Heart as a Pump

Prof. K. Sivapalan

2013 Mechanical properties 2

Mechanical properties of cardiac muscle.

• Contraction is sliding of actin on myosin filaments.

• This depends on presence of calcium ions in the ICF of the myocardium.

• Force generated depends on the number of cross linkages bound.

2013 Mechanical properties 3

Contraction.

• Ca++ enters ICF from terminal cystern and ECF with depolarization.

• Intercalated discs, at Z line transfer the force generated across the cells.

• Even though cells branch, continuity of the myofibrils is maintained.

2013 Mechanical properties 4

Contraction.

• Actin and myosin do not react in a relaxed muscle.• Calcium release by action potential initiates sliding.• Contraction can not reduce length to zero.• In heart, there will be residual blood after maximal

contraction.

2013 Mechanical properties 5

Sequencial Contraction.

• The pace maker is the point that comes to firing level first after the previous impulse.

• Usually it is SA node.• Atrial contraction follows excitation and lasts for 0.1

second.• The impulse is delayed in the AV node for 0.1 second

and then spreads rapidly over the ventricle.• The ventricles contract after Atria and last for about 0.3

second.• If the heart is beating at 72 / min, each beat takes ,

– 60 / 72 = 0.8 second.

2013 Mechanical properties 6

Contraction and relaxation.

• Contraction follows depolarization.

• Removal of calcium causes relaxation.

• The muscle relaxes before the refractory period is over.

• Alternating contraction and relaxation is essential for heart as pump.

• It cannot be tetanised.

2013 Mechanical properties 7

Starling’s law of the heart.

• Initial length is proportional to the force generated up to a level and then declines.

• Degree of filling with blood determines the force generated.