Figure 27.4 – Internal Features of the Heart
An in‑depth look at the structures that make the heart work
The human heart is a muscular organ roughly the size of a closed fist, yet its internal architecture is remarkably complex. 4 in many anatomy and physiology textbooks provides a detailed schematic of the heart’s interior, highlighting the chambers, septa, valves, muscular ridges, conduction pathways, and coronary vessels that together enable efficient blood circulation. Plus, below we walk through the internal features shown in Figure 27. That's why understanding each of these components is essential for grasping how the heart pumps oxygen‑rich blood to the body and returns deoxygenated blood to the lungs. That said, figure 27. 4, explaining their anatomy, function, and clinical relevance Less friction, more output..
1. Cardiac Chambers and Septa
1.1 Atria
The heart contains two atria (singular: atrium) – the right atrium and the left atrium.
- Right atrium receives deoxygenated blood from the superior and inferior vena cavae and the coronary sinus. Its interior shows the crista terminalis, a vertical ridge that separates the smooth posterior wall from the rough anterior wall containing the pectinate muscles.
- Left atrium receives oxygenated blood from the four pulmonary veins. Its wall is smoother than the right atrium, with only a few pectinate muscles near the atrial appendage.
1.2 Ventricles
Below the atria lie the two ventricles – the right ventricle and the left ventricle The details matter here..
- The right ventricle pumps blood to the pulmonary circulation. Its lumen is triangular in cross‑section, and the inner surface displays prominent trabeculae carneae (muscular ridges) and papillary muscles that anchor the tricuspid valve.
- The left ventricle generates the highest pressure to drive systemic circulation. Its wall is up to three times thicker than that of the right ventricle, reflecting the greater workload. The left ventricular interior also contains trabeculae carneae and two large papillary muscles that support the mitral (bicuspid) valve.
1.3 Septa
Separating the chambers are two septa:
- Interatrial septum divides the left and right atria. In the fetal heart, the foramen ovale allows blood to bypass the lungs; after birth it closes, leaving the fossa ovalis as a shallow depression.
- Interventricular septum separates the ventricles. It is thick and muscular, contributing significantly to the heart’s ability to generate pressure. Defects in this septum (ventricular septal defects) are among the most common congenital heart anomalies.
2. Valvular Apparatus
2.1 Atrioventricular (AV) Valves
The AV valves prevent backflow of blood from the ventricles into the atria during systole Most people skip this — try not to..
- Tricuspid valve (right AV valve) consists of three leaflets (anterior, posterior, septal) attached to the ventricular wall via chordae tendineae that arise from the anterior and posterior papillary muscles.
- Mitral valve (left AV valve) has two leaflets (anterior and posterior) also secured by chordae tendineae from the two left papillary muscles.
Both valves open during ventricular diastole, allowing passive filling, and close tightly during ventricular systole to ensure unidirectional flow.
2.2 Semilunar Valves
Located at the exits of the ventricles, the semilunar valves prevent reflux into the ventricles during diastole Easy to understand, harder to ignore..
- Pulmonary valve guards the pulmonary trunk; it has three crescent‑shaped cusps.
- Aortic valve guards the aorta; similarly tricuspid, it endures the highest pressure gradients in the cardiac cycle.
The semilunar valves open when ventricular pressure exceeds arterial pressure and close when the pressure gradient reverses, producing the characteristic “lub‑dub” heart sounds.
3. Internal Muscular Features
3.1 Trabeculae Carneae
These are irregular muscular ridges that line the ventricular walls. They increase the surface area for contraction and help prevent suction that could impede ventricular filling. In the right ventricle, a prominent moderator band (also called the septomarginal trabecula) extends from the interventricular septum to the anterior papillary muscle, carrying part of the right bundle branch of the conduction system Worth keeping that in mind. Which is the point..
3.2 Papillary Muscles and Chordae Tendineae
Papillary muscles project inward from the ventricular wall and anchor the AV valve leaflets via the chordae tendineae. During ventricular contraction, these muscles tense, preventing valve prolapse and maintaining valve competence. Dysfunction or rupture of papillary muscles (e.g., after myocardial infarction) can lead to acute mitral regurgitation.
3.3 Moderator Band
Unique to the right ventricle, the moderator band carries the right bundle branch of the His‑Purkinje system and helps coordinate ventricular contraction. Its presence is a useful anatomic landmark when identifying the right ventricle in cross‑sectional images.
4. Cardiac Conduction System
Figure 27.4 often illustrates the specialized conductive tissues that initiate and propagate the electrical impulse.
- Sinoatrial (SA) Node – located in the upper part of the right atrium near the opening of the superior vena cava; it is the heart’s natural pacemaker.
- Internodal Pathways – conduct the impulse from the SA node to the atrioventricular (AV) node.
- AV Node – situated in the lower part of the interatrial septum, just above the tricuspid valve; it introduces a brief delay (~0.1 s) to allow atrial contraction to complete before ventricular activation.
- Bundle of His (AV Bundle) – emerges from the AV node and traverses the interventricular septum.
- Left and Right Bundle Branches – diverge from the bundle of His to travel down the respective ventricles.
- Purkinje Fibers – spread throughout the ventricular myocardium, ensuring rapid and synchronized ventricular depolarization.
Disruption anywhere in this pathway (e.g., AV block, bundle branch block) alters the ECG and can compromise cardiac output.
5. Coronary Vasculature Within the Chambers
Although the coronary arteries primarily run on the heart’s exterior, Figure 27.4 sometimes shows their ostia and early branches visible from within the aortic root.
- Coronary Ostia – two openings just behind the aortic valve leaflets: the left coronary artery (LCA) arises from the left aortic sinus, and the right coronary artery (RCA) from the right aortic sinus.
- Early Branches – the LCA quickly divides into the left anterior descending (LAD) and circumflex (CX) arteries; the RCA gives off the
right ventricular branches and often the posterior descending artery (PDA).
The venous drainage occurs via
