atrial systole

• Contraction of the atria propels some additional blood into the ventricles.
• Contraction of the atrial muscle?


  narrows the orifices of the superior and inferior vena cava and pulmonary veins, and the inertia of the blood moving toward the heart tends to keep blood in it.
• Heart failure occurs ?


  when the heart is unable to put out an amount of blood that is adequate for the needs of the tissues
• Heart failure may also be systolic or diastolic
• In systolic failure, stroke volume is reduced because ventricular contraction is weak
• diastolic failure, the ejection fraction is initially maintained, but the elasticity of the myocardium is reduced so filling during diastole is reduced. This leads to inadequate stroke volume
• At the start of ventricular systole, the AV valves close.
• This period of isovolumetric (isovolumic, isometric) ventricular contraction lasts about 0.05 s, until the pressures in the left and right ventricles exceed the pressures in the aorta (80 mm Hg; 10.6 kPa) and pulmonary artery (10 mm Hg)
• When the aortic and pulmonary valves open, the phase of ventricular ejection begins.
• The amount of blood ejected by each ventricle per stroke at rest is 70–90 mL.
• The end-diastolic ventricular volume is about 130 mL
• Isovolumetric relaxation ends when the ventricular pressure falls below the atrial pressure and the AV valves open, permitting the ventricles to fill.
• Right atrial systole precedes left atrial systole, and contraction of the right ventricle starts after that of the left
• since pulmonary arterial pressure is lower than aortic pressure, right ventricular ejection begins before that of the lef
• During expiration?

  the pulmonary and aortic valves close at the same time;
• during inspiration, ?

  the aortic valve closes slightly before the pulmonary. The slower closure of the pulmonary valve is due to lower impedance of the pulmonary vascular tree
• The action potentials in the SA and AV nodes are largely due to Ca2+, with no contribution by Na+ influx.