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 (hypertension )
Lower blood pressure reduces blood flow (hypotension )
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 (atherosclerosis )
Decreased resistance from dilated blood vessels lowers blood pressure and increases flow (vasodilation )
Cardiac output , a product of heart rate and stroke volume , 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)
Afterload , the pressure that the ventricle must overcome to eject blood, affects cardiac output
Phases of cardiac cycle
Atrial systole
Atria contract, pumping blood into ventricles
Atrioventricular (AV) valves open, allowing atrial-to-ventricular blood flow
Ventricular systole
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 aorta and pulmonary artery
Begins with isovolumetric contraction phase
Atrial diastole
Atria relax and fill with blood from venae cavae and pulmonary veins
AV valves remain closed
Ventricular diastole
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 isovolumetric relaxation phase
Systole vs diastole in heart chambers
Atrial systole
Atrial contraction phase
Occurs concurrently with ventricular diastole
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 (S1 , "lub")
Results from AV valve (mitral and tricuspid) closure
Marks the beginning of ventricular systole
Second heart sound (S2 , "dub")
Results from semilunar valve (aortic and pulmonary) closure
Marks the end of ventricular systole and start of ventricular diastole
Third heart sound (S3 , "lub-dub-dee")
May be audible in some individuals (children, athletes)
Occurs during early ventricular filling due to rapid blood inflow
Fourth heart sound (S4 , "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
End-systolic volume : The volume of blood remaining in the ventricle after contraction
End-diastolic volume : The volume of blood in the ventricle just before contraction
Preload : The initial stretching of cardiac muscle fibers before contraction, typically represented by end-diastolic volume
Frank-Starling law : Describes how the heart automatically adjusts to changes in preload , increasing stroke volume as end-diastolic volume increases