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The Heartbeat

Page history last edited by Gregory Hylander 15 years, 3 months ago



Gregory Hylander



March 2, 2009

English 140: Fundamentals of Technical Writing

Technical Description Project


The Heartbeat: The Conducting System



Cardiac Physiology. 1

Initiation of an Action Potential 1

Electrocardiogram.. 1

Autorythmicity. 2

The Sino Atrial Node. 2

The Ventricle Conduction System.. 2

A Shortcut to the Right Ventricle. 2

Regurgitation. 3


Cardiac Physiology

Cardiac physiology is the rhythmic beating of the heart.  It involves the contraction of the heart.  This is also referred to as the “heart beat.”  It progresses in a series of steps that coordinate the movement of blood from the heart into the arteries, capillaries, and lastly into the veins.  At the beginning of the cardiac cycle the atria contract first and then the ventricles contract.  There are two types of cardiac muscle cells that are involved in the process; conducting cells  and contractile cells.  The conducting cells are incorporated into the conducting system of the heartbeat, which controls and coordinates the heartbeat.  The contractile cells produce the powerful contraction necessary to create enough pressure to propel blood through the aortic semilunar and pulmonary semilunar valve.


Initiation of an Action Potential

Each heartbeat is initiated with an action potential generated by special pacemaker cells  begins at the sino atrial node, which is the first part of the conducting system.  This is the first step in the heartbeat.  This action potential is referred to as an electrical impulse.  The impulse is then propagated by the conducting system and distributed in a manner that stimulates the contractile cells to push blood in the right direction. 



These electrical impulses can be monitored and evaluated from the surface of the body through the procedure known as an electrocardiography.  The printed recording of an electrocardiography is referred to as an electrocardiogram (ECG or EKG).   AN ECG is labeled by a P wave, QRX Complex, and T wave.  The height of the ventricle contraction is the R portion.  Through the coordinated action of the conducting system, the right and left atria contract first, which drives blood into the right and left ventricles through the atrioventricular valves, and then the ventricles contract next, which drives blood out of the heart through the semilunar valves.  The sino atrial node generates electrical impulses at regular intervals, resulting in one heartbeat immediately following the previous heartbeat through the course of life.  On the conclusion of each heartbeat there is a very brief period, less than half a second, preceding the next heartbeat.



This property of generating its own electrical impulse independent of the central nervous system is known as “automaticity or autorhymicity.”  The specialized conducting  cells responsible for distributing the electrical impulse are referred to as the cardiac conducting system or the nodal system.  The contraction of specialized contractile cardiac muscle cells lags behind the distribution of the electrical impulse. 



This lapse time represents the time it takes for calcium ions to enter the sarcoplasm and activate the contraction of the cardiac myofibrils.

The Sino Atrial Node

The conducting system involves a series a compartmentalized structures that are responsible for distributing the electrical impulse to the myocardium of the ventricles.  The sino atrial node (SA Node) is located in the posterior wall of the right atrium.  The atrioventricular node (AV Node) is located at the junction between the atria and ventricles on the anterior, inferior portion of the right atria.  Conducting cells are responsible for interconnecting the two nodes and distribute the electrical impulse throughout the myocardium.  The conducting cells connecting the SA Node and the AV node are located in the intermodal pathway.  The intermodal pathway is responsible for distributing the contractile electrical impulse from the SA Node to the AV Node.


The Ventricle Conduction System

The ventricle conducting system involves a different series of conducting structures, cells, and fibers.  These include the: atrioventricular bundle, bundle branches, and purkinje fibers.  The connection between the AV node and the AV bundles, which is also referred to as the bundle of His, is the only electrical impulse connecting the atria to the ventricles.  The impulse generated as the SA Node must travel through this specific site to reach the ventricles to result in coordinated heartbeat.  Once the electrical impulse enters the AV bundle it then enters the interventricular septum and enters the right and left bundle branches. 

The left bundle branch supplies the left ventricle and is much larger than the right bundle branch.  These two branches proceed toward the apex of the heart, make a u-turn and fan out to the deep endocardial surface.   As these branches diverge the electrical impulse is propagated to the purkinje fibers, and through the moderator band to the papillary muscles of the right ventricle.  The entire process of generation an action potential at the SA node to the complete depolarization of the ventricular myocardium, on average, takes 225 msec. 

A Shortcut to the Right Ventricle

By this time the atria have contracted and ventricular contraction occur without disrupting the coordinated flow of blood.  Interesting to note, because the right bundle branch delivers the impulse across the moderator band to the papillary muscle of the right ventricle, rather than by the purkinje fibers, the papillary muscles begin contracting, before the rest of the ventricular myocardium.  The contraction of the papillary muscle applies tension of the chordate tendineae, which braces the atrioventricular valves for closure, denying a prolapsed and regurgitation of blood.


By limiting the movement of the cusps, the tension in the choradae tendeneae prevents the backflow to blood into the atria (regurgitation) when the ventricles begin contracting.  The rest of the purkinje fibers fan out toward the base of the heart, where the great vessels of the heart are connected.  This results in the electrical impulse of the heartbeat spreading out from the apex toward the base.  Blood is propelled toward the base of the heart into the ascending aorta and pulmonary trunk.


If the conducting pathways are damaged or diseased, the normal autorhymicity of the heart with be disturbed.  The resulting problems are referred to as conduction deficits and can result in series clinical problems.  If the SA node is damaged the AV node will assume command of generated cardiac action potential in sequential order (heartbeat).  Although the heart will contract in the appropriate order the beats will occur at a slower rate, roughly 40 60 beats per minute.  If an abnormal state of conducing cells or ventricular cell begin generating action potentials at a higher rate, the impulse could override the correct circuit of the SA and AV node.  The place of origin of these abnormal signals is referred to as ectopic pacemakers.  The activity of these ectopic pacemaker have the potential to partially or completely bypass the conducting system, disrupting the timing of ventricular contraction.

This could result in a dangerous reduction in the pumping efficiency of the heart.  From a clinical applications approach, such conditions are commonly diagnosed with the aid of an electrocardiogram, which is the recording of the electrical activity of the heart.




Comments (3)

Isabel said

at 9:40 am on Mar 31, 2009

You can tell immediately that this document is geared toward an audience with a higher reading level. The terms are extremely technical. In order to focus this toward a more general audience, you could simplify a few of the terms you use; i.e. from “The specialized conducting cells responsible for distributing the electrical impulse are referred to as the cardiac conducting system or the nodal system” to “The specific conducting cells that distribute electrical impulses are called the cardiac conducting system or the nodal system.”

This is a good process decription, but I think that it should be titled something else besides simply “The Heartbeat.” I understand that it focuses on the heart, but perhaps it could be labeled better for a lay audience; something like “Blood Circulation Through the Heart” is more descriptive and grabbing.

There are no visuals with this document—it would be extremely helpful to have a flowchart to help illustrate what you’re explaining. Even pictures at the top or in the middle of the page to help break up the text would make your process description avoid looking like a textbook. Using larger fonts and different colors for the subtitles are good; it helps visually separate each step for the audience. When you introduce a new idea or topic, you could also italicize or bold these words. They help your document’s readability.

Great job describing the process in paragraph form versus simply numbering each step. This helps your document avoid looking like instructions.

I would run a Spell Check again over the document: some words are misspelled and a few sentences are missing punctuation marks.

Deanna_C said

at 10:06 am on Mar 31, 2009

Your document is indeed very technical and written for a group of people who already know a lot about your subject matter. Simpler terms can help the general audience to understand the processes you have mentioned and perhaps changing the format to be more engaging. You may also want to explain and expound on why it is important to learn about these processes and provide a concluding paragraph that can tie everything together instead of the document ending on the last step in the process.

You can also use bullets or simply break down the paragraphs to make your document more easy to read. The use of visuals will also be useful in explaining your processes better so the readers can have a more specific idea of the parts and areas you are detailing.

I would also read through it again and re-write sentences that are too short and use more transitional words. Some punctuations are missing and there are misspelled words in the document.

Great job on the technical description though, you seem to really know a lot about the subject you chose.

Joaquin Harvey said

at 9:45 pm on Apr 15, 2009

This is a highly technical document filled with a lot of substances that demonstrates that you are in tuned with your topic and that you can articulate that information well with someone who is familiar with this subject matter. Although I agree that it would help a more general audience to follow what is actually going on if there were more simplified terms.

In this process description I enjoy the way each process is broken into their own paragraph to be explained thoroughly and individually I admit if you may have added a photo or two of each process that would make it an easier transition from one process to the next.

The last thing I notice was that when introducing a new topic everything was in a larger red font but the title of the document was left in a small plain font I encourage that you make the title more visible and appealing to the audience.

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