blood to cells and absorb waste products such as CO 2 , urea, creatinine, and
ammonia. The deoxygenated blood is returned to the circulation by the venules
and veins for elimination of waste products by the lungs and kidneys. The pump-
ing action of the heart circulates blood through blood vessels.
The heart is divided into four chambers.1.Right atrium. The right atrium receives deoxygenated blood from the
circulation.
2.Right ventricle. The right ventricle pumps unoxygenated blood to the pul-
monary artery to the lungs for gas exchange (CO 2 for O 2 ).
3.Left atrium. The left atrium receives oxygenated blood via the pulmonary
vein.
4.Left ventricle. The left ventricle pumps the blood into the aorta for sys-
temic circulation.THE HEART
The heart muscle, called the myocardium, has a fibrous covering called the peri-
cardium. The endocardium is a three-layered membrane that lines the interior of
the heart chambers.
The heart has four valves—two atrioventricular (tricuspid and mitral) and two
semilunar (pulmonic and aortic). Atrioventricular valves control bloodflow
between the atria and ventricles. Semilunar valves control the bloodflow between
the ventricles and the pulmonary artery and the aorta.
There are three major coronary arteries: the right, left, and circumflex. Each
provides nutrients to the myocardium. Blockage in one or more of these arteries
can result in a myocardial infarction (“heart attack.”)
The myocardium is capable of generating and conducting its own electrical
impulses. The impulse begins in the sinoatrial (SA) node and moves to the atri-
oventricular (AV) node. The heart beats about 60 to 80 beats a minute. The ven-
tricle can beat independently at a rate of about 30 to 40 beats per minute.
Drugs can affect cardiac contraction by stimulating or inhibiting the heart.
Contractions are also influenced by the autonomic nervous system (ANS). The
sympathetic nervous system increases heart rate and the parasympathetic ner-
vous system decreases heart rate. (See Chapter 15.)BLOOD PRESSURE AND CARDIAC PERFORMANCE
Resistance develops as the blood travels through the circulatory system. This
resistance is known as blood pressure. The average systemic arterial pressure(^348) CHAPTER 19 Cardiac Circulatory Medications