Profile:Admin jan 2023

Graph of the heart

The topic on Transport in Humans is a long one and by the time we reach the sub-topic of the heart, some students find it a tad tough to follow... one of the more complicated concepts that students take a bit longer to grasp is the one on pressure changes in the heart. And so, I thought I should dedicate one blog post on this.

This graph is not frequently tested, a flip through the pure Biology TYS showed that it appeared four times in the past 10 years: three in Paper 1 and once in Paper 2. Usually, the question is testing your knowledge on the closing/opening of the valves in the heart at different points in one cardiac cycle. Sometimes, the deduction of the length of a phase in the cardiac cycle (e.g. ventricular systole) is also tested.

Most of my explanations have been annotated in the diagrams below, I think it's easier to understand with direct reference to the different parts of the graph.

  • atrial systole (when muscles of atrium relax)
  • ventricular systole (when muscles of ventricle contract)
  • ventricular diastole and atrial diastole (when muscles of both atrium and ventricle relax)

Electrocardiography is the process of producing an electrocardiogram a recording of the heart's electrical activity through repeated cardiac cycles. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including cardiac rhythm disturbances such as atrial fibrillation and ventricular tachycardia inadequate coronary artery blood flow such as myocardial ischemia and myocardial infarction, and electrolyte disturbances such as hypokalemi and hyperkalemie.


Traditionally, "ECG" usually means a 12-lead ECG taken while lying down as discussed below. However, other devices can record the electrical activity of the heart such as a Holter monitor but also some models of smartwatch are capable of recording an ECG. ECG signals can be recorded in other contexts with other devices. In a conventional 12-lead ECG, ten electrodes are placed on the patient's limbs and on the surface of the chest. The overall magnitude of the heart's electrical potential is then measured from twelve different angles and is recorded over a period of time (usually ten seconds). In this way, the overall magnitude and direction of the heart's electrical depolarization is captured at each moment throughout the cardiac cycle.