Why are grizzled anatomists so sure on the parts of the heart, while the beginning student is sometimes baffled by a fairly simple organ?
You can quickly learn what you need of the theory of all this, and then apply it to feeling your way through some hearts to pick up the experience - the only real advantage the anatomist has over you.
Now examine the cut-off vessels attached to/near the atria. You may need to put fingers into the atria to fill them out and restore something of their natural shape. Look for three vessels lined up close to each other. The one with the really thin wall is the vena cava. [The wall is thin because venous pressures are low.] The thick-walled (high-pressure) one in the middle is the aorta (actually the ascending aorta) looking rather like pale garden hose. The other outside one has a wall like the aorta's, but not quite as thick: this is the pulmonary trunk, cut off before it could turn into R & L pulmonary arteries. You will probably have to look at two or three hearts to find one where at least one of these vessels has not been sliced open. Now hold the heart correctly tipped in front of your chest with the vena cava on the right, & the pulmonary trunk on the left; the aorta has to be central. Now the heart is positioned so that its front (anterior) surface is in fact facing forward and the three vessels are in the correct order left to right. [You will need to return the heart to this position and orientation every so often.]
Now hold the heart by the ventricles and put whatever finger fits down into the pulmonary trunk until it reaches into a ventricle. You can identify the ventricle, knowing that you have gone against the direction of blood flow back into the chamber that ejects blood into the pulmonary trunk - the right ventricle. If the ventricle has been slit open, look in to see how your finger has arrived there from the pulmonary trunk. Note the relatively thick wall of the right ventricle, and that this ventricle lies to the front. Withdraw your finger and insert it or a better fitting one into the superior vena cava. Now try to get the right atrium that you have just entered (with the blood flow) into its proper filled shape. Opposite where the superior vena cava entered from above should be a symmetrical hole (just above the coronary sulcus) where the inferior vena cava came in, but has been cut off. Hold the heart in its normal position in front of you to appreciate that the inferior VC brings back blood from the lower body (anything below the diaphragm), and the superior VC drains blood from the upper body.
Next, put a finger into the right atrium and push it down towards the apex. It will enter the right ventricle having passed through the right atrio- ventricular valve (save fancier names for later). Examine the valve. It consists of cusps/leaflets/flaps attached to strings (chordae tendinae), themselves anchored in grey worm-like projections (papillary muscles) of the muscle in the ventricle's wall. [When the right ventricle contracts, the papillary muscles shorten and help restrain the AV valve from everting or flopping inside out, and letting blood back into the right atrium. [Think of the landing parachutist pulling on her lines to rein in the canopy.] Run your finger up from the right ventricle into the pulmonary trunk. You are going with the flow, and you pass the pulmonary semilunar valve. This comprises three halfmoon-shaped (semilunar) flaps arranged to catch blood trying to get back into the ventricle, and balloon out to block backflow. If neither the pulmonary trunk nor the aorta is slit open so that you can see these flaps, search on some other hearts, and look into the vessels from outside and with a little finger try to lift one or more of them away from the wall. [Do not pull hard on valves or smaller heart vessels - they will tear!]
Imagine the blood going from the pulmonary trunk to both lungs, serving the lungs' capillaries and returning via pulmonary veins to the left atrium. Put your fingers in the right atrium (follow the superior VC down). Having got this atrium identified, what is left (to the left and posteriorly) is the left atrium. Manipulate it into a filled shape and look for two pairs of holes where the right pulmonary veins and left pulmonary veins entered.[You may find the atrium has been cut to show less than four holes.] Push a finger from the left atrium down into the left ventricle, going past the left atrioventricular valve. Open the left ventricle, or find an opened one, and note the similar construction of this AV valve (cusps, chordae tendinae, & papillary muscles), and the thick wall of this ventricle - thick because it does most of the heart's work. Push a fairly long finger up from the left ventricle into the aorta, passing the aortic semilunar valve as you go. Look at the three flaps of the aortic valve by peering back in against the blood flow, or in an opened aorta. Find the two small openings of the coronary arteries just above the valve. [The heart muscle gets first crack at the oxygenated blood.]
Push a thin probe into one of the arterial openings. Hold the heart against you in its proper position. See whether the probe is going forward and slightly to the right - the right coronary artery, or back and leftwards and less easily seen - the left coronary artery. Dissect out the left coronary a. a little with your fingers and see it branch almost immediately one part goes off to the left to be the anterior interventricular/ descending artery (LAD); the other branch bends around (circumflexes) to serve the large posterior muscle of the left ventricle. While you are at this posterior surface of the heart, find the left atrium, and look just below it in the coronary sulcus for a vessel running crosswise and ending at the corner of the right atrium. This vessel is the coronary sinus, draining venous blood from the heart into the right atrium. This route provides a check that the structure is the sinus. Thus, open the right atrium, find the opening, and push a probe in against the flow and it should enter the sinus, where the probe can be felt, or wiggled and seen. Another check is that coronary arteries have a firm stringy feel when you roll them between your fingers, the sinus is less robust.
Now go back to the ventricles. Put an index finger in one ventricle and your thumb in the other and hold up the heart. What you have hold of is the interventricular septum - the partition between the ventricles. Look in either ventricle and note that the papillary muscles have strings attached to them, but elsewhere there are branched ridges in the muscle. These ridges sometimes almost lift free, and are called trabeculae carnae, meaning fleshy struts [There is a similar area of rough textured muscle inside the right atrium given another, best ignored name.] Finally, repeat the trick of putting a finger and thumb in two chambers of the heart, but this time do it for the atria to identify the inter-atrial septum. Feel along this septum for a thin region, then having felt it, look into either atrium to see the thin patch on the wall (holding the heart up against the light may show the thin area). This patch is the fossa ovale (does it look oval?). Before birth there was a hole there - the foramen ovale - to allow blood to bypass the lung circulation and go from right atrium directly to left atrium, and then on to the left ventricle. At birth a thin membrane flops onto the hole and more or less seals it, creating the fossa ovale. A sometimes distinctive feature of the interventricular septum is the grey worm-like ridge that runs down the right-ventricular side of the septum to meet the base of the papillary muscle at a right-angle. This worm is the moderator band that conducts the impulse for the papillary muscle to contract just before the main ventricular muscle.
Another remnant of fetal times is the ligamentum arteriosus, which is a thin string connecting the pulmonary trunk with the aorta, but at some distance from the heart, and not to be found for now. Before birth it was an open tube - the ductus arteriosus, again serving to keep blood out of the unopened fetal lungs. [In most hearts in the lab. the vessels have been cut too close to the heart for the ligamentum arteriosus to be present.]
Names for the AV valves are a pest: the right has three leaflets and is called the tricuspid; the left has two, and is the bicuspid or mitral valve. The main thickness of each heart-chamber wall is muscle - the myocardium. Endocardium is a thin membrane lining the chambers and covering the valves; visceral pericardium is the slippery outside coat, stuck onto the outside of the muscle to allow it to move in its sac of parietal pericardium.
Suggestions for improvement please send to: William A
Department of Anatomy
School of Medicine
West Virginia University
MORGANTOWN WV 26506-9128, USA
Other teaching pieces are 'Medical Cytology Module' at http://wberesford.hsc.wvu.edu/cytol.htm and 'Extracellular Matrix' at /ecm.htm