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The cardiac cycle is the rhythmic sequence of contractions and relaxations that keeps blood flowing through your body every second of your life. You're being tested on more than just "the heart pumps blood"—exams focus on pressure-volume relationships, valve mechanics, and the precise timing that makes the heart an efficient pump. Understanding why valves open and close when they do, and how pressure gradients drive blood flow, connects directly to concepts like Starling's Law, cardiac output, and clinical conditions like heart murmurs.
Think of the cardiac cycle as a pressure story: blood always moves from high pressure to low pressure, and valves are passive gates that respond to these pressure differences. When you study these phases, focus on what's happening to pressure, which valves are open or closed, and whether volume is changing. Don't just memorize phase names—know what mechanical principle each phase demonstrates and how it contributes to efficient circulation.
Systole refers to contraction—the active, energy-requiring phase where the myocardium generates force. The key principle here is that contraction increases pressure, and pressure gradients determine valve behavior and blood movement.
Compare: Isovolumetric Contraction vs. Rapid Ejection—both occur during ventricular systole, but isovolumetric contraction builds pressure with all valves closed, while rapid ejection moves blood with semilunar valves open. If asked about pressure-volume loops, isovolumetric contraction is the vertical rise on the left side.
Diastole is relaxation—the passive phase where chambers refill. The principle here is that relaxation decreases pressure, creating gradients that pull blood into the chambers without requiring energy.
Compare: Isovolumetric Relaxation vs. Isovolumetric Contraction—both have all valves closed and constant volume, but relaxation drops pressure (beginning diastole) while contraction raises pressure (beginning systole). FRQs love asking about these "isovolumetric" phases because they test your understanding of valve mechanics.
Understanding how pressure and volume change together reveals why the cardiac cycle works. These relationships are often tested through pressure-volume loops and Wiggers diagrams.
Compare: Ventricular Systole vs. Ventricular Diastole—systole is shorter (~0.3 sec) and active, while diastole is longer (~0.5 sec) and passive. At higher heart rates, diastole shortens more than systole, which can compromise filling time and cardiac output.
| Concept | Best Examples |
|---|---|
| Valves closed, volume constant | Isovolumetric Contraction, Isovolumetric Relaxation |
| AV valves open | Atrial Systole, Rapid Ventricular Filling, Reduced Ventricular Filling |
| Semilunar valves open | Rapid Ejection, Reduced Ejection |
| Pressure building | Isovolumetric Contraction, Atrial Diastole |
| Pressure falling | Isovolumetric Relaxation, Reduced Ejection |
| Active contraction | Atrial Systole, Ventricular Systole |
| Passive filling | Rapid Ventricular Filling, Reduced Ventricular Filling |
| Heart sounds | S1 (Isovolumetric Contraction), S2 (Isovolumetric Relaxation) |
Which two phases share the characteristic of having all four heart valves closed simultaneously, and what distinguishes them from each other?
If a patient has a heart murmur heard during ventricular systole, which valves might be malfunctioning, and during which specific phases would you expect to hear it?
Compare and contrast rapid ventricular filling with atrial systole—how do their contributions to ventricular filling differ, and why does the "atrial kick" become more important during tachycardia?
On a pressure-volume loop, which phases correspond to the vertical segments (where volume doesn't change), and what is happening to pressure during each?
Why does the second heart sound (S2) occur at the beginning of isovolumetric relaxation rather than at the end of reduced ejection? What pressure relationship triggers semilunar valve closure?