Cardiac Cycle and Cardiac Output

NCERT grounding

The cardiac cycle is described in NCERT Class 11 Biology, Chapter 18 — Body Fluids and Circulation, Section 15.3.2. The chapter fixes the heart rate of a normal adult at 70–75 beats per minute, with 72 taken as the working average. Stroke volume is given as approximately 70 mL per ventricle per beat and cardiac output as roughly 5 litres per minute. The same section credits the SA node as the pacemaker, explains atrial top-up of ventricular filling at about thirty per cent, and pairs the two heart sounds (lub, dub) with closure of the AV and semilunar valves respectively.

“This sequential event in the heart which is cyclically repeated is called the cardiac cycle and it consists of systole and diastole of both the atria and ventricles… the duration of a cardiac cycle is 0.8 seconds.”

NCERT Class 11 Biology — §15.3.2 Cardiac Cycle

The 0.8-second cardiac cycle

Begin with all four chambers relaxed. This state is called joint diastole. Because the atrial and ventricular pressures are both low, the tricuspid and bicuspid (mitral) valves — the two atrioventricular or AV valves — sit passively open. Blood returning to the heart through the vena cavae and pulmonary veins therefore pours through the atria into the ventricles without any active pumping. The two semilunar valves, guarding the bases of the pulmonary artery and aorta, are closed because the arterial pressure beyond them is higher than the ventricular pressure inside.

The sino-atrial node (SAN) in the upper right atrium then fires an action potential. The wave of depolarisation sweeps across the atrial myocardium and produces a simultaneous atrial systole. Atrial pressure briefly exceeds ventricular pressure, pushing about thirty per cent of the final ventricular volume across the open AV valves. NCERT phrases this as “increases the flow of blood into the ventricles by about 30 per cent” — meaning passive filling during joint diastole already accounts for roughly 70%, and the atrial kick tops the ventricles up.

The impulse next reaches the atrio-ventricular node (AVN), is delayed there for around 0.1 second to let the atria complete their squeeze, and is then relayed through the AV bundle (bundle of His), down the right and left bundle branches, and out into the ventricular muscle via the Purkinje fibres. This route guarantees that the ventricles depolarise from apex upward, contracting as a unified pump.

Valve and pressure events in order

Following the valves through one beat is the cleanest way to memorise the cycle, because every valve event is driven by a pressure crossing. The four switches are: (i) AV valves close when ventricular pressure first exceeds atrial pressure at the start of ventricular systole; (ii) semilunar valves open when ventricular pressure exceeds aortic / pulmonary arterial pressure; (iii) semilunar valves close when ventricular pressure falls back below arterial pressure as the ventricle relaxes; (iv) AV valves reopen when ventricular pressure falls below atrial pressure during late diastole.

First and second heart sounds

The familiar stethoscope sequence lub-dub… lub-dub is mechanical, not electrical. The first heart sound (S1, “lub”) is produced as the tricuspid and bicuspid valves slam shut at the start of ventricular systole. The second heart sound (S2, “dub”) is produced as the pulmonary and aortic semilunar valves close at the end of ventricular systole. Both sounds are vibrations of the valve leaflets and surrounding blood, and both have clinical diagnostic significance — for instance, abnormal splitting of S2 or muffling of S1 are entry points to valve-disease diagnosis.

Stroke volume and cardiac output

Each ventricle does not empty completely with every contraction. Of the roughly 130 mL it holds at the end of diastole (end-diastolic volume, EDV), it ejects about 70 mL and retains the remaining ~60 mL (end-systolic volume, ESV). The ejected amount is the stroke volume (SV): the volume of blood pumped by one ventricle in one beat. NCERT fixes SV at approximately 70 mL.

Cardiac output (CO) is the volume of blood pumped by each ventricle in one minute. It is a simple product of two numbers:

The body adjusts CO either by changing SV — through Frank–Starling stretching of the ventricular wall, sympathetic boost to contractility, and adrenaline from the adrenal medulla — or by changing HR through autonomic drive at the SAN. Both levers are summarised in the sibling article on cardiac regulation and the ECG; here, just remember the multiplicative form, because every NEET numerical on this topic ultimately resolves to CO = SV × HR or to the SV identity SV = EDV − ESV.

Pressure–volume loop, qualitatively

Plotting left-ventricular pressure on the y-axis against left-ventricular volume on the x-axis traces a closed loop for every cardiac cycle. The loop has four limbs corresponding to the four valve events. Although NCERT does not draw this loop, the concept consolidates the mechanics in a single picture and explains how stroke volume emerges as the horizontal width of the loop.

Correlation with the ECG

Every mechanical event in the cycle has an electrical herald on the ECG. The P-wave is depolarisation of the atria and immediately precedes atrial systole. The QRS complex is depolarisation of the ventricles and marks the onset of ventricular systole — NCERT specifically notes that ventricular contraction starts shortly after Q. The T-wave is ventricular repolarisation; its end marks the end of ventricular systole, after which joint diastole resumes. The T-P gap (electrically silent stretch between T-wave end and the next P-wave) maps onto most of joint diastole. For a complete dissection of the ECG, see the sibling subtopic on regulation and ECG; here it is enough to remember the wave-to-phase pairings, because NEET converts them directly into match-the-column items.

Worked examples

Common confusion & NEET traps