Muscle Physics and Heart as a Pump Flashcards
Cardiac output (CO): define. when does it change?
volume of blood pumped per minute by the left ventricle.
o At rest: 4-6 L/min. Can be increased up to 8-fold during strenuous exercise
Stroke volume (SV): define. What is it determined by?
volume of blood pumped per beat.
o Determined by the strength of contraction of the heart and by venous return and vascular resistance (“preload” and “afterload”).
CO = (equation)
heart rate (HR) x stroke volume
4 phases of the cardiac cycle
filling phase
isovolumetric contraction phase
ejection phase
isovolumetric relaxation phase
volume changes of the cardiac cycle
o First, the ventricle passively fills, with a slight hump toward the end of diastole when the atrium contracts.
o Then, during the isovolumetric contraction phase, there is no change in volume, because the aortic and mitral valves are closed.
o When the aortic valve opens and blood can leave the ventricle, the volume decreases.
pressure changes of the cardiac cycle
o After diastole and passive filling of the left atrium with blood, contraction of the atrium results in increased atrial pressure, followed by an increase in ventricular pressure (while the mitral valve is open).
o Once the mitral valve is closed and ventricular contraction commences, ventricular pressure increases rapidly until the ventricular pressure exceeds that in the aorta and the aortic valve is pushed open.
o This immediately results in a slow decrease in ventricular pressure followed by a much faster drop in ventricular pressure once the ventricular pressure drops below the aortic pressure and the aortic valve. The ventricle continues to relax with both valves closed, so the pressure falls rapidly.
End-diastolic pressure volume relationship (EDPVR):
pressure-volume relationship during filling of the heart BEFORE contraction:passive elastic properties of ventricle (compliance).
Systolic pressure volume relationship (SPVR):
pressure-volume relationship at the peak of isometric contraction
Active tension:
the difference in the force between the peak systolic pressure and the end diastolic pressure, that is, the tension developed by the contraction.
compare the slopes of the EDPVR and SPVR
EDPVR is shallow in the normal physiologic range (it steepens at very high volumes where the heart is so full it is bounded by the epicardium)
SPVR is much steeper than EDPVR—pressure increases even at low volume.
o SPVR includes the passive properties of the heart.
Frank-Starling Law of the Heart
• Intrinsic mechanism by which the heart adapts to changes in preload (in the normal physiologic range):
o Heart response to an ↑EDV by ↑force of contraction.
o The heart always functions on the ascending limb of the ventricular function curve.
o What goes in, must come out. Cardiac output must equal venous return (on average)
Molecular basis for the Frank-Starling Law of the Heart
o Cardiac titin Isoform is very stiff, resists stretch.
o Ca2+ sensitivity of myofilaments increases as sarcomeres are stretched. So the same intracellular Ca2+ produces a greater force of contraction.
o Closer lattice spacing—stretched sarcomeres have altered spacing between actin and myosin which may result in more force generated per cross-bridge)
