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2. HEART Blood supply to the heart muscle
Blood, which supplies oxygen and nutrients to the myocardium of the heart, flows through the right and left coronary arteries (Figure 12-5); called coronary circulation
Blockage of blood flow through the coronary arteries is called myocardial infarction (heart attack)
Angina pectoris—chest pain caused by inadequate oxygen to the heart
Coronary bypass surgery—veins from other parts of the body are used to bypass blockages in coronary arteries (Figure 12-6)

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5. HEART
Cardiac cycle
Heartbeat is regular and rhythmic—each complete beat is called a cardiac cycle—average is about 72 beats per minute (normal heart rate BPM)
Each cycle, about 0.8 seconds long, is subdivided into systole (contraction phase) and diastole (relaxation phase)
Stroke volume—volume of blood ejected from one ventricle with each beat
Cardiac output—amount of blood that one ventricle can pump each minute; average is about 5 L per minute at rest

6. HEART Conduction system of the heart (Figure 12-7)
Intercalated disks are electrical connectors that join all the cardiac muscle fibers in a region together so that they receive their impulse, and thus contract, at about the same time
SA (sinoatrial) node, the pacemaker—located in the wall of the right atrium near the opening of the superior vena cava
AV (atrioventricular) node—located in the right atrium along the lower part of the interatrial septum
AV bundle (bundle of His)—located in the septum of the ventricle
Purkinje fibers—located in the walls of the ventricles

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8. HEART Electrocardiogram [ECG or EKG] (Figure 12-8)
Specialized conduction system structures generate and transmit the electrical impulses that result in contraction of the heart
These tiny electrical impulses can be picked up on the surface of the body and transformed into visible tracings by a machine called an electrocardiograph
The visible tracing of these electrical signals is called an electrocardiogram, or ECG
The normal ECG has three deflections or waves
P wave—associated with depolarization of the atria
QRS complex—associated with depolarization of the ventricles
T wave—associated with repolarization of the ventricles

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10. BLOOD VESSELS Types Arteries—carry blood away from the heart
Veins—carry blood toward the heart
Capillaries—carry blood from the arterioles to the venules

11. BLOOD VESSELS Structure (Figure 12-9) Arteries
Tunica intima—inner layer of endothelial cells
Tunica media—smooth muscle with some elastic tissue, thick in arteries; important in blood pressure regulation
Tunica externa—thin layer of fibrous elastic connective tissue
Capillaries—microscopic vessels with only one layer—tunica intima
Veins
Tunica intima—inner layer; valves prevent retrograde movement of blood
Tunica media—smooth muscle; thin in veins
Tunica externa—heavy layer of fibrous connective tissue in many veins