Electrical and Mechanical Events of the Heart Cycle

Comprehensive vocabulary flashcards covering the electrical conduction system, electrocardiogram waves, valve dynamics, and mechanical phases of the cardiac cycle.

Sinoatrial (SA) node

The natural pacemaker of the heart situated in the right atrium that exhibits an intrinsic firing rate of approximately $100\text{ beats per minute}$ and initiates cardiac contractions without external nerve or hormonal input. made up by pacemaker cells

Pacemaker potential

An unstable resting membrane potential characteristic of autorhythmic cells in the SA node that continuously drifts upward toward threshold (this means that the potential will always be reached it just depends on how fast it will be reached).

Sympathetic stimulation of the SA node

Autonomic input that increases the heart rate by steepening the slope of the pacemaker drift toward threshold through increased permeability of the $Na^+_{(F)}$ (funny) inward current. (releases norepinephrine and epinephrine into the atrium and ventricles from the thoric spinal nerves)

Parasympathetic stimulation of the SA node

Autonomic input via the vagus nerves that lowers the heart rate by decreasing the inward current slope and hyperpolarising the cell membrane through increased $K^+$ permeability. (from vagus nerves release acetylcholine (ach), into atria)

Atrioventricular (AV) node delay

A brief pause in the transmission of electrical impulses through the AV node, ensuring the atria complete their contraction and empty blood into the ventricles prior to ventricular systole.

Purkinje fibres

Specialised myocardial conducting fibres branching throughout the ventricular walls that rapidly distribute electrical depolarisation upward through the ventricular myocardium.

P wave

The deflection on an electrocardiogram representing atrial depolarisation/contraction and excitation spreading across both atria from the SA node.

QRS complex

The rapid waveform on an electrocardiogram reflecting ventricular depolarisation/contraction and excitation, which electrically initiates ventricular contraction and conceals atrial repolarisation.

T wave

The electrocardiogram deflection corresponding to ventricular repolarisation/relaxation, marking electrical recovery during ventricular diastole.

Tricuspid valve

The right atrioventricular valve that separates the right atrium and right ventricle; it opens when right atrial pressure exceeds right ventricular pressure and closes when right ventricular pressure exceeds right atrial pressure.

Mitral (bicuspid) valve

The left atrioventricular valve that separates the left atrium and left ventricle; it opens when left atrial pressure exceeds left ventricular pressure and closes when left ventricular pressure exceeds left atrial pressure.

Aortic valve

The left semilunar valve separating the left ventricle from the aorta; it opens when left ventricular pressure exceeds aortic pressure and closes when left ventricular pressure falls below aortic pressure.

Pulmonary valve

The right semilunar valve separating the right ventricle from the pulmonary artery; it opens when right ventricular pressure exceeds pulmonary artery pressure and closes when right ventricular pressure drops below pulmonary artery pressure.

Isovolumetric ventricular contraction

The early phase of systole where the ventricles contract with both AV and semilunar valves closed, causing ventricular pressure to rise rapidly while ventricular volume remains unchanged.

Ventricular ejection

The second phase of systole wherein ventricular pressure surpasses arterial pressure, forcing open the aortic and pulmonary valves to expel blood into circulation.

Isovolumetric ventricular relaxation

The initial phase of diastole wherein the ventricles relax and their internal pressure drops below arterial pressure, closing the semilunar valves while the AV valves remain shut, keeping volume constant.

Ventricular filling

The diastolic phase occurring when ventricular pressure falls below atrial pressure, causing the AV valves to open and blood to fill the ventricles, concluding with atrial contraction.

End-diastolic volume (EDV)

The volume of blood contained within a ventricle at the conclusion of diastole, typically around $135\,\text{mL}$.

End-systolic volume (ESV)

The residual volume of blood remaining in a ventricle following ejection at the end of systole, typically around $65\,\text{mL}$.

Stroke volume

The volume of blood pumped out of the ventricle per contraction, equal to the difference between end-diastolic volume and end-systolic volume ($\text{EDV} - \text{ESV}$, approximately $70\,\text{mL}$ at rest).

Dicrotic notch

A brief rebound spike observed on the aortic pressure tracing caused by blood rebounding against the closed aortic valve leaflets at the onset of ventricular diastole.

First heart sound (lub)

The heart sound produced by the sudden closure of the atrioventricular (mitral and tricuspid) valves at the onset of ventricular systole.

Second heart sound (dub)

The heart sound produced by the closure of the semilunar (aortic and pulmonary) valves at the beginning of ventricular diastole.

heart beat phases shown on pressure graph and left ventricle vol?

pressure volume curve

it shows pressure changes

left → right the heart is filling up with blood

down → up pressure increases

atria contraction adds a bit more blood and stretches the ventricles so the pressure begins to rise

what does it mean if the peak on the ecg is bigger?

the heart muscle is bigger and stronger

the electrical events of a heart beat

what 2 things does the rate of firing from the cells of the sa node depend on?

• on the initial value of the membrane potential

• on the slope of the drift towards threshold

threshold potential for firing rate

a = control

b = during sympathetic stimulation

c = during parasympathetic stimulation