19.8 The Cardiac Cycle

Cardiac cycle

all events in heart from the start of one heart beat to start of the next

• Includes both systole (contraction) and diastole (relaxation)

Contraction increases pressure; relaxation decreases it

• Blood moves down its pressure gradient (high to low)

• Valves ensure that flow is forward (closure prevents backflow)

ventricular activity

• Ventricular contraction raises ventricular pressure

  • AV valves pushed closed

  • Semilunar valves pushed open and blood ejected to artery

• Ventricular relaxation lowers ventricular pressure

  • Semilunar valves close

  • AV valves open

As the cardiac cycle begins

• Four chambers at rest

• Blood returning to both atria

• Passive filling of ventricles

• AV valves open

• Atrial pressure > ventricular pressure

• Semilunar valves closed

• Pressure in ventricles < arterial trunk pressure

Atrial contraction and ventricular filling

• SA node starts atrial excitation

• Atria contract pushing remaining blood into ventricles

• Ventricles filled to end-diastolic volume (EDV)

• Atria relax for remainder of cardiac cycle

Isovolumic contraction

• Purkinje fibers initiate ventricular excitation

• Ventricles contract, pressure rises, and AV valves are pushed closed

• Ventricular pressure is still less than arterial trunk pressure, so semilunar valves still closed

Ventricular ejection

• Ventricles continue to contract so that ventricular pressure rises above arterial pressure

• Semilunar valves forced open as blood moves from ventricles to arterial trunks

• Stroke volume (SV)

  • blood ejected by ventricle

• End systolic volume (ESV)

  • blood remaining in ventricle after contraction finishes

• ESV = EDV − SV

Isovolumic relaxation

• Ventricles relax and start to expand, lowering pressure

• Arterial pressure greater than ventricular pressure

• By sliding back toward ventricles, blood closes semilunar valves

• AV valves remain closed

• When all valves are closed, blood neither enters nor leaves and the time is called “isovolumic”

Ventricular balance

• Equal amounts of blood are pumped by left and right sides of the heart

• Left heart pumps blood farther and so must be stronger (thicker)

• But ejected blood volumes must be the same or edema (swelling) can occur