Anatomy and Physiology 2e · The Cardiovascular System: The Heart
Heart Anatomy
On this page 9 sections
In 30 seconds
The heart is a hollow, muscular pump about the size of a fist, located in the Mediastinum Central thoracic space between the lungs that houses the heart Full entry → — the space between the lungs behind the sternum — with about two-thirds of its mass left of the midline. Its base (where the great vessels attach) faces posteriorly and superiorly, while its apex (the tip of the left Ventricle Lower pumping chamber Full entry →) points down, forward, and left. Functionally, the heart is two pumps in series: the right side sends deoxygenated blood to the lungs (pulmonary circuit), and the left side sends oxygenated blood to the body (systemic circuit). Four chambers, four one-way valves, and the heart's own Coronary circulation The heart's own arteries and veins (coronary arteries → cardiac veins → coronary sinus) Full entry → keep flow moving one way.
Why this matters
Heart anatomy is the foundation for the rest of the chapter: the cardiac cycle, electrical activity, and cardiac output all build on knowing chambers and valves. Clinically, "lub-dub" sounds are valves closing, murmurs are valves that do not open or close cleanly, and coronary blockages cause heart attacks (myocardial infarction). Knowing which artery feeds which region explains where damage occurs.
The college version
Core Concepts
Location and orientation
The heart sits in the mediastinum, protected by the rib cage and sternum. The base faces posteriorly and slightly superiorly, where the great vessels enter or leave; the apex points anteriorly, inferiorly, and left, producing the apical beat (point of maximal impulse) felt on the chest wall. This orientation explains left-sided chest sensations and common cardiac procedure approach routes.
The pericardium
The heart is wrapped in the Pericardium Sac around the heart: fibrous outer layer + two-layer serous membrane Full entry →. The tough outer fibrous pericardium anchors the heart to the diaphragm and great vessels and prevents overstretching. Inside, the serous pericardium has a parietal layer lining the sac and a visceral layer (the epicardium) on the heart; between them, the pericardial cavity holds a small amount of lubricating fluid so the heart beats with minimal friction. Inflammation of this sac (pericarditis) is the classic example of these layers rubbing painfully.
Layers of the heart wall
From outside to inside: epicardium (outer layer, identical to the visceral serous pericardium; carries vessels and fat), myocardium (the cardiac muscle that pumps; thickest in the left ventricle, which faces the high resistance of the systemic circuit, thinnest in the atria), and endocardium (smooth endothelium lining chambers and valves, reducing friction and discouraging clots).
Chambers, septa, and great vessels
The interatrial septum and interventricular septum divide the heart into right and left halves, each with an upper receiving chamber and a lower pumping chamber. The right Atrium Upper receiving chamber Full entry → receives deoxygenated blood from the venae cavae and coronary sinus; the right ventricle pumps it through the pulmonary trunk to the lungs. The left atrium receives oxygenated blood from the four pulmonary veins; the left ventricle pumps it through the aorta to the body. Surface grooves (sulci) — the coronary sulcus between atria and ventricles and the anterior/posterior interventricular sulci over the septum — mark the divisions and carry the coronary vessels.
Valves and one-way flow
Two atrioventricular (AV) valves sit between each atrium and ventricle: the tricuspid (three cusps) on the right, the mitral/bicuspid (two cusps) on the left. Chordae tendineae Cords tethering AV valve cusps to papillary muscles Full entry → tether the AV cusps to papillary muscles, so the valves cannot flip backward into the atria during ventricular contraction. Two Semilunar valves Pulmonary and aortic valves with crescent-shaped cusps Full entry → guard the ventricular exits: the pulmonary valve (right ventricle → pulmonary trunk) and the aortic valve (left ventricle → aorta); their three pocket-like cusps snap shut when ventricular pressure falls. Valve closures produce the heart sounds: S1 ("lub") = AV valves closing at the start of ventricular contraction; S2 ("dub") = semilunar valves closing at the start of relaxation.
Coronary circulation
The heart cannot survive on blood passing through its chambers, so it has its own supply. The coronary arteries arise from the aorta just above the aortic valve. The left coronary artery branches into the anterior interventricular (LAD) and circumflex arteries; the right coronary artery gives off the right marginal artery and, in most hearts, the posterior interventricular artery. Cardiac veins drain blood into the coronary sinus, which empties into the right atrium. Because the contracting myocardium squeezes these arteries shut, they fill mainly during diastole.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Right heart pumps to the body | Right heart pumps to the lungs | Right = pulmonary circuit; left = systemic circuit |
| Arteries always carry oxygenated blood | Pulmonary arteries carry deoxygenated blood | "Artery" means direction away from the heart, not oxygen content |
| Mitral valve is on the right | Mitral valve is on the left | Right = tricuspid; left = bicuspid/mitral |
| Semilunar valves have chordae tendineae | Only AV valves have chordae | Semilunar cusps snap shut from blood pressure alone |
| Heart sounds = muscle contracting | Heart sounds = valves closing | "Lub" and "dub" are valve closures, not contraction |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your heart is a two-story pump house with four rooms. The top rooms (the atria) catch blood and drop it into the big rooms below (the ventricles), which do the heavy pumping. Every doorway has a one-way door, so blood never sloshes backward — and the pump house has its own private water pipes (the coronary arteries) so it never runs out of fuel.
Worked example
Follow one red blood cell through the heart. It returns from the body in the inferior vena cava with little oxygen, enters the right atrium, slips through the open tricuspid valve into the relaxed right ventricle, and is squeezed through the pulmonary valve into the pulmonary trunk. In the lungs it exchanges gases, then returns through a pulmonary vein to the left atrium, passes the mitral valve into the left ventricle, and is launched through the aortic valve into the aorta at high pressure. The trip shows the logic: the right heart handles blood before the lungs, the left heart after the lungs, and the valves never let blood flow backward.
Why is the left ventricle so thick? Compare workloads: the right ventricle pushes blood a short distance through the low-resistance lungs; the left ventricle must push through the entire body against high resistance. Muscle thickness follows workload — which is why the apex beat you feel on the chest comes from the left ventricle, and why left and right ventricular failure differ.
Key takeaways
- The heart is a double pump: right side → pulmonary circuit; left side → systemic circuit; the two pumps work in series.
- Flow route: venae cavae → RA → tricuspid → RV → pulmonary valve → pulmonary trunk → lungs → pulmonary veins → LA → mitral → LV → aortic valve → aorta.
- Atria receive; ventricles pump. The left ventricle has the thickest myocardium.
- Four valves: tricuspid and mitral (AV); pulmonary and aortic (semilunar). Chordae tendineae + papillary muscles exist only on AV valves.
- Pericardium: fibrous (tough sac) + serous (parietal and visceral layers) with a lubricated cavity.
- Wall layers: epicardium → myocardium → endocardium.
- Coronary arteries arise from the aorta and fill mostly during diastole; veins drain to the coronary sinus → right atrium.
- S1 "lub" = AV valve closure; S2 "dub" = semilunar valve closure.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
List, in order, the structures a drop of blood passes through from the superior vena cava to the aorta.
Show answer
Superior vena cava → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary trunk → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta.
Which chamber has the thickest myocardium, and why?
Show answer
The left ventricle, because it must generate enough pressure to pump through the entire systemic circuit against high resistance.
What structures prevent the AV valves from everting into the atria during ventricular contraction?
Show answer
The chordae tendineae, which tether the valve cusps to the papillary muscles of the ventricular wall.
Where do the coronary arteries arise, and when do they fill most?
Show answer
They arise from the aorta just above the aortic valve and fill mostly during diastole, when the myocardium is relaxed.
Which heart sounds do the AV and semilunar valves produce, and when?
Show answer
S1 ("lub") = AV valve closure at the start of ventricular systole; S2 ("dub") = semilunar valve closure at the start of ventricular diastole.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Mediastinum
- Central thoracic space between the lungs that houses the heart
- Pericardium
- Sac around the heart: fibrous outer layer + two-layer serous membrane
- Epicardium / myocardium / endocardium
- Outer, muscular, and inner lining layers of the heart wall
- Atrium
- Upper receiving chamber
- Ventricle
- Lower pumping chamber
- Tricuspid / mitral valves
- Right (3 cusps) and left (2 cusps) AV valves
- Semilunar valves
- Pulmonary and aortic valves with crescent-shaped cusps
- Chordae tendineae
- Cords tethering AV valve cusps to papillary muscles
- Coronary circulation
- The heart's own arteries and veins (coronary arteries → cardiac veins → coronary sinus)
Sources & references
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