Cardiac cycle
MCAT glossary · Biological & Biochemical Foundations of Living Systems
Definition
Systole (ventricular contraction, S1 closes AV valves) → diastole (filling, S2 closes semilunar valves); P wave atrial depol, QRS ventricular depol, T wave ventricular repol.
Cardiac cycle in plain English
The cardiac cycle is the repeating two-beat rhythm your heart follows every second of your life: squeeze, then relax. In systole the ventricles (the big pumping chambers) contract and shove blood out, and the valves between the upper and lower chambers snap shut — that's the 'lub' (S1) you hear; in diastole the heart relaxes and refills, and the exit valves close to make the 'dub' (S2). An ECG traces the electrical signals that trigger all this: the P wave is the atria charging up to squeeze, the QRS spike is the powerful ventricles charging up, and the T wave is the ventricles resetting. Picture a hand rhythmically squeezing and releasing a water balloon, with a heart monitor beeping out the electrical 'go' signal just before each squeeze.
Sign up free: all 171 MCAT terms as flashcards, plus practice questions with full explanations →
Related MCAT terms
MCAT practice questions · MCAT flashcards · MCAT study guide