McKenna's Pharmacology for Nursing, 2e

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

CIRCULATION The purpose of the heart’s continual pumping action is to keep blood flowing to and from all of the body’s tissues and cells. Blood delivers oxygen and much- needed nutrients to the cells for producing energy, and it carries away carbon dioxide and other waste products of metabolism. The steady circulation of blood is essential for the proper functioning of all of the body’s organs, including the heart. The circulation of the blood follows two courses: • Heart–lung or pulmonary circulation : The right side of the heart sends blood to the lungs, where carbon dioxide and some waste products are removed from the blood and oxygen is picked up by the red blood cells. • Systemic circulation : The left side of the heart sends oxygenated blood out to all of the cells in the body. In addition, the heart muscle, like any other muscle, requires adequate oxygen and nutrients to function. This is accomplished via coronary circulation. The blood moves from areas of high pressure to areas of lower pressure. The system is a “closed” system;

that is, it has no openings or holes that would allow blood to leak out. The closed nature of the system is what keeps the pressure differences in the proper rela­ tionship so that blood always flows in the direction in which it is intended to flow (Figure 42.6). Pulmonary circulation The right atrium is a very low pressure area in the car­ diovascular system. All of the deoxygenated blood from the body flows into the right atrium from the inferior and superior venae cavae (see Figure 42.1) and from the great cardiac vein, which returns deoxygenated blood from the heart muscle. As the blood flows into the atrium, the pressure increases. When the pressure becomes greater than the pressure in the right ventricle, most of the blood flows into the right ventricle; this is called the rapid-filling phase. At this point in the cardiac cycle, the atrium is stimulated to contract and pushes the remaining blood into the right ventricle. The ventri­ cle is then stimulated to contract; it generates pressure that opens the pulmonic valve (see Figure 42.1) and sends blood into the pulmonary artery, which takes the blood into the lungs, a very low pressure area. The

PULMONARY CIRCULATION

Veins— distensible, thin walls

Pulmonary vessels— distensible, thin walls

Arteries— elastic, thick walls

SYSTEMIC CIRCULATION

FIGURE 42.6  Blood flow through the systemic and pulmonary vasculature circuits.