McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 4 2 Introduction to the cardiovascular system

troponin: chemical in heart muscle that prevents the reaction between actin and myosin, leading to muscle relaxation; it is inactivated by calcium during muscle stimulation to allow actin and myosin to react, causing muscle contraction veins: vessels that return blood to the heart; distensible tubes ventricle: bottom chamber of the heart, which contracts to pump blood out of the heart

T he cardiovascular system is responsible for delivering oxygen and nutrients to all of the cells of the body and for removing waste products for excretion. The cardio­ vascular system consists of a pump—the heart—and an interconnected series of vessels that continually move blood throughout the body.

Aorta

Pulmonary arteries

Superior vena cava

Pulmonic valve

Pulmonary veins

STRUCTURE AND FUNCTION OF THE HEART

Right atrium

Left atrium

The heart is a hollow, muscular organ that is divided into four chambers. The heart may actually be viewed as two joined hearts: a right heart and a left heart, each of which is divided into two parts, an upper part called the atrium (literally “porch” or entryway) and a lower part called the ventricle . Attached to each atrium is an appendage called the auricle , which collects blood that is then pumped into the ventricles by atrial contraction. The right auricle is quite large; the left auricle is very small. The ventri­ cles pump blood out of the heart to the lungs or the body. Between the atria and ventricles are two cardiac valves—thin tissues that are anchored to an annulus, or fibrous ring, which also gives the hollow organ some structure and helps to keep the organ open and divided into distinct chambers. A partition called a septum separates the right half of the heart from the left half. The right half receives deoxygenated blood from everywhere in the body through the veins (vessels that carry blood towards the heart) and directs that blood into the lungs. The left half receives the now-oxygenated blood from the lungs and directs it into the aorta. The aorta delivers blood into the systemic circulation by way of arteries (vessels that carry blood away from the heart) (Figure 42.1). The aorta delivers blood into the systemic circulation by way of arteries. The circulatory system is composed of about 96,500 kilometres of interconnecting blood vessels that carry the needed oxygen and nutrients to the cells and carry away the metabolic waste products from the tissues. Cardiac cycle The heart, a muscle that contracts thousands of millions of times in a lifetime, possesses structural and func­ tional properties that are different from those of other muscles. The fibres of the cardiac muscle, or myocard­ ium , form two intertwining networks called the atrial

Tricuspid valve

Mitral valve

Inferior vena cava

Left ventricle

Right ventricle

and ventricular syncytia . These interlacing structures enable the atria and then the ventricles to contract syn­ chronously when excited by the same stimulus. Cardiac: Cardiac cycle Simultaneous contraction is a necessary property for a muscle that acts as a pump. A hollow pumping mecha­ nism must also pause long enough in the pumping cycle to allow the chambers to fill with fluid. The heart muscle relaxes long enough to ensure adequate filling; the more completely it fills, the stronger is the subsequent con­ traction. This occurs because the muscle fibres of the heart, stretched by the increased volume of blood that has returned to them, spring back to normal size. This is similar to the stretching of a rubber band, which returns to its normal size after it is stretched—the further it is stretched, the stronger is the spring back to normal. This property is defined through Starling’s law of the heart . During diastole —the period of cardiac muscle relax­ ation—blood returns to the heart from the systemic and pulmonary veins, flowing into the right and left atria, respectively. When the pressure generated by the blood FIGURE 42.1  Blood flow into and out of the heart. Deoxygenated blood enters the right atrium from the great cardiac vein and the superior and inferior venae cavae and flows through the tricuspid valve into the right ventricle, which contracts and sends the blood through the pulmonic valve into the pulmonary artery and to the lungs. Oxygenated blood from the lungs enters the left atrium through the pulmonary veins and passes through the mitral valve into the left ventricle, which contracts and ejects the blood through the aortic valve into the aorta and out to the systemic circulation.