<P> Endothelium is a thin layer of simple squamous epithelial cells that line the interior of both blood and lymph vessels . The endothelium that lines blood vessels is known as vascular endothelium, which is subject to and must withstand the forces of blood flow and blood pressure from the cardiovascular system . To withstand these cardiovascular forces, endothelial cells must simultaneously have a structure capable of withstanding the forces of circulation while also maintaining a certain level of plasticity in the strength of their structure . This plasticity in the structural strength of the vascular endothelium is essential to overall function of the cardiovascular system . Endothelial cells within blood vessels can alter the strength of their structure to maintain the vascular tone of the blood vessel they line, prevent vascular rigidity, and even help to regulate blood pressure within the cardiovascular system . Endothelial cells accomplish these feats by using depolarization to alter their structural strength . When an endothelial cell undergoes depolarization, the result is a marked decrease in the rigidity and structural strength of the cell by altering the network of fibers that provide these cells with their structural support . Depolarization in vascular endothelium is essential not only to the structural integrity of endothelial cells, but also to the ability of the vascular endothelium to aid in the regulation of vascular tone, prevention of vascular rigidity, and the regulation of blood pressure . </P> <P> Depolarization occurs in the four chambers of the heart: both atria first, and then both ventricles . </P> <Ol> <Li> The sinoatrial (SA) node on the wall of the right atrium initiates depolarization in the right and left atria, causing contraction, which is symbolized by the P wave on an electrocardiogram . </Li> <Li> The SA node sends the depolarization wave to the atrioventricular (AV) node which--with about a 100 ms delay to let the atria finish contracting--then causes contraction in both ventricles, seen in the QRS wave . At the same time, the atria re-polarize and relax . </Li> <Li> The ventricles are re-polarized and relaxed at the T wave . </Li> </Ol> <Li> The sinoatrial (SA) node on the wall of the right atrium initiates depolarization in the right and left atria, causing contraction, which is symbolized by the P wave on an electrocardiogram . </Li>

The wave of depolarization across the heart means