Heart Layers & Coverings
From the outside in: a protective sac and three wall layers. Know the order — and the two emergencies that come from fluid building up in the wrong space.
Pericardium — the protective sac
- Fibrous pericardium — tough outer layer that anchors the heart in place.
- Serous pericardium — two thin layers:
- Parietal layer — lines the fibrous pericardium.
- Visceral layer (epicardium) — sits directly on the heart surface.
- Pericardial fluid — between the parietal and visceral layers; lubricates and reduces friction with each beat. Normally 15–50 mL.
- Pericarditis — inflammation of the sac. Causes a friction rub, sharp chest pain that's worse with inspiration and lying flat, relieved by sitting up and leaning forward.
- Cardiac tamponade — excess fluid compresses the heart → falling cardiac output. Look for Beck's triad (muffled heart sounds, JVD, hypotension). Emergency — needs pericardiocentesis.
The three wall layers
- Epicardiumouter
- Same as the visceral pericardium. Protective covering — holds the coronary arteries and fat.
- Myocardiummiddle · thickest
- The muscular layer that contracts and pumps. The left ventricle's myocardium is the thickest — it pumps to the whole body.
- Endocardiuminner
- Smooth lining that reduces friction as blood flows through. Continuous with the lining of the blood vessels. Site of endocarditis (infection).
From outside in: Epi (on top) → Myo (muscle) → Endo (inner lining). The myocardium does all the heavy lifting, and the left ventricle has the thickest myocardium because it works against the highest pressure.
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Chambers & Valves
Four chambers, four valves. The whole system splits into a right side that handles deoxygenated blood and a left side that handles oxygenated blood.
The four chambers
- Right AtriumRA
- Receives deoxygenated blood from the body via the superior & inferior vena cava; sends it to the right ventricle through the tricuspid valve.
- Right VentricleRV
- Pumps deoxygenated blood to the lungs through the pulmonic valve and pulmonary artery.
- Left AtriumLA
- Receives oxygenated blood from the lungs via the pulmonary veins; sends it to the left ventricle through the mitral valve.
- Left VentricleLV
- Pumps oxygenated blood to the entire body through the aortic valve. Thickest wall, strongest chamber — it works against the highest pressure.
The four valves
- TricuspidAV valve
- Between RA → RV. Three leaflets. Prevents backflow into the right atrium.
- MitralAV valve
- Between LA → LV. Two leaflets (a.k.a. bicuspid). The most commonly diseased valve.
- Pulmonicsemilunar
- Between RV → pulmonary artery. Prevents backflow into the right ventricle.
- Aorticsemilunar
- Between LV → aorta. Aortic stenosis is the most common valve disease in the elderly.
Toilet Paper My Ass
Tricuspid → Pulmonic → Mitral → Aortic. The two AV valves (Tricuspid, Mitral) sit between atria and ventricles; the two semilunar valves (Pulmonic, Aortic) guard the ventricle exits.
Blood Flow Pathway
Follow one drop of blood from the body, through the lungs, and back out to the body. This sequence is tested on every nursing exam.
Everywhere else, arteries carry oxygenated blood and veins carry deoxygenated. The pulmonary circuit is reversed — and it's commonly tested.
Cardiac Cycle & Heart Sounds
One heartbeat is one cardiac cycle — a squeeze (systole) and a refill (diastole). The valves snapping shut are the sounds you hear.
Systole
Ventricles contract and pump blood out (RV → lungs, LV → body).
AV valves (tricuspid & mitral) close → S1 "lub", the start of systole.
Systolic pressure = the highest arterial pressure, during contraction.
Diastole
Ventricles relax and refill from the atria.
Semilunar valves (pulmonic & aortic) close → S2 "dub", the start of diastole.
Diastolic pressure = the lowest arterial pressure, during relaxation.
Abnormal heart sounds
- S3 ("Ken-tuck-y") — early diastole. Normal in children and young adults; in older adults it signals heart failure (ventricular overload).
- S4 ("Ten-nes-see") — late diastole. A stiff, noncompliant ventricle (hypertension, aortic stenosis, MI). Generally abnormal in adults.
- Murmur — a swooshing sound from turbulent flow through a diseased valve (stenosis or regurgitation).
- Friction rub — a scratchy, grating sound = pericarditis.
? Which extra heart sound points to a stiff, noncompliant ventricle in an adult — S3 or S4?
Cardiac Conduction System
The heart's electrical wiring fires in a fixed order — and each backup pacemaker is slower than the one before it.
EKG waveform correlation
- P wave
- Atrial depolarization (contraction). Should appear before every QRS.
- PR interval
- SA firing to ventricular depolarization, including the AV delay. Normal 0.12–0.20 sec. Prolonged = heart block.
- QRS complex
- Ventricular depolarization. Normal 0.06–0.12 sec. Wide = bundle branch block or hyperkalemia.
- T wave
- Ventricular repolarization. Peaked = hyperkalemia; flat/inverted = hypokalemia or ischemia.
- QT interval
- Total depolarization + repolarization. Prolonged QT = Torsades de Pointes risk.
If the SA node fails, the AV node takes over at 40–60 bpm (junctional rhythm). If that fails too, the Purkinje fibers fire at 20–40 bpm (ventricular escape — dangerously slow). The further down the conduction system the problem, the slower the heart.
Cardiac Output
How much blood the heart moves per minute. Everything that changes a patient's perfusion traces back to this one equation.
Three factors that change stroke volume
- Preload
- How stretched the ventricle is before it pumps. ↑ Preload → ↑ stroke volume (Frank–Starling). Fluid overload raises it; dehydration, hemorrhage, diuretics, and nitroglycerin lower it.
- Afterload
- The resistance the ventricle pushes against. ↑ Afterload → ↓ stroke volume. Hypertension and aortic stenosis raise it; ACE inhibitors, ARBs, and vasodilators lower it.
- Contractility
- How hard the muscle squeezes. Digoxin, dopamine, and dobutamine raise it; heart failure and beta-blockers lower it.
Preload = how far the rubber band is stretched before you let go (blood volume filling the ventricle). Afterload = how tight the tube is that the rubber band has to shoot through (arterial resistance). Contractility = how strong the rubber band itself is (heart-muscle strength). All three set how much blood leaves with each beat.
? A patient on a new beta-blocker has a dropping cardiac output. Why?
Blood Vessels
Arteries carry blood away from the heart; veins carry it back. Capillaries are where the actual exchange happens.
- Arteries
- Thick, muscular, elastic walls; no valves. Carry blood away from the heart under high pressure. Bright red (oxygenated — except the pulmonary arteries). Palpable pulses; arterial bleeding spurts.
- Arterioles
- The smallest arteries. Control flow into capillary beds by constricting/dilating — the primary site of blood-pressure regulation.
- Capillaries
- Walls one cell thick. The only vessel where gas, nutrient, and waste exchange occurs.
- Venules
- The smallest veins. Collect blood from the capillary beds.
- Veins
- Thinner walls than arteries, with valves to prevent backflow. Carry blood toward the heart under low pressure. Dark red (deoxygenated — except the pulmonary veins); venous bleeding is a steady flow. Varicose veins = incompetent valves.
"Arteries carry Away" (both start with A). "Veins have Valves" (both start with V). Capillaries = exchange. The only exception: pulmonary arteries carry deoxygenated blood and pulmonary veins carry oxygenated blood.
Blood Pressure Regulation
Blood pressure is cardiac output × resistance — and the body has both a fast and a slow system to defend it.
The regulatory systems
- Baroreceptors (short-term) — pressure sensors in the carotid sinus and aortic arch. When BP drops, they signal the brain to raise heart rate and constrict vessels within seconds.
- RAAS (long-term) — the kidney-driven cascade:
- ↓ BP or ↓ renal perfusion → kidneys release renin
- Renin → angiotensinogen → Angiotensin I
- ACE (in the lungs) → Angiotensin II, a powerful vasoconstrictor
- Angiotensin II → aldosterone → kidneys retain Na⁺ and water → ↑ volume → ↑ BP
- ADH (antidiuretic hormone) — from the posterior pituitary; makes the kidneys retain water → ↑ volume → ↑ BP.
- ANP / BNP — released when the heart is stretched (volume overload); cause Na⁺ and water excretion → ↓ BP. BNP is a lab marker for heart failure.
ACE inhibitors (-pril) block ACE → less Angiotensin II. ARBs (-sartan) block it at the receptor. Aldosterone antagonists (spironolactone) make you excrete Na⁺/water but retain K⁺. Understand RAAS and the whole cardiac drug list starts to make sense.
Coronary Arteries
The heart feeds itself through the coronary arteries. Block one and that muscle starts to die — that's a myocardial infarction.
- Right Coronary Artery (RCA) — supplies the right atrium, right ventricle, inferior wall of the left ventricle, and the SA and AV nodes. Blockage → inferior wall MI and bradycardia (SA/AV node ischemia).
- Left Main Coronary Artery — branches into:
- Left Anterior Descending (LAD) — "the widow-maker." Supplies the anterior wall and septum of the LV. The most common MI site.
- Left Circumflex (LCx) — supplies the lateral and posterior LV walls. Blockage → lateral wall MI.
- Coronary arteries fill during diastole (when the heart relaxes). Tachycardia shortens diastole → less coronary perfusion.
The LAD supplies the largest area of the left ventricle, so a complete blockage causes a massive anterior wall MI with high mortality. If a question mentions an anterior wall MI, think LAD territory.
Fetal Circulation
Three shunts route blood around the baby's non-working lungs — because oxygen comes from the placenta, not the air.
- Ductus Venosusbypasses liver
- Umbilical vein → inferior vena cava. Closes when the cord is clamped; becomes the ligamentum venosum.
- Foramen Ovalebypasses lungs
- Right atrium → left atrium. Functionally closes within hours of birth (pressure changes); permanently by ~3 months. Becomes the fossa ovalis.
- Ductus Arteriosusbypasses lungs
- Pulmonary artery → aorta. Functionally closes within ~15 hours (oxygen constricts it); permanently by ~3 weeks. Becomes the ligamentum arteriosum.
Fetal lungs are fluid-filled and not yet working — gas exchange happens at the placenta. These shunts redirect blood away from the lungs. After birth, the first breath inflates the lungs, pulmonary resistance drops, and the shunts close.
The umbilical cord
- 2 arteries — carry deoxygenated blood from the fetus to the placenta.
- 1 vein — carries oxygenated blood from the placenta to the fetus.
- Protected by Wharton's jelly; normal length ~55 cm (22 inches).
If the ductus arteriosus fails to close, blood shunts from the aorta back to the pulmonary artery → increased pulmonary blood flow → heart failure over time. Sign: a continuous "machinery" murmur. Treatment: indomethacin (an NSAID that promotes closure) or surgical ligation.
Age-Related Cardiovascular Changes
The aging heart stiffens, slows, and loses its quick reflexes — which changes how you assess and treat older patients.
- Arteriosclerosis — arteries stiffen and lose elasticity → ↑ systolic BP (isolated systolic hypertension is common in the elderly).
- Decreased cardiac output — the heart muscle weakens slightly and responds less to stress.
- Decreased baroreceptor sensitivity → higher risk of orthostatic hypotension (teach patients to rise slowly).
- Valves thicken and calcify → murmurs become more common (aortic stenosis).
- SA node fibrosis → slower resting heart rate, more arrhythmias, sick sinus syndrome.
- Veins dilate and valves weaken → higher risk of varicose veins and DVT.
- Reduced beta-adrenergic response → maximum heart rate falls with age (≈ 220 − age).
Frequently Asked Questions
Common student questions on this topic.
What is the difference between systole and diastole?
Where does the heartbeat originate in a normal heart?
Why are the pulmonary arteries and veins different from all other vessels?
What is the difference between the right and left sides of the heart?
How do you calculate cardiac output?
Which electrolyte imbalance most affects cardiac rhythm?
Quick-Reference Cheatsheet
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Cardiovascular System Cheatsheet
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