159
Structure and Function
Sinoatrial (SA) node
Left atrium
Right atrium
Atrioventricular (AV) node
Bundle of His
Left ventricle
Right ventricle
Purkinje fibers
FIGURE 6-3 Electrical conduction system of the heart.
Learning Tip Trace the conduction path- way of the heart with your finger while you verbally identify all of its structures and describe their function.
to the outside. In response, all the individual cardiac muscle cells in your atria contract in unison. The term for this normal rhythm of the heart is normal sinus rhythm , named for the SA node (Figure 6-4). An abnormal rhythm is called an arrhythmia ( a- = without). Arrhythmia can be atrial or ventricular. It can be a serious condition that can cause sudden death if left untreated. Within the floor of the right atrium is a second pacemaker, the atrioventricu- lar (AV) node . It receives the impulse from the SA node and transmits it down- ward to both ventricles via the bundle of His located within the septum and then to Purkinje fibers distributed through the septum and the inner walls of the ven- tricles. When the electrical impulse is transmitted throughout your ventricles, all ventricular muscle fibers contract in unison. This contraction occurs just slightly after the contraction of the atria, and the combination of the two results in one complete heartbeat. This entire process is repeated with each heartbeat. The contraction and relaxation of the four heart chambers, known as the cardiac cycle , creates each heartbeat. Each cardiac cycle is quite rapid, taking an average of just 0.8 seconds. Blood pressure, created by the pumping of blood, is written as two numbers, one over the other: 120/80. The upper number, the systolic pressure, reflects the highest pressure exerted against artery walls during ventricular contraction, or systole . The lower number, the diastolic pres- sure, reflects the lowest pressure exerted against artery walls during ventricular relaxation, or diastole . Pulse points are locations where an artery with a strong pulse is near the skin and the pulse can be felt easily (Figure 6-5). Pulse is also known as heart rate
Flashpoint
Blood-pressure readings reflect the amount of pressure exerted against the arterial walls during the ventricular contraction and ventricular relaxation phases of the cardiac cycle. A reading of 130/80 or higher is considered high blood pressure by the American Heart Association.
Powered by FlippingBook