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Cardiology Physiology
Interactive Tutor

A step-by-step guide through cardiac physiology — from key terms to advanced concepts. Work through each module in order, or jump to any topic.

9 modules Interactive graphs Cardiac Cycle Quiz at the end

Core Rules

Five principles that govern how the heart works. Master these first.

Pressure drives flow. Blood moves from high → low pressure. This governs when every valve opens and closes.

Venous return = cardiac output. More blood in → more blood out, assuming normal valves and pulmonary circuit.

Afterload reduces SV. Greater resistance (hypertension, aortic stenosis) → lower stroke volume if uncompensated.

More resistance → less flow. Vasoconstriction reduces blood flow throughout the circuit.

Increased preload → increased cardiac work. Via Frank-Starling: more stretch → stronger contraction → more O₂ demand.

Your Learning Path

Work through each module in order for the best understanding.

Step 1

📖

Key Terms

Flip cards to learn definitions

Step 2

🫀

Anatomy

Label cardiac structures

Step 3

🔴

Pressure & Flow

The core principle — interactive valve

Step 4

🔄

Cardiac Cycle

Four phases, valve states

Step 5

🌀

Circulation

Full circuit with live diagram

Step 6

📊

PV Loop

Interactive pressure–volume

Step 7

📈

Frank-Starling

Preload & contractility

Step 8

🔃

Venous Return

Psf, RAP, and curves

Step 9

⚡

ECG

Electrical vectors & waveforms

Step 10

📋

ECG Rhythms

Reference guide for all rhythms

Step 11

🩺

ECG Characteristics

Rhythm diagnosis tool

Step 12

💊

Drug MOAs

Pharmacology matcher

Step 13

🔬

Disease Classifier

Match signs to diagnosis

Step 14

🩻

Radiology

Radiographic findings

Step 15

🎯

Quiz

Comprehensive test

Key Terms

Tap any card to reveal its definition.

Pressure–Volume Loop

Adjust preload, afterload, and contractility. Watch the loop — and the heart's metrics — change in real time.

Current loop

Baseline

ESPVR

SV (mL)

EF (%)

ESV (mL)

150

EDV (mL)

0.93

Stroke Work (J)

Adjust sliders to explore how each variable shifts the loop.

Controls

Preload (EDV)150 mL

Afterload (AoDP)80 mmHg

Contractility100%

Reading the loop: Bottom-right = EDV. Top = peak ejection pressure. Bottom-left = ESV. Horizontal width = stroke volume.

AoDP line — the afterload the LV must overcome. Raised by vasoconstriction (↑ SVR). Shifts the height of the loop.

Preload slider — shifts the loop right (↑ EDV via ↑ venous return / venoconstriction).

Contractility (ESPVR) — rotates the upper-left corner of the loop. ↑ inotropy → steeper slope → lower ESV.

ESPVR — End-Systolic Pressure-Volume Relationship

The top-left corner of every PV loop always hits this line — regardless of preload. It's the heart's contractile ceiling: P_es = E_es × (V_es − V₀)

↑ Afterload (vasoconstriction) → loop climbs higher → hits ESPVR further right → ↑ ESV

↓ Afterload (vasodilation) → loop hits ESPVR lower → further left → ↓ ESV

↑ Contractility → steeper ESPVR slope → same pressure reached at smaller volume → ↓ ESV

↓ Contractility → flatter slope → heart leaves more blood behind → ↑ ESV

Cardiac Cycle

Step through the four phases and see valve states at each moment.

🚪

Mitral valve

Left atrium → Left ventricle

🚪

Aortic valve

Left ventricle → Aorta

The door analogy: Think of each valve as a one-way door with two people pushing from opposite sides. The mitral valve opens when LA pressure > LV pressure. The aortic valve opens when LV pressure > aortic pressure. Afterload is the "person" pushing from the aortic side.

Frank-Starling Relationship

The heart pumps out whatever volume it receives — the more it fills, the harder it contracts. Drag the sliders to explore.

Preload (EDV)150 mL

Under-filled Optimal Over-stretched

ContractilityNormal

↓ Heart failureNormal↑ Catecholamines

Adjust preload to explore the relationship.

Sarcomere overlap at current EDV

Frank-Starling Law

As EDV ↑, myofibrils stretch → more cross-bridge overlap → greater force → ↑ SV.

📉 Under-filled

Poor overlap → weak contraction → low SV

✅ Optimal (~150 mL)

Max cross-bridge overlap → peak SV

📈 Over-stretched

Actin/myosin pulled apart → SV declines

Venous Return Curves

Adjust blood volume and vascular resistance. See how venous return to the right heart changes.

Current

Normal baseline

Normal state: Psf 7 mmHg, normal SVR.

Variables

Blood volume (Psf)7 mmHg

SVR multiplier1×

Mean systemic filling pressure (Psf)

Psf reflects the pressure in the systemic circulation when flow stops — determined by venous tone (compliance) and the volume of blood in the veins. It is the driving pressure for venous return. Importantly, ↑ blood volume alone does not equal ↑ Psf — venous tone must also be considered.

📈 The relationship:

Venous Return ∝ (Psf − RAP)

Where Psf = mean systemic filling pressure | RAP = right atrial pressure

✅ What this means:

If Psf ↑ (e.g. venoconstriction → ↓ venous compliance → ↑ pressure) → the gradient (Psf − RAP) ↑ → venous return ↑. Venous return can be maintained or even increased despite higher RAP, as long as Psf increases more than RAP.

⚠️ Important nuance:

Venous return does not "overcome" RAP directly — it depends on the difference between Psf and RAP. Venous return stops when RAP = Psf (zero gradient).

👉 Precise version: Increased Psf can maintain or increase venous return even when RAP is elevated, provided the pressure gradient (Psf − RAP) increases.

⚠ Key distinction:

Venoconstriction — squeezes veins → ↓ resistance + ↑ Psf → ↑ venous return → ↑ preload

↑ Pure venous resistance (blockage/narrowing) → Psf unchanged → ↓ venous return → ↓ preload

Circulation Loop

Adjust the sliders to see how arteriolar tone, venous tone, and inotropy propagate through the circuit.

Core principle — pressure drives flow

Blood (like any fluid) moves from high pressure → low pressure. A valve only opens when the upstream pressure exceeds the downstream pressure. Drag the slider to see this in action — this is exactly how every cardiac valve works.

Chamber A pressure 20 mmHg

Arteriolar Tone (SVR)

VasodilationVasoconstriction

Venous Tone (Compliance)

VenodilationVenoconstriction

Inotropy (Contractility)

↓ Inotropy↑ Inotropy

Calculated values

800

SVR

Psf (mmHg)

Preload (mmHg)

Afterload (mmHg)

150

EDV (mL)

ESV (mL)

SV (mL)

5.0

CO (L/min)

What's happening

ECG — Understanding the Electrocardiogram

Start with the basics of how electrodes detect electrical vectors, then step through the full cardiac cycle.

Part 1 — How does an ECG detect electrical activity?

The ECG detects currents flowing through extracellular fluids as action potentials propagate through heart muscle. When some cells are depolarised (negative outside) while others are at rest (positive outside), a dipole is created. This establishes an electric force vector. Sensors on the body surface detect changes in this field. The key rule: depolarisation moving toward the positive electrode = upward deflection. Moving away = downward deflection. No movement = isoelectric (flat).

Step

ABCDEF

Part 2 — Vector direction determines deflection size and direction

The direction of the depolarisation wavefront relative to the positive electrode determines the deflection. The size of the deflection depends on how aligned the vector is with the electrode axis AND how much muscle mass is involved. Step through the 5 examples to see how changing the vector changes the ECG trace.

Vector direction

max↑(+)⊥ flatslight↑slight↓max↓(−)

Part 3 — Action potential propagation through the heart (Lead II)

Step through each phase of the cardiac cycle. The bold arrow shows the net electrical vector at each moment — its direction relative to the Lead II axis determines the ECG deflection shown on the right.

① SA Node fires Before P wave

AtrialEarly ventricularVentricularLate ventricularRepolarization

Mechanical event

Ion channels

Cardiac Anatomy

Study the labelled diagram, then test yourself by typing the name of each numbered structure.

Pressure & Flow

The single most important principle in cardiology. Everything else follows from this.

Pressure drives flow.

Blood moves from high pressure → low pressure. This governs when every valve opens and closes.

Valve opens when…

Upstream pressure exceeds downstream pressure

Valve closes when…

Downstream pressure exceeds upstream pressure

Flow rate depends on…

The pressure gradient — larger difference = faster flow

Interactive: drag to change Chamber A pressure

Chamber B is fixed at 80 mmHg. Raise Chamber A above 80 to open the valve and drive flow. This is exactly how mitral, aortic, tricuspid and pulmonary valves work.

Chamber A: 20 mmHg

Applied to cardiac valves

Mitral valve opens when…

LA pressure > LV pressure → blood fills the LV (diastole). Closes when LV pressure rises above LA pressure (isovolumic contraction).

Aortic valve opens when…

LV pressure > aortic pressure → blood is ejected (systole). Closes when aortic pressure exceeds LV pressure (isovolumic relaxation).

Tricuspid valve opens when…

RA pressure > RV pressure → right ventricular filling (diastole).

Pulmonary valve opens when…

RV pressure > pulmonary artery pressure → right ventricular ejection (systole).

💡 Every sound you hear with a stethoscope (S1, S2) is a valve closing — the sudden deceleration of blood when a valve snaps shut. Murmurs occur when turbulent flow passes through a narrowed or leaking valve.

ECG Rhythms — Reference Guide

Tap any rhythm card to expand its full diagnostic criteria, key distinguishing features, and clinical significance.

ECG Characteristics

The rhythm is named — select all of its key characteristics. Some options are distractors from similar rhythms.

Drug MOA Matcher

Drag each drug to its correct mechanism of action category, or flip cards to study.

Disease Classifier

Match clinical findings to the correct cardiac disease. Select all features that apply, then check your answer.

Radiograph Findings

Interactive reference for cardiac radiographic abnormalities. Click a chamber to reveal its radiographic signs.

Comprehensive Quiz

Questions drawn from physiology, ECG, disease classification, drugs, and clinical findings.

Score: 0 / 0

Question 1 of 8

Cardiology PhysiologyInteractive Tutor

Core Rules

Your Learning Path

Key Terms

Pressure–Volume Loop

Controls

Cardiac Cycle

Frank-Starling Relationship

Venous Return Curves

Variables

Circulation Loop

ECG — Understanding the Electrocardiogram

Cardiac Anatomy

Pressure & Flow

ECG Rhythms — Reference Guide

ECG Characteristics

Drug MOA Matcher

Disease Classifier

Radiograph Findings

Comprehensive Quiz

Cardiology Physiology
Interactive Tutor