The Cardiac Cycle (Srčani ciklus)

These flashcards cover the physiological events of the cardiac cycle, including the timings of atrial and ventricular systole/diastole, heart sounds, volumetric measurements, and ejection phases.

Cardiac Cycle

The sequence of events representing the function of the heart as a pump, occurring from the beginning of one contraction to the start of the next, encompassing one systole and one diastole.

Heart Rate and Cycle Duration Relationship

The duration of the cardiac cycle is inversely proportional to the heart rate; higher frequencies result in shorter cycles and vice versa.

Average Cardiac Cycle Duration

Based on a normal average heart rate of $75/min$, the cardiac cycle lasts approximately $0.8\,s$.

Atrial Cycle Timings

A cycle lasting $0.8\,s$ where atrial systole takes $0.1\,s$ and atrial diastole takes $0.7\,s$.

Ventricular Cycle Timings

A cycle lasting $0.8\,s$ where ventricular systole takes $0.3\,s$ and ventricular diastole takes $0.5\,s$.

Effects of Increased Heart Rate on Cardiac Function

Shortens the cycle mainly at the expense of diastole, which leads to inadequate ventricular filling and reduced myocardial nourishment via coronary arteries.

Atrial Systole

A phase lasting $0.1\,s$ occurring after excitation, responsible for transferring $10-30\%$ of blood into the ventricles during rest, while the majority of blood enters passively.

Fourth Heart Sound ($S_4$)

Also known as the atrial heart sound, it is caused by the turbulent flow of blood during atrial systole.

Atrial Diastole

A phase lasting $0.7\,s$ where the atria act as reservoirs, filling with blood from the veins while the AV valves are closed.

Isovolumetric Contraction

The initial phase of ventricular systole where cardiomyocytes contract and pressure rises rapidly without a change in volume, leading to the closure of AV valves.

First Heart Sound ($S_1$)

The sound produced by the closure of the AV valves when ventricular pressure exceeds atrial pressure.

Rapid Ejection Phase

Occurs when ventricular pressure exceeds aortic ($80\,mmHg$) or pulmonary ($10\,mmHg$) pressure, causing semilunar valves to open and blood to exit quickly with maximum velocity.

Slow Ejection Phase

Follows repolarization ($0.2\,s$ after contraction begins); active tension and pressure drop, but blood continues to flow forward briefly due to inertia.

Protodiastole

The brief introductory period of diastole where blood flow backward toward the heart closes the semilunar valves.

Isovolumetric Relaxation

A phase where all valves are closed, volume remains constant, and ventricular pressure drops rapidly due to $Ca^{2+}$ removal by the SERCA pump.

Second Heart Sound ($S_2$)

The sound produced by the closure of the semilunar valves when ventricular pressure falls below aortic and pulmonary pressures.

Rapid (Early) Ventricular Filling

Occurs in the first third of diastole immediately after AV valves open, where blood accumulated in the atria flows passively and quickly into the ventricles.

Third Heart Sound ($S_3$)

A sound that may occur during the rapid (early) ventricular filling phase.

Diastasis (Slow Filling Phase)

The middle third of diastole where atrial and ventricular pressures equalize, and blood from veins flows slowly and directly into the ventricles.

Presystole (Late Filling Phase)

Corresponds to atrial systole, providing additional blood filling to the ventricles; may generate a 4th heart sound as it fills an already full ventricle.

End-Diastolic Volume (EDV)

The volume of blood in the ventricle at the end of diastole, typically between $120-140\,mL$.

End-Systolic Volume (ESV)

The volume of blood remaining in the ventricle at the end of systole, typically between $40-70\,mL$.

Stroke Volume (SV)

The amount of blood pumped out of the heart per beat, calculated as $EDV - ESV$, normally between $70-80\,mL$.

Ejection Fraction (EF)

A clinical indicator of contractility calculated as the ratio $SV/EDV$; normal values are $50-70\%$ at rest.