Your sinoatrial node is sometimes called your heart’s natural pacemaker. It sends the electrical impulses that start the heartbeat. The left atrium contracts and pushes the blood into the left ventricle through the left atrioventricular valve. These steps make up one full contraction of your heart muscle. Your heart conduction system sends out thousands of signals per day to keep your heart beating. How does electrical conduction perform with the rest of your heart?

A normal healthy ECG trace shows what is known as the “PQRST shape” (Figure 5). Wave P is the stage that corresponds to atrial systole, excitation and contraction of the atria. Ventricular systole forms the “QRS” complex and the start of ventricular diastole corresponds to the “T” wave. Figure 5: The start is diastole where both the atria and ventricles are relaxed. The P wave shows the depolarisation of the atria and is followed by atrial systole (contraction). Atrial systole continues to the end of the QRS complex and the point where the atria relax. The QRS complex shows the relaxation of the ventricles and is followed by ventricular systole (contraction). The T wave represents a further depolarization (re- polarization) and marks the start of ventricular relaxation. Deoxygenated blood returning from the body first enters the heart from the superior and inferior vena cava. The blood enters the right atrium and is pumped through the tricuspid valve into the right ventricle. From the right ventricle, the blood is pumped through the pulmonary semilunar valve into the pulmonary trunk. The first part of the wave, called the P wave, is a small increase in voltage of about 0.1 mV that corresponds to the depolarization of the atria during atrial systole. The next part of the EKG wave is the QRS complex which features a small drop in voltage (Q) a large voltage peak (R) and another small drop in voltage (S). The QRS complex corresponds to the depolarization of the ventricles during ventricular systole. The atria also repolarize during the QRS complex, but have almost no effect on the EKG because they are so much smaller than the ventricles. Blood will then travel from the right atrium, through the tricuspid valve, and enter the right ventricle. Now when I was a little chap I had a passion for maps. I would look for hours at South America, or Africa, or Australia, and lose myself in all the glories of exploration. At that time there were many blank spaces on the earth , and when I saw one that looked particularly inviting on a map (but they all look that) I would put my finger on it and say, 'When I grow up I will go there.' The North Pole was one of these places, I remember. Well, I haven't been there yet, and shall not try now. The glamour's off. Other places were scattered about the hemispheres. I have been in some of them, and . . . well, we won't talk about that. But there was one yet -- the biggest, the most blank, so to speak -- that I had a hankering after.

Case Discussion

I smiled, --for what had I to fear? I bade the gentlemen welcome. The shriek, I said, was my own in a dream. The old man, I mentioned, was absent in the country. I took my visitors all over the house. I bade them search --search well. I led them, at length, to his chamber. I showed them his treasures, secure, undisturbed. In the enthusiasm of my confidence, I brought chairs into the room, and desired them here to rest from their fatigues, while I myself, in the wild audacity of my perfect triumph, placed my own seat upon the very spot beneath which reposed the corpse of the victim.

i Identify the atria and ventricles. Try to describe any differences in the structures of the walls of the atria and ventricles.

Heart anatomy

The sounds of a normal heartbeat are known as “lubb” and “dupp” and are caused by blood pushing on the valves of the heart. The “lubb” sound comes first in the heartbeat and is the longer of the two heart sounds. The “lubb” sound is produced by the closing of the AV valves at the beginning of ventricular systole. The shorter, sharper “dupp” sound is similarly caused by the closing of the semilunar valves at the end of ventricular systole. During a normal heartbeat, these sounds repeat in a regular pattern of lubb-dupp-pause. Any additional sounds such as liquid rushing or gurgling indicate a structure problem in the heart. The most likely causes of these extraneous sounds are defects in the atrial or ventricular septum or leakage in the valves. Cardiac Output