You do not have to be involved in medicine to know something about the heart. It is very likely that one of your relatives has suffered a heart attack or has undergone heart surgery. We are all constantly confronted with new information about how best to treat our own hearts, from quitting smoking to changing our diets--I'm sure you have eaten at a restaurant whose menus specifically advertise "heart-healthy" entrees, for example. It is important that we all know the basic facts about how the heart works and how it becomes diseased. Hopefully this period will both expand your knowledge base and fuel your growing interests in this fascinating subject.

By the end of today's lab period, you should be able to discuss the following points:

1. Organization of the heart's chambers.
2. How blood flows through the heart.
3. Why blood travels in one direction through the body.
4. The anatomy of the chamber walls.
5. The blood supply to the heart itself.
6. How the heart beats.
7. Some common heart ailments.

I. The Chambers of the Heart

The heart is a muscular organ which is divided into four chambers. There are two atria (right and left) and two ventricles (also right and left). The atrial walls are only a few millimeters thick, while the ventricular walls are over a centimeter thick. In a normal heart, the wall of the left ventricle is wider than the right ventricle.

The blood flows through the heart in the following sequence: Superior and Inferior Vena Cavae -> Right Atrium -> Right Ventricle -> Pulmonary Artery 4 Lungs -> Pulmonary Veins 4 Left Atrium -> Left Ventricle -> Aorta. The blood leaves the heart through the aorta (the largest artery in the body) and reaches tissues via arteries. It flows through capillary beds and then into veins, returning to the heart through the inferior and superior vena cavae, the largest veins in the body.

1. Identify the 4 heart chambers. Appreciate the differences in wall thickness and anatomy.

Function: The veins deliver deoxygenated blood (blood with little oxygen) to the right side of the heart. The right side of the heart is responsible for pumping this deoxygenated blood to the lungs, where oxygen is added to the blood. This newly oxygenated blood is returned from the lungs to the left side of the heart. The job of the left side of the heart is to pump this oxygenated blood to the body. The left ventricle is thicker than the right ventricle because it must work harder to pump blood to the whole body, whereas the right ventricle only pumps blood to the lungs. Also, the atrial walls are thinner because they collect blood rather than pump blood out of the heart.

2. Probe with fingers the eight great vessels of the heart:

Superior Vena Cava
Inferior Vena Cava
Pulmonary Artery
Pulmonary Veins (four total)
Aorta

Relate each great vessel to its appropriate chamber. Trace blood flow through these structures with your finger.

Although these chambers are termed "right and left", anatomically their position in the body does not show this. The heart is actually skewed to the left and to its side, so that the right ventricle is most anterior (front) and the left atrium is most posterior (back).

3. With table leader, visualize how the heart sits in the thorax (chest cavity).

Examine the interior of the heart more carefully. Do you notice the mesh-like texture of the chamber walls? In the ventricles, this meshwork is called the trabeculae carnae and in the atria it is called the musculi pectinati (pectinate muscle). Feel free to throw these terms around at your next social event. The finger-like muscle knobs in the ventricles are the papillary muscles, which you can see are attached to the valves by stringy cords called the chordae tendinae. Their functions are discussed below.

4. Identify these anatomical structures in the heart.

II. Valves

Valves in the heart are "one-way doors" between the chambers of the heart and between the chambers and the vessels. There are four valves in the heart. Conceptualize the beating heart. Can you imagine why the valves are vital anatomical features? (Hint: does blood flow in only one direction?)

Answer: blood flows in only one direction due to the function of the valves. When the left ventricle contracts, for example, the papillary muscles in this chamber also contract. Because they are attached to the chordae tendinae (which you saw earlier) which are in turn attached to the valves, the mitral valve is closed and blood cannot be regurgitated (ejected back) into the left atrium. Instead, the blood only has one way to travel--through the aortic semilunar valves and out to the rest of the body.

The other valves in the heart perform the same function--to prevent regurgitation into a previous chamber. If heart valves begin to work incorrectly, they can be replaced, as you will see in a video later on.

5. Identify these valves in the heart:

• Tricuspid valve--separates right atrium and ventricle
• Mitral valve--separates left atrium and ventricle
• Pulmonic valve--separates right ventricle and pulmonary artery
• Aortic valve--separates left ventricle and aorta

Ill. Coronary circulation

Now, we have been talking about how the heart pumps blood to nourish the tissues of the body. The myocardium, (heart muscle) needs oxygen and nutrients like any other muscle. Let's stop and think about how blood gets to the heart muscle.

Well, the coronary circulation is responsible for nourishing the heart itself. Although the heart pumps five liters of blood per minute, the muscular walls of the heart are too thick to receive nutrients from the blood which is being pumped through the heart.

The first two branches off the aorta are the right and left coronary arteries. The right coronary artery winds around the heart to supply the posterior (back) of the heart. The left coronary artery is short and branches to supply the anterior (front) of the heart.

And, like any other tissue in the body, the heart must have veins to return the blood supplying it to the right atrium for reoxygenation. The coronary sinus is the major vein returning blood from the coronary arteries which were nourishing the heart.

The coronary arteries can become lined inside by fatty deposits called plaques that contain cholesterol. More plaques are deposited over a period of decades if there is a relatively high level of cholesterol in the blood and can block the passage of blood through these vessels. This process is called atherosclerosis, which we will discuss further a little later.

You have most likely heart of a "coronary bypass" operation. So, what is a "coronary bypass", exactly? The vessel blocked is literally "bypassed" by connecting (grafting) a vein from the leg or chest around the blocked region. The vein is actually cut out of these other regions of the body and attached to the heart.

We will discuss both of these problems further later in this lab.

6. Identify these coronary vessels:

Right coronary artery
Posterior interventricular artery
Left coronary artery
Anterior interventricular artery
Circumflex artery
Cardiac Sinus

Review the 4 chambers, the valves, and the coronary circulation before we continue.

IV. Conduction system

One of the fascinating-if not eerie!-facts about the heart is that it beats on its own: it does not need stimulation from the brain. For instance, a transplanted heart continues to pump even though its nerves have obviously been cut. However, your quadriceps or biceps cannot contract without direction from the brain. What allows the heart to do this?

The heart has its own system to generate and spread electrical signals from one end of the heart to the other, making the muscle contract. This happens 60-100 times a minute. The electrical impulse begins at a bundle of cells called the sinoatrial (SA) node. This node is a small pacemaker in the right atrium near the entrance of the superior vena cava. It is difficult to see this structure with the naked eye.

7. Look for the entrance of the superior vena cava into the heart. Pinch along the front edge where it meets up with the atrium. There you will find a thickened area. This is the sulcus terminalis with the sinoatrial node inside.

The rest of the electrical conduction system cannot easily be seen without a microscope. However, you should be familiar with the path of the electrical signals in the heart. Please refer to the diagram of the conduction pathway below and discuss with your lab leader. The sequence is as follows:

1. SA node (pacemaker)
2. Atrial muscle (contracts)
3. Atrioventricular (AV) node
4. AV bundle
5. Left and right bundle branches (in wall between the ventricles)
6. Purkinje fibers within the ventricular muscle
7. Ventricular muscle (contracts)

If this sequence is disturbed, a pacemaker can be surgically implanted that serves as an "artificial SA node". If this sequence is really disturbed, defibrillators (as seen on "ER") are used to shock the heart back into working correctly, Don't try this at home!

The electrocardiogram (EKG) is the way that a physician can see the electrical signals through the heart. There are three waves to the EKG, each of which corresponds to a stage of the heart's contraction. These waves are called P, QRS, and T. The stages of contraction for each wave are listed below:

P: Atrial muscle contraction after the SA node fires
QRS: Ventricular contraction T: Ventricular refilling and recharging after contraction

Is there a wave missing here? (Hint: Do the atria also have to refill with blood?)

Answer: The wave showing the refilling of the atria is too small to be seen and is actually under the QRS wave!

The relationship between the heart's contractions and the EKG can often be difficult to understand at first--make sure you bug your table leader if it seems a little unclear!