constant, tension increases with increasing radius. T = PR.

Question 21

What is the physiological relevance of the law of Laplace with regards to the structure of the right and left ventricles?

Answer 21

The left ventricle has a smaller radius of curvature than the right ventricle meaning that the left ventricle is able to generate higher pressures with similar wall tension.

Question 22

What is the clinical significance of the law of Laplace?

Answer 22

In dilated cardiomyopathy, the radius of curvature increases and hence the pressure generated decreases.