Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
The Cardiac Cycle is the sequential event in the heart which is cyclically repeated and consists of systole (contraction) and diastole (relaxation) of both the atria and ventricles. The total duration for one cycle is approximately seconds.
Joint Diastole: All four chambers of the heart are in a relaxed state. The tricuspid and bicuspid valves are open, allowing blood from pulmonary veins and vena cava to flow into the left and right ventricles respectively through the atria. Semilunar valves are closed at this stage.
Atrial Systole ( s): The SA node generates an action potential which stimulates both the atria to undergo a simultaneous contraction. This increases the flow of blood into the ventricles by about .
Ventricular Systole ( s): The action potential is conducted to the ventricular side by the AV node and AV bundle. Ventricular contraction increases ventricular pressure, causing the closure of tricuspid and bicuspid valves (producing the first heart sound: 'Lubb'). This is followed by the opening of semilunar valves as pressure rises, forcing blood into the circulatory pathways.
Ventricular Diastole ( s): The ventricles relax and ventricular pressure falls, causing the closure of semilunar valves (producing the second heart sound: 'Dupp').
Stroke Volume (SV): Each ventricle pumps out approximately mL of blood during a cardiac cycle, which is called the stroke volume.
Cardiac Output (CO): It is the volume of blood pumped out by each ventricle per minute. For a healthy individual, it is roughly mL or L.
Electrocardiogram (ECG): A graphical representation of the electrical activity of the heart. The P-wave represents atrial depolarisation (excitation). The QRS complex represents ventricular depolarisation. The T-wave represents ventricular repolarisation (return to excited to normal state).
📐Formulae
💡Examples
Problem 1:
An athlete has a resting heart rate of beats per minute and a stroke volume of mL. Calculate his cardiac output and compare it with an ordinary man having a heart rate of beats per minute and a stroke volume of mL.
Solution:
For the athlete: . For the ordinary man: .
Explanation:
Even though the athlete's heart rate is lower (bradycardia typical of trained athletes), the increased efficiency represented by a higher stroke volume ( mL vs mL) allows the athlete to maintain a similar cardiac output as a normal person.
Problem 2:
During a clinical checkup, a patient's ECG shows an abnormally high number of QRS complexes within a minute interval. What does this indicate?
Solution:
A higher number of complexes indicates a higher heart rate ( beats/min), a condition known as tachycardia.
Explanation:
Since each complex corresponds to one ventricular contraction (one heartbeat), counting the number of complexes in a given time period is the standard method to determine the heart rate of an individual.