You're staring at a preserved sheep heart, scalpel in hand, and the review sheet asks you to identify the left atrioventricular valve. Again Not complicated — just consistent..
Sound familiar? 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. 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.
Let's walk through it properly. 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.
The sheep heart vs. the human heart
Most labs use sheep hearts. They're cheaper, bigger than rat hearts, and the anatomy is nearly identical. But the chambers, valves, and coronary pattern are functionally the same. But for Exercise 20 purposes? 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. Your instructor will flag any species-specific quirks on the practical.
Why This Lab Trips People Up
Three reasons, and none of them are "the material is too hard."
First: passive labeling. In real terms, two days later, the unlabeled practical heart looks like alien anatomy. So naturally, students stare at a labeled model, nod, move to the next station. Recognition isn't recall.
Second: ignoring the "why." The review sheet asks what and where. Even so, 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 The details matter here..
Third: **treating dissection like butchery.A good dissection reveals. So ** Hacking through the atria to "see inside" destroys the very structures you need to identify — the valve cusps, the chordae, the delicate fossa ovalis. A bad one just makes a mess.
How to Actually Learn This Stuff
Start with the big picture: orientation and external landmarks
Before you pick up a probe, hold the heart. Find the apex — it points down and left. The left border? Also, the base is the broad posterior surface where the great vessels attach. Rotate it. The right border is formed by the right atrium. Also, the anterior surface faces the sternum; the diaphragmatic surface rests on the diaphragm. Mostly left ventricle Surprisingly effective..
Now find the grooves. The anterior interventricular sulcus runs vertically toward the apex on the front — that's the LAD territory. It carries the right coronary artery and the circumflex branch of the left. The coronary sulcus (atrioventricular groove) circles the heart like a belt, separating atria from ventricles. The posterior interventricular sulcus does the same on the back — usually the right coronary's posterior interventricular branch.
These aren't just lines. On the flip side, they're roadmaps for the coronaries. And they mark where the internal septa sit Most people skip this — try not to..
The auricles — don't skip them
Those wrinkled, ear-like flaps on each atrium? Day to day, under the right auricle, you'll see the pectinate muscles — muscular ridges lining the atrial wall. They increase atrial volume. The left atrium has them too, but only in its auricle. Now, on a dissected heart, peel them back gently. The right auricle is larger, more prominent. The left is smaller, tucked against the pulmonary trunk. Still, Auricles (or atrial appendages). 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. Plus, don't cut through the septum. Spread it open.
You're looking at two distinct zones. Plus, posterior and inferior: smooth wall. On top of that, this is the sinus venarum — receives blood from the SVC, IVC, and coronary sinus. Because of that, the coronary sinus opening sits between the IVC and the tricuspid valve, guarded by the valve of the coronary sinus (Thebesian valve). The fossa ovalis — the remnant of the foramen ovale — is an oval depression in the interatrial septum. On the flip side, its raised border is the limbus fossa ovalis. And in about 25% of adults, a probe can still pass through a patent foramen ovale. Clinically relevant. Worth knowing That's the part that actually makes a difference..
Anterior and superior: rough wall with pectinate muscles. The crista terminalis is the vertical ridge marking the boundary between smooth and rough. It corresponds externally to the sulcus terminalis Easy to understand, harder to ignore..
The tricuspid valve guards the entrance to the right ventricle. Plus, the septal papillary muscle sometimes connects directly to the septum via chordae (the septal chordae). They're tethered by chordae tendineae to papillary muscles — anterior, posterior, and septal. Three cusps: anterior (largest), posterior, septal. That's a common practical trap That's the whole idea..
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.
Three things jump out. That's why Trabeculae carneae — irregular muscular ridges covering the inner wall. Here's the thing — 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. And 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**. Unlike the tricuspid valve, this is a semilunar valve. So it has three thin, crescent-shaped cusps: anterior, posterior, and anteriorly located. 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 Practical, not theoretical..
People argue about this. Here's where I land on it.
The left ventricle: the powerhouse
Now, move to the left side of the heart. So the myocardium of the left ventricle is three times thicker than that of the right. This is where the real work happens. Also, as you open it, you will immediately notice a massive difference in wall thickness. Cut into the left ventricle, starting from the aortic valve and moving toward the apex. This isn't just for show; it provides the high-pressure output necessary to drive blood through the entire systemic circulation And that's really what it comes down to. No workaround needed..
The internal landscape is dominated by strong trabeculae carneae. You will find three large papillary muscles: anterior, posterior, and septal. Unlike the right ventricle, which has the distinct moderator band, the left ventricle’s trabeculae are more complex and densely packed. 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.
The mitral (bicuspid) valve is your next landmark. And 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. The aortic valve sits at the base of the aorta. So 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 And that's really what it comes down to. Took long enough..
This is the bit that actually matters in practice The details matter here..
Conclusion
Dissecting the heart reveals more than just a collection of chambers and valves; it reveals a masterpiece of biological engineering. Here's the thing — 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 Turns out it matters..
