Download Cardiovascular 3 – Mechanical Properties of the Heart I

Cardio 3 – Mechanical Properties of the Heart I
Anil Chopra
1. Describe the relationship between ventricular wall tension, chamber radius and
chamber pressure. (Law of Laplace)
Law of Laplace
“When pressure in a cylinder is constant, the tension ion the walls increases with
increasing chamber pressure”
 Ventricular volume increases causing circumference to increase and length of
cells to increase.
 This increases intraventricular pressure and so increases tension in the
ventricular cells.
 Larger forces are needed to put pressure on the ventricles as their volume
increases.
 This means that the L.V. is able to generate higher pressures with similar wall
stress on the R.V.
 In dilated cardiomyopathy, ventricles dilate and so the pressure generated with
each pulse is low.
2. List the sequence of events from excitation that bring about contraction and
relaxation of a ventricular cell.
 Ventricular cells are 100μm wide and 15 μm across with small finger-like
invaginations called T-tubules (200nm) which carry depolarisation into the
cell. T-tubules lie along each Z-line.
 Contraction of cardiac muscle is dependent on the amount of extracellular Ca2+
and intracellular Ca2+ stores i.e. calcium couples electrical and mechanical
events.
 Ca2+ ions enter through L-type Ca2+ channels and induce the opening of the
sarcoplasmic reticulum Ca2+ release channels.
 These cause Ca2+ ions to flood out and consequently cause muscle contraction.
 ATP is used to pump Ca2+ ions back into the sarcoplasmic reticulum (SR-CaATPase) and an Na/Ca exchanger mediates Ca efflux.
 The amount of calcium entering the cell must be equal to that leaving with
each beat if a steady state is to be maintained.
3. State Starlings Law of the Heart.
“The more a ventricle is filled during diastole, the more blood is pumped out in
that stroke as the fibres increase in length”
4. Understand the mechanisms underlying Starling’s law of the heart.
Due to 2 factors:
(1) Changes in the number of myofilament cross-bridges that interact.
The length of the sarcomeres determines the force of
contraction in that the more interactions between the
thin actin and thick myosin filaments, the greater the
force of contraction. Increase in ventricular volume
increases sarcomeres length and so produces more
interactions. Cardiac muscle is much more resistant to
stretch than skeletal muscle.
(2) Changes in the Ca sensitivity of myofilaments
As the sarcomeres length increases, the Troponin C has a greater affinity for Ca2+ ,
therefore less Ca2+ is required for the same amount of force.
5. Use a graph to compare the length-tension relationships for cardiac and
skeletal muscle.
Cardiac muscle has a much greater stiffness due to properties of the extracellular
matrix and cytoskeleton.
6. Understand the concepts of preload and afterload.
Preload: weight that stretches a muscle before it contracts i.e. the filling of ventricles
in diastole causing cells to stretch. This produces isometric contraction. ( Measured by
end diastolic volume, end diastolic pressure, & right atrial pressure )
Afterload: weight not apparent when muscle is resting but encountered when muscle
begins to contract. i.e. the load against which the left ventricle ejects blood after
opening the aortic valve. Produces isotonic contraction. ( Measured by diastolic
atrial pressure )