Difficulty: Intermediate | Prerequisites: Basic tissue types (epithelial, connective, muscle), thoracic cavity orientation
Tags: heart anatomy, cardiac chambers, pericardium, heart valves, coronary circulation, cardiac conduction system, SA node, AV node, pulmonary circuit, systemic circuit, APK2100c, University of Florida
The heart is the central pump of the cardiovascular system, and this chapter is where you learn its physical structure before tackling how it works physiologically. You need a solid grasp of the four chambers, the valves that keep blood moving in one direction, the layers of the heart wall, and the electrical conduction pathway that coordinates each heartbeat. If you are behind, make sure you are comfortable with basic tissue types (especially cardiac muscle and serous membranes) before diving in. Everything here feeds directly into blood vessel anatomy (Ch. 20) and respiratory gas exchange (Ch. 22).
The heart sits in the mediastinum, tilted so the apex points left. It has four chambers separated by valves that enforce one-way flow, a layered wall wrapped in a protective pericardial sac, and its own built-in electrical wiring (the conduction system) that fires in sequence from the SA node down through the Purkinje fibres. The right side serves the pulmonary circuit; the left side serves the systemic circuit.
Pulmonary circuit
The loop of blood flow from the right ventricle to the lungs and back to the left atrium. Its job is gas exchange: dropping off CO₂ and picking up O₂.
In simple terms, this is the "lung loop."
Systemic circuit
The loop from the left ventricle out to the body's tissues and back to the right atrium. It delivers oxygenated blood and collects deoxygenated blood.
In simple terms, this is the "body loop."
Pericardium
The multi-layered sac surrounding the heart. It has a tough outer fibrous pericardium and an inner serous pericardium (itself split into parietal and visceral layers).
Think of it as a double-walled bag with a thin film of fluid between the walls, reducing friction.
Epicardium (visceral pericardium)
The outermost layer of the heart wall, which is also the innermost layer of the pericardial sac. This is the layer shared by the heart wall and the pericardium.
Myocardium
The thick middle layer of the heart wall, composed of cardiac muscle. Its thickness varies by chamber depending on workload.
Endocardium
The smooth inner lining of the heart chambers and valves, made of endothelium over a thin connective tissue layer.
Atrioventricular (AV) valves
The tricuspid valve (right) and bicuspid/mitral valve (left), positioned between the atria and ventricles. They are anchored by chordae tendineae and papillary muscles to prevent eversion.
Semilunar valves
The pulmonary valve and aortic valve, located at the exits of the right and left ventricles, respectively. They have three cusps each and prevent backflow into the ventricles.
Sinoatrial (SA) node
The heart's primary pacemaker, located in the right atrial wall near the superior vena cava. It sets the rhythm.
Atrioventricular (AV) node
Located in the interatrial septum near the floor of the right atrium. It is the only electrical connection between the atria and ventricles.
Bundle of His (AV bundle)
The conduction pathway that carries the electrical signal from the AV node into the interventricular septum, where it splits into right and left bundle branches.
Purkinje fibres (subendocardial conducting network)
The terminal branches of the conduction system that spread the impulse rapidly through the ventricular myocardium, triggering contraction from the apex upward.
Coronary sinus
The large venous channel on the posterior surface of the heart that collects most of the deoxygenated blood from the heart wall and empties into the right atrium.
The right side of the heart (right atrium + right ventricle) handles the pulmonary circuit
Receives deoxygenated blood from the body, sends it to the lungs
The left side (left atrium + left ventricle) handles the systemic circuit
Receives oxygenated blood from the lungs, sends it to the entire body
A useful mnemonic: Right = Receives from the body, routes to the lungs
The heart sits in the mediastinum (the central compartment of the thoracic cavity), between the lungs
It rests on the diaphragm, posterior to the sternum
The base (broad, superior portion) is directed posteriorly and superiorly; the great vessels attach here
The apex (inferior, pointed tip) points anteriorly, inferiorly, and to the left, roughly at the 5th intercostal space, midclavicular line
The anterior surface is mostly the right ventricle; the left ventricle forms most of the left lateral and posterior surfaces
Fibrous pericardium: tough, dense connective tissue outer sac; anchors the heart in the mediastinum and prevents overfilling
Serous pericardium:
Parietal layer: lines the inner surface of the fibrous pericardium
Visceral layer (epicardium): adheres to the heart surface
Pericardial cavity: the thin, fluid-filled space between parietal and visceral layers; reduces friction during beating
Heart wall (outside in):
Epicardium (visceral pericardium) – serous membrane with fat and coronary vessels
Myocardium – cardiac muscle; bulk of the wall
Endocardium – smooth endothelial lining
The shared layer: the epicardium is simultaneously the visceral layer of the serous pericardium and the outermost layer of the heart wall. Exam favourite.
Atria have thin walls because they only need to push blood a short distance into the ventricles below
Ventricles have thicker walls because they must generate enough pressure to push blood through entire circuits
The left ventricle is the thickest chamber, roughly three times the wall thickness of the right ventricle, because the systemic circuit is far longer and has greater resistance than the pulmonary circuit
The right ventricle is thinner because the lungs are immediately adjacent and offer low resistance
Right atrium:
Receives blood via the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus
Pectinate muscles line the anterior wall and the auricle (ear-shaped appendage)
Fossa ovalis: a depression in the interatrial septum, remnant of the fetal foramen ovale
Right ventricle:
Trabeculae carneae (muscular ridges) on the inner walls
Papillary muscles and chordae tendineae anchor the tricuspid valve
Blood exits via the pulmonary trunk through the pulmonary semilunar valve
Left atrium:
Receives oxygenated blood from four pulmonary veins (two from each lung)
Smoother walls than the right atrium; pectinate muscles mostly confined to the auricle
Left ventricle:
Thickest myocardium
Trabeculae carneae, papillary muscles, and chordae tendineae for the bicuspid (mitral) valve
Blood exits via the aorta through the aortic semilunar valve
To the right atrium: SVC, IVC, coronary sinus
From the right ventricle: pulmonary trunk (splits into left and right pulmonary arteries)
To the left atrium: four pulmonary veins
From the left ventricle: aorta (ascending aorta)
AV valves (tricuspid on the right, bicuspid/mitral on the left):
Open when atrial pressure exceeds ventricular pressure (during filling)
Close when ventricular pressure rises during contraction, preventing backflow into the atria
Chordae tendineae (fibrous cords) tethered to papillary muscles prevent the valve leaflets from everting into the atria
Semilunar valves (pulmonary and aortic):
Open when ventricular pressure exceeds arterial pressure (during ejection)
Close when blood in the artery tries to fall back, filling the valve cusps and snapping them shut
No chordae tendineae; the three pocket-like cusps are self-supporting
The combination of these four valves ensures blood can only travel: atria → ventricles → arteries
The heart's conduction system is a network of specialised cardiac cells that generate and propagate electrical impulses without neural input.
SA node (right atrium, near SVC) → sets the pace (~75 bpm at rest)
Impulse spreads across both atria via gap junctions (atrial contraction)
AV node (interatrial septum, near the tricuspid valve) → brief delay (~0.1 s) so atria finish contracting before ventricles start
Bundle of His (AV bundle) → only electrical bridge between atria and ventricles (the fibrous skeleton insulates them otherwise)
Splits into right and left bundle branches running down the interventricular septum
Terminates in Purkinje fibres that spread through the ventricular walls, triggering contraction from apex to base so blood is pushed upward toward the outflow valves
Left coronary artery (LCA): arises from the aorta just above the aortic valve
Anterior interventricular artery (LAD): runs in the anterior interventricular sulcus; supplies the anterior walls of both ventricles and the interventricular septum
Circumflex artery: wraps around the left side in the coronary sulcus; supplies the left atrium and the posterior left ventricle
Right coronary artery (RCA): arises from the aorta on the right side
Right marginal artery: runs along the right border; supplies the lateral right side
Posterior interventricular artery (posterior descending artery): runs in the posterior interventricular sulcus; supplies the posterior ventricular walls
Venous drainage:
Great cardiac vein: runs alongside the anterior interventricular artery
Middle cardiac vein: runs alongside the posterior interventricular artery
Small cardiac vein: runs alongside the right marginal artery
All drain into the coronary sinus, which empties into the right atrium
Blockage of the left anterior descending artery is sometimes called the "widow-maker" heart attack because it supplies such a large portion of the left ventricular wall. Understanding coronary artery anatomy is how cardiologists decide where to place a stent or perform bypass surgery. The conduction pathway also explains why an artificial pacemaker is wired to the ventricle, not the atrium: it replaces the function of the SA and AV nodes when they fail.
Students often think the right and left sides of the heart contain the same type of blood. They do not: the right side carries deoxygenated blood, the left side carries oxygenated blood.
The "shared layer" question trips people up. Epicardium and visceral pericardium are the same layer, not two separate structures sitting next to each other.
Students sometimes confuse which valve is where. Remember: the tricuspid has three leaflets and sits on the right; the bicuspid (mitral) has two leaflets and sits on the left. Tri before Bi alphabetically, Right before Left.
The AV node's delay is not a malfunction. It is a deliberate pause so the atria can finish emptying before the ventricles fire.
⚠️ Know which layer is shared between the heart wall and the pericardium (epicardium = visceral pericardium).
⚠️ Be able to trace the path of electrical conduction: SA node → atria → AV node → Bundle of His → bundle branches → Purkinje fibres.
⚠️ Left ventricle thickness vs. right ventricle thickness, and the reason (systemic resistance vs. pulmonary resistance).
⚠️ Know every vessel entering and leaving each chamber.
⚠️ Understand how chordae tendineae and papillary muscles prevent valve eversion during ventricular contraction.
True or False: The right ventricle pumps blood into the aorta.
Fill in the blank: The layer shared by the heart wall and the pericardium is the __________.
True or False: The AV node is the only electrical connection between the atria and the ventricles.
Fill in the blank: The coronary sinus drains into the __________.
True or False: The left ventricle wall is thinner than the right ventricle wall.
Answers: 1. False (pulmonary trunk). 2. Epicardium (visceral pericardium). 3. True. 4. Right atrium. 5. False (left is thicker).
Q: Which side of the heart is associated with the pulmonary circuit, and which with the systemic circuit?
A: The right side (right atrium and right ventricle) serves the pulmonary circuit. The left side (left atrium and left ventricle) serves the systemic circuit.
Q: Name the layers of the heart wall from outermost to innermost.
A: Epicardium (visceral pericardium), myocardium, endocardium.
Q: Why is the left ventricular wall so much thicker than the right?
A: The left ventricle must generate enough pressure to push blood through the entire systemic circuit, which has much greater resistance and distance than the pulmonary circuit served by the right ventricle.
Q: Trace the electrical conduction pathway through the heart.
A: SA node → across both atria → AV node (brief delay) → Bundle of His → right and left bundle branches → Purkinje fibres → ventricular myocardium contracts from apex upward.
Q: What prevents the AV valve leaflets from flipping into the atria during ventricular contraction?
A: The chordae tendineae (fibrous cords) anchor the valve leaflets to papillary muscles on the ventricular wall. When the ventricles contract, the papillary muscles also contract, pulling the cords taut and holding the leaflets in place.
Q: Name the blood vessels that deliver blood to the right atrium.
A: Superior vena cava, inferior vena cava, and coronary sinus.
Q: Which coronary artery is sometimes called the "widow-maker" and why?
A: The left anterior descending (anterior interventricular) artery, because it supplies a large portion of the left ventricular myocardium and its blockage can cause a massive, often fatal, heart attack.
This connects to Blood Vessels (Ch. 20) because the great vessels entering and leaving the heart are the starting points for the pulmonary and systemic circuits you will map in detail there. It also connects to the Respiratory System (Ch. 22), since the entire point of the pulmonary circuit is to deliver blood to the alveoli for gas exchange. The coronary circulation is itself a miniature systemic circuit, so understanding it reinforces how arteries, capillaries, and veins work generally.
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