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The cardiac cycle is the heart's rhythmic dance of contraction and relaxation. It's a crucial process that keeps blood flowing through our bodies, delivering oxygen and nutrients to every cell. Understanding this cycle helps us grasp how our hearts work tirelessly to keep us alive.

Blood pressure, flow, and resistance play key roles in this intricate ballet. As the heart's chambers contract and relax, valves open and close, creating the familiar "lub-dub" sounds we associate with a heartbeat. These phases and sounds reveal the complex coordination required for efficient blood circulation.

Cardiac Cycle

Blood pressure and cardiovascular flow

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  • Blood pressure drives blood flow through the cardiovascular system
    • Higher blood pressure leads to greater blood flow ()
    • Lower blood pressure reduces blood flow ()
  • Pressure gradient determines direction and rate of blood flow
    • Blood flows from high pressure to low pressure areas
    • Larger pressure differences result in faster blood flow
  • Resistance affects blood pressure and flow rate
    • Increased resistance from narrowed blood vessels raises blood pressure and reduces flow ()
    • Decreased resistance from dilated blood vessels lowers blood pressure and increases flow ()
  • , a product of heart rate and , influences blood pressure and flow
    • Higher cardiac output raises blood pressure and enhances flow (exercise)
    • Lower cardiac output lowers blood pressure and reduces flow (heart failure)
  • , the pressure that the ventricle must overcome to eject blood, affects cardiac output

Phases of cardiac cycle

    • Atria contract, pumping blood into ventricles
    • Atrioventricular (AV) valves open, allowing atrial-to-ventricular blood flow
    • Ventricles contract, ejecting blood from the heart
    • AV valves close, preventing blood backflow into atria
    • Semilunar valves (aortic and pulmonary) open, enabling blood flow into and
    • Begins with phase
    • Atria relax and fill with blood from and
    • AV valves remain closed
    • Ventricles relax and fill with blood from atria
    • Semilunar valves close, preventing blood backflow into ventricles
    • AV valves open, facilitating atrial-to-ventricular blood flow
    • Begins with phase

Systole vs diastole in heart chambers

  • Atrial
    • Atrial contraction phase
    • Occurs concurrently with ventricular
  • Atrial diastole
    • Atrial relaxation and filling phase
    • Occurs concurrently with ventricular systole
  • Ventricular systole
    • Ventricular contraction phase
    • Occurs concurrently with atrial diastole
  • Ventricular diastole
    • Ventricular relaxation and filling phase
    • Occurs concurrently with atrial systole

Heart sounds and valve actions

  • First heart sound (, "lub")
    • Results from AV valve (mitral and tricuspid) closure
    • Marks the beginning of ventricular systole
  • Second heart sound (, "dub")
    • Results from semilunar valve (aortic and pulmonary) closure
    • Marks the end of ventricular systole and start of ventricular diastole
  • Third heart sound (, "lub-dub-dee")
    • May be audible in some individuals (children, athletes)
    • Occurs during early ventricular filling due to rapid blood inflow
  • Fourth heart sound (, "dee-lub-dub")
    • May be audible in individuals with stiff ventricles (reduced compliance)
    • Occurs during late ventricular filling due to forceful atrial contraction

Ventricular volumes and cardiac function

  • : The volume of blood remaining in the ventricle after contraction
  • : The volume of blood in the ventricle just before contraction
  • : The initial stretching of cardiac muscle fibers before contraction, typically represented by end-diastolic volume
  • : Describes how the heart automatically adjusts to changes in , increasing stroke volume as end-diastolic volume increases
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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Glossary
Glossary