Wk11 iWorx Circulation Lab(2) - iWorx Circulation Lab: The...

iWorx Circulation Lab: The Heart, ECG, Pulse, and Heart Sounds
The heart is a pump that pushes blood around the body. Blood enters the heart at a low pressure and
leaves at a higher pressure, and this high pressure provides the force to propel the blood through the
circulatory system. The organization of the human heart and the circulatory system is shown in Figure 1.
Blood returning from the body is sent to the right side of the heart and then to the lungs to pick up
oxygen and release carbon dioxide. This oxygenated blood is sent to the left side of the heart and back to
the body, where oxygen is released and carbon dioxide is collected. The complete division of
the heart insures that there is no mixing of deoxygenated blood (in the right side) with oxygenated blood
(in the left side).
The mammalian heart is autorhythmic, since it will continue to beat if removed from the body (and kept
in an appropriate solution). Heart contractions are, therefore, not dependent upon the brain, rather the
rhythm comes from within the heart itself. The heart is composed almost entirely of large, strong muscle
fibers (myocardium), which are responsible for the pumping action of the heart. Other cardiac muscle
cells are weakly contractile and produce or conduct the rhythm for the rest of the heart. A group of
these weak muscle cells is located in the sinoatrial (SA) node and acts as the pacemaker for the heart
(Figure 2). These cells rhythmically produce action potentials, which spread via gap junctions to fibers of
both atria. The resulting contraction pushes blood into the ventricles. While adjacent atrial fibers are
connected by gap junctions, the only electrical connection between the atria and the
ventricles is via the atrioventricular (AV) node. The action potential spreads slowly through the AV
node and then rapidly through the Bundle of His and Purkinje fibers to excite both ventricles.
Figure 1: A diagram to show the circulation of blood around the human body and its
association
with the heart, composed of a right atrium (RA), a left atrium (LA), a right ventricle (RV), and a left
ventricle (LV).
Amended from iWorx Experiment HH-1: The Electrocardiogram and Peripheral Circulation Page 1

Figure 2: A diagram of the human heart to show the location of the sinoatrial (SA)
and atrioventricular (AV) nodes.
The semilunar valves are located between the ventricle and the main artery on each side of the heart. In
the relaxed heart, the high arterial pressure shuts the semilunar valves and prevents blood flow from the
artery back into the ventricle. Ventricular contraction increases the pressure of the blood in the
ventricle. When the ventricular pressure is greater than the arterial pressure, the semilunar valves open
and blood flows into the artery. Then, the myocardium relaxes, the ventricular pressure declines, and the
semilunar valves close.