Anatomy & Physiology · Chapter 19

The Cardiovascular System: The Heart

Somewhere behind your breastbone, a muscle the size of your fist is squeezing about once a second, and it has never once taken a break. Over an average lifetime it will beat more than two and a half billion times and move enough blood to fill a small lake. This chapter opens that pump up. You will learn how the heart is built as two pumps in one, trace a single drop of blood on its full loop through the lungs and the body, watch the four valves keep everything flowing forward, and follow the tiny electrical spark the heart makes for itself — the spark that a machine at the bedside can read straight off the skin as an ECG.

By the end of this chapter you can

Locate the heart in the mediastinum, name the layers of its wall and the sac around it, and explain why the left ventricle is the thickest chamber.
Identify the four chambers and the great vessels, and state which blood (oxygen-rich or oxygen-poor) each one carries.
Trace blood flow through the heart in order and distinguish the pulmonary from the systemic circuit.
Name the four valves, place each one, and explain how chordae tendineae and pressure keep flow one-way.
Describe the conduction system from SA node to Purkinje fibers and explain the purpose of the AV node delay.
Break one heartbeat into the phases of the cardiac cycle and say what makes the “lub-dub” heart sounds.
Match each wave of the ECG — P, QRS, and T — to the electrical event it records.
Calculate cardiac output from heart rate and stroke volume, and predict how it changes.

Section 1The heart as a double pump

Anterior frontal section of the heart reference figure — **Reference.** The heart in frontal section, anterior view — chambers, valves, great vessels, and wall layers — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A fist-sized pump in the middle of the chest

The heart sits in the mediastinum, the space between the two lungs, resting on the diaphragm and tilted so that about two-thirds of it lies to the left of the midline. Its broad top, where the great vessels attach, is the base; its blunt, downward-and-leftward tip is the apex — the spot where you feel the beat against the chest wall. Around it is a tough double-walled sac, the pericardium. The outer fibrous pericardium anchors the heart and keeps it from overfilling; inside that, a two-layered serous pericardium secretes a slippery fluid into the pericardial cavity so the beating heart glides without friction. When that space fills with fluid or blood and squeezes the heart, the result — cardiac tamponade — is a true emergency.

Two pumps, four chambers

The single most useful idea in this whole chapter is that the heart is really two pumps bolted together, side by side, working in perfect step. The right heart receives oxygen-poor blood returning from the body and pushes it a short distance to the lungs. The left heart receives the freshly oxygenated blood coming back from the lungs and drives it out to the entire rest of the body. A solid muscular wall, the septum, keeps the two sides from mixing. Each pump has two chambers: a thin-walled atrium on top that receives blood, and a thick-walled ventricle below that ejects it. Blood always travels atrium → ventricle → out an artery.

Chamber	Receives blood from	Sends blood to
Right atrium	Body, via the superior & inferior venae cavae	Right ventricle
Right ventricle	Right atrium	Lungs, via the pulmonary trunk & arteries
Left atrium	Lungs, via the four pulmonary veins	Left ventricle
Left ventricle	Left atrium	Whole body, via the aorta

Three layers, and a heart that feeds itself

The wall itself has three layers. The inner endocardium is a smooth lining continuous with the vessels, so blood never touches rough tissue. The bulky middle myocardium is the cardiac muscle that actually does the pumping — and it is far thicker on the left. The outer epicardium is the visceral layer of the serous pericardium. Notice on the reference figure how the left ventricle wall dwarfs the right: it must generate roughly five times the pressure to push blood through the long systemic circuit, while the right ventricle only has to reach the nearby lungs. Finally, all that hard-working muscle needs its own blood supply, delivered by the coronary arteries that branch off the aorta. Block one and the muscle downstream starves — that is a heart attack, a myocardial infarction.

Thickness follows workload. The left ventricle is the thickest chamber not because it is more important, but because it faces the highest pressure. Anything that raises that pressure long-term — chronic hypertension, a stiff aortic valve — makes the left ventricle thicken further (hypertrophy), just as any muscle grows against a heavier load.

Tap through the chambers, vessels, and wall layers in the interactive below.

Interactive The heart as a double pump — tap each chamber and vessel; blue is low O₂, red is high O₂. Open full ↑

Key points

The heart lies in the mediastinum, wrapped in a pericardial sac whose fluid lets it beat friction-free.
It is two pumps in one: the right heart serves the lungs, the left heart serves the body, separated by the septum.
Atria receive, ventricles eject; the wall is endocardium, myocardium, epicardium; the left ventricle is thickest because it faces the highest pressure.
The coronary arteries feed the myocardium; blocking one causes a myocardial infarction.

Section 2Blood flow through the heart

Pulmonary and systemic circulation reference figure — **Reference.** The pulmonary and systemic circuits — red vessels carry oxygenated blood, blue carry deoxygenated, purple are sites of gas exchange — OpenStax, CC BY 3.0 (via Wikimedia Commons)

One drop, one full loop

Let us follow a single drop of blood all the way around. Returning from the body tired and oxygen-poor, it empties into the right atrium through the superior and inferior venae cavae. It drops through the tricuspid valve into the right ventricle, which squeezes it through the pulmonary valve into the pulmonary trunk and on to the lungs. In the lungs it unloads carbon dioxide and picks up a fresh load of oxygen. Now bright red, it flows back through the pulmonary veins into the left atrium, passes the mitral valve into the powerful left ventricle, and is launched through the aortic valve into the aorta to begin its journey around the body again.

Step	Structure	Blood here is
1	Venae cavae → right atrium	Oxygen-poor
2	Tricuspid valve → right ventricle	Oxygen-poor
3	Pulmonary valve → pulmonary arteries → lungs	Oxygen-poor (arriving)
4	Pulmonary veins → left atrium	Oxygen-rich
5	Mitral valve → left ventricle	Oxygen-rich
6	Aortic valve → aorta → body	Oxygen-rich

Two circuits, run in series

That single loop is really two loops chained together. The pulmonary circuit runs from the right heart to the lungs and back; its whole purpose is gas exchange. The systemic circuit runs from the left heart out to every other tissue — brain, gut, kidneys, muscles — and back. Because they are wired in series, the same drop must pass through both, and the two ventricles must pump the exact same volume over time. If the right side pumped even a little more than the left for long, blood would pile up in the lungs; if the left outran the right, it would pile up in the body. That balance is why heart failure on one side eventually drags down the other.

The great exception

Here is a rule worth burning in, along with the one place it seems to break. Arteries carry blood away from the heart; veins carry blood back toward it. Notice the definition says nothing about oxygen. Almost everywhere, arteries happen to be oxygen-rich and veins oxygen-poor — but the pulmonary vessels flip that. The pulmonary arteries carry oxygen-poor blood (away from the heart, toward the lungs), and the pulmonary veins carry oxygen-rich blood (back toward the heart). They are the exception that proves the real rule is about direction, not color.

Direction, not color. If a test question shows a blue pulmonary artery, do not call it a mistake. “Artery” means away from the heart — and the pulmonary artery is carrying used blood away, toward the lungs to be refreshed. The same logic makes the pulmonary vein the one vein full of bright red blood.

Play or step a single drop through the whole loop in the interactive.

Interactive Follow one drop of blood on its full loop through the right heart, the lungs, and the left heart. Open full ↑

Key points

The forward path: venae cavae → RA → tricuspid → RV → pulmonary valve → lungs → pulmonary veins → LA → mitral → LV → aortic valve → aorta.
The pulmonary circuit (right heart ↔ lungs) and systemic circuit (left heart ↔ body) run in series, so both ventricles move equal volumes.
Arteries lead away, veins lead back — a rule about direction; the pulmonary vessels are the oxygen-content exception.

Section 3The four one-way valves

Superior view of the four heart valves reference figure — **Reference.** The four heart valves seen from above with the atria removed — two AV valves and two semilunar valves — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Four doors that only open one way

A pump is only as good as its valves, and the heart has four, each a flap-door that opens under forward pressure and slams shut against backward pressure. Two of them, the atrioventricular (AV) valves, sit between each atrium and its ventricle: the tricuspid valve on the right (three cusps) and the mitral or bicuspid valve on the left (two cusps). The other two, the semilunar valves, guard the exits where the great arteries leave: the pulmonary valve at the mouth of the pulmonary trunk and the aortic valve at the mouth of the aorta. Together they make sure that with every squeeze, blood can only go forward.

Valve	Type	Location	Open when
Tricuspid	AV	Right atrium → right ventricle	Ventricles relaxed (filling)
Mitral (bicuspid)	AV	Left atrium → left ventricle	Ventricles relaxed (filling)
Pulmonary	Semilunar	Right ventricle → pulmonary trunk	Ventricles contracting (ejecting)
Aortic	Semilunar	Left ventricle → aorta	Ventricles contracting (ejecting)

What holds them shut

The two valve types are built to resist backflow in different ways. When the ventricles contract hard, the pressure could blow the AV valve cusps up into the atria — so each cusp is tethered by tough strings, the chordae tendineae (“heart strings”), to little muscle bumps called papillary muscles on the ventricle floor. The papillary muscles tighten just before the squeeze, and the chords hold the cusps like the ropes on a parachute, letting them close flat but not turn inside out. The semilunar valves need no chords: their three cup-shaped pockets simply catch the blood that tries to fall back after ejection, and the weight snaps them shut.

When valves go wrong

Valves fail in exactly two ways, and the words are worth knowing. A stenotic valve is stiff and narrowed — it will not open all the way, so the heart must strain to push blood through a smaller hole. A regurgitant (or insufficient) valve is leaky — it will not close all the way, so blood sloshes backward with each beat. Either fault sets up turbulence, and turbulence you can hear through a stethoscope as a murmur. The normal, healthy sounds — the familiar “lub-dub” — are simply the valves snapping shut, which is the bridge into the cardiac cycle.

LAB · RAT. To remember which AV valve is which: Left Atrium has the Bicuspid; Right Atrium has the Tricuspid. And the heart sounds map cleanly: S1 (“lub”) is the AV valves closing as the ventricles start to squeeze; S2 (“dub”) is the semilunar valves closing as the ventricles relax.

Tap each valve to watch it open and close in the interactive.

Interactive Four one-way valves keep blood moving forward — tap each valve to see it open and close. Open full ↑

Key points

Two AV valves (tricuspid right, mitral/bicuspid left) sit between atria and ventricles; two semilunar valves (pulmonary, aortic) guard the artery exits.
Chordae tendineae and papillary muscles anchor the AV cusps so they cannot flip backward during the squeeze.
Valves fail as stenosis (won’t open) or regurgitation (won’t close), producing murmurs; normal S1/S2 are valves snapping shut.

Section 4The conduction system

Spread of the cardiac impulse reference figure — **Reference.** The electrical impulse spreading through the heart in sequence, step by step — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A heart that writes its own rhythm

Unlike the muscles in your arm, the heart does not wait for the brain to tell it to beat. A small population of specialized cells can depolarize all on their own, over and over, a property called autorhythmicity. The fastest of these cells — the sinoatrial (SA) node, tucked in the wall of the right atrium — fires about 60 to 100 times a minute and sets the pace for everyone else. That is why the SA node earns the nickname the heart’s pacemaker. Cut every nerve to the heart and it would keep right on beating; the nerves only speed it up or slow it down, as we will see.

The path of the spark

The impulse spreads in a precise, deliberate order so the chambers squeeze in the right sequence — atria first, then ventricles. From the SA node the signal sweeps across both atria, making them contract and top off the ventricles. It then reaches the atrioventricular (AV) node, the only electrical doorway between atria and ventricles, where it deliberately pauses for about a tenth of a second. From the AV node it races down the AV bundle (bundle of His), splits into the right and left bundle branches along the septum, and fans out through the Purkinje fibers into the ventricle walls. The ventricles then contract from the apex up, wringing blood toward the exits like squeezing a tube of toothpaste from the bottom.

Component	Role	Intrinsic rate
SA node	Pacemaker; starts each beat, fires the atria	60–100 / min
AV node	Gateway to the ventricles; delays the signal	40–60 / min
AV bundle & branches	Carry the signal down the septum	—
Purkinje fibers	Spread the signal through ventricle walls	20–40 / min

Why the pause, and who controls the tempo

That AV-node delay is not a flaw — it is essential. It buys the atria just enough time to finish emptying into the ventricles before the ventricles slam shut and eject. Without it, all four chambers would contract at once and pump far less blood. Notice, too, the built-in backups: if the SA node quits, the AV node can take over at a slower 40 to 60 beats, and the Purkinje fibers slower still — a safety net that keeps a failing heart beating, just too slowly. Finally, the autonomic nervous system is the volume dial: sympathetic signals speed the SA node and strengthen contractions (fear, exercise), while the parasympathetic vagus nerve slows it back down at rest.

The delay is the point. The tenth-of-a-second pause at the AV node is what lets the atria finish filling the ventricles before they fire. On an ECG you can literally measure this pause — it is the flat stretch of the PR segment between the P wave and the QRS complex.

Follow the spark from the SA node all the way down in the interactive.

Interactive Follow the electrical spark from the pacemaker down through the heart in order. Open full ↑

Key points

The heart is autorhythmic; the SA node is the pacemaker, firing 60–100 times a minute.
The path is SA node → AV node → AV bundle → bundle branches → Purkinje fibers, so atria contract before ventricles.
The AV node delay lets the atria finish filling the ventricles; slower backup pacemakers protect against SA failure.
Sympathetic nerves speed the heart, parasympathetic (vagus) nerves slow it.

Section 5The cardiac cycle

Phases of the cardiac cycle reference figure — **Reference.** The phases of one heartbeat, from atrial systole around through ventricular ejection to ventricular filling — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Squeeze and rest, over and over

One heartbeat is a repeating rhythm of two states. Systole is contraction — a chamber squeezing to push blood out. Diastole is relaxation — a chamber loosening to fill back up. At a resting rate of about 75 beats a minute, the whole cycle takes roughly 0.8 seconds, and, importantly, the heart spends more of that time relaxing than contracting. That rest is not laziness: it is when the ventricles refill, and it is also the only time the coronary arteries can push blood into the thick heart muscle. Race the heart too fast and you steal from diastole — less filling, less coronary flow.

The phases, in order

Walk around the reference wheel and the sequence tells itself. During atrial systole the atria give a gentle squeeze that tops off the already-mostly-full ventricles. Then comes ventricular systole: first an isovolumetric contraction where all four valves are shut and pressure builds with no blood moving, then ventricular ejection as the semilunar valves pop open and blood surges into the arteries. Finally ventricular diastole: an isovolumetric relaxation with all valves shut again, then ventricular filling as the AV valves open and blood pours in — most of it passively, before the atria even contract.

Phase	What happens	AV valves	Semilunar valves
Atrial systole	Atria squeeze; final 20–30% of filling	Open	Closed
Isovolumetric contraction	Ventricles tense; pressure builds, no ejection	Closed	Closed
Ventricular ejection	Ventricles push blood into the arteries	Closed	Open
Isovolumetric relaxation	Ventricles relax; pressure falls, no filling	Closed	Closed
Ventricular filling	Blood pours passively into ventricles	Open	Closed

Lub-dub, and the numbers that matter

The two heart sounds are made by valves closing, not opening. S1, the low “lub,” is the AV valves snapping shut as ventricular systole begins; S2, the sharper “dub,” is the semilunar valves shutting as diastole begins. Between them the heart is ejecting; after S2 it is filling. A few numbers frame the whole thing. The volume in a ventricle at the end of filling is the end-diastolic volume (EDV); what remains after ejection is the end-systolic volume (ESV); the amount actually pumped per beat is the stroke volume (EDV − ESV). Multiply stroke volume by heart rate and you get cardiac output — the liters of blood the heart moves each minute, about 5 L at rest.

Cardiac output = heart rate × stroke volume. Every way the body has of moving more blood — exercise, fear, blood loss — works through one of these two levers. Beat faster, or pump more per beat. And because most ventricular filling is passive, a resting heart can often tolerate losing the atrial “kick” (as in atrial fibrillation) far better than it tolerates a racing rate.

Step through each phase and hear the lub-dub in the interactive.

Interactive Step through the phases of one heartbeat and hear where the lub-dub comes from. Open full ↑

Key points

Systole is contraction, diastole is relaxation; the resting cycle is about 0.8 s, mostly diastole.
Order of phases: atrial systole → isovolumetric contraction → ejection → isovolumetric relaxation → filling.
S1 (“lub”) = AV valves closing; S2 (“dub”) = semilunar valves closing.
Stroke volume = EDV − ESV; cardiac output = heart rate × stroke volume (≈5 L/min at rest).

Section 6The ECG

Normal electrocardiogram tracing reference figure — **Reference.** A normal ECG tracing — the P wave, QRS complex, T wave, and the intervals between them — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Reading the heart’s electricity off the skin

Every time the heart depolarizes, the wave of electrical change is large enough to reach the body surface, where electrodes on the skin can pick it up. Plotted over time, that signal is an electrocardiogram (ECG or EKG) — one of the most useful, painless, and inexpensive tests in all of medicine. It does not record the heartbeat itself but the electrical events that trigger it, so each bump on the tracing lines up with a step in the conduction system you just met. Learn to read three simple deflections and the whole picture opens up.

P, QRS, and T

The tracing repeats three main shapes. The small P wave is atrial depolarization — the SA node’s signal spreading across the atria just before they contract. The tall, spiky QRS complex is ventricular depolarization — big and sharp because the ventricles are so muscular. (Atrial repolarization happens here too, but it is buried inside the QRS and never gets its own wave.) The rounded T wave is ventricular repolarization, the ventricles electrically resetting so they can relax and fill again. Remember: depolarization triggers contraction, so on the tracing the electrical wave always comes a beat ahead of the mechanical squeeze.

Feature	Electrical event	Mechanical partner
P wave	Atrial depolarization	Atria contract
PR interval	SA firing through the AV-node delay	Signal waits at the AV node
QRS complex	Ventricular depolarization	Ventricles contract
T wave	Ventricular repolarization	Ventricles relax

Intervals, rate, and trouble

The flat stretches between the waves carry as much information as the waves themselves. The PR interval spans the start of the P wave to the start of the QRS — it is the atria firing plus that all-important AV-node pause; stretch it too long and you have a heart block. The QRS duration tells you how fast the ventricles are being activated, and the QT interval covers the whole span of ventricular electrical activity. You can even read the rate straight off the paper: the tracing runs at a fixed speed, so the spacing between two QRS peaks gives beats per minute. A tracing that is too fast is tachycardia, too slow is bradycardia, and a chaotic, quivering line with no real QRS at all — ventricular fibrillation — means the ventricles are twitching uselessly and no blood is moving. That is the rhythm a defibrillator is built to shock back into order.

Watch the flat stretch. The ST segment — the level stretch between the QRS and the T wave, when the whole ventricle is depolarized — normally sits right on the baseline. When a coronary artery is blocked and heart muscle is dying, that segment lifts off the baseline (“ST elevation”), the classic ECG fingerprint of a heart attack in progress.

Tap each wave to link it to its conduction event in the interactive.

Interactive Each bump of the ECG matches an event in the conduction system — tap a wave. Open full ↑

Key points

An ECG records the heart’s electrical events from the skin; each deflection maps to a conduction step.
P = atrial depolarization, QRS = ventricular depolarization, T = ventricular repolarization; the electrical wave leads the squeeze.
The PR interval holds the AV-node delay; a long one signals heart block.
Rhythm names: tachycardia (fast), bradycardia (slow), ventricular fibrillation (chaotic, no output); ST elevation flags a heart attack.

The chapter in one breath

The heart is a fist-sized double pump in the mediastinum, wrapped in a pericardial sac; the left ventricle is thickest because it faces the highest pressure.
Blood flows atrium → ventricle → artery on each side: the right heart serves the pulmonary circuit (lungs), the left heart the systemic circuit (body), and the two run in series.
Arteries lead away, veins lead back — a rule about direction; the pulmonary vessels flip the usual oxygen content.
Four one-way valves — two AV (tricuspid, mitral), two semilunar (pulmonary, aortic) — keep flow forward; chordae tendineae hold the AV cusps shut.
The heart is autorhythmic: the SA node paces it, the AV node delays the signal, and the Purkinje fibers deliver it to the ventricles.
One beat = systole and diastole; S1/S2 are valves closing, and cardiac output = heart rate × stroke volume.
The ECG reads the electrical events from the skin: P (atrial), QRS (ventricular depolarization), T (ventricular repolarization).

ReferenceWords worth knowing

A quick reference for the terms in this chapter — skim it now, or circle back whenever a word trips you up.

Aorta: The largest artery in the body; it carries oxygen-rich blood out of the left ventricle to begin the systemic circuit.
Atrioventricular (AV) node: The electrical gateway between the atria and ventricles; it delays the impulse about a tenth of a second so the atria can finish filling the ventricles.
Atrioventricular (AV) valve: A valve between an atrium and its ventricle — the tricuspid on the right, the mitral (bicuspid) on the left — held shut by chordae tendineae.
Atrium: An upper, thin-walled receiving chamber of the heart; the right atrium takes blood from the body, the left from the lungs.
Cardiac cycle: One complete heartbeat, made of contraction (systole) and relaxation (diastole) of the chambers.
Cardiac output: The volume of blood pumped by a ventricle each minute; equal to heart rate multiplied by stroke volume (about 5 L/min at rest).
Chordae tendineae: Tough fibrous cords that tie the AV valve cusps to the papillary muscles, keeping the cusps from flipping backward during contraction.
Diastole: The relaxation phase of a heart chamber, during which it fills with blood.
Electrocardiogram (ECG): A recording of the heart’s electrical activity taken from the skin; its P, QRS, and T deflections map to conduction events.
Endocardium: The smooth inner lining of the heart, continuous with the lining of the blood vessels.
Myocardium: The thick middle layer of cardiac muscle that does the heart’s pumping; thickest in the left ventricle.
Pericardium: The double-walled sac around the heart; its inner serous layers secrete fluid so the beating heart moves without friction.
Pulmonary circuit: The loop of circulation from the right heart to the lungs and back, where blood picks up oxygen and drops off carbon dioxide.
Purkinje fibers: Fast-conducting fibers that spread the electrical impulse through the ventricle walls, triggering contraction from the apex up.
QRS complex: The tall, spiky deflection of the ECG marking ventricular depolarization, just before the ventricles contract.
Semilunar valve: A cup-shaped valve guarding a ventricle’s exit into a great artery — the pulmonary and aortic valves — that snaps shut against backflow.
Sinoatrial (SA) node: The heart’s natural pacemaker, in the wall of the right atrium; it fires 60–100 times a minute and sets the rhythm.
Stroke volume: The amount of blood a ventricle ejects in one beat; the end-diastolic volume minus the end-systolic volume.
Systemic circuit: The loop of circulation from the left heart out to all the body’s tissues and back to the right heart.
Systole: The contraction phase of a heart chamber, during which it ejects blood.
Ventricle: A lower, thick-walled pumping chamber of the heart; the right ejects blood to the lungs, the left to the whole body.

Check yourself

1. A classmate calls the heart “one pump.” Correct them, and explain what the septum has to do with it.

The heart is really two pumps in one — a right heart and a left heart working in step. The right heart pushes oxygen-poor blood to the lungs; the left heart pushes oxygen-rich blood to the body. The muscular septum divides them so the two blood supplies never mix.

2. Why is the wall of the left ventricle so much thicker than the wall of the right ventricle?

Because the left ventricle must generate far higher pressure. It pushes blood through the long, high-resistance systemic circuit to the whole body, while the right ventricle only has to reach the nearby, low-pressure lungs. Thickness follows workload.

3. Trace a drop of blood in order from the superior vena cava to the aorta, naming every chamber and valve it passes.

SVC → right atrium → tricuspid valve → right ventricle → pulmonary valve → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta. The lungs sit in the middle, where the blood is reoxygenated.

4. The pulmonary artery carries dark, oxygen-poor blood. Does this break the rule “arteries carry oxygen-rich blood”? Explain.

No — the real rule is about direction, not oxygen. An artery carries blood away from the heart. The pulmonary artery is carrying oxygen-poor blood away from the heart toward the lungs to be refreshed, so it fits the true definition perfectly.

5. Name the four heart valves, sort them into their two types, and say which type has chordae tendineae.

AV valves: tricuspid and mitral (bicuspid). Semilunar valves: pulmonary and aortic. Only the AV valves have chordae tendineae. The chords tie the AV cusps to the papillary muscles so they cannot flip into the atria during the ventricular squeeze.

6. Distinguish valve stenosis from valve regurgitation.

A stenotic valve is narrowed and will not open fully; a regurgitant (insufficient) valve is leaky and will not close fully. Stenosis forces the heart to strain against a small opening; regurgitation lets blood slosh backward. Both cause turbulence heard as a murmur.

7. Which structure is the heart’s pacemaker, and what does “autorhythmic” mean?

The SA (sinoatrial) node; autorhythmic means the heart generates its own beat without needing a signal from the brain. The SA node fires 60–100 times a minute; nerves only speed it up or slow it down.

8. Why does the impulse pause at the AV node, and what would happen without that delay?

The delay gives the atria time to finish emptying into the ventricles before the ventricles contract. Without it, atria and ventricles would fire almost together, the ventricles would eject before they were fully filled, and each beat would pump much less blood.

9. Put the phases of the cardiac cycle in order, starting from atrial systole.

Atrial systole → isovolumetric contraction → ventricular ejection → isovolumetric relaxation → ventricular filling. During the two “isovolumetric” phases all four valves are shut and no blood moves — the ventricle only changes pressure, not volume.

10. What exactly makes the “lub” (S1) and the “dub” (S2) heart sounds?

S1 (“lub”) is the AV valves snapping shut as ventricular systole begins; S2 (“dub”) is the semilunar valves snapping shut as diastole begins. The sounds come from valves closing, not opening.

11. A patient’s heart rate is 80 beats per minute and each beat ejects 70 mL. What is the cardiac output, and which two levers change it?

Cardiac output = 80 × 70 mL = 5,600 mL/min (about 5.6 L/min). The two levers are heart rate and stroke volume — the body raises output by beating faster, pumping more per beat, or both.

12. On an ECG, what electrical event does each of the P wave, QRS complex, and T wave represent?

P = atrial depolarization; QRS = ventricular depolarization; T = ventricular repolarization. Atrial repolarization also occurs, but it is hidden inside the large QRS complex and has no separate wave. Depolarization triggers contraction, so each wave slightly precedes its mechanical squeeze.

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