heart lecture

Overview: Pulmonary vs Systemic Circuits

The pulmonary circuit is responsible for receiving deoxygenated blood returning from the body and pumping it to the lungs for oxygenation.
The left heart is associated with the systemic circuit: it receives oxygenated blood from the lungs and pumps it out to the rest of the body.

The heart is more superficial than many expect; it sits just behind the sternum and is not deeply located in the chest.
It is essentially in the midline, not off to the left, even though the pulse is most easily felt on the left side of the chest.
The common misconception from everyday gesture (hand on heart) stems from feeling the heartbeat on the left chest, but the organ itself is in the midline.

The heart is surrounded by serous layers: the pericardium and its layers.
Fibrous pericardium (parietal pericardium) is the tough outer layer; in cadavers it appears like a thin leather bag around the heart.
The visceral layer of the serous pericardium adheres to the heart surface and is called the epicardium; it is extremely thin, like a thin layer of Saran wrap, and often hard to visualize without histology.
The bulk of the heart is the myocardium (the muscular layer).
The innermost lining is the endocardium (inside the heart chambers).

Cardiac muscle fibers are arranged in a spiral (wrapping) pattern around the ventricles.
This spiral arrangement causes the heart to twist or ring out as it contracts, rather than merely shortening linearly.
In living hearts, this twisting action contributes to the pumping motion.

If a coronal section is cut to view into the heart, the following chambers are visible: right ventricle, left ventricle, right atrium, left atrium.
With a well-executed cut, you can also see the valves.
A reliable lab landmark for orientation (especially when reviewing anatomy specimens) is the four pulmonary veins entering the left atrium (two on each side):
- There are 4 pulmonary veins, with 2 on each side.
The general plan of valves seen from an elegant view is:
- Atrioventricular (AV) valves: tricuspid (right) and mitral/bicuspid (left).
- Semilunar valves: aortic (systemic) and pulmonary (pulmonary circuit).
The AV valves are attached to the papillary muscles via chordae tendineae (heartstrings).
Papillary muscles contract shortly before the ventricles contract, pulling on the chordae tendineae to prevent the AV valve cusps from swinging back toward the atria.
The mitral valve is commonly mentioned as the valve most often replaced in heart disease.
The purpose of all valves (AV and semilunar) is to prevent backflow and ensure blood moves in one direction through the heart.

Tricuspid valve is on the right side; it prevents backflow from the right ventricle to the right atrium.
Bicuspid/Mitral valve is on the left side; it prevents backflow from the left ventricle to the left atrium.
The chordae tendineae anchor the valve cusps to the papillary muscles; the papillary muscles tense to keep the flaps of the AV valves from flipping back toward the atria during ventricular contraction. Think of holding an umbrella in a windstorm to prevent it from turning the wrong way.
The ventricles contracting generates pressure that opens the AV valves to allow blood to flow from the atria into the ventricles.

The semilunar valves consist of the aortic valve (leading to the aorta) and the pulmonary valve (leading to the pulmonary artery).
These valves are set up so that the ventricles’ contraction pushes blood through them.
When the ventricles relax, the blood in the arteries tries to flow back toward the ventricles; this backflow fills the cusps of the semilunar valves, causing them to snap shut and prevent backflow into the ventricles.
The key principle is preventing backflow, maintaining unidirectional blood movement.

In anatomy labs, it’s common for the heart to look different from textbook images, especially if there is fat around the epicardium or disease changes.
A practical strategy is to pick a landmark you can reliably identify to orient yourself (e.g., the pulmonary veins entering the left atrium) and build from there.
A lean heart will show clearer features; heavily fat-laden hearts may obscure landmarks.
The interior view of the heart (e.g., a nice coronal slice) can reveal the right atrium, left atrium, right ventricle, left ventricle, and the valves if the cut is well aligned.

The mitral valve is often the valve that undergoes replacement in disease because it is particularly prone to calcification and dysfunction.
In lab or dissection, the fibrous pericardium appears as a tough, leather-like layer around the heart; the epicardium is the very thin outer surface of the heart itself.
The heart’s superficial location and midline position are important concepts for clinical approaches and imaging.

The next discussion will quickly cover the flow of blood through the heart, building on the structural details described above.
Expect to integrate chambers, valves, and the sequence of events during the cardiac cycle, including how atrial contraction contributes to filling and how ventricular contraction drives ejection through the semilunar valves.

Pulmonary veins: 4 total, 2 per side.
AV valves: tricuspid (right), mitral/bicuspid (left);
- chordae tendineae = heartstrings; attach to cusps.
- papillary muscles tense just before ventricular contraction to prevent valve backflow.
Semilunar valves: aortic and pulmonary; close when ventricles relax to prevent backflow.
Myocardium is the muscular wall; epicardium is the thin outer surface; endocardium lines the interior.
The heart’s muscle fibers form a spiral arrangement, giving a twisting contraction.
The fibrous pericardium is the tough external layer; the visceral layer is the epicardium; the space between them is the pericardial cavity.
The heart is positioned directly behind the sternum and is largely midline, not situated high in the chest as the common pledge-of-allegiance depiction might suggest.