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Heart Anatomy
and the Cardiac Cycle

Explore the four-chambered heart, trace the path of blood flow, and understand how the cardiac cycle powers every heartbeat.

Anatomy & Physiology

Why the Heart Matters

Your heart beats approximately 100,000 times per day, pumping about 5 liters of blood per minute at rest. This muscular organ is the central pump of the cardiovascular system, delivering oxygen and nutrients to every cell while removing metabolic waste.

Essential Vocabulary

Atrium
Upper receiving chamber that collects blood returning to the heart
Ventricle
Lower pumping chamber that ejects blood out of the heart
Septum
Muscular wall dividing the left and right sides of the heart
Valve
Flap-like structure preventing backflow of blood between chambers

The Four Chambers

Right Atrium Left Atrium Right Ventricle Left Ventricle Septum Tricuspid Mitral

The Four Heart Valves

Tricuspid
Between right atrium and right ventricle. Has three cusps.
Pulmonary
Between right ventricle and pulmonary artery. A semilunar valve.
Mitral (Bicuspid)
Between left atrium and left ventricle. Has two cusps.
Aortic
Between left ventricle and aorta. A semilunar valve.

Blood Flow Pathway

Vena Cava (deoxy blood) R. Atrium R. Ventricle Pulmonary Artery LUNGS Gas exchange Pulmonary Veins (oxy blood) L. Atrium L. Ventricle Aorta (to body) DEOXYGENATED PATHWAY OXYGENATED PATHWAY

Pulmonary vs Systemic Circulation

Pulmonary Circuit Systemic Circuit
Pump Right ventricle Left ventricle
Destination Lungs Entire body
Carries Deoxygenated blood away Oxygenated blood away
Returns Oxygenated blood via pulmonary veins Deoxygenated blood via vena cava
Pressure Low pressure High pressure
Distance Short (heart to lungs) Long (heart to whole body)

The Cardiac Cycle

The cardiac cycle is the complete sequence of contraction and relaxation events that occurs with each heartbeat. At a resting heart rate of 75 beats per minute, one cycle lasts approximately 0.8 seconds.

Systole
Contraction phase: chambers squeeze to eject blood. Pressure rises.
Diastole
Relaxation phase: chambers fill with blood. Pressure drops.

Three Phases of One Heartbeat

1
Atrial Systole (~0.1 s): Both atria contract, pushing remaining blood through AV valves into the ventricles. Ventricles are in diastole (relaxed and filling).
2
Ventricular Systole (~0.3 s): Both ventricles contract. AV valves snap shut (first heart sound: S1, "lub"). Semilunar valves open, blood is ejected into arteries.
3
Relaxation / Diastole (~0.4 s): All chambers relax. Semilunar valves close (second heart sound: S2, "dub"). Blood flows passively from veins into atria and then into ventricles.

Pressure Changes in the Left Ventricle

0 40 80 120 Pressure (mmHg) Time (seconds) 0.1 0.3 0.7 0.8 Atrial Sys. Ventricular Systole Diastole (Relaxation) LV Pressure Aortic Pressure S1 S2

Electrical Conduction System

1
SA Node (Sinoatrial): The natural pacemaker in the right atrium. Fires at ~60-100 bpm, initiating each heartbeat.
2
AV Node (Atrioventricular): Receives the signal and delays it ~0.1 s so the atria finish contracting before the ventricles begin.
3
Bundle of His: Carries the electrical impulse from the AV node into the interventricular septum.
4
Bundle Branches and Purkinje Fibers: Split into left and right branches, then into a network of fibers that cause the ventricles to contract from the apex upward.

Conduction Pathway Diagram

SA SA Node (pacemaker) AV AV Node (0.1s delay) Bundle of His Left Bundle Right Bundle Purkinje Fibers

How to Trace Blood Flow

1
Start at the vena cava. Deoxygenated blood enters the right atrium from the superior and inferior vena cava.
2
Cross the tricuspid valve into the right ventricle.
3
Exit via the pulmonary valve into the pulmonary artery, heading to the lungs for gas exchange.
4
Return via pulmonary veins (now oxygenated) into the left atrium.
5
Cross the mitral valve into the left ventricle.
6
Exit via the aortic valve into the aorta, delivering oxygenated blood to the entire body.

Worked Example: Heart Sounds

Question: A clinician hears a "lub-dub" during auscultation. What structural events produce each sound?

S1: "Lub"
AV valves (tricuspid + mitral) close at the start of ventricular systole. Blood pushes against closed valves, creating the first sound.
S2: "Dub"
Semilunar valves (pulmonary + aortic) close at the start of diastole. Backflow pressure snaps them shut, creating the second sound.

Worked Example: Cardiac Output

Problem: An athlete has a resting heart rate of 55 bpm and a stroke volume of 85 mL. Calculate cardiac output. During peak exercise, heart rate rises to 180 bpm and stroke volume to 120 mL. What is the exercise cardiac output?

Common Mistakes

✕ Arteries always carry oxygenated blood
Wrong: The pulmonary arteries carry deoxygenated blood to the lungs. Arteries are defined by direction (away from heart), not oxygen content.
✕ Blood goes: heart, lungs, body, heart
Incomplete: Blood passes through the heart twice per circuit: once before the lungs, once before the body. It is a double circulation.
✕ Left ventricle is on the left side of a diagram
Careful: Anatomical diagrams often show the heart as if facing you. The left ventricle appears on the right side of the image.

Question 1 of 5

Which valve prevents backflow from the aorta into the left ventricle?

Question 2 of 5

During ventricular systole, which of the following occurs?

Question 3 of 5

The SA node is sometimes called the "pacemaker" of the heart because it:

Question 4 of 5

A patient has a heart rate of 80 bpm and a stroke volume of 65 mL. What is their cardiac output?

Question 5 of 5

Which vessel carries oxygenated blood from the lungs back to the heart?