The cardiac cycle in ONE table (plus my ECG cheat sheet)

Hi notes people!!! This is my favourite kind of post, a table that took me three weekends and two highlighters to make, and that I now actually remember in tests. The topic is the cardiac cycle from Body Fluids and Circulation, class 11, which is everyone's "I know this" chapter and also everyone's lost-marks chapter.

First, the numbers (NCERT)

• Heart rate in a resting adult is about 72 beats per minute
• So one cardiac cycle lasts about 0.8 seconds
• Stroke volume is about 70 mL
• Cardiac output = stroke volume × heart rate, so about 70 × 72 ≈ 5000 mL, i.e. 5 litres per minute

Write these on a sticky note. They are the easiest marks in the chapter.

The cycle in order

Here's the table I made. Colours: yellow for atria, pink for ventricles, blue for valves.

Phase	Approx. time	What happens	AV valves	Semilunar valves
Joint diastole	~0.4 s	Atria and ventricles both relaxed. Blood flows from the veins into atria and on into ventricles through the open AV valves.	Open	Closed
Atrial systole	~0.1 s	SA node fires, atria contract, extra blood pushed into ventricles to finish filling them.	Open	Closed
Ventricular systole	~0.3 s	Impulse reaches the ventricles via AV node, AV bundle, bundle branches, Purkinje fibres. Ventricles contract. Pressure rises. AV valves close (lub), then semilunar valves open and blood goes out to the pulmonary artery and aorta.	Closed	Open
Ventricular diastole (start of joint diastole)		Ventricles relax, pressure falls, semilunar valves close (dup), AV valves open again.	Open	Closed

Notice the timings: 0.1 + 0.3 + 0.4 = 0.8 s. When I realised the pieces add up, I stopped doubting them.

Sounds: lub and dub

The first heart sound (lub) is the closing of the tricuspid and bicuspid (mitral) valves at the start of ventricular systole. It's longer and a bit lower. The second heart sound (dub) is the closing of the semilunar valves at the start of ventricular diastole. Short and sharp. A doctor uses a stethoscope to listen to these. Fun mental image: lub = "the doors between rooms close", dub = "the exit doors close".

The conducting system, in order of firing

SAN → AVN → AV bundle (bundle of His) → bundle branches → Purkinje fibres

• SAN (sinoatrial node): in the right upper corner of the right atrium. The pacemaker, since it generates the maximum number of action potentials (70-75/min).
• AVN (atrioventricular node): in the lower left corner of the right atrium, near the atrio-ventricular septum.
• The AV bundle passes through the septum and divides into right and left bundle branches, which go into the ventricular muscle as Purkinje fibres.

Heart is myogenic, meaning the beat starts in the muscle itself. The nervous system only adjusts it: sympathetic fibres speed it up and increase the strength of contraction, parasympathetic ones slow it down. Adrenal medullary hormones (adrenaline and noradrenaline) increase cardiac output.

ECG cheat sheet

An electrocardiograph records the electrical activity of the heart. The graph is the ECG. To get a standard ECG the patient is connected to the machine with three electrical leads, one to each wrist and one to the left ankle, and the machine records the electrical activity of the heart through the cycle.

Wave	What it represents
P wave	Depolarisation of the atria (leads to atrial contraction)
QRS complex	Depolarisation of the ventricles (start of ventricular systole)
T wave	Repolarisation (return to normal) of the ventricles (end of ventricular systole)

Where's the atrial repolarisation? It happens at the same time as ventricular depolarisation, so it's hidden inside the QRS complex. You won't see a separate wave for it. Trick questions love this.

You can find the heart rate by counting the number of QRS complexes in a given time period. Any deviation in the shape of the ECG indicates a possible abnormality or disease.

The mistakes list (mine, in order of pain)

1. Saying the P wave is ventricular. P for P-atria? I link it with "P = Primary chamber, atria". Don't judge.
2. Thinking the ventricles fill only when the atria contract. Most filling happens during joint diastole, passively. Atrial systole is the finishing touch.
3. Writing "semilunar valves open during joint diastole". No, those are shut during diastole. They open during ventricular systole.
4. Mixing up AVN and SAN positions. SAN right upper corner of right atrium, AVN lower left corner of right atrium.
5. Saying "heart rate equals count of P waves". It's QRS complexes in NCERT's description.

How I use the table

I print it, cover the last four columns with a sheet of paper, and fill them in by memory. Then I do the same with the ECG table. Every time I get one wrong I put a small star in the margin. After three rounds the table is mostly stars and I can see exactly what to fix.

Another thing: I draw a tiny version of the ECG with P, QRS, T in three colours and write beside each one the matching event in the cycle. P is yellow (atria), QRS is pink (ventricles contract), T is purple (ventricles relax). Visual cues are a big part of why this stuck.

Next up: the double circulation and the disorders (hypertension, coronary artery disease, angina, heart failure). If you want that table, say so in the comments!!!