Heart Anatomy, Pacemaker Tissue, and Valves of the Heart Help
Introduction to Heart Anatomy, Pacemaker Tissue, and Valves of the Heart
The powerful four-chambered heart of humans and other mammals has a complex internal anatomy (Figure 16.3). Consider, for example, the myocardium ( my -oh- CAR-dee -um) or “heart” (cardi) “muscle” (my). The cardiac muscle fibers of the myocardium are arranged in a fairly circular pattern around the heart, so when they contract, they squeeze the blood out of the heart chambers like a noose tightening around a bag. The blood from the atria is pushed down into the ventricles, through a pair of one-way valves. Not surprisingly, these valves are called the right and left atrioventricular ( ay -tree-oh-ven- TRIK -you-lar) valves. These two A-V (atrioventricular) valves essentially act as one-way doors. They are actually flaps of connective tissue that are pushed open from above by blood in the atria.
The atria contract because they are excited by cardiac pacemaker cells. These pacemaker cells are actually modified cardiac muscle fibers that are self-exciting. Sodium (Na + ) and other charged particles are automatically let into the pacemaker cells at a certain rate or rhythm. This happens because the proteins in the membranes of the pacemaker cells tend to shift around, allowing ions to enter and excite the pacemaker cells or “turn themselves on.”
The main cardiac pacemaker area is called the sinoatrial ( sigh-no-AY -tree-al) or S-A node (NOAD). This region is called a node because it is somewhat rounded, like a “knot.” The word sinoatrial indicates its anatomical location. The S-A node lies in the outer wall of the right “atrium,” just below the entrance of the superior vena cava – a major venous ( VEE -nus) sinus ( SIGH -nus). (A sinus, in general, is a large vein that is shaped like a “hollow bay” [ sin ], holding a considerable volume of blood.)
As the primary cardiac pacemaker, the sinoatrial node excites itself first. The excitation then spreads from one cardiac muscle fiber to another, due to the presence of intercalated ( in -ter- KAY -lay-ted) discs between them. These intercalated discs appear as thick dark lines when viewed through a compound microscope. They are actually intercellular bridges “inserted between” (intercalat) adjacent cardiac muscle fibers. Thus, when the S-A node fires off an action potential, this traveling wave of excitation moves from one cardiac muscle fiber to another, across the intercalated discs connecting them end-to-end. Eventually, both atria become excited and contract. And finally, both ventricles become excited.
After the ventricles are excited, their walls contract. The blood in the right ventricle is pushed up into the common pulmonary artery through the right semilunar ( sem -eye- LOO -nar) valve. Likewise, the blood contained within the left ventricle is pumped up into the aortic arch through the left semilunar valve. Both of these valves get their names from the shapes of their flaps, which look like “partial or half” (semi-) “moons” (lun). This is reflected in their abbreviation, S-L.
Note in Figure 16.3 that the myocardium is thicker in the walls of the ventricles, compared to the atria. And observe that the myocardium is thickest of all in the wall of the left ventricle.
Study suggestion: Ask yourself, “Why is the myocardium thicker in the wall of the left ventricle, compared to that of the right ventricle?” Hint: What happens to a skeletal muscle when you repeatedly exercise it?
Practice problems for these concepts can be found at: Blood And The Circulatory System Test
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