Lecture 2.1a - Cardiac Cycle PDF

Cardiac cycle: ◦The cardiac cycle is defined as a sequence of alternating contraction and relaxation of the atria and ventricles with every heartbeat ◦Systolic phase (~300ms): ‣ Heart chambers are contracted ‣ Heart pumps the blood ◦Diastolic phase (~500ms)...

Cardiac cycle: ◦The cardiac cycle is defined as a sequence of alternating contraction and relaxation of the atria and ventricles with every heartbeat ◦Systolic phase (~300ms): ‣ Heart chambers are contracted ‣ Heart pumps the blood ◦Diastolic phase (~500ms) ‣ Heart chambers are relaxed ‣ Heart fills with the blood ◦Both atria and ventricles alternating states of systole and diastole Conduction system: ◦Area of sub-specialised cells: SA node ‣ Near the opening of the superior vena cava ‣ SA node is able to contract faster than the rest of the heart tissue ‣ Sets the pace of cardiac contraction ‣ Known as the pacemaker of the heart ‣ The impulse is spread to the rest of the heart through conductive pathways by so contraction and relaxation Cardiac cycle: ◦Atrial diastole ◦Atrial systole (mid to late ventricular diastole) ◦Early ventricular systole (isovolumetric contraction) ◦Mid to late ventricular systole (ventricular ejection) ◦Early ventricular diastole (isovolumetric relaxation) Atrial diastole: ◦Blood is filled in the right and left atrium ◦As blood fills, atrial pressure rises above ventricular pressure ◦AV valves open -> ~80% blood passively flows to ventricles due to gravity ◦This process lasts 0.7 seconds Atrial systole/mid-to-late ventricular diastole: ◦SA node fires -> atrial systole/contraction ◦Remaining ~20% blood flows to ventricles ◦All blood is now in ventricles: End diastolic volume (EDV) or also known as pre-load Early ventricular systole (isovolumetric contraction): ◦Depolarisation passed down to ventricles ◦Ventricles contract ◦Pressure within ventricles rises above atrial pressure ◦AV valves close to prevent backflow of blood-> S1 or lub sound or 1st heart sound created ◦Ventricles are contracting but no change in blood volume, hence isovolumetric contraction  Mid-to-late ventricular systole: ◦Ventricular pressure very high ◦Semilunar valves open ◦Blood pushed out to the outflow tracts ◦Some blood remains in ventricles: End systolic volume (ESV) or after load ◦Known as the rapid ejection phase ◦AV valves closed Early ventricular diastole/isovolumetric relaxation: ◦Blood is in aorta ◦Aorta is elastic and recoils -> dichrotic notch ◦Semilunar valves closed -> dub sound/2nd heart sound/S2 ◦Ventricular pressure is reducing but still higher than atrial pressure ◦AV valves closed Stroke volume: ◦Can be calculated by EDV-ESV