McKenna's Pharmacology for Nursing, 2e

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

in a pattern resembling a crown, which is why they are called “coronary” arteries. Cardiac: Myocardial blood flow The left coronary artery arises from the left side of the aorta and bifurcates, or divides, into two large vessels called the left circumflex artery (which travels down the left side of the heart and feeds most of the left ventricle) and the left anterior descending coronary artery (which travels down the front of the heart and feeds the septum and anterior areas, including much of the conduction system). The artery arising from the right side of the aorta, called the right coronary artery, supplies most of the right side of the heart, including the SA node. The coronary arteries receive blood during diastole, when the muscle is at rest and relaxed so that blood can flow freely into the muscle. When the ventricle contracts, it forces the aortic valve open, which in turn causes the leaflets of the valve to cover the openings of the coronary arteries. When the ventricles relax, the blood is no longer pumped forwards and starts to flow back towards the ventricle. The blood flowing down the sides of the aorta closes the aortic valve and fills the coronary arteries. The pressure that fills the coronary arteries is the difference between the systolic (ejection) pressure and the diastolic (resting) pressure. This is called the pulse pressure (systolic minus diastolic blood pressure readings). The pulse pressure is monitored clinically to evaluate the filling pressure of the coronary arteries. The oxygenated blood that is fed into the heart by the coronary circulation reaches every cardiac muscle fibre as the vessels divide and subdivide throughout the myo­ cardium (Figure 42.8). The heart has a pattern of circulation called end- artery circulation. The arteries go into the muscle and

end without a great deal of backup or collateral circula­ tion. Normally this is an efficient system and is able to meet the needs of the heart muscle. The heart’s supply of and demand for oxygen are met by changes in the delivery of oxygen through the coronary artery system. Problems can arise, however, when an imbalance develops between the supply of oxygen delivered to the heart muscle and the myocardial demand for oxygen. The main forces that determine the heart’s use of oxygen or oxygen consumption include the following: • Heart rate : The more the heart has to pump, the more oxygen it requires. • Preload (amount of blood that is brought back to the heart to be pumped throughout the body) : The more blood that is returned to the heart, the harder it will have to work to pump the blood around. The volume of blood in the system is a determinant of preload. • Afterload (resistance against which the heart has to beat) : The higher the resistance in the system, the harder the heart will have to contract to force open the valves and pump the blood along. Blood pressure is a measure of afterload. • Stretch on the ventricles : If the ventricular muscle is stretched before it is stimulated to contract, more actomyosin bridges will be formed, which will take more energy; alternatively, if the muscle is stimulated to contract harder than usual (which happens with The muscle can be stretched, as in ventricular hyper­ trophy related to chronic hypertension or cardiac muscle damage, or in heart failure when the ventricle does not empty completely and blood backs up in the system. The supply of blood to the myocardium can be altered if the heart fails to pump effectively and cannot deliver blood to the coronary arteries. This happens in sympathetic stimulation), more bridges will be formed, which also will require more energy.

Right pulmonary veins Aortic valve

Superior vena cava

Aortic arch

Coronary sinus Left pulmonary veins

Superior vena cava

Right atrium

Left auricle Circumflex branch of left coronary artery

Right auricle

Inferior vena cava

Left circumflex branch Anterior descending branch of left coronary artery Left ventricle

Right coronary artery

Right ventricle Posterior descending

Right ventricle

branch of right coronary artery

FIGURE 42.8  Coronary arteries and veins. A. Anterior view. B. Posterior view.

A

B