Mechanical Properties of the Heart 1 Flashcards
What is needed for contraction of a single ventricular cell?
External calcium
What is the approximate size of a ventricular myocyte and T tubules?
100 micrometres
Describe the excitation-contraction coupling of the heart including the receptors involved.
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.
What is the shape of the force-calcium relationship?
SIGMOIDAL
What concentration of calcium is sufficient to generate maximum contraction?
10 micromolar
Compare the length-tension relationship in skeletal and cardiac muscle.
Cardiac muscle is much more resistant to stretch. Cardiac muscle exerts a lot more passive force.
What are the two types of contraction used by the heart?
Isometric and Isotonic
What is Preload?
The weight that stretches the muscle before it is stimulated to contract
What is Afterload?
The weight that is not apparent to the muscle in the resting state and is only encountered once the muscle has started to contract
What is the effect of increasing preload?
Increasing preload increases the force exerted by the muscle fibres
What are the effects of increasing afterload?
Increasing afterload decreases the amount of shortening of muscle fibres and decreases the velocity of shortening of the fibres.
What are the in vivo correlates of preload?
End diastolic volume (this is the venous return to the heart that stretches the muscle fibres)
What are the in vivo correlates of afterload?
Blood pressure in the vessels leaving the ventricles.
What is a simple measure of afterload?
Diastolic arterial blood pressure
State Starling’s Law.
Increase in diastolic fibre length increases ventricular contraction.
Starling’s law is caused by what two factors?
Changes in the number of myofilament cross-bridges
Changes in the calcium sensitivity of the myofilaments
State two possible explanations for increasing calcium sensitivity of the myofilaments.
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.
What is Stroke Work?
Work done by the heart to eject blood under pressure into the aorta and pulmonary artery.
State the equation for stroke work.
Stroke Work = Stroke Volume x Pressure (at which the blood is ejected)
Stroke volume is greatly affected by preload, afterload and contractility.
State the law of Laplace.
When the pressure within a cylinder is kept constant, tension increases with increasing radius. T = PR.
What is the physiological relevance of the law of Laplace with regards to the structure of the right and left ventricles?
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.
What is the clinical significance of the law of Laplace?
In dilated cardiomyopathy, the radius of curvature increases and hence the pressure generated decreases.
