You're staring at a preserved sheep heart, scalpel in hand, and the review sheet asks you to identify the left atrioventricular valve. Again.
Sound familiar? Because of that, if you've taken an A&P lab, you know Exercise 20. It's the one where everyone crowds around the demonstration table, squinting at chordae tendineae the size of dental floss, wondering if that pale strip of tissue is the moderator band or just a chunk of fat you accidentally tore.
Here's the thing — this lab isn't about memorizing labels. Still, it's about understanding how structure dictates function. The heart doesn't pump because you colored the arrows red and blue on a diagram. It pumps because the fibrous skeleton anchors the valves, because the papillary muscles tense at just the right millisecond, because the coronary arteries hug the epicardium in a pattern that keeps the muscle fed even during systole Small thing, real impact. But it adds up..
Real talk — this step gets skipped all the time And that's really what it comes down to..
Let's walk through it properly. That's why no textbook definitions. Just what you actually need to see, understand, and remember.
What Exercise 20 Actually Covers
The review sheet hits every major anatomical feature of the heart — external and internal, gross and microscopic. You'll identify structures on models, diagrams, dissected specimens, and microscope slides. The typical checklist includes:
- External landmarks: apex, base, auricles, coronary sulcus, anterior and posterior interventricular sulci
- Internal chambers: right and left atria, right and left ventricles, interatrial and interventricular septa
- Valves: tricuspid, pulmonary, mitral (bicuspid), aortic — plus their cusps, chordae tendineae, and papillary muscles
- Major vessels: superior/inferior vena cava, pulmonary trunk and veins, aorta
- Coronary circulation: left and right coronary arteries, circumflex, anterior interventricular (LAD), posterior interventricular, cardiac veins, coronary sinus
- Microscopic: cardiac muscle tissue, intercalated discs, Purkinje fibers
That's a lot. But it's not random. Every structure on that list exists for a mechanical or electrical reason Worth keeping that in mind..
The sheep heart vs. the human heart
Most labs use sheep hearts. In practice, key differences worth knowing: the sheep's brachiocephalic trunk splits differently, the azygos vein drains differently, and the heart sits more vertically in the thorax. But for Exercise 20 purposes? They're cheaper, bigger than rat hearts, and the anatomy is nearly identical. Because of that, the chambers, valves, and coronary pattern are functionally the same. Your instructor will flag any species-specific quirks on the practical That's the part that actually makes a difference. Surprisingly effective..
Easier said than done, but still worth knowing.
Why This Lab Trips People Up
Three reasons, and none of them are "the material is too hard."
First: passive labeling. Students stare at a labeled model, nod, move to the next station. That said, two days later, the unlabeled practical heart looks like alien anatomy. Recognition isn't recall.
Second: ignoring the "why." The review sheet asks what and where. It rarely asks why the left ventricular wall is three times thicker than the right or why the aortic valve has no chordae tendineae. But those questions are the difference between a B and an A — and between passing the practical and actually understanding cardiac physiology next semester.
Third: **treating dissection like butchery.Now, ** Hacking through the atria to "see inside" destroys the very structures you need to identify — the valve cusps, the chordae, the delicate fossa ovalis. So a good dissection reveals. A bad one just makes a mess.
People argue about this. Here's where I land on it.
How to Actually Learn This Stuff
Start with the big picture: orientation and external landmarks
Before you pick up a probe, hold the heart. Here's the thing — find the apex — it points down and left. The base is the broad posterior surface where the great vessels attach. Rotate it. The anterior surface faces the sternum; the diaphragmatic surface rests on the diaphragm. The right border is formed by the right atrium. The left border? Mostly left ventricle But it adds up..
Now find the grooves. The coronary sulcus (atrioventricular groove) circles the heart like a belt, separating atria from ventricles. Worth adding: it carries the right coronary artery and the circumflex branch of the left. On the flip side, the anterior interventricular sulcus runs vertically toward the apex on the front — that's the LAD territory. The posterior interventricular sulcus does the same on the back — usually the right coronary's posterior interventricular branch Simple, but easy to overlook..
These aren't just lines. Now, they're roadmaps for the coronaries. And they mark where the internal septa sit.
The auricles — don't skip them
Those wrinkled, ear-like flaps on each atrium? The left is smaller, tucked against the pulmonary trunk. Under the right auricle, you'll see the pectinate muscles — muscular ridges lining the atrial wall. On top of that, Auricles (or atrial appendages). Plus, they increase atrial volume. The right auricle is larger, more prominent. On a dissected heart, peel them back gently. The left atrium has them too, but only in its auricle. The rest of the left atrial wall is smooth — because it develops from the pulmonary veins, not the primitive atrium.
That developmental difference? It's why the fossa ovalis sits where it does. Which brings us inside.
Inside the right atrium: the venous end of the heart
Open the right atrium by cutting along the right border, starting at the superior vena cava and continuing toward the IVC. Don't cut through the septum. Spread it open Took long enough..
You're looking at two distinct zones. Posterior and inferior: smooth wall. In practice, this is the sinus venarum — receives blood from the SVC, IVC, and coronary sinus. On the flip side, the coronary sinus opening sits between the IVC and the tricuspid valve, guarded by the valve of the coronary sinus (Thebesian valve). Think about it: the fossa ovalis — the remnant of the foramen ovale — is an oval depression in the interatrial septum. Its raised border is the limbus fossa ovalis. Practically speaking, in about 25% of adults, a probe can still pass through a patent foramen ovale. Clinically relevant. Worth knowing Worth keeping that in mind. Turns out it matters..
Anterior and superior: rough wall with pectinate muscles. Now, the crista terminalis is the vertical ridge marking the boundary between smooth and rough. It corresponds externally to the sulcus terminalis.
The tricuspid valve guards the entrance to the right ventricle. Now, the septal papillary muscle sometimes connects directly to the septum via chordae (the septal chordae). Still, three cusps: anterior (largest), posterior, septal. On top of that, they're tethered by chordae tendineae to papillary muscles — anterior, posterior, and septal. That's a common practical trap Took long enough..
The right ventricle: volume over pressure
Cut into the right ventricle from the pulmonary trunk downward, staying left of the anterior interventricular sulcus. Open it like a book And that's really what it comes down to. Which is the point..
Three things jump out. Trabeculae carneae — irregular muscular ridges covering the inner wall. Papillary muscles — the anchor points. And the moderator band (septomarginal trabecula) — a distinct muscular bridge crossing from the interventricular septum to the anterior wall. It carries the right bundle branch of the conduction system. That's its real job. It's not just a random ridge.
The pulmonary valve (pulmonic valve) sits at the apex of the **conus arterios
us arteriosus**. Think about it: it has three thin, crescent-shaped cusps: anterior, posterior, and anteriorly located. Unlike the tricuspid valve, this is a semilunar valve. Note the lack of chordae tendineae here; semilunar valves rely on their shape and the backpressure of blood to snap shut, rather than being tethered by strings Took long enough..
The left ventricle: the powerhouse
Now, move to the left side of the heart. The myocardium of the left ventricle is three times thicker than that of the right. This is where the real work happens. So cut into the left ventricle, starting from the aortic valve and moving toward the apex. As you open it, you will immediately notice a massive difference in wall thickness. This isn't just for show; it provides the high-pressure output necessary to drive blood through the entire systemic circulation Worth knowing..
The internal landscape is dominated by solid trabeculae carneae. Consider this: unlike the right ventricle, which has the distinct moderator band, the left ventricle’s trabeculae are more complex and densely packed. You will find three large papillary muscles: anterior, posterior, and septal. These are much more substantial than their right-sided counterparts, providing the necessary tension to the chordae tendineae to prevent the mitral valve from prolapsing during high-pressure systole.
And yeah — that's actually more nuanced than it sounds And that's really what it comes down to..
The mitral (bicuspid) valve is your next landmark. It consists of two large, thick cusps: the anterior and the posterior. These are much more delicate and mobile than the tricuspid cusps, designed to seal perfectly against the high pressures of the left ventricle.
Finally, look at the exit. Still, the aortic valve sits at the base of the aorta. But like the pulmonary valve, it is a semilunar valve with three cusps: left, right, and posterior. As you inspect the base of these cusps, look for the small, shallow recesses known as the aortic sinuses (Sinuses of Valsalva). These tiny pockets are critical; they are the origins of the left and right coronary arteries. If these sinuses were to become obstructed, the heart's own blood supply would fail.
Conclusion
Dissecting the heart reveals more than just a collection of chambers and valves; it reveals a masterpiece of biological engineering. Understanding these anatomical nuances—the difference between a pectinate muscle and a trabecula carneae, or the significance of the moderator band—is not merely an academic exercise. From the thin-walled, low-pressure right side designed for pulmonary circulation to the thick, muscular left side built for systemic endurance, every ridge, chordae, and sinus serves a specific hemodynamic purpose. It is the fundamental foundation required to understand the pathology of heart disease, the mechanics of valvular dysfunction, and the very rhythm of life.
