Anatomy and Physiology: The Heart's Own Blood
The Heart's Own Blood
Why does the heart need its own circuit if blood is already going through it? The answer to that rests with our constant companion, surface area to volume ratio. Remember the issue of SA/V when I talked about capillaries? Well, the lumen of the chambers just doesn't have enough surface area to nourish the heart, especially with the thick ventricular walls, so there needs to be a separate blood flow to the heart, one that occurs on the capillary level. Without that, the heart would quickly die, and so would you!
Branching off opposite sides of the aorta, almost immediately after it leaves the left ventricle, are the left and right coronary arteries. These arteries pass between the atria and the ventricles (in case you ever have to find them in a practicum exam!), and I bet you can find the anterior and posterior interventricular branches. (They're between the ventricles, of course! Don't forget to keep using the names to figure things out!) This is the start of the coronary circuit, which nourishes the heart.
The return of blood to the heart from the heart—which should be thought of as a return of blood to the chambers of the heart from the tissues of the heart—starts by draining the tissues in the back of the heart at the middle cardiac vein, and at the front through the great cardiac vein. The blood finally empties into the right atrium from the posterior coronary sinus.
In a Heartbeat!
A heartbeat, that little twist of anatomical fate that keeps the vice president as a last-resort tiebreaker in the Senate, is a complicated thing. Despite all the emphasis given above to the left and right sides of the heart, the heart doesn't beat that way; the paired beat of your heart involves contraction from top to bottom. The first portion of the beat is the simultaneous contraction of the two atria pumping their contacts into the ventricles. The second part of the beat is the simultaneous contraction of the two ventricles pumping their blood into the pulmonary trunk and the aorta.
Remember those valves I told you about? When you listen to a heartbeat you are listening to the sounds of the valves slamming shut. Listening to the sounds of the heart, or any body sounds in fact, is called auscultation, and it is usually done with a stethoscope. If you close your eyes and imagine the sound of a heartbeat you can easily remember the rhythm of the beat since it's so distinctive—“lubb dupp, lubb dupp, lubb dupp,” and so on. The first sound (S1 = “lubb”) is the sound of the AV valves slamming shut during the contraction of the ventricle (ventricular systole). The second sound (s2 = “dupp”) is the sound of the semilunar valves slamming shut when the ventricles relax (ventricular diastole).
Remember how atrial systole needs to happen during ventricular diastole (think about the rhythm of the beat)? Only then can the ventricles remain open to receive the blood from the atria. Now think about the last time you touched something really hot; didn't you pull your hand away incredibly fast? That rapid response time is due on part to the efficiency of nerve conduction (see The Nervous System) and to the speed of muscle cell stimulation (see The Structure of the Muscles and Muscle Cells).
Now think about the rhythm of the heartbeat. The two parts of a heartbeat come from the same initial impulse—which originates at the sinoatrial (SA) node, familiarly called the pacemaker. The length of the heart is rather short (about the size of your fist) when compared to the length of your arm, so why the delay between the first half of your heartbeat to the second half?
First off, there is a certain characteristic of cardiac cells that can be beneficial in the right hands, but fatal in the wrong hands (sinister music swells!). Cardiac cells have the capacity to be pacemakers, or autorhythmic (self-stimulating). The SA node is made of these autorhythmic cells; this means the heart, unlike skeletal muscle, does not need stimulation from nerves in order to contract. There is, nonetheless, a connection to the brain, but this connection, part of the autonomic nervous system (see The Senses), only serves to regulate the rate of the contraction (not your rhythm, but your tempo!).
The existence of a pacemaker makes it possible for a person to survive a severed spinal cord, for the heart can continue to contract on its own. The autorhythmic capability of other cells can sometimes cause a problem, for if multiple parts of the heart start to try to contract on their own, the overall contraction of the heart will be too weak to effectively pump the blood. A defibrillator works by depolarizing (see The Structure of the Muscles and Muscle Cells) all the muscle cell membranes (sarcolemmas) so that when they repolarize, the cells can now contract in sync.
A heartbeat is the result of the intrinsic conducting system of the heart (see Figure 11.4). There are three basic parts of the conducting system: the SA node, the AV node (from its name, atrioventricular node, you should know its location—between the atria and the ventricle!), and conducting cells, which connect the two nodes, as well as sending the message to the ventricles. From the SA node's pacemaker cells (located at the top of the right ventricle, near the superior vena cava) the stimulus begins, and then spreads outward through the atrial walls via the internodal pathways (yet another name that makes sense) to the AV node. This takes about 50 milliseconds.
At the AV node something essential happens, for the narrower cells in the AV node, as well as the lower efficiency at sending messages from cell to cell, creates an essential delay (about 100 milliseconds) in the message being sent to the AV bundle, allowing for ventricular diastole to occur during ventricular systole. The AV bundle (or bundle of His) then sends the message on to the right and left bundle branches, which carry the message down through the ventricular septum, where they then travel up the outer ventricular walls via the Purkinje fibers, ultimately causing the ventricles to contract.
Just prior to ventricular systole the papillary muscle contracts, being stimulated directly from the bundle branches, thus bracing the cusps of the valves prior to ventricular contraction. The message travels very fast along the Purkinje fibers, taking only about 75 milliseconds. The full time for the message to travel along the conducting system from the SA node to the end of the Purkinje fibers takes only 225 milliseconds, but the essential 100 milliseconds delay made the filling of the ventricles possible. Lastly, a relatively long refractory period (the period during which a second stimulus cannot be received) of 200 milliseconds ensures the spacing between successive heartbeats.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.