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The cardiovascular system is composed of the heart and blood vessels that carry blood to and from the body’s organs. There are 2 major circuits: pulmonary and systemic. The pulmonary goes out to the lungs while the systemic goes to the rest of the body. These circuits are closed and continuous loops.

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The heart is located in the thoracic cavity between the lungs and deep to the sternum. It is tilted slightly to the left, about 2/3 of the heart lies left to the mid-sagital plane.

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The heart has a covering called the pericardium. The pericardium is broken into the parietal pericardium (top portion) and the visceral pericardium (bottom portion). The parietal pericardium has an outer fibrous layer and inner serous layer (think back to our tissues chapter and the discussion on membranes). The visceral pericardium is formed by the serous layer of the parietal pericardium turning inward at the base of the heart (top portion). There is a space between the parietal and visceral pericardiums called the pericardial cavity. It is filled with pericardial fluid made by the serous membrane and lubricates the layers and allows heart to beat with little to no friction against surrounding tissues

The wall of the heart has three layers. The Epicardium is the outer layer and made of a thin layer of epithelium and areolar tissue (serous membrane). It also contains adipose tissue. It is kind of one in the same as the visceral pericardium. If you talk about the wall of the heart, you call it epicardium. If you talk about the coverings it is called visceral pericardium. The next layer is the myocardium. It is made of cardiac muscle and is the thickest, middle layer. It is the performance layer of the heart. The Endocardium is the inner layer. It is similar in structure to the epicardium, minus the adipose.

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The coverings and the wall of the heart. Notice how thick the myocardium is in comparison to the endocardium and epicardium. You can also see the pericardial cavity and the visceral and parietal pericardiums. The visceral pericardium is the same layer as the epicardium and the parietal is the continuation of that peach colored layer and forms the outer wall of the pericardial cavity.

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There are 4 chambers in the heart, 2 atria (right and left) and 2 ventricles (right and left).

The atria are the top chambers and receive blood when it returns to the heart. Ventricles are the bottom chambers and pump blood out of the heart to the lungs or the rest of the body.

The chambers are separated by septa. The interatrial septum is a wall of muscle that separates the atria, and the interventricular septum is a wall of muscle that separates the ventricles.

The passage of blood from the various chambers is regulated by valves. Valves make sure blood does not flow backward, keeping it going in one direction. The atrioventricular (AV) valves regulate blood flow from the atria to ventricles. They have tendinous chords that connect the valves to papillary muscles in the floor of the ventricle. The chords keep the AV valves from caving in. The tricuspid is on the right and also known as the right AV valve, while the bicuspid is on the left and also known as the left AV valve, or mitral valve.

Semilunar valves regulate blood flow from the ventricles to vessels leaving the heart. They are sort of pie/pizza shaped. There are two semilunar valves: the pulmonary valve (off the right ventricle) and the aortic valve (off the left ventricle).

There are no valves in the vena cavae or pulmonary veins (the veins that bring blood back to the heart). During atrial contraction the walls of the atrium close in on the

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vessels, forming a pseudo valve.

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You need to be able to know the steps of blood flow through the heart forward and backward. One way to learn it is to focus on the structures. Start on the pulmonary side and work your way through the systemic side of the heart.

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Heart tissue is capable of producing its own electrical signals and rhythm (autorythmic).

You can take a heart out and place it in saline, and if enough O2 is present, it will keep beating. Without nerve control, the heart would beat 100 times a minute. However, the vagus nerve inhibits the pace of the heart, keeping it at about 75 beats/min. So, while the heart can stimulate itself, we need the nervous system to provide the brakes. The system within the heart that provides the electrical stimulation for contraction is the Cardiac Conduction System. There are a few key structures involved in the system.

1. The Sinoatrial Node (SA Node) is located in upper right corner of the right atrium. It is a group of specialized cells that act as an electrical generator. It is also called the pacemaker. As mentioned, part of the vagus nerve controls some of its activity and acts like a brake. It does so by interacting with the SA Node.

2. The Atrioventricular Node (AV node) is located in the lower left portion of the right atrium. It receives signals from the SA Node and then distributes that signal.

3. The AV bundle is a nerve that comes off the AV node. This is how the AV node passes on the signal. The bundle also delays the impulse slightly. This is important because if it did not, the signals travel so fast that the atria and ventricles would end up contracting simultaneously.

4. The bundle branches come off the AV bundle and branch to the left and right to

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carry the message to the ventricles.

5. Purkinje fibers are multiple branches off of the bundle branches that carry the impulse to the ventricle cells and stimulate them to contract.

The pacemaker is the most common area with which to have a problem in the cardiacconduction system. If it fails, the AV node can also send signals to compensate, but the rhythm is not completely accurate.

One thing to recognize about the system is that the AV Node is the source of impulse for the ventricles, but the action is done by Purkinje fibers. The Purkinje fibers do the actual stimulation of cardiac muscle cells.

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As with blood flow, you need to be able to go through these 5 steps of how the electrical signal flows through the heart.

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EKG (electrocardiogram)

The heart beat is both muscular and electric. An EKG detects the electrical currents in the heart by placing electrodes on the skin.

EKGs are broken down and read in 3 primary segments:

1. The P wave is produced when the SA Node sends out the signal throughout the atria and depolarizes them. It shows up as a small little bump on the EKG.

2. The QRS complex is caused by the signal from the AV Node that spreads throughout the ventricles and causes them to depolarize and contract at the same time the atria are repolarizing.

3. The T wave is generated by the repolarization of the ventricles.

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This is the EKG. The spikes in blue represent the electrical impulses being sent and the depolarizing and repolarizing events occurring in the heart. The line in grey underneath demonstrates when the muscle contraction is occurring. The P wave represents the depolarizing and contracting of the atria. The QRS complex represents the repolarizing of the atria and depolarizing of the ventricles. The atria will relax at the beginning of the complex and ventricular contraction begins with the spike or “R” portion of the complex and continues until the beginning of the T wave. In the T wave we have ventricular repolarization and relaxation. There is a brief moment when neither the atria nor the ventricles are contracting, at which point the cycle starts again.

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Similar to our skeletal muscles, cardiac muscles are stimulated to contract by action potentials. There is an absolute and relative refractory period for the impulses in the cardiac system. Where the two muscle types differ is the involvement of calcium in cardiac action potentials. Calcium enters the cell and prolongs the absolute refractory period. This makes the contraction more intense, not like the twitch that is in skeletal muscle.

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Notice how the green line causes the action potential to drop slowly and almost flatten. This is when the absolute refractory period is lengthened.

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This is one of my favorite visual aids from your book. You can see what is happening electrically as well as what the muscle itself is doing. This combines what we see on the EKG and what is actually happening in the heart. You can also consider how this is affecting blood flow through the heart.

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The Cardiac cycle is one complete contraction and relaxation of all four chambers.

Systole refers to the contraction of heart muscle.

Diastole refers to the relaxation of heart muscle.

So a full cycle, would be described in the following manner:

1. Atrial systole, ventricular diastole.

2. Atrial diastole, ventricular systole.

3. Atrial diastole, ventricular diastole.

The one thing we will not see is atrial systole with ventricular systole. That would mean both are contracting simultaneously. The contractions also make an audible noise, as we should know from visiting the doctor. They are listening to make sure the heart is cycling through correctly. The sounds (lubb-dubb) are actually the result of the valves closing and turbulence in the blood and movement of the heart wall. The “lubb” is the first sound and is due to the AV valves closing. The “dubb” is the second sound and is the result of the semilunar valves closing.

Go back to the previous slide and the picture of the heart depolarizing and repolarizing. Can you walk through the diagram now using the terms systole and diastole? Can you see how the terms correspond with the depolarizing and repolarizing and the electrical events of the EKG?

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As we cycle through, the atria contract, pushing blood into the relaxed ventricles. Once their contraction is complete, they relax, and the ventricles contract, pushing blood out into the vessels. For a moment, both will be in a relaxed state, and then the cycle starts over again.

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