Atlas MCAT Course
Systems & Physiology → The Cardiac Cycle
The Cardiac Cycle
How the heart generates pulsatile pressure.
01The idea
Blood flow through the body is continuous, but the heart’s action is intermittent. This seems inefficient — until you consider the physics.
The heart doesn’t maintain pressure — it periodically restores it.
THREAD · Energy vs. rate
02Build the model
AWhat the cardiac cycle represents
The cardiac cycle describes repeating changes in pressure, repeating changes in volume, and coordinated opening and closing of valves. Each cycle ensures blood enters the heart, blood is pressurized, and blood is ejected into circulation.
BThe two global phases
| Diastole: filling | Systole: ejection |
|---|---|
| Ventricles relax | Ventricles contract |
| Pressure is low | Pressure rises sharply |
| Blood flows into ventricles | Blood is forced into arteries |
CValves: enforcing one-way flow
Heart valves open and close passively, respond to pressure differences, and prevent backflow. They do not generate pressure or actively pump blood.
DPressure–volume logic (no memorization required)
You can reason through the entire cycle by tracking ventricular pressure and ventricular volume.
| Phase | Volume | Pressure |
|---|---|---|
| 1. Filling | Increases | Remains low |
| 2. Isovolumetric contraction | Constant | Rises sharply |
| 3. Ejection | Decreases | Initially high, then falls |
| 4. Isovolumetric relaxation | Constant | Falls rapidly |
EWhy pressure must exceed arterial pressure
Ejection only occurs when ventricular pressure is greater than arterial pressure. This explains why increased arterial pressure increases cardiac workload, and why pressure generation precedes flow.
FStroke volume: what gets pumped each cycle
Stroke volume is the volume ejected per beat. It depends on how much blood fills the ventricle, how forcefully it contracts, and the resistance it ejects against. No formula required — just logic.
GCardiac output: flow at the system level
Cardiac output reflects total blood flow per unit time. It is determined by stroke volume and heart rate.
HCompliance revisited: why arteries matter
Large arteries stretch during systole and recoil during diastole. This stores energy, smooths pulsatile pressure, and maintains flow between beats. Without compliance, pressure would spike dangerously and flow would be erratic.
IEnergy cost of the cardiac cycle
Each cycle consumes ATP and must overcome resistance. Increased resistance increases pressure demand and raises energy cost. This directly links vascular resistance and cardiac workload.
03See the connections
The lesson names these links itself. Read the map left to right.
Muscle
- Contracts: pressure rises
- Relaxes: chamber fills
Valves
- Open and close passively
- Follow pressure differences
Output
- Stroke volume × heart rate
- Smoothed by arterial compliance
Next: vascular regulation.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
During isovolumetric contraction, why doesn’t ventricular volume change?
Think first.
What causes a heart valve to open?
Think first.
Arterial pressure rises (for example, from higher vascular resistance). What happens to the heart’s job?
Think first.
How does the ventricle fill during diastole?
05MCAT lens
The MCAT emphasizes
Pressure–volume reasoning
Valve logic
Effects of resistance changes
You are rarely asked to memorize timing percentages or recall ECG wave labels. You are often asked to predict effects of pressure or resistance changes.
06Common traps
Trap 1: The heart pulls blood in
Misconception
Filling is pressure-driven.
Correct model
Relaxation allows flow.
Trap 2: Valves actively pump blood
Misconception
They respond passively.
Correct model
Pressure controls them.
Trap 3: Flow occurs throughout systole equally
Misconception
Pressure must exceed arterial pressure.
Correct model
Ejection depends on gradient.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why the heart operates cyclically
Check against 01 and 02 · B: it periodically restores pressure, and compliance smooths the gaps.
Why pressure generation precedes flow
Check against 02 · E: ejection waits until ventricular pressure exceeds arterial pressure.
Why valves enforce direction
Check against 02 · C: they open and close passively with pressure differences.
Why resistance increases cardiac work
Check against 02 · E and I: more resistance, more pressure needed each beat.
If something feels unclear, the missing idea is usually thisThe cardiac cycle is pressure management over time.
08Remember
REMEMBER · if you keep only a few things
- The heart periodically restores pressure; it doesn’t hold it.
- Diastole fills passively; systole ejects.
- Valves follow pressure; muscle makes pressure.
- Isovolumetric = all valves closed: pressure changes, volume doesn’t.
- Ejection starts only when ventricular pressure beats arterial pressure.
- Cardiac output = stroke volume × heart rate.
09Forward hook
Now that you understand how pressure is generated, the next question is:
How does the body regulate flow to different tissues moment-to-moment?
That leads directly to vascular control, vasodilation and vasoconstriction, and regional blood flow regulation.
Next lessonVascular Regulation→